Showing 1–50 of 89 results for author: Rawat, R

Microscopic Origin of Random Singlet Behavior in B-site Disordered Spin-1/2 Perovskite BaCu_1/3Nb_2/3O_3 Revealed by EXAFS and Thermodynamics

Authors: Sagar Mahapatra, Francesco De Angelis, Martin Etter, Edmund Welter, M. P. Saravanan, Rajeev Rawat, Carlo Meneghini, Surjeet Singh

Abstract: We report a combined structural and thermodynamic study of the ABO$_3$-type disordered perovskite BaCu$_{1/3}$Nb$_{2/3}$O$_3$ (BCNO), whose B site is jointly occupied by Cu and Nb in the $1:2$ ratio. Using synchrotron powder x-ray diffraction (XRD) and x-ray absorption fine structure (XAFS) spectroscopy, we investigate the microscopic nature of Cu$^{2+}$/Nb$^{5+}$ disorder on the pseudo-cubic B-su… ▽ More We report a combined structural and thermodynamic study of the ABO$_3$-type disordered perovskite BaCu$_{1/3}$Nb$_{2/3}$O$_3$ (BCNO), whose B site is jointly occupied by Cu and Nb in the $1:2$ ratio. Using synchrotron powder x-ray diffraction (XRD) and x-ray absorption fine structure (XAFS) spectroscopy, we investigate the microscopic nature of Cu$^{2+}$/Nb$^{5+}$ disorder on the pseudo-cubic B-sublattice and its relation to the emergent random-singlet (RS) behavior evidenced at low temperatures. While XRD reveals no long-range Cu/Nb ordering and average site occupancy consistent with stoichiometry, XAFS reveals a peculiar local chemical order characterized by preferential heteroatomic Cu$:$Nb correlations. This local arrangement strongly suppresses direct Cu$:$Cu linkages, despite the Cu concentration being close to the percolation threshold of a cubic lattice. The resulting exchange network explains the absence of spin-glass freezing or long-range magnetic order in the presence of substantial antiferromagnetic interactions, as indicated by a Curie-Weiss temperature $Θ_{CW}\approx -50$ K. Instead, the magnetic susceptibility $χ(T)$ and specific heat $c_p(T)$ exhibit power-law behavior and characteristic single-parameter $T/H$ scaling over broad temperature and magnetic-field ranges, consistent with random-singlet phenomenology. Notably, at very low temperatures, the specific heat behavior transitions from $T^{1-γ}$ ($γ\approx 0.6$ from the $T/H$ scaling) in zero-field to a T-linear dependence under high field, indicating a crossover to a distinct low-energy regime whose microscopic origin remains to be established. △ Less

Submitted 4 August, 2026; originally announced August 2026.

Comments: Under review

Probing Interfacial Spin Dynamics and Temperature Dependent Asymmetry in Spin Pumping Across Ni80Fe20/Cu/Cr1.12Te2 Interfaces

Authors: Mahammad Tahir, Swati Pandey, Sourabh Manna, Rajdeep Singh Rawat, Rohit Medwal, Soumik Mukhopadhyay

Abstract: Spin transfers in magnetic multilayers offers a promising pathway toward ultrafast, energy-efficient spintronic devices. In this study, we investigate the interfacial spin pumping and temperature-dependent spin current exchange in a Cr1.12Te2/Cu/Ni80Fe20 (Py)(FM1/NM/FM2) trilayer structure. Using broadband and cryogenic ferromagnetic resonance (FMR) measurements, we investigate key magnetization d… ▽ More Spin transfers in magnetic multilayers offers a promising pathway toward ultrafast, energy-efficient spintronic devices. In this study, we investigate the interfacial spin pumping and temperature-dependent spin current exchange in a Cr1.12Te2/Cu/Ni80Fe20 (Py)(FM1/NM/FM2) trilayer structure. Using broadband and cryogenic ferromagnetic resonance (FMR) measurements, we investigate key magnetization dynamical parameters, including the effective Gilbert damping factor, effective magnetic fields, interfacial spin mixing conductance, and spin current density. Efficient spin angular momentum transfers from Py to Cr1.12Te2 are observed at room temperature. At lower temperatures, the enhanced linewidth reflects temperature dependent spin pumping effects occurring at distinct precession frequencies of the ferromagnetic layers. Notably, the absence of interfacial Damping indicates that spin pumping can be modulated by controlling the net spin current flow. These findings offer critical insight into temperature-dependent tunable spin transport mechanisms in magnetic multilayers, highlighting their potential for next-generation spintronic applications. △ Less

Submitted 30 January, 2026; originally announced January 2026.

Comments: 10 pages, 7 figures

Emergent Random Spin Singlets in Disordered Spin-1/2 perovskite BaCu$_{1/3}$Ta$_{2/3}$O$_3$

Authors: Sagar Mahapatra, Francesco De Angelis, Dibyata Rout, Priyanshi Tiwari, Martin Etter, Edmund Welter, M. P. Saravanan, Rajeev Rawat, Satoshi Nishimoto, Carlo Meneghini, Surjeet Singh

Abstract: We investigate the disordered perovskite BaCu$_{1/3}$Ta$_{2/3}$O$_3$, where Cu (spin-1/2) and Ta randomly occupy a pseudo-cubic lattice. Synchrotron X-ray diffraction and X-ray absorption spectroscopy establish the local nature of the disorder, revealing the presence of structurally constrained magnetic exchange paths. No magnetic ordering or spin freezing is observed down to 0.1 K. The low-temper… ▽ More We investigate the disordered perovskite BaCu$_{1/3}$Ta$_{2/3}$O$_3$, where Cu (spin-1/2) and Ta randomly occupy a pseudo-cubic lattice. Synchrotron X-ray diffraction and X-ray absorption spectroscopy establish the local nature of the disorder, revealing the presence of structurally constrained magnetic exchange paths. No magnetic ordering or spin freezing is observed down to 0.1 K. The low-temperature magnetic and thermodynamic behavior is captured by a broad but non-singular distribution $P(J)$ of exchange couplings $J$. These results open the possibility of realizing a disordered quantum ground state where the exchange randomness is broad yet intrinsically bounded, departing from the conventional infinite-randomness fixed point driven random-singlet phase. △ Less

Submitted 24 January, 2026; originally announced January 2026.

Electrical- and magneto-transport across the thermo-elastic martensitic transformation in anti-site-disordered off-stoichiometric Co-Fe-Ti-Si Heusler alloy thin films

Authors: Mainur Rahaman, Lanuakum A Longchar, Rajeev Joshi, R. Rawat, M. Manivel Raja, S. N. Kaul, S. Srinath

Abstract: In this work, we systematically investigate the effect of Anti-site Disorder (ASD) on electrical resistivity $ρ(T)$ and transverse magnetoresistance $MR_{\perp}$ in off-stoichiometric Co-Fe-Ti-Si (CFTS) thin films across the thermo-elastic martensitic phase transformation (MPT). The CFTS films with A2 ASD exhibit a negative temperature coefficient of resistivity (n-TCR) and an upturn below… ▽ More In this work, we systematically investigate the effect of Anti-site Disorder (ASD) on electrical resistivity $ρ(T)$ and transverse magnetoresistance $MR_{\perp}$ in off-stoichiometric Co-Fe-Ti-Si (CFTS) thin films across the thermo-elastic martensitic phase transformation (MPT). The CFTS films with A2 ASD exhibit a negative temperature coefficient of resistivity (n-TCR) and an upturn below $\sim 30\,$K. In sharp contrast, the partially L2$_1$-ordered films are metallic in nature, characterized by a resistivity minimum at low temperatures ($T_{\min} \cong 30\,$K) and a positive TCR for $T > T_{\min}$. The change in the sign of TCR finds a straightforward explanation in terms of the competition between the quantum corrections (weak localization, electron-diffuson scattering) and the ballistic scattering mechanisms (electron-magnon and electron-phonon). We find that, stronger the atomic ASD, more prominent the quantum corrections and the weaker the scattering of $e-m$ and $e-p$ scattering. All the CFTS films exhibit a distinct thermal hysteresis and a significant drop in resistivity, symptomatic of a MPT, near the characteristic temperatures: martensite-end $T_{Me} \cong 300\,$K and austenite-begin $T_{\mathrm{Ab}} \cong 325\,$K. Regardless of the strength of ASD, in the martensite phase the anti-symmetric (ASMR) component of $\mathrm{MR}_{\perp}(H)$ dominates over the symmetric (SMR) counterpart, whereas the reverse is true (i.e. SMR $\gg$ ASMR) for the austenite phase at temperatures $T_{\mathrm{Ab}} \cong 325\,$K $\le T \le 375\,$K, where $\mathrm{MR}_{\perp}$ increases very sharply with temperature as the austenite phase grows rapidly at the expense of the martensite phase. The present results assert that the CFTS Heusler alloy thin films are promising candidates for shape-memory devices and for spintronic applications such as spin valves. △ Less

Submitted 25 November, 2025; v1 submitted 8 May, 2025; originally announced May 2025.

Evidence of magnetoelastic coupling and magnetic phase coexistence in Mn$_{1.7}$Fe$_{1.3}$Si Heusler Alloy

Authors: Kulbhushan Mishra, Elaine T. Dias, Rajeev Joshi, A. D. Fortes, Christopher M. Howard, Rajeev Rawat, P. A. Bhobe

Abstract: Noncollinear metallic antiferromagnets, with their rapid spin dynamics, efficient spin transport, and distinctive spin textures, play a pivotal role in advancing the field of spintronics. In this study, we report a comprehensive investigation of the structural, magnetic, and transport properties of cubic Mn$_{1.7}$Fe$_{1.3}$Si Heusler compound. Temperature-dependent magnetization measurement revea… ▽ More Noncollinear metallic antiferromagnets, with their rapid spin dynamics, efficient spin transport, and distinctive spin textures, play a pivotal role in advancing the field of spintronics. In this study, we report a comprehensive investigation of the structural, magnetic, and transport properties of cubic Mn$_{1.7}$Fe$_{1.3}$Si Heusler compound. Temperature-dependent magnetization measurement reveals a paramagnetic to ferromagnetic transition at $T_C$ = 85 K, followed by a spin reorientation transition. Neutron diffraction data, analyzed as a function of temperature, demonstrates that the occurrence of a spin-reorientation transition is accompanied by magnetoelastic coupling, as evidenced by a change in unit cell volume below $T_C$. Magnetic structure refinement of the low-temperature neutron powder diffraction data confirms the canted antiferromagnetic ordering below 55 K. The metallic nature of the sample is confirmed by the gradual decrease in the $ρ$(T) with decreasing temperature. At low temperatures, a field-induced metamgnetic transition is observed in both, magnetization and magneto-transport measurements. The $H-T$ phase diagram shows a phase-coexistence region emerging at low temperatures for H $<$ 2.5 T. These findings provide valuable insights into the magnetic and transport behavior of the Heusler compounds, underscoring their potential for spintronic applications. △ Less

Submitted 13 February, 2025; originally announced February 2025.

Comments: 11 pages, 10 figures

Half-metallicity and wandering axis ferromagnetism in Fe$_2$Ti$_{1-x}$Mn$_x$Sn (0 $< x \leq$ 0.3) Heusler Alloys

Authors: Kulbhushan Mishra, Shishir Kumar Pandey, S. Chaudhuri, Rajeev Rawat, P. A. Bhobe

Abstract: We investigate the effect of Mn substitution in Fe$_2$Ti$_{1-x}$Mn$_x$Sn on electronic structure and magnetic and electrical transport properties. The spin-polarized density of states calculations using density-functional theory (DFT) yields a half-metallic ground state in Mn-rich compositions. Localized magnetic moments at Mn sites interacting through the cloud of conduction electrons formed by F… ▽ More We investigate the effect of Mn substitution in Fe$_2$Ti$_{1-x}$Mn$_x$Sn on electronic structure and magnetic and electrical transport properties. The spin-polarized density of states calculations using density-functional theory (DFT) yields a half-metallic ground state in Mn-rich compositions. Localized magnetic moments at Mn sites interacting through the cloud of conduction electrons formed by Fe and Ti atoms are also predicted. Electrical resistivity and magneto-transport measurements reveal a Kondo-like ground state at low temperatures and a peculiar linear negative temperature coefficient of resistance in the high-temperature regime with a predominant electron-phonon scattering mechanism. Analysis of room temperature powder X-ray diffraction data reveals a highly ordered L2$_1$ structure and reduction of antisite disorder upon Mn substitution. The temperature-dependent magnetization measurements reveal distinct features indicative of weak anisotropy in the system. Isothermal magnetization measured as a function of the applied field helps identify the unique magnetic ground state of the half-metallic Fe$_2$Ti$_{1-x}$Mn$_x$Sn composition as a ferromagnet with a wandering axis that distinctively orients in the direction of the applied magnetic field. The measurement of X-ray absorption fine structure (XAFS) reveals that the random anisotropy arises due to the local lattice distortion around Mn atoms in the prepared compositions. Our findings thus provide a new perspective for studying the mechanism of half-metallicity and associated magnetic order in Heuslers. △ Less

Submitted 14 February, 2025; v1 submitted 16 July, 2024; originally announced July 2024.

Comments: 13 pages, 13 figures

Journal ref: Phys. Rev. B 111, 235146 (2025)

Atypical antiferromagnetic ordering in single crystalline quasi-2D honeycomb magnet YbI$_3$

Authors: Nashra Pistawala, Luminita Harnagea, Sitaram Ramakrishnan, Priyanshi Tiwari, M. P. Saravanan, Rajeev Rawat, Surjeet Singh

Abstract: Here, we study YbI$_3$, a quasi-2D layered material with Yb atoms arranged on an ideal honeycomb network of edge-sharing YbI$_6$ octahedra, analogous to the low-temperature phase of $α-$RuCl$_3$. High quality single crystals of YbI$_3$ are grown from Yb and I as starting precursors, using the vapor transport technique. The grown crystals are characterized by single crystal x-ray diffraction, Raman… ▽ More Here, we study YbI$_3$, a quasi-2D layered material with Yb atoms arranged on an ideal honeycomb network of edge-sharing YbI$_6$ octahedra, analogous to the low-temperature phase of $α-$RuCl$_3$. High quality single crystals of YbI$_3$ are grown from Yb and I as starting precursors, using the vapor transport technique. The grown crystals are characterized by single crystal x-ray diffraction, Raman spectroscopy, magnetization, and heat capacity probes. The crystal-field split ground state of Yb$^{3+}$ in \Yb~ is a well-isolated Kramers doublet with an effective moment $\rm J_{eff} = 1/2$. Upon cooling, the low-temperature heat capacity of \Yb~ reveals a broad peak at $\rm T_1 = 0.95$~K due to short-range ordering of the Yb moments, followed by a sharp peak at $\rm T_2 = T_N = 0.6$~K due to long-range ordering. The magnetic behavior is found to be weakly anisotropic with $χ^\parallel > χ^\perp$, where $χ^\parallel$ and $χ^\perp$ refers to the in-plane ($H \parallel ab$) and out-of-plane ($H \perp ab$) susceptibilities. The 2~K isothermal magnetization saturates at $\rm \approx~1.5~μ_B/Yb^{3+}$ (in-plane) and $\rm \approx~1~μ_B/Yb^{3+}$ (out-of-plane), suggesting the anisotropy to be easy-plane type. Low-temperature heat capacity, well below T$_N$, is found to vary as T$^α$ with $α~\approx~2.5$, indicating a possible unconventional magnetic ground state for YbI$_3$. △ Less

Submitted 2 July, 2024; originally announced July 2024.

Comments: 10 pages, 5 figures, Supplemental Material available on request

Investigation of Spin-Pumping and -Transport in the Ni80Fe20/Pt/Co Asymmetric Trilayer

Authors: Shilpa Samdani, Yaqi Rong, Birte Coester, Amit Kumar Shukla, Lew Wen Siang, Yumeng Yang, Rajdeep Singh Rawat

Abstract: FM1/NM/FM2 trilayers have garnered considerable attention because of their potential in spintronic applications. A thorough investigation of the spin transport properties of these trilayers is therefore important. Asymmetric trilayers, particularly those including Platinum (Pt) as a spacer are less explored. Pt mediates exchange coupling between the two FM layers and thus offers a unique platform… ▽ More FM1/NM/FM2 trilayers have garnered considerable attention because of their potential in spintronic applications. A thorough investigation of the spin transport properties of these trilayers is therefore important. Asymmetric trilayers, particularly those including Platinum (Pt) as a spacer are less explored. Pt mediates exchange coupling between the two FM layers and thus offers a unique platform to investigate the spin-transport properties under indirect exchange coupling conditions through the spin-pumping mechanism. Our analytical focus on the acoustic mode of the ferromagnetic resonance spectrum, facilitated by the distinct magnetizations of the Ni80Fe20 and Co layers, allows for the isolation of individual layer resonances. The derived spin-pumping induced damping of the Ni80Fe20 and Co layers reveals a direct dependence on the Pt spacer thickness. Furthermore, fitting of the weighted average of the damping parameters to the spin-pumping induced damping of acoustic mode reveals that the observed FMR spectra is indeed a result of the in-phase precession of the magnetizations in two FM layers. The extracted effective spin-mixing conductance varies with the FM/NM interface, specifically 1.72x10^19 m^(-2) at the Ni80Fe20/Pt and 4.07x10^19 m^(-2) at the Co/Pt interface, indicating a strong correlation with interfacial characteristics. Additionally, we deduce the spin diffusion length in Pt to be between 1.02 and 1.55 nm and calculate the interfacial spin transparency and spin current densities, highlighting significant disparities between the Ni80Fe20/Pt and Co/Pt interfaces. This detailed analysis enhances our understanding of spin transport in Ni80Fe20/Pt/Co trilayers. It offers insights important for advancing spintronic device design and lays the groundwork for future theoretical investigations of trilayer system. △ Less

Submitted 18 April, 2024; originally announced April 2024.

Comments: 16 pages, 11 figures

Phase Binarization in Mutually Synchronized Bias Field-free Spin Hall Nano-oscillators for Reservoir Computing

Authors: Sourabh Manna, Rohit Medwal, John Rex Mohan, Yasuhiro Fukuma, Rajdeep Singh Rawat

Abstract: Mutually coupled spin Hall nano-oscillators (SHNO) can exhibit binarized phase state, offering pathways to realize Ising machines and efficient neuromorphic hardware. Conventionally, phase binarization is achieved in coupled SHNOs via injecting an external microwave at twice of the oscillator frequency in presence of a biasing magnetic field. However, this technology poses potential challenges of… ▽ More Mutually coupled spin Hall nano-oscillators (SHNO) can exhibit binarized phase state, offering pathways to realize Ising machines and efficient neuromorphic hardware. Conventionally, phase binarization is achieved in coupled SHNOs via injecting an external microwave at twice of the oscillator frequency in presence of a biasing magnetic field. However, this technology poses potential challenges of higher energy consumption and complex circuit design. Moreover, fabrication-induced mismatch in SHNO dimensions may hinder mutual synchronization. Addressing these challenges, we demonstrate purely DC current-driven mutual synchronization and phase binarization of two non-identical nanoconstriction SHNOs without biasing magnetic field and microwave injection. We thoroughly investigate these phenomena and underlying mechanisms using micromagnetic simulation. We further demonstrate the bias field-free synchronized SHNO pair efficiently performing a reservoir computing benchmark learning task: sin and square wave classification, utilizing current tunable phase binarization. Our results showcase promising magnetization dynamics of coupled bias field-free SHNOs for future computing applications. △ Less

Submitted 5 April, 2024; originally announced April 2024.

Comments: Supplementary data is attached as an appendix to the main manuscript. Submitted to Physical Review B

Direct visualization of local magnetic domain dynamics in a 2D Van der Walls material/ferromagnet interface

Authors: Joseph Vimal Vas, Rohit Medwal, Sourabh Manna, Mayank Mishra, Aaron Muller, John Rex Mohan, Yasuhiro Fukuma, Martial Duchamp, Rajdeep Singh Rawat

Abstract: Exploring new strategies for controlling the magnetic domain propagation is the key to realize ultrafast, high-density domain wall-based memory and logic devices for next generation computing. These strategies include strain modulation in multiferroic devices, geometric confinement and area-selective pinning of domain wall. 2D Van der Waals materials introduce localized modifications to the interf… ▽ More Exploring new strategies for controlling the magnetic domain propagation is the key to realize ultrafast, high-density domain wall-based memory and logic devices for next generation computing. These strategies include strain modulation in multiferroic devices, geometric confinement and area-selective pinning of domain wall. 2D Van der Waals materials introduce localized modifications to the interfacial magnetic order, enabling control over the propagation of magnetic domains. Here, using Lorentz-Transmission Electron Microscopy (L-TEM) along with the Modified Transport of Intensity equations (MTIE), we demonstrate controlled domain expansion with in-situ magnetic field in a ferromagnet (Permalloy, NiFe) interfacing with a 2D Van der Waals material Graphene (Gr). The Gr/NiFe interface exhibits distinctive domain expansion rate with magnetic field selectively near the interface which is further analyzed using micromagnetic simulations. Our findings are crucial for comprehending direct visualization of interface controlled magnetic domain expansion, offering insights for developing future domain wall-based technology. △ Less

Submitted 3 April, 2024; originally announced April 2024.

Comments: Merged Manuscript and supplementary file. Submitted to Communications Physics (under review)

Investigating the cause of crossover from charge/spin stripe insulator to correlated metallic phase in layered T' nickelates -- R$_4$Ni$_3$O$_8$

Authors: Dibyata Rout, Sanchayeta Ranajit Mudi, Suman Karmakar, Rajeev Rawat, Surjeet Singh

Abstract: The $T^{\prime}$ infinite layered nickelates have recently garnered significant attention owing to the discovery of superconductivity in hole-doped RNiO$_2$ (R $=$ La, Pr, or Nd), which is the $n = \infty$ member of the series R$_{n+1}$Ni$_n$O$_{2n+2}$. Here, we investigate the $n = 3$ member, namely R$_4$Ni$_3$O$_8$ (R $=$ La, Pr, or Nd) of this family. The compound La$_4$Ni$_3$O$_8$ exhibits sim… ▽ More The $T^{\prime}$ infinite layered nickelates have recently garnered significant attention owing to the discovery of superconductivity in hole-doped RNiO$_2$ (R $=$ La, Pr, or Nd), which is the $n = \infty$ member of the series R$_{n+1}$Ni$_n$O$_{2n+2}$. Here, we investigate the $n = 3$ member, namely R$_4$Ni$_3$O$_8$ (R $=$ La, Pr, or Nd) of this family. The compound La$_4$Ni$_3$O$_8$ exhibits simultaneous charge/spin-stripe ordering at T$_N^\ast$ $=$ 105 K, which is also concomitant with the onset of metal-to-insulator (MIT) transition upon lowering the temperature below T$_N^\ast$. We investigate the conspicuous absence of this transition in the Pr and Nd analogues of La$_4$Ni$_3$O$_8$. To achieve this purpose, we synthesized solid-solutions of the form (La, Pr)$_4$Ni$_3$O$_8$ and (La, Nd)$_4$Ni$_3$O$_8$ and examined the behavior of T$_N^\ast$ as a function of the average R-site ionic radius ($r_{\overline{R}}$). We show that after an initial quasilinear decrease with decreasing $r_{\overline{R}}$, T$_N^\ast$ suddenly vanishes in the narrow range 1.134 $Å$ $\leq$ $r_{\overline{R}}$ $\leq$ 1.143 $Å$. In the same range, we observed the emergence of a new transition below T$^\ast$, whose onset temperature increases as $r_{\overline{R}}$ further decreases. We, therefore, argue that the sudden vanishing of charge/spin-stripe/MIT ordering upon decreasing $r_{\overline{R}}$ is due to the appearance of a new competing phase. The point $r_{\overline{R}}$ $\approx$ $r_c$, where T$_N^\ast$ vanishes and T$^\ast$ appears -- a quantum critical point -- should be investigated further. In this regard, Pr$_4$Ni$_3$O$_8$ and Pr-rich samples should be useful due to the weak magnetization response associated with the Pr-sublattice, as shown here. △ Less

Submitted 9 November, 2023; originally announced November 2023.

Comments: 18 pages, 16 figures

Reconfigurable neural spiking in bias field-free spin Hall nano oscillator

Authors: Sourabh Manna, Rohit Medwal, Rajdeep Singh Rawat

Abstract: In this study, we theoretically investigate neuron-like spiking dynamics in an elliptic ferromagnet/heavy metal bilayer-based spin Hall nano oscillator (SHNO) in bias field-free condition, much suitable for practical realization of brain inspired computing schemes. We demonstrate regular periodic spiking with tunable frequency as well as the leaky-integrate-and-fire (LIF) behavior in a single SHNO… ▽ More In this study, we theoretically investigate neuron-like spiking dynamics in an elliptic ferromagnet/heavy metal bilayer-based spin Hall nano oscillator (SHNO) in bias field-free condition, much suitable for practical realization of brain inspired computing schemes. We demonstrate regular periodic spiking with tunable frequency as well as the leaky-integrate-and-fire (LIF) behavior in a single SHNO by manipulating the pulse features of input current. The frequency of regular periodic spiking is tunable in a range of 0.5 GHz to 0.96 GHz (460 MHz bandwidth) through adjusting the magnitude of constant input dc current density. We further demonstrate the reconfigurability of spiking dynamics in response to a time varying input accomplished by continuously increasing the input current density as a linear function of time. Macrospin theory and micromagnetic simulation provide insights into the origin of bias field-free auto-oscillation and the spiking phenomena in our SHNO. In addition, we discuss how the shape anisotropy of the elliptic ferromagnet influence the bias field-free auto oscillation characteristics, including threshold current, frequency and transition from in-plane to out-of-plane precession. The SHNO operates below $10^{12} A/m^2$ input current density and exhibits a large auto-oscillation amplitude, ensuring high output power. We show that the threshold current density can be reduced by decreasing the ellipticity of the ferromagnet layer as well as enhancing the perpendicular magnetic anisotropy. These findings highlight the potential of bias field-free elliptic SHNO in designing power-efficient spiking neuron-based neuromorphic hardware. △ Less

Submitted 14 September, 2023; originally announced September 2023.

Journal ref: Physical Review B 108, 184411 (2023)

Dense plasma irradiated platinum with improved spin Hall effect

Authors: Sachin Kumar, Sourabh Manna, John Rex Mohan, Utkarsh Shashank, Jospeh Vimal, Mayank Mishra, Surbhi Gupta, Hironori Asada, Yasuhiro Fukuma, Rajdeep Singh Rawat, Rohit Medwal

Abstract: The impurity incorporation in host high-spin orbit coupling materials like platinum has shown improved charge-to-spin conversion by modifying the up-spin and down-spin electron trajectories by bending or skewing them in opposite directions. This enables efficient generation, manipulation, and transport of spin currents. In this study, we irradiate the platinum with non-focus dense plasma to incorp… ▽ More The impurity incorporation in host high-spin orbit coupling materials like platinum has shown improved charge-to-spin conversion by modifying the up-spin and down-spin electron trajectories by bending or skewing them in opposite directions. This enables efficient generation, manipulation, and transport of spin currents. In this study, we irradiate the platinum with non-focus dense plasma to incorporate the oxygen ion species. We systematically analyze the spin Hall angle of the oxygen plasma irradiated Pt films using spin torque ferromagnetic resonance. Our results demonstrate a 2.4 times enhancement in the spin Hall effect after plasma treatment of Pt as compared to pristine Pt. This improvement is attributed to the introduction of disorder and defects in the Pt lattice, which enhances the spin-orbit coupling and leads to more efficient charge-to-spin conversion without breaking the spin-orbit torque symmetries. Our findings offer a new method of dense plasma-based modification of material for the development of advanced spintronic devices based on Pt and other heavy metals. △ Less

Submitted 22 July, 2023; originally announced July 2023.

Room temperature charge-to-spin conversion from q-2DEG at SrTiO3-based interfaces

Authors: Utkarsh Shashank, Angshuman Deka, Chen Ye, Surbhi Gupta, Rohit Medwal, Rajdeep Singh Rawat, Hironori Asada, X. Renshaw Wang, Yasuhiro Fukuma

Abstract: Interfacial two-dimensional electron gas (2DEG), especially the SrTiO3-based ones at the unexpected interface of insulators, have emerged to be a promising candidate for efficient charge-spin current interconversion. In this article, to gain insight into the mechanism of the charge-spin current interconversion at the oxide-based 2DEG, we focused on conducting interfaces between insulating SrTiO3 a… ▽ More Interfacial two-dimensional electron gas (2DEG), especially the SrTiO3-based ones at the unexpected interface of insulators, have emerged to be a promising candidate for efficient charge-spin current interconversion. In this article, to gain insight into the mechanism of the charge-spin current interconversion at the oxide-based 2DEG, we focused on conducting interfaces between insulating SrTiO3 and two types of aluminium-based amorphous insulators, namely SrTiO3/AlN and SrTiO3/Al2O3, and estimated their charge-spin conversion efficiency, θ_cs. The two types of amorphous insulators were selected to explicitly probe the overlooked contribution of oxygen vacancy to the θ_cs. We proposed a mechanism to explain results of spin-torque ferromagnetic resonance (ST-FMR) measurements and developed an analysis protocol to reliably estimate the θ_cs of the oxide based 2DEG. The resultant θ_cs/t, where t is the thickness of the 2DEG, were estimated to be 0.244 nm-1 and 0.101 nm-1 for the SrTiO3/AlN and SrTiO3/Al2O3, respectively, and they are strikingly comparable to their crystalline counterparts. Furthermore, we also observe a large direct current modulation of resonance linewidth in SrTiO3/AlN samples, confirming its high θ_cs and attesting an oxygen-vacancy-enabled charge-spin conversion. Our findings emphasize the defects' contribution to the charge-spin interconversion, especially in the oxide-based low dimensional systems, and provide a way to create and enhance charge-spin interconversion via defect engineering. △ Less

Submitted 7 November, 2022; originally announced November 2022.

Disentanglement of intrinsic and extrinsic side-jump scattering induced spin Hall effect in N-implanted Pt

Authors: Utkarsh Shashank, Yoji Nakamura, Yu Kusaba, Takafumi Tomoda, Razia Nongjai, Asokan Kandasami, Rohit Medwal, Rajdeep Singh Rawat, Hironori Asada, Surbhi Gupta, Yasuhiro Fukuma

Abstract: The rapidly evolving utilization of spin Hall effect (SHE) arising from spin-orbit coupling in 5d transition metals and alloys have made giant strides in the development of designing low-power, robust and non-volatile magnetic memory. Recent studies, on incorporating non-metallic lighter elements such as oxygen, nitrogen and sulfur into 5d transition metals, have shown an enhancement in damping-li… ▽ More The rapidly evolving utilization of spin Hall effect (SHE) arising from spin-orbit coupling in 5d transition metals and alloys have made giant strides in the development of designing low-power, robust and non-volatile magnetic memory. Recent studies, on incorporating non-metallic lighter elements such as oxygen, nitrogen and sulfur into 5d transition metals, have shown an enhancement in damping-like torque efficiency θ_DL due to the modified SHE, but the mechanism behind this enhancement is not clear. In this paper, we study θ_DL at different temperatures (100-293 K) to disentangle the intrinsic and extrinsic side-jump scattering induced spin Hall effect in N-implanted Pt. We observe a crossover of intrinsic to extrinsic side-jump mechanism as the implantation dose increases from 2*10^16 ions/cm2 to 1*10^17 ions/cm2. A sudden decrease in the intrinsic spin Hall conductivity is counterbalanced by the increase in the side-jump induced SHE efficiency. These results conclude that studying θ_DL as a function of implantation dose, and also as a function of temperature, is important to understand the physical mechanism contributing to SHE, which has so far been unexplored in incorporating non-metallic element in 5d transition metals. △ Less

Submitted 7 November, 2022; originally announced November 2022.

Kinetically Decoupled Electrical and Structural Phase Transitions in VO2

Authors: S. R. Sahu, S. S. Majid, A. Ahad, A. Tripathy, K. Dey, S. Pal, B. K. De, Wen-Pin Hsieh, R. Rawat, V. G. Sathe, D. K. Shukla

Abstract: Vanadium dioxide (VO2) has drawn significant attention for its near room temperature insulator to metal transition and associated structural phase transition. The underlying Physics behind the temperature induced insulator to metal and concomitant structural phase transition in VO2 is yet to be fully understood. We have investigated the kinetics of the above phase transition behaviors of VO2 with… ▽ More Vanadium dioxide (VO2) has drawn significant attention for its near room temperature insulator to metal transition and associated structural phase transition. The underlying Physics behind the temperature induced insulator to metal and concomitant structural phase transition in VO2 is yet to be fully understood. We have investigated the kinetics of the above phase transition behaviors of VO2 with the help of resistivity measurements and Raman spectroscopy. Resistance thermal hysteresis scaling and relaxation measurements across the temperature induced insulator to metal transition reveal the unusual behaviour of this first-order phase transition, whereas Raman relaxation measurements show that the temperature induced structural phase transition in VO2 follows usual behaviour and is consistent with mean field prediction. At higher temperature sweeping rates decoupling of insulator to metal transition and structural phase transition have been confirmed. The observed anomalous first order phase transition behavior in VO2 is attributed to the unconventional quasi particle dynamics, i.e. significantly lowered electronic thermal conductivity across insulator to metal transition, which is confirmed by ultrafast optical pump-probe time domain thermoreflectance measurements. △ Less

Submitted 6 October, 2022; originally announced October 2022.

Comments: 25 pages, 8 figures

Observation of Fermi liquid phase with broken symmetry in a single crystalline nanorod of Pr$_2$Ir$_2$O$_7$

Authors: Bikash Ghosh, Abhishek Juyal, Sourav Biswas, R. Rawat, Arijit Kundu, Soumik Mukhopadhyay

Abstract: We report experimental evidence of emergent broken symmetry Fermi liquid state in an isolated single crystalline nanorod of $\rm Pr_2 Ir_2 O_7$. We find clear signature of the onset of the Fermi liquid behavior at low temperature marked by the sign inversion of magnetoresistance from negative at high temperature, characteristic of incoherent Kondo scattering, to positive as well as a $\rm T^2$ dep… ▽ More We report experimental evidence of emergent broken symmetry Fermi liquid state in an isolated single crystalline nanorod of $\rm Pr_2 Ir_2 O_7$. We find clear signature of the onset of the Fermi liquid behavior at low temperature marked by the sign inversion of magnetoresistance from negative at high temperature, characteristic of incoherent Kondo scattering, to positive as well as a $\rm T^2$ dependence of resistivity at low temperature. A resistive anomaly is observed, which is accompanied by thermal hysteresis in the presence of magnetic field, suggesting itinerant metamagnetism. The observed high field negative magnetoresistance with quadratic field dependence at low temperature, which is most likely due to suppression of itinerant spin fluctuation, and the irreversibility of the magneto-resistive properties in the Fermi liquid regime suggest existence of an unusual state with broken spin rotation and time reversal symmetry, hallmark of `hastatic' order. The major features of such temperature dependence of resistivity and magnetoresistance can be explained in a phenomenological model incorporating two distinct hybridization channels, which is physically consistent with the possibility of the formation of the `hastatic' Fermi liquid phase. △ Less

Submitted 18 August, 2022; originally announced August 2022.

Comments: 10 pages with appendix

Crystal growth, magnetic, and magnetocaloric properties of J_eff = 1/2 quantum antiferromagnet CeCl_3

Authors: Nashra Pistawala, Suman Karmakar, Rajeev Rawat, Surjeet Singh

Abstract: We report growth of high-quality single crystals of CeCl3 using a modified Bridgman Stockbarger method in an infrared image furnace. The grown crystals are characterized using single-crystal/powder X-ray diffraction, Laue X-ray diffraction, Raman spectroscopy, magnetization, and heat capacity probes. CeCl3 crystallizes in a hexagonal structure with a weak trigonal distortion. The Raman spectrum at… ▽ More We report growth of high-quality single crystals of CeCl3 using a modified Bridgman Stockbarger method in an infrared image furnace. The grown crystals are characterized using single-crystal/powder X-ray diffraction, Laue X-ray diffraction, Raman spectroscopy, magnetization, and heat capacity probes. CeCl3 crystallizes in a hexagonal structure with a weak trigonal distortion. The Raman spectrum at 300 K showcases five, clearly resolvable, phonon modes at 106.8, 181.2, 189, 213, and 219.7 wavenumbers. The magnetic susceptibility show a large anisotropy with a broad peak in the perpendicular orientation, which is explained using the crystal field theory. The crystal field in CeCl3 splits the J = 5/2 manifold of Ce3+ into three Kramers doublets, resulting in a well-isolated ground state. In the specific heat, no magnetic ordering is detected above 2 K. However, in non-zero fields the low-temperature specific heat changes dramatically, showcasing a peak at 2.5 K under a moderate field of 30 kOe. The weak Ce-Ce exchange, large Ce moment in the crystal field ground state, and significant anisotropy are ingredients for realizing a high magnetocaloric effect. A maximum entropy change of 23 J/Kg/K is observed near 2.5 K in fields ranging from 50 kOe to 60 kOe. These values are comparable to some of the best Gd-based magnetocaloric materials, signifying the potential of CeCl3 as a magnetic coolant. △ Less

Submitted 2 July, 2024; v1 submitted 1 July, 2022; originally announced July 2022.

Comments: Nine figures (Supplemental Material is included at the end

Coexistence of local structural heterogeneities and long-range ferroelectricity in Pb-free (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

Authors: Koushik Dey, Abdul Ahad, Kamini Gautam, Abinash Tripathy, Sofi Suhail Majid, Sonia Francoual, Carsten Richter, MN Singh, Archna Sagdeo, Edmund Welter, Naratip Vittayakorn, Vasant G. Sathe, Rajeev Rawat, Dinesh Kumar Shukla

Abstract: Environmentally benign (1-x)Ba(Ti$_{0.8}$Zr$_{0.2}$)O$_3$-x(Ba$_{0.7}$Ca$_{0.3}$)TiO$_3$ (BZT-BCT) ceramics are promising materials due to their remarkable high piezoresponse [Liu and Ren, Phys. Rev. Lett. \textbf{103}, 257602 (2009)]. In this Letter, by focusing on local and average structure in combination with macroscopic electromechanical and dielectric measurements we demonstrate the structur… ▽ More Environmentally benign (1-x)Ba(Ti$_{0.8}$Zr$_{0.2}$)O$_3$-x(Ba$_{0.7}$Ca$_{0.3}$)TiO$_3$ (BZT-BCT) ceramics are promising materials due to their remarkable high piezoresponse [Liu and Ren, Phys. Rev. Lett. \textbf{103}, 257602 (2009)]. In this Letter, by focusing on local and average structure in combination with macroscopic electromechanical and dielectric measurements we demonstrate the structure property relationship in the tetragonal BZT-BCT ceramic. During high-temperature cubic to tetragonal phase transformation, polar nanoregions are manifested through the spontaneous volume ferroelectrostriction at temperatures below $\sim$ 477 K. Temperature-dependent local structural investigations across the Zr K edge extended x-ray absorption fine structure spectroscopy reveal an anomalous collaboration between the ZrO$_{6}$ and TiO$_6$ octahedra. These octahedra compromise their individuality during polarization development. The presence of domains of submicron size embedded inside the macroscopic ferroelectric regions below T$_{m}$, as well as their hierarchical arrangement, is observed by piezo-response force microscopy. Effects of the existence of the structural/polar heterogeneities below T$_{m}$ are observed also when polarizibilities of the poled and the unpoled samples are compared; the poled sample is found to be more susceptible to the electric field. In addition, by using electric field dependent x-ray diffraction studies we also show that this ceramic under field exhibits reduction of tetragonal distortion, which is consistent with earlier reports. △ Less

Submitted 12 May, 2022; originally announced May 2022.

Comments: 9 pages, 6 figures

Controlling degeneracy and magnetization switching in an artificial spin ice system of peanut-shaped nanomagnets

Authors: Avinash Chaurasiya, Manish Anand, Rajdeep Singh Rawat

Abstract: Using extensive numerical simulations, we probe the magnetization switching in two-dimensional artificial spin ice (ASI) system consisting of peanut-shaped nanomagnets. We also investigated the effect of the external magnetic field on the degeneracy of the magnetic states in such a system. The switching field is found to be one order smaller in the proposed ASI system with peanut-shaped nanomagnet… ▽ More Using extensive numerical simulations, we probe the magnetization switching in two-dimensional artificial spin ice (ASI) system consisting of peanut-shaped nanomagnets. We also investigated the effect of the external magnetic field on the degeneracy of the magnetic states in such a system. The switching field is found to be one order smaller in the proposed ASI system with peanut-shaped nanomagnets as compared to the conventionally used highly-anisotropic nanoisland such as elliptically shaped nanomagnets. The metastable 2-in/2-out (Type II) magnetic state is robust at the remanence. We are also able to access the other possible microstates corresponding to Type II magnetic configurations by carefully varying the external magnetic field. It implies that one can control the degeneracy of the magnetic state by the application of a suitable magnetic field. Interestingly, the magnetic charge neutrality breaks due to the defects induced by removing nanomagnets. In such a case, the system also appears to have a 1-out/3-in or 3-out/1-in (Type III) spin state, reminiscent of a magnetic monopole. We believe that our study is highly desirable in the context of developing the next-generation spintronics-based devices for future technologies. △ Less

Submitted 19 February, 2022; originally announced February 2022.

Comments: 21 pages; 08 Figures

Active Magnetoelectric Control of Terahertz Spin Current

Authors: Avinash Chaurasiya, Ziqi Li, Rohit Medwal, Surbhi Gupta, John Rex Mohan, Yasuhiro Fukuma, Hironori Asada, Elbert E. M. Chia, Rajdeep Singh Rawat

Abstract: Electrical control of photogenerated THz spin current pulses from a spintronic emitter has been at the forefront for the development of scalable, cost-efficient, wideband opto-spintronics devices. Artificially combined ferroelectric and ferromagnet heterostructure provides the potential avenue to control the spin dynamics efficiently utilizing the magnetoelectric coupling. The demonstration of the… ▽ More Electrical control of photogenerated THz spin current pulses from a spintronic emitter has been at the forefront for the development of scalable, cost-efficient, wideband opto-spintronics devices. Artificially combined ferroelectric and ferromagnet heterostructure provides the potential avenue to control the spin dynamics efficiently utilizing the magnetoelectric coupling. The demonstration of the electric field control of spin dynamics has so far been limited up to gigahertz frequencies. Here, we demonstrate the electric field mediated piezoelectric strain control of photogenerated THz spin current pulse from a multiferroic spintronic emitter. The phase reversal of the THz spin current pulse is obtained from the combined effect of piezoelectric strain and a small constant magnetic field applied opposite to the initial magnetization of the ferromagnet. The piezoelectric strain-controlled phase switching of THz spin current thus opens a door to develop efficient strain engineered scalable on-chip THz spintronics devices. △ Less

Submitted 27 November, 2021; originally announced November 2021.

Comments: 17 pages, 4 figures

Robust evidence for the stabilization of the premartensite phase in Ni-Mn-In magnetic shape memory alloys by chemical pressure

Authors: Anupam K. Singh, Sanjay Singh, B. Dutta, K. K. Dubey, Boby Joseph, R. Rawat, Dhananjai Pandey

Abstract: The thermodynamic stability of the premartensite (PM) phase has been a topic of extensive investigation in shape memory alloys as it affects the main martensite phase transition and the related physical properties. In general, the PM phase is stable over a rather narrow temperature-composition range. We present here evidence for chemical pressure induced suppression of the main martensite transiti… ▽ More The thermodynamic stability of the premartensite (PM) phase has been a topic of extensive investigation in shape memory alloys as it affects the main martensite phase transition and the related physical properties. In general, the PM phase is stable over a rather narrow temperature-composition range. We present here evidence for chemical pressure induced suppression of the main martensite transition and stabilization of the PM phase over a very wide temperature range from 300 K to around 5 K in a magnetic shape memory alloy (MSMA) Ni50Mn34In16 using magnetic susceptibility, synchrotron X-ray powder diffraction (SXRPD) studies and first-principles calculations. The ac-susceptibility studies show a highly skewed and smeared peak around 300 K without any further transition up to the lowest temperature of our measurement (5 K) for around 5% Al substitution. The temperature evolution of the SXRPD patterns confirms the appearance of the PM phase related satellite peaks at and below 300 K without any splitting of the main austenite (220) peak showing preserved cubic symmetry. This is in marked contrast to the temperature evolution of the SXRPD patterns of the martensite phase of the Al free as well as around 3% Al substituted compositions where the austenite (220) peak shows a clear splitting due to Bain distortion/symmetry breaking transition. Our theoretical calculations support the experimental findings and reveal that the substitution at the In site by a smaller size atom, like Al, can stabilize the PM phase with preserved cubic symmetry. Our results demonstrate that Al-substituted Ni-Mn-In MSMAs provide an ideal platform for investigating the physics of various phenomena related to the PM state. △ Less

Submitted 4 December, 2021; v1 submitted 23 October, 2021; originally announced October 2021.

Journal ref: Physical Review MATERIALS 5, 113607 (2021)

Angle Selective Piezoelectric Strain Controlled Magnetization Switching in Artificial Spin Ice Based Multiferroic System

Authors: Avinash Chaurasiya, Manish Anand, Rajdeep Singh Rawat

Abstract: The prospect of all electrically controlled writing of ferromagnetic bits is highly desirable for developing scalable and energy-efficient spintronics devices. In the present work, we perform micromagnetic simulations to investigate the electric field-induced strain mediated magnetization switching in artificial spin ice (ASI) based multiferroic system, which is proposed to have a significant decr… ▽ More The prospect of all electrically controlled writing of ferromagnetic bits is highly desirable for developing scalable and energy-efficient spintronics devices. In the present work, we perform micromagnetic simulations to investigate the electric field-induced strain mediated magnetization switching in artificial spin ice (ASI) based multiferroic system, which is proposed to have a significant decrease in Joule heating losses compared to electric current based methods. As the piezo electric strain-based system cannot switch the magnetization by $180^\circ$ in ferromagnets, we propose an ASI multiferroic system consisting of the peanut-shaped nanomagnets on ferroelectric substrate with the angle between the easy axis and hard axis of magnetization less than $90^\circ$. Here the piezoelectric strain-controlled magnetization switching has been studied by applying the electric field pulse at different angles with respect to the axes of the system. Remarkably, magnetization switches by $180^\circ$ only if the external electric field pulse is applied at some specific angles, close to the anisotropy axis of the system ( $\sim 30^\circ - 60^\circ$). Our detailed analysis of the demagnetization energy variation reveals that the energy barrier becomes antisymmetric in such cases, facilitating the complete magnetization reversal. Moreover, we have also proposed a possible magnetization reversal mechanism with two sequential electric field pulses of relatively smaller magnitude. We believe that the present work could pave the way for future ASI-based multiferroic system for scalable magnetic field-free low power spintronics devices. △ Less

Submitted 21 August, 2021; originally announced August 2021.

Comments: 23 pages, 9 figures

Microscopic Origin of Structural Disorder in $δ$-NbN: Correlation of Superconductivity and Electronic Structure

Authors: Shailesh Kalal, Sanjay Nayak, Akhil Tayal, Jens Birch, Rajeev Rawat, Mukul Gupta

Abstract: Rock-salt type niobium nitride ($δ$-NbN) is a well-known superconductor having superconducting transition temperature (Tc) $\approx$ 18\,K and a large superconducting gap $\approx$3\,meV. The Tc of $δ$-NbN thin film exhibits a large scattering irrespective of the growth conditions and lattice parameter. In this work, we investigate the atomic origin of suppression of Tc in $δ$-NbN thin film by emp… ▽ More Rock-salt type niobium nitride ($δ$-NbN) is a well-known superconductor having superconducting transition temperature (Tc) $\approx$ 18\,K and a large superconducting gap $\approx$3\,meV. The Tc of $δ$-NbN thin film exhibits a large scattering irrespective of the growth conditions and lattice parameter. In this work, we investigate the atomic origin of suppression of Tc in $δ$-NbN thin film by employing combined methods of experiments and ab-initio simulations. Sputtered $δ$-NbN thin films with different disorder were analyzed through electrical resistivity and x-ray absorption spectroscopy. A strong correlation between the superconductivity and the atomic distortion induced electronic reconstruction was observed. The theoretical analysis revealed that under N-rich growth conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously and are responsible for the suppression of Tc in $δ$-NbN by smearing its electronic densities of states around Fermi level. △ Less

Submitted 9 October, 2020; originally announced October 2020.

Comments: 5 Pages, 4 figures

Electronic properties in itinerant ferromagnet SrRu$_{1-x}$Ti$_x$O$_3$

Authors: Renu Gupta, R. Rawat, A. K. Pramanik

Abstract: Here, we study the electrical transport and specific heat in 4$d$ based ferromagnetic material SrRuO$_3$ and its Ti substituted SrRu$_{1-x}$Ti$_x$O$_3$ series ($x$ $\le$ 0.7). The SrRuO$_3$ is a metal and shows itinerant ferromagnetism with transition temperature $T_c$ $\sim$ 160 K. The nonmagnetic Ti$^{4+}$ (3$d^0$) substitution would not only weaken the active Ru-O-Ru channel but is also expecte… ▽ More Here, we study the electrical transport and specific heat in 4$d$ based ferromagnetic material SrRuO$_3$ and its Ti substituted SrRu$_{1-x}$Ti$_x$O$_3$ series ($x$ $\le$ 0.7). The SrRuO$_3$ is a metal and shows itinerant ferromagnetism with transition temperature $T_c$ $\sim$ 160 K. The nonmagnetic Ti$^{4+}$ (3$d^0$) substitution would not only weaken the active Ru-O-Ru channel but is also expected to tune the electronic density and electron correlation effect. A metal to insulator transition has been observed around $x$ $\sim$ 0.4. The nature of charge transport in paramagnetic-metallic state ($x$ $\leq$ 0.4) and in insulating state ($x$ $>$ 0.4) follows modified Mott's variable range hopping model. In ferromagnetic-metallic state, resistivity shows a $T^2$ dependence below $T_c$ which though modifies to $T^{3/2}$ dependence at low temperature. In Ti substituted samples, temperature range for $T^{3/2}$ dependence extends to higher temperature. Interestingly, this $T^{3/2}$ dependence dominates in whole ferromagnetic regime in presence of magnetic field. This evolution of electronic transport behavior can be explained within the framework of Fermi liquid theory and electron-magnon scattering mechanism. The negative magnetoresistance exhibits a hysteresis and a crossover between negative and positive value with magnetic field which is connected with magnetic behavior in series. The decreasing electronic coefficient of specific heat with $x$ supports the increasing insulating behavior in present series. We calculate a high Kadowaki-Woods ratio ($x$ $\leq$ 0.3) for SrRuO$_3$ which increases with substitution concentration. This signifies an increasing electronic correlation effect with substitution concentration. △ Less

Submitted 31 August, 2020; originally announced September 2020.

Comments: 21 pages, 10 figures

Journal ref: J. Phys.: Condens. Matter 32, 285502 (2020)

Enhanced charge-transfer character in the monoclinic phase of Mott-insulator LaVO3 thin film

Authors: Anupam Jana, Rajamani Raghunathan, Ritu Rawat, R. J. Choudhary, D. M. Phase

Abstract: Electronic structure of pulsed laser deposited epitaxial LaVO3 (LVO) thin film grown on LaAlO3 (001) (LAO) substrate, has been studied at room temperature and at 130 K, which is below the structural (141 K) and magnetic transition temperatures (143 K) of single crystal LVO. Significant modification in spectral intensity, largely around the V 3d - O 2p hybridized region, is observed in valence band… ▽ More Electronic structure of pulsed laser deposited epitaxial LaVO3 (LVO) thin film grown on LaAlO3 (001) (LAO) substrate, has been studied at room temperature and at 130 K, which is below the structural (141 K) and magnetic transition temperatures (143 K) of single crystal LVO. Significant modification in spectral intensity, largely around the V 3d - O 2p hybridized region, is observed in valence band spectrum (VBS) of LVO film at 130 K. Resonant photoemission study at 130 K confirms the presence of charge transfer screened 3dnL (L: hole in the O 2p) final state along with the dominant 3dn-1 final state at 1.5 eV binding energy in the valence band. On the contrary, in the room temperature VBS dominant V 3d sates with only 3dn-1 kind final state is accentuated. To understand this difference, density functional theory (DFT) calculations are employed. The changes in crystal structure from room temperature orthorhombic (O-LVO) to low temperature monoclinic (M-LVO) symmetry leads to remarkable change in the electronic structure around the Fermi level, including transition from direct to indirect nature of band gap. Our calculations also confirm an enhanced O 2p character in the valence band edge of M-LVO that is hybridized with V 3d. These results are further corroborated with octahedral distortions associated with the structural transition. △ Less

Submitted 8 July, 2020; originally announced July 2020.

Comments: 26 pages, 6 figures

Mixed ground state in Fe-Ni Invar alloys

Authors: S. S. Acharya, V. R. R. Medicherla, Komal Bapna, Khadiza Ali, Deepnarayan Biswas, Rajeev Rawat, Kalobaran Maiti

Abstract: We investigate the ground state properties of Invar alloys via detailed study of the electronic structure of Fe$_{1-x}$Ni$_x$ alloys ($x$ = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9) employing $x$-ray photoelectron spectroscopy (XPS). While all the alloys exhibit soft ferromagnetic behavior with Curie temperature much higher than the room temperature, the results for invar alloy, Fe$_{0.6}$Ni$_{0.4}$ exhi… ▽ More We investigate the ground state properties of Invar alloys via detailed study of the electronic structure of Fe$_{1-x}$Ni$_x$ alloys ($x$ = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9) employing $x$-ray photoelectron spectroscopy (XPS). While all the alloys exhibit soft ferromagnetic behavior with Curie temperature much higher than the room temperature, the results for invar alloy, Fe$_{0.6}$Ni$_{0.4}$ exhibit anomalous behavior. Moreover, the magneto-resistance of the Invar alloy becomes highly negative while the end members possess positive magneto-resistance. The core level spectra of the Invar alloy exhibit emergence of a distinct new feature below 20~K while all other Fe-Ni alloys exhibit no temperature dependence down to 10~K. Interestingly, the shallow core level spectra (3$s$, 3$p$) of Fe and Ni of the Invar alloy reveal stronger deviation at low temperatures compared to the deep core levels (2$s$, 2$p$) indicating crystal field effect. It appears that there is a large precipitation of antiferromagnetic $γ^\prime$ phase below 20 K possessing low magnetic moment (0.5$μ_B$) on Fe within the $α$ phase. The discovery of negative magneto-resistance, anomalous magnetization at low temperature and the emergence of unusual new features in the core levels at low temperature provide an evidence of mixed phase in the ground state of Invar alloys. △ Less

Submitted 23 May, 2020; originally announced May 2020.

Comments: 9 pages, 11 figures

Magneto-transport and Berry phase in magnetically doped Bi$_{0.97-x}$Sb$_{0.03}$ single crystals

Authors: V. K. Maurya, Manju Mishra Patidar, Anita Dhaka, R. Rawat, V. Ganesan, R. S. Dhaka

Abstract: We report large magnetoresistance (MR) and Shubnikov-de Haas (SdH) oscillations in single crystals of magnetically (M= Ni and Fe) doped M$_x$Bi$_{0.97-x}$Sb$_{0.03}$ ($x=$ 0, 0.02) topological insulators. The R$\bar{3}$m symmetry and phase have been confirmed by the Rietveld refinement of x-ray diffraction data. Interestingly, a magnetic field induced phase transition from semi-metallic to semi-co… ▽ More We report large magnetoresistance (MR) and Shubnikov-de Haas (SdH) oscillations in single crystals of magnetically (M= Ni and Fe) doped M$_x$Bi$_{0.97-x}$Sb$_{0.03}$ ($x=$ 0, 0.02) topological insulators. The R$\bar{3}$m symmetry and phase have been confirmed by the Rietveld refinement of x-ray diffraction data. Interestingly, a magnetic field induced phase transition from semi-metallic to semi-conducting type is found with the energy gap around 80 meV at 15 Tesla in the $x=$ 0 sample. Moreover, we observe linear behavior of MR up to 15 Tesla in transverse mode and SdH oscillations in longitudinal mode where the field direction is with respect to the current and crystal plane. For the parent sample, we found the coherence length L$_φ=$ 12.7 nm through the fitting of MR data in transverse mode with modified H-L-N equation. The extracted frequencies of SdH oscillations using the fast Fourier transform method and Landau level (LL) fan diagram are found to be consistent for the parent and Ni doped samples. The determined Fermi surface area is found to be slightly larger in Ni doped as compared to the parent sample possibly due to change in the Fermi energy. The Kohler's plot indicates a single scattering mechanism below 100 K. More importantly, the analysis with the help of LL fan diagram reveals the non-zero Berry phase $φ_{\rm B}= -$(1$\pm$0.1)$π$, which demonstrates the non-trivial topological states near the Dirac point in the parent and Ni doped samples. △ Less

Submitted 22 July, 2020; v1 submitted 28 April, 2020; originally announced April 2020.

Comments: submitted, added fig2c, corrected fig4

Journal ref: Phys. Rev. B 102, 144412 (2020)

Critical behavior and magnetocaloric effect across the magnetic transition in Mn$_{1+x}$Fe$_{4-x}$Si$_{3}$

Authors: Vikram Singh, Pallab Bag, R. Rawat, R. Nath

Abstract: The nature of the magnetic transition, critical scaling of magnetization, and magnetocaloric effect in Mn$_{1+x}$Fe$_{4-x}$Si$_{3}$ ($ x =$ 0 to 1) are studied in detail. Our measurements show no thermal hysteresis across the magnetic transition for the parent compound which is in contrast with the previous report and corroborate the second order nature of the transition. The magnetic transition c… ▽ More The nature of the magnetic transition, critical scaling of magnetization, and magnetocaloric effect in Mn$_{1+x}$Fe$_{4-x}$Si$_{3}$ ($ x =$ 0 to 1) are studied in detail. Our measurements show no thermal hysteresis across the magnetic transition for the parent compound which is in contrast with the previous report and corroborate the second order nature of the transition. The magnetic transition could be tuned continuously with Mn substitution at the Fe site. The Mn substitution leads to a linear increase in the unit cell volume and a slight reduction in the effective moment. A detailed critical analysis of the magnetization data for $x = 0.0$ and 0.2 is performed in the critical regime using the modified Arrott plots, Kouvel-Fisher plot, universal curve scaling, and scaling analysis of magnetocaloric effect. The magnetization isotherms follow modified Arrott plots with critical exponent ($β\simeq 0.308$, $γ\simeq 1.448$, and $δ\simeq 5.64$) for the parent compound ($x=0.0$) and ($β\simeq 0.304$, $γ\simeq 1.445$, and $δ\simeq 5.64$) for $x = 0.2$. The Kouvel-Fisher and universal scaling plots of the magnetization isotherms further confirm the reliability of our critical analysis and values of the exponents. These values of the critical exponents are found to be same for both the parent and doped samples which do not fall under any of the standard universality classes. A reasonable magnetocaloric effect $ΔS_{\rm m}\simeq-6.67$~J/Kg-K and -5.84~J/Kg-K for $x= 0.0$ and 0.2 compounds, respectively, with a huge relative cooling power ($RCP \sim 700$~J/Kg) for 9~T field change is observed. The universal scaling of magnetocaloric effect further mimics the second order character of the magnetic transition. The obtained critical exponents from the critical analysis of magnetocaloric effect agree with the values deduced from the magnetic isotherm analysis. △ Less

Submitted 27 April, 2020; originally announced April 2020.

Comments: 18 pages, 13 figures, 1 table

Journal ref: Sci. Rep. 10, 6981 (2020)

Structural correlations in the enhancement of ferroelectric property of Sr doped BaTiO3

Authors: Satish Yadav, Mohit Chandra, R. Rawat, Vasant Sathe, A. K. Sinha, Kiran Singh

Abstract: The effect of Sr doping in BaTiO3 (BTO) with nominal compositions Ba0.80Sr0.20TiO3 (BSTO) have been explored in its structural, lattice vibration, dielectric, ferroelectric and electrocaloric properties. The temperature dependent dielectric results elucidate the enhancement in dielectric constant and exhibit three frequency independent transitions around 335, 250 and 185 K which are related to dif… ▽ More The effect of Sr doping in BaTiO3 (BTO) with nominal compositions Ba0.80Sr0.20TiO3 (BSTO) have been explored in its structural, lattice vibration, dielectric, ferroelectric and electrocaloric properties. The temperature dependent dielectric results elucidate the enhancement in dielectric constant and exhibit three frequency independent transitions around 335, 250 and 185 K which are related to different structural transitions. All these transitions occur at lower temperature as compared with pristine BTO, however; remnant electric polarization (P) of BSTO is much higher than in BTO. The value of P is around 5 microC/cm2 at room temperature and the maximum P around 8 microC/cm2 is observed at tetragonal to orthorhombic and orthorhombic to rhombohedral transitions. The electro-caloric effect shows the maximum adiabatic change in temperature deltaT approx 0.24 K at cubic to tetragonal transition. The temperature dependent synchrotron X-ray diffraction and Raman results shows correlations between P, crystal structure and lattice vibrations. Our results demonstrate the enhancement in ferroelectric properties of BTO with Sr doping. The origin of the enhancement in ferroelectric property is also discussed which is related to the appearance of superlattice peak around room temperature due to TiO6 octahedral distortion. These enhanced properties would be useful to design lead free high quality ferroelectric and piezoelectric materials. △ Less

Submitted 28 February, 2020; originally announced February 2020.

Comments: 22 pages, 7 figures

Imaging of Strain Driven Magnetic Domains and Strong Spin-Phonon Coupling in Epitaxial Thin Films of SrRuO3

Authors: Shekhar Tyagi, V. G. Sathe, Gaurav Sharma, Rajeev Rawat, Mael Guennou, Jens Kreisel

Abstract: Epitaxial thin films of SrRuO3 with large strain disorder were grown using pulsed laser deposition method which showed two distinct transition temperatures in Magnetic measurements. For the first time, we present visual evolution of magnetic domains across the two transitions using Magnetic force microscopy on these films. The study clearly showed that the magnetic anisotropy corresponding to the… ▽ More Epitaxial thin films of SrRuO3 with large strain disorder were grown using pulsed laser deposition method which showed two distinct transition temperatures in Magnetic measurements. For the first time, we present visual evolution of magnetic domains across the two transitions using Magnetic force microscopy on these films. The study clearly showed that the magnetic anisotropy corresponding to the two transitions is different. It is observed that the perpendicular magnetic anisotropy is dominating in films which results in domain spin orientation preferably in out of plane direction. The Raman studies showed that the lattice is highly influenced by the magnetic order. The analysis of the phonon spectra around magnetic transition reveals the existence of strong spin-phonon coupling and the calculations resulted in spin-phonon coupling strength (λ) values of λ ~ 5 cm-1 and λ ~ 8.5 cm-1, for SrRuO3 films grown on LSAT and SrTiO3 single crystal substrates, respectively. △ Less

Submitted 12 January, 2020; originally announced January 2020.

Comments: 6 pages, 4 figures

Enhancement of magnetoelectric coupling in Cr doped Mn2O3

Authors: Mohit Chandra, Satish Yadav, Rajeev Rawat, Kiran Singh

Abstract: The effect of Cr doping with nominal compositions Mn2-xCrxO3 (0 less than equal to x less than equal to 0.10) has been undertaken to investigate its effect on structural, magnetic, dielectric and magnetoelectric properties. The Cr doping transformed the room temperature crystal structure from orthorhombic to cubic symmetry. Similar to alpha-Mn2O3, two magnetic transitions have been observed in the… ▽ More The effect of Cr doping with nominal compositions Mn2-xCrxO3 (0 less than equal to x less than equal to 0.10) has been undertaken to investigate its effect on structural, magnetic, dielectric and magnetoelectric properties. The Cr doping transformed the room temperature crystal structure from orthorhombic to cubic symmetry. Similar to alpha-Mn2O3, two magnetic transitions have been observed in the Cr doped samples. The effect of Cr doping is significant on the low temperature transition i.e. the lower magnetic transition shifted towards higher temperature (25 K for pristine to 40 K for x=0.10) whereas the high temperature transition decreases slightly with increasing Cr content. A clear frequency independent transition is observed in complex dielectric measurements for all compositions around high temperature magnetic ordering. Interestingly, the magnetodielectric behaviour enhanced enormously approx 21% with Cr substitution as compared to pristine Mn2O3. △ Less

Submitted 27 November, 2019; originally announced November 2019.

Comments: 13 pages, 2 tables, 4 figures

Magneto-dielectric coupling and non-ergodic electrical behaviour in hexagonal Sr0.6Ba0.4MnO3 via local strain driven magnetic ordering

Authors: Ritu Rawat, R. J. Choudhary, A. M. Awasthi, Rajamani Raghunathan, Archna Sagdeo, A. K. Sinha, S. Chaudhary, S. Patnaik, D. M. Phase

Abstract: The crystal structure of hexagonal-Sr0.6Ba0.4MnO3 allows various competing superexchange interactions, leading to intriguing magnetic properties. Local structural changes modify overlapping between Mn and oxygen ions with temperature. Calculations based on our model spin-Hamiltonian reveal that the dominant linear antiferromagnetic superexchange interaction between the oxygen-linked Mn4+ ions resu… ▽ More The crystal structure of hexagonal-Sr0.6Ba0.4MnO3 allows various competing superexchange interactions, leading to intriguing magnetic properties. Local structural changes modify overlapping between Mn and oxygen ions with temperature. Calculations based on our model spin-Hamiltonian reveal that the dominant linear antiferromagnetic superexchange interaction between the oxygen-linked Mn4+ ions results in short range correlations (SRC), manifesting a smooth drop in magnetization below 325K. Dominance of superexchange interaction changes its allegiance towards the non-linear oxygen-linked Mn-O-Mn interactions, onsetting long-range correlations (LRC) below 225K. Below the SRC-LRC crossover temperature, electrical response arising from the interacting dipoles exhibits power-law divergent behaviour of relaxation time, upon cooling. Non-ergodic character of the dipole-cluster glass state is examined via the indispensable aging and rejuvenation effects, similar to the spin glasses. Competitive-frustration among spin-exchange and local-strain is reckoned as responsible for the electrical glass origin. △ Less

Submitted 30 October, 2019; originally announced October 2019.

Comments: 20 pages, 6 figures, 32 references. arXiv admin note: substantial text overlap with arXiv:1807.01303

Journal ref: Journal of Magnetism and Magnetic Materials 497, 165972 (2020)

Magneto-dielectric Effect in Relaxor Dipolar Glassy Tb2CoMnO6 Film

Authors: R. Mandal, M. Chandra, V. Roddatis, M. Tripathi, R. Rawat, R. J. Choudhary, V. Moshnyaga

Abstract: We report magneto-dielectric properties of partially B-site ordered monoclinic Tb2CoMnO6 double perovskite thin film epitaxially grown by metalorganic aerosol deposition technique. Transmission electron microscopy and electron energy loss spectroscopy mapping shows the presence and distribution of both Co2+ and Co3+ ions in the film, evidencing a partial B-site disorder, which was further confirme… ▽ More We report magneto-dielectric properties of partially B-site ordered monoclinic Tb2CoMnO6 double perovskite thin film epitaxially grown by metalorganic aerosol deposition technique. Transmission electron microscopy and electron energy loss spectroscopy mapping shows the presence and distribution of both Co2+ and Co3+ ions in the film, evidencing a partial B-site disorder, which was further confirmed by the observation of reduced saturation magnetization at low temperatures. The ferromagnetic Curie temperature, TC=110 K, is slightly higher as compared to the bulk value (100 K) probably due to an in plane tensile strain. Remarkably, a short range ordering of spins at T*~190 K>>TC was established and assigned to the B-site disorder in the film. Two different dielectric relaxation peaks have been observed; they merge at the same temperature T* of short range spin correlations. Moreover, an unexpected high temperature dipolar relaxor-glass-like transition at T~T* was observed, at which a coupling to short range magnetic correlations results in a 4% magneto-dielectric coupling. △ Less

Submitted 17 October, 2019; originally announced October 2019.

Comments: 19 pages, 6 figures

Journal ref: Phys. Rev. B 101, 094426 (2020)

Structural, magnetic and electric polarization properties of geometrically frustrated YBaCo4O7 and DyBaCo4O7 cobaltites

Authors: C. Dhanasekhar, A. K Das, A. Das, S. K. Mishra, R. Rawat, A. Venimadhav

Abstract: In RBaCo4O7 (R = Ca, Y and Rare earth) cobaltite family, only CaBaCo4O7 shows 3D long range ferrimagnetic and spin driven electric polarization properties. In the present study, we have investigated the structural, magnetic and electric polarization properties in YBaCo4O7 and DyBaCo4O7 members of this family and the obtained results are compared with the CaBaCo4O7. The compound YBaCo4O7 showed a s… ▽ More In RBaCo4O7 (R = Ca, Y and Rare earth) cobaltite family, only CaBaCo4O7 shows 3D long range ferrimagnetic and spin driven electric polarization properties. In the present study, we have investigated the structural, magnetic and electric polarization properties in YBaCo4O7 and DyBaCo4O7 members of this family and the obtained results are compared with the CaBaCo4O7. The compound YBaCo4O7 showed a series of magnetic transitions in agreement with reported literature, additionally, a cluster glass behavior below 5 K is observed. Powder neutron diffraction studies on DyBaCo4O7 cobaltite showed an orthorhombic Pbn21 symmetry and signature of short-range 120 degree magnetic correlations of kagome layer and a spin glass behavior below 65 K. Dielectric measurements on both the samples showed strong frequency dispersion at high temperature and frequency independent behavior at low temperature without dielectric anomaly. Pyroelectric current measurement has showed a broad peak around 50 K in both the samples; however, a careful analysis relates the peak to thermally stimulated depolarization current. This study signifies that the giant structural distortions and cobalt charge ordering at kagome layer are the key factors to drive both long range magnetic ordering and spin driven electric polarization in this cobaltite family. △ Less

Submitted 17 September, 2019; v1 submitted 13 September, 2019; originally announced September 2019.

Comments: accepted in EPL

Non thermal isostructural electrically driven insulator-metal transition and electro-strain in layered cobaltate

Authors: Abdul Ahad, R. Rawat, F. Rahman, D. K. Shukla

Abstract: We report here a discovery of electrically driven insulator to metal transition (IMT) and concomitant isostructural volume expansion in the layered cobaltates which otherwise do not exhibit temperature dependent IMT. These findings are demonstrated at macroscopic, microscopic and atomic scales. With application of voltage growth of metallic regions have been observed in the 2D layered La2-xSrxCoO4… ▽ More We report here a discovery of electrically driven insulator to metal transition (IMT) and concomitant isostructural volume expansion in the layered cobaltates which otherwise do not exhibit temperature dependent IMT. These findings are demonstrated at macroscopic, microscopic and atomic scales. With application of voltage growth of metallic regions have been observed in the 2D layered La2-xSrxCoO4. Growth of metallic regions is associated with volume increase (strain as high as 0.3%). Non thermal IMT and electro-strain are proposed to be caused by electro-proliferation of the Co3+ high spin states. △ Less

Submitted 3 August, 2019; originally announced August 2019.

Comments: 13 pages, 7 figures

Generalized Brewster-angle effect in thin-film optical absorbers and its application for graphene hydrogen sensing

Authors: Kandammathe Valiyaveedu Sreekanth, Mohamed ElKabbash, Rohit Medwal, Jihua Zhang, Theodore Letsou, Giuseppe Strangi, Michael Hinczewski, Rajdeep S. Rawat, Chunlei Guo, Ranjan Singh

Abstract: Generalized Brewster angle (GBA) is the incidence angle at which polarization by reflection for p- and s-polarized light takes place. Realizing s-polarization Brewster effect requires a material with magnetic response which is challenging at optical frequencies since the magnetic response of materials at these frequencies is extremely weak. Here, we experimentally realize GBA effect in the visible… ▽ More Generalized Brewster angle (GBA) is the incidence angle at which polarization by reflection for p- and s-polarized light takes place. Realizing s-polarization Brewster effect requires a material with magnetic response which is challenging at optical frequencies since the magnetic response of materials at these frequencies is extremely weak. Here, we experimentally realize GBA effect in the visible using a thin-film absorber system consisting of a dielectric film on an absorbing substrate. Polarization by reflection is realized for both p- and s- polarized light at different angles of incidence and multiple wavelengths. We provide a theoretical framework for the generalized Brewster effect in thin-film light absorbers. We demonstrate hydrogen gas sensing using a single layer graphene film transferred on a thin-film absorber at the GBA with ~1 fg/mm2 aerial mass sensitivity. The ultrahigh sensitivity stems from the strong phase sensitivity near point of darkness, particularly at the GBA, and the strong light-matter interaction in planar nanocavities. These findings depart from the traditional domain of thin-films as mere interference optical coatings and highlight its many potential applications including gas sensing and biosensing. △ Less

Submitted 22 April, 2019; originally announced April 2019.

Comments: 5 figures and 12 Supporting figures

Magneto-structural and dynamic susceptibility studies on rare-earth rich intermetallic compound

Authors: Srikanta Goswami, P. D. Babu, R. Rawat

Abstract: Complex magneto structural behaviour of rare rich intermetallic Tb$_3$Co is reported in this study. Below the 84K (T$_N$), it undergoes a first order magnetic transition 72K confirmed from specific heat and magnetization measurements. Detailed study using magnetization, specific heat, neuron diffraction and ac-susceptibility measurements suggests that the compound in the question possesses canted… ▽ More Complex magneto structural behaviour of rare rich intermetallic Tb$_3$Co is reported in this study. Below the 84K (T$_N$), it undergoes a first order magnetic transition 72K confirmed from specific heat and magnetization measurements. Detailed study using magnetization, specific heat, neuron diffraction and ac-susceptibility measurements suggests that the compound in the question possesses canted magnetic structure. Neutron diffraction reveals that Tb moments are aligned antiferromagnetically (AFM) in ab plane and ferromagnetically (FM) along c direction. This study focusses on the temperature evolution of magnetic order at low temperatures. Neutron Diffraction clearly shows that magnetic structure remains more or less same up to 70K although low field magnetization exhibits a transition like feature around 30K. Field dependent neutron diffraction suggests that a strong spin-lattice coupling is present at lower temperature region (below 20K) compared to that for T above 40K. This is responsible for the observed drop in ZFC magnetization around 30K and is not due to any change in magnetic structure. Another important but surprising result that observed is the signature of magnetic glass below 72K in frequency dependent ac susceptibility measurements. Real part of ac-susceptibility data shows dispersion with frequency which on detailed analysis provides evidence for spin glass behaviour in the compound riding on top of non-collinear AFM order. Higher order harmonics in ac non-linear) susceptibilities, which are expected to show a well-defined behaviour for pure FM or AFM or spin glass or cluster glass systems, were also measured. However, the non-linear susceptibilities do not exhibit any well-known variation but show complex behaviour thus indicating Tb$_3$Co is neither a pure AFM or FM or spin glass system but provides another evidence for non-collinear magnetic structure. △ Less

Submitted 14 January, 2019; originally announced January 2019.

Comments: 10 pages, 15 figures being submited as regular paper

Journal ref: J. Phys. Condens. Mater. 31, 445801 (2019)

Giant negative magnetoresistance and kinetic arrest of first-order ferrimagnetic-antiferomagnetic transition in Ge doped Mn$_2$Sb

Authors: Vikram Singh, R Rawat, Pallavi Kushwaha

Abstract: Effect of Ge substitution on first order ferrimagnetic (FRI) - antiferromagnetic (AFM) transition in Mn$_2$Sb has been studied. It shows that transition temperature (T$_t$) can be tuned between 119~K - 271~K by substituting 2.5-10\% Ge at Sb site in Mn$_2$Sb. The variation of density of state at Fermi level N(E$_f$) with Ge substitution shows that dN(E)/dE is positive at E$_f$ in the AFM state. Wi… ▽ More Effect of Ge substitution on first order ferrimagnetic (FRI) - antiferromagnetic (AFM) transition in Mn$_2$Sb has been studied. It shows that transition temperature (T$_t$) can be tuned between 119~K - 271~K by substituting 2.5-10\% Ge at Sb site in Mn$_2$Sb. The variation of density of state at Fermi level N(E$_f$) with Ge substitution shows that dN(E)/dE is positive at E$_f$ in the AFM state. With the application of magnetic field T$_t$ shifts to low temperature, which results in a giant negative magnetoresistance (MR) reaching a value of 70\% for 2.5\% substitution. Our results show that FRI to AFM transformation during cooling stops around 35 K, even though it remains incomplete. It along with non-monotonic variation of lower critical field, open loop in isothermal MR and increasing difference in zero field cooled warming (ZFCW) and field cooled warming (FCW) resistivity with increasing magnetic field shows that FRI to AFM transition is kinetically arrested in the case of 2.5\% Ge substitution. △ Less

Submitted 2 January, 2019; originally announced January 2019.

Hexagonal Sr0.6Ba0.4MnO3: Spin and Dipole Coupling via Local Structure

Authors: Ritu Rawat, R. J. Choudhary, A. M. Awasthi, Archna Sagdeo, A. K. Sinha, Rajamani Raghunathan, V. G. Sathe, D. M. Phase

Abstract: Hexagonal Sr0.6Ba0.4MnO3 (SBMO) follows P63/mmc symmetry where MnO6 octahedra are both face-shared (Mn2O9 bi-octahedra) and corner-shared via oxygen anion. It undergoes ferroelectric (FE) and antiferromagnetic (AFM) orderings close to the room temperature. Magnetic properties appear to be governed by intricate exchange interactions among Mn4+ ions within and in adjacent Mn2O9 bi-octahedra, conting… ▽ More Hexagonal Sr0.6Ba0.4MnO3 (SBMO) follows P63/mmc symmetry where MnO6 octahedra are both face-shared (Mn2O9 bi-octahedra) and corner-shared via oxygen anion. It undergoes ferroelectric (FE) and antiferromagnetic (AFM) orderings close to the room temperature. Magnetic properties appear to be governed by intricate exchange interactions among Mn4+ ions within and in adjacent Mn2O9 bi-octahedra, contingent upon the local structural changes. Calculations based on our model spin-Hamiltonian reveal that the dominant linear AFM fluctuations between the Mn4+ ions of two oxygen-linked bi-octahedra result in short range correlations, manifest as a smooth drop in magnetization below 325 K. Competition between spin-exchange and local-strain is reckoned as responsible for the atypical magneto-electricity, obtained near the room temperature. △ Less

Submitted 25 June, 2019; v1 submitted 3 July, 2018; originally announced July 2018.

Comments: 15 pages, 4 figures, 1 table, and 28 references

Journal ref: Journal of Alloys and Compounds 796 (2019) 237

Multiferroicity and magnetoelastic coupling in alpha-Mn2O3: A binary perovskite

Authors: Mohit Chandra, Satish Yadav, R. J. Choudhary, R. Rawat, A. K. Sinha, Marie-Bernadette Lepetit, Kiran Singh

Abstract: Multiferroics where at least two primary ferroic orders are present and coupled in a single system constitute an important class of materials. They attracted special consideration as they present both intriguing fundamental physics problems and technological importance for potential multifunctional devices. Here, we present the evidence of multiferroicity and magnetoelectric (ME) coupling in alpha… ▽ More Multiferroics where at least two primary ferroic orders are present and coupled in a single system constitute an important class of materials. They attracted special consideration as they present both intriguing fundamental physics problems and technological importance for potential multifunctional devices. Here, we present the evidence of multiferroicity and magnetoelectric (ME) coupling in alpha-Mn2O3; a unique binary perovskite. Corresponding to the antiferromagnetic (AFM) ordering around 80K, a clear frequency independent transition is observed in the dielectric permittivity. We showed that electric polarization emerges near AFM regime that can be modulated with magnetic field. The detailed structural analysis using synchrotron radiation X-ray diffraction demonstrates the increase in structural distortion with decreasing temperature, as well as changes in the unit cell parameters and bond lengths across the ferroelectric and magnetic ordering temperatures. This observation of multiferroicity and magnetoelastic coupling in alpha-Mn2O3 provides insights for the exploration of ME coupling in related materials. △ Less

Submitted 29 June, 2018; originally announced June 2018.

Comments: Page 20 and 6 figures

Journal ref: Phys. Rev. B 98, 104427 (2018)

Structural response to the magnetic pre ordering in LiFeSi2O6

Authors: R. K. Maurya, Priyamedha Sharma, R. Rawat, R. S. Singh, R. Bindu

Abstract: We investigate the temperature evolution of the structural parameters of potential ferrotoroidic LiFeSi2O6 compound across structural and magnetic phase transitions. The structural transition (TS)is around 220K and the paramagnetic to antiferromagnetic transition (TN) is around 18K. The lattice parameters exhibit unusual temperature dependence and based on its behaviour, the exper- imental results… ▽ More We investigate the temperature evolution of the structural parameters of potential ferrotoroidic LiFeSi2O6 compound across structural and magnetic phase transitions. The structural transition (TS)is around 220K and the paramagnetic to antiferromagnetic transition (TN) is around 18K. The lattice parameters exhibit unusual temperature dependence and based on its behaviour, the exper- imental results can be divided into 3 regions.In region I (300K to 240K), the cell parameters are mainly governed by mere thermal effect. As the compound enters region II (below 240K to 50K), the lattice parameters show non linear behaviour. In this region, the exchange pathways that lead to the magnetic interactions within and between the Fe-Fe chains do not show significant response.The region III (below 50K) is dominated by the magnetic contribution where we observe setting up of intra and inter-chain magnetic interaction. This behaviour is unlike other low dimensional com- pounds like Ca3Co2O6, Sr3NiRhO6, MnTiO3 etc. thereby suggesting the magnetism in LiFeSi2O6 is of three dimensional nature. The present results will be helpful in understanding the evolution of the spin rings that give rise to net toroidal moment and hence its multiferroic behaviour. △ Less

Submitted 16 April, 2018; originally announced April 2018.

Comments: 6 pages, 7 figures

Strong electron-phonon coupling and multiband effects in the superconducting $β$-phase Mo$_{1-x}$Re$_x$ alloys

Authors: Shyam Sundar, L S Sharath Chandra, M K Chattopadhyay, Sudhir K Pandey, D Venkateshwarlu, R Rawat, V Ganesan, S B Roy

Abstract: Superconducting transition temperature $T_C$ of some of the cubic $β$-phase Mo$_{1-x}$Re$_x$ alloys with x > 0.10 is an order of magnitude higher than that in the elements Mo and Re. We investigate this rather enigmatic issue of the enhanced superconductivity with the help of experimental studies of the temperature dependent electrical resistivity ($ρ$(T)) and heat capacity (C$_P$(T)), as well as… ▽ More Superconducting transition temperature $T_C$ of some of the cubic $β$-phase Mo$_{1-x}$Re$_x$ alloys with x > 0.10 is an order of magnitude higher than that in the elements Mo and Re. We investigate this rather enigmatic issue of the enhanced superconductivity with the help of experimental studies of the temperature dependent electrical resistivity ($ρ$(T)) and heat capacity (C$_P$(T)), as well as the theoretical estimation of electronic density of states (DOS) using band structure calculations. The $ρ$(T) in the normal state of the Mo$_{1-x}$Re$_x$ alloys with x > 0.15 is distinctly different from that of Mo and the alloys with x < 0.10. We have also observed that the Sommerfeld coefficient of electronic heat capacity $γ$, superconducting transition temperature $T_C$ and the DOS at the Fermi level show an abrupt change above x > 0.10. The analysis of these results indicates that the value of electron-phonon coupling constant λep required to explain the $T_C$ of the alloys with x > 0.10 is much higher than that estimated from $γ$. On the other hand the analysis of the results of the $ρ$(T) reveals the presence of phonon assisted inter-band s-d scattering in this composition range. We argue that a strong electron-phonon coupling arising due to the multiband effects is responsible for the enhanced $T_C$ in the $β$-phase Mo$_{1-x}$Re$_x$ alloys with x > 0.10. △ Less

Submitted 28 March, 2018; originally announced March 2018.

Journal ref: New J. Phys. 17, 053003 (2015)

Diffuson contribution to anomalous Hall effect in disordered Co2FeSi thin films

Authors: Binoy Krishna Hazra, S. N. Kaul, S. Srinath, M. Manivel Raja, R. Rawat, Archana Lakhani

Abstract: A wide variation in the disorder strength, as inferred from an order of magnitude variation in the longitudinal resistivity of Co2FeSi (CFS) Huesler alloy thin films of fixed (50 nm) thickness, has been achieved by growing these films on Si(111) substrates at substrate temperatures ranging from room temperature (RT) to 600 C. An in-depth study of the influence of disorder on anomalous Hall resisti… ▽ More A wide variation in the disorder strength, as inferred from an order of magnitude variation in the longitudinal resistivity of Co2FeSi (CFS) Huesler alloy thin films of fixed (50 nm) thickness, has been achieved by growing these films on Si(111) substrates at substrate temperatures ranging from room temperature (RT) to 600 C. An in-depth study of the influence of disorder on anomalous Hall resistivity,longitudinal resistivity(LR) and magnetoresistance, enabled by this approach, reveals the following. The side-jump mechanism gives a dominant contribution to anomalous Hall resistivity (AHR) in the CFS thin films, regardless of the degree of disorder present. A new and novel contribution to both LR and AHR characterized by the logarithmic temperature dependence at temperatures below the minimum, exclusive to the amorphous CFS films, originates from the scattering of conduction electrons from the diffusive hydrodynamic modes associated with the longitudinal component of magnetization, called diffusons. In these amorphous CFS films, the electron-diffuson, e d, scattering and weak localization (WL) mechanisms compete with that arising from the inelastic electron magnon, e m, scattering to produce the minimum in longitudinal resistivity, whereas the minimum in AHR is caused by the competing contributions from the e d and e m scattering, as WL does not make any contribution to AHR. In sharp contrast, in crystalline films, enhanced electron electron Coulomb interaction (EEI), which is basically responsible for the resistivity minimum, makes no contribution to AHR with the result that AHR does not exhibit a minimum. △ Less

Submitted 15 March, 2018; originally announced March 2018.

Comments: 7 pages, 7 figures

Measurement independent magnetocaloric effect in Mn-rich Mn-Fe-Ni-Sn(Sb/In) Heusler alloys

Authors: Arup Ghosh, Rajeev Rawat, Arpan Bhattacharyya, Guruprasad Mandal, A. K. Nigam, Sunil Nair

Abstract: We report a systematic study on the magneto-structural transition in Mn-rich Fe-doped Mn-Fe-Ni-Sn(Sb/In) Heusler alloys by keeping the total valence electron concentration (e/a ratio) fixed. The martensitic transition (MT) temperature is found to shift by following a proportional relationship with the e/a ratio of the magnetic elements alone. The magnetic entropy change across MT for a selected sa… ▽ More We report a systematic study on the magneto-structural transition in Mn-rich Fe-doped Mn-Fe-Ni-Sn(Sb/In) Heusler alloys by keeping the total valence electron concentration (e/a ratio) fixed. The martensitic transition (MT) temperature is found to shift by following a proportional relationship with the e/a ratio of the magnetic elements alone. The magnetic entropy change across MT for a selected sample (Mn49FeNi40Sn9In) has been estimated from three different measurement methods (isofield magnetization (M) vs temperature (T), isothermal M vs field (H) and heat capacity (HC) vs T). We observed that though the peak value of magnetic entropy change changes with the measurement methods, the broadened shape of the magnetic entropy change vs T curves and the corresponding cooling power (~140 Jkg-1) remains invariant. The equivalent adiabatic temperature change ~ -2.6 K has been obtained from indirect measurements of temperature change. Moreover, an exchange bias field ~ 783 Oe at 5 K and a magnetoresistance of -30% are also obtained in one of these materials. △ Less

Submitted 25 November, 2018; v1 submitted 5 March, 2018; originally announced March 2018.

Comments: 21 pages, 8 figures, 3 tables

Vortex-glass transformation within the surface superconducting state of $β$-phase Mo$_{1-x}$Re$_x$ alloys

Authors: Shyam Sundar, M. K. Chattopadhyay, L. S. Sharath Chandra, R. Rawat, S. B. Roy

Abstract: We have performed an experimental study on the temperature dependence of electrical resistivity $ρ$($T$) and heat capacity $C$($T$) of the Mo$_{1-x}$Re$_x$ $(x = 0.20, 0.25)$ alloy superconductors in different magnetic fields. In the presence of applied magnetic field, the electrical resistivity of these alloys go to zero at a temperature well above the bulk superconducting transition temperature… ▽ More We have performed an experimental study on the temperature dependence of electrical resistivity $ρ$($T$) and heat capacity $C$($T$) of the Mo$_{1-x}$Re$_x$ $(x = 0.20, 0.25)$ alloy superconductors in different magnetic fields. In the presence of applied magnetic field, the electrical resistivity of these alloys go to zero at a temperature well above the bulk superconducting transition temperature obtained with the help of heat capacity measurements in the same magnetic field. Our study indicates the presence of surface superconducting state in these alloys, where the flux lines are pinned in the surface sheath of the superconductor. The configuration of the flux-lines (2d pancake-like) in the surface sheath is understood in the realm of the flux-spot model. Experimental evidence in support of the surface mixed-state state or "Kulik vortex-state" and the occurrence of a vortex-liquid to vortex-glass transition is presented. △ Less

Submitted 15 January, 2018; originally announced January 2018.

Comments: 15 pages, 9 figures

Journal ref: Supercond. Sci. Technol. 30, 025003 (2017)

Comment on "Reversibility of magnetic field driven transition from electronic phase separation state to single-phase state in manganites: A microscopic view"

Authors: R. Rawat, V G. Sathe, P. Chaddah

Abstract: It is a comment on a paper titled "Reversibility of magnetic field driven transition from electronic phase separation state to single-phase state in manganites: A microscopic view" by Liu et al. Phys. Rev. B 96 (2017) 195154. It is a comment on a paper titled "Reversibility of magnetic field driven transition from electronic phase separation state to single-phase state in manganites: A microscopic view" by Liu et al. Phys. Rev. B 96 (2017) 195154. △ Less

Submitted 18 December, 2017; originally announced December 2017.

Comments: Comment on Phys. Rev. B 96 (2017) 195154

Low field magnetoelectric effect in Fe substituted Co4Nb2O9

Authors: C. Dhanasekhar, S. K. Mishra, R. Rawat, A. K. Das, A. Venimadhav

Abstract: Co4Nb2O9 (CNO) having α-Al2O3 crystal structure with Co chains along c-direction shows gigantic magnetoelelctric coupling below antiferromagnetic ordering temperature of 27 K but above a spin flop field of 1.6 T. We have investigated structural, magnetic and magnetoelectric properties of Fe substituted (10% and 20%) samples and compared with the parent one. In fact magnetic and specific heat measu… ▽ More Co4Nb2O9 (CNO) having α-Al2O3 crystal structure with Co chains along c-direction shows gigantic magnetoelelctric coupling below antiferromagnetic ordering temperature of 27 K but above a spin flop field of 1.6 T. We have investigated structural, magnetic and magnetoelectric properties of Fe substituted (10% and 20%) samples and compared with the parent one. In fact magnetic and specific heat measurements have revealed an additional magnetic transition below 10 K and presence of short range magnetic ordering above ~ 50 K in parent as well as in Fe substituted samples. Linear magnetoelelctric and ferroelectric behaviours are evidenced in the Fe substituted samples where an electric field of 5 kV/m is sufficient to align the dipoles and the magnetoelelctric coupling is ensured for magnetic fields as low as 0.25 T, far below the spin flop field. △ Less

Submitted 15 June, 2016; originally announced June 2016.

Comments: 13 pages

Structural and magnetic properties of S = 3/2 chemically disordered perovskite Sr2TiMnO6

Authors: Shivani Sharma, R. Rawat, N. P. Lalla

Abstract: Single phase polycrystalline sample of perovskite Sr2TiMnO6 (STMO) has been successfully synthesized via solid state reaction route. Its detailed structural and physical properties have been studied using x-ray diffraction, transmission electron microscopy, magnetization, specific heat, and dielectric measurements. Rietveld analysis of the powder XRD data and transmission electron microscopy studi… ▽ More Single phase polycrystalline sample of perovskite Sr2TiMnO6 (STMO) has been successfully synthesized via solid state reaction route. Its detailed structural and physical properties have been studied using x-ray diffraction, transmission electron microscopy, magnetization, specific heat, and dielectric measurements. Rietveld analysis of the powder XRD data and transmission electron microscopy studies confirm that STMO is not a double perovskite structure with Fm-3m space group. But it is a disordered perovskite structure with Pm-3m space group where Ti4+ and Mn4+ ions are randomly distributed at the 1b site. The dc magnetization (\c{hi}dc-vs-T) measurements in the temperature range 2-300K reveals a single anomaly at ~13K with predominant long range canted antiferromagnetic (AFM) ordering. The frequency invariant maxima at 13K in the ac susceptibility (\c{hi}ac-vs-T) discards the spin-glass behaviour in STMO. Despite the presence of sharp feature in M-T data, the specific heat data (Cp-vs-T) shows only a broad hump around 13K, which has been attributed to the magneto-structural ordering giving rise to canted AFM state in chemically disorder STMO. Matching with the magnetic and specific heat anomaly, the dielectric permittivity also shows a broad anomaly around 14K, indicating the possible magneto-dielectric coupling in STMO. △ Less

Submitted 3 September, 2016; v1 submitted 23 February, 2016; originally announced February 2016.

Comments: this paper has been withdrawn due to minor error in the calculation of dielectric constant

Path dependent resistivity study across field induced paramagnetic to ferromagnetic transition in Se doped CoS$_2$

Authors: Saroj Kumar Mishra, R Rawat

Abstract: A systematic study of thermomagnetic irreversibility CoS$_{1.76}$Se$_{0.24}$ has been carried out. Our study shows that the resistivity at low temperature can be tuned by cooling in different magnetic fields and the critical field required for paramagnetic (PM) to ferromagnetic (FM) transition varies non-monotonically with temperature. The field induced PM to FM transition results in giant positiv… ▽ More A systematic study of thermomagnetic irreversibility CoS$_{1.76}$Se$_{0.24}$ has been carried out. Our study shows that the resistivity at low temperature can be tuned by cooling in different magnetic fields and the critical field required for paramagnetic (PM) to ferromagnetic (FM) transition varies non-monotonically with temperature. The field induced PM to FM transition results in giant positive magnetoresistance (MR) of about 160\% at 5 K. Measurements under CHUF (cooling and heating in unequal magnetic field) protocol show reentrant transition on warming under higher magnetic field (than that applied during cooling). It indicates that the glass like behaviour in this system can be explained in the framework of the kinetic arrest of first order transition. Among the growing list of diverse system showing glass like arrested magnetic states, the present system is the first example where, kinetic arrest is observed for a disordered (here PM) to ordered (here FM) first order transition. △ Less

Submitted 26 January, 2016; originally announced January 2016.

Comments: 10 pages, 4 figures, Kinetic arrest, CHUF, Magnetoresistance, Metamagnetism, First order magnetic transition, Di-chalcogenides, Sulphides

Journal ref: Solid State Communications 244 (2016) 33