Showing 1–14 of 14 results for author: Mallick, S

Giant Thermal Amplification via Engineered Dissipation in a Sierpinski-Gasket Aharonov-Bohm Interferometer

Authors: Shubhra Shubhadarshini Mallick, Salil Bedkihal, Mattias Fitzpatrick, Malay Bandyopadhyay

Abstract: We propose a three-terminal thermal amplifier based on a Sierpinski-gasket Aharonov-Bohm interferometer, where the third (base) terminal is realized as a floating Buttiker probe that acts as an engineered dissipative reservoir, exchanging energy with the conductor while carrying no net charge current. Using the nonequilibrium Green's function formalism, we demonstrate that the interplay of quantum… ▽ More We propose a three-terminal thermal amplifier based on a Sierpinski-gasket Aharonov-Bohm interferometer, where the third (base) terminal is realized as a floating Buttiker probe that acts as an engineered dissipative reservoir, exchanging energy with the conductor while carrying no net charge current. Using the nonequilibrium Green's function formalism, we demonstrate that the interplay of quantum coherence and engineered dissipation gives rise to giant, magnetic-flux-controlled thermal amplification, whereas purely coherent transport exhibits little or no amplification. We show that the amplification originates from a flux-induced cancellation of the energy-resolved thermal response of the base terminal, causing its differential heat current to vanish while finite heat currents continue to flow through the emitter and collector terminals. As a result, the thermal gain diverges without requiring resonant transmission. This interference-driven cancellation gives rise to an emergent thermal transparency, closely analogous to electromagnetically induced transparency in optical systems, where destructive quantum interference suppresses the thermal response of the base reservoir while maintaining finite heat transport through the remaining terminals. Our results establish the interplay of engineered dissipation and quantum interference as a powerful mechanism for controlling heat flow and realizing high-performance thermal amplifiers in mesoscopic conductors. △ Less

Submitted 18 August, 2026; v1 submitted 17 August, 2026; originally announced August 2026.

Effects of antiferromagnetic coupling and pinning on domain wall dynamics in synthetic ferrimagnets

Authors: Sougata Mallick, Nicolas Reyren, André Thiaville, Philippe Ohresser, Nicolas Jaouen, Vincent Cros, Vincent Jeudy

Abstract: Domain wall (DW) dynamics in antiferromagnetic (AFM) systems offer the advantages over their ferromagnetic counterparts of having faster and more energy efficient manipulation due to the absence of net magnetization, leading to reduced magnetic crosstalk and improved performance in spintronic devices. A comprehensive analysis of DW dynamics across regimes such as creep, depinning, and flow is well… ▽ More Domain wall (DW) dynamics in antiferromagnetic (AFM) systems offer the advantages over their ferromagnetic counterparts of having faster and more energy efficient manipulation due to the absence of net magnetization, leading to reduced magnetic crosstalk and improved performance in spintronic devices. A comprehensive analysis of DW dynamics across regimes such as creep, depinning, and flow is well established in ferromagnetic systems but remains lacking in AFM-coupled systems. In this study, we explore the nature of DW dynamics in synthetic ferrimagnetic multilayers composed of Pt|Co|Tb|Al for different Tb thickness, focusing on the underlying pinning parameters, and on the different regimes of DW dynamics driven by spin-orbit torques (SOTs). We find that due to the AFM coupling between Co and Tb, the magnetic moment of Tb increases with Tb thickness resulting in a reduced saturation magnetization and an enhanced depinning field. The DW disorder interaction is found to vary weakly with the AFM coupling between Co and Tb, while the complete withdrawal of the Tb layer strongly increases the anisotropy and the DW pinning. Furthermore, we propose a novel approach to measure effective SOTs by comparing depinning transitions in current and field-induced DW motion. This research reveals new insights into DW dynamics in coupled AFM systems, highlighting enhancements in mobility through optimized SOTs and pinning landscapes. △ Less

Submitted 17 February, 2025; originally announced February 2025.

Comments: 12 pages, 9 figures

Driving skyrmions in flow regime in synthetic ferrimagnets

Authors: Sougata Mallick, Yanis Sassi, Nicholas Figueiredo Prestes, Sachin Krishnia, Fernando Gallego, Thibaud Denneulin, Sophie Collin, Karim Bouzehouane, André Thiaville, Rafal E. Dunin-Borkowski, Vincent Jeudy, Albert Fert, Nicolas Reyren, Vincent Cros

Abstract: Despite significant advances in the last decade regarding the room temperature stabilization of skyrmions or their current induced dynamics, the impact of local material inhomogeneities still remains an important issue that impedes to reach the regime of steady state motion of these spin textures. Here, we study the spin-torque driven motion of skyrmions in synthetic ferrimagnetic multilayers with… ▽ More Despite significant advances in the last decade regarding the room temperature stabilization of skyrmions or their current induced dynamics, the impact of local material inhomogeneities still remains an important issue that impedes to reach the regime of steady state motion of these spin textures. Here, we study the spin-torque driven motion of skyrmions in synthetic ferrimagnetic multilayers with the aim of achieving high mobility and reduced skyrmion Hall effect. We consider Pt|Co|Tb multilayers of various thicknesses with antiferromagnetic coupling between the Co and Tb magnetization. The increase of Tb thickness in the multilayers allows to reduce the total magnetic moment and increases the spin-orbit torques allowing to reach velocities up to 400 m.s-1 for skyrmions with diameters of about 160 nm. We demonstrate that due to reduced skyrmion Hall effect, combined with the edge repulsion of the magnetic track making the skyrmions moving along the track without any transverse deflection. Further, by comparing the field-induced domain wall motion and current-induced skyrmion motion, we demonstrate that the skyrmions at the largest current densities present all the characteristics of a dynamical flow regime. △ Less

Submitted 1 June, 2023; v1 submitted 30 May, 2023; originally announced May 2023.

Comments: 14 pages, 4 figures

Current-induced nucleation and motion of zero field skyrmion

Authors: Sougata Mallick, Sujit Panigrahy, Gajanan Pradhan, Stanislas Rohart

Abstract: We study the stabilization and electrical manipulation of skyrmions in magnetic ultrathin films in the absence of an applied magnetic field. We show that this requires an increased magnetic anisotropy, controlled by the sample thickness, as compared to usual skyrmionic samples, so that the uniform state corresponds to the zero field ground state and the skyrmions to metastable excitations. Althoug… ▽ More We study the stabilization and electrical manipulation of skyrmions in magnetic ultrathin films in the absence of an applied magnetic field. We show that this requires an increased magnetic anisotropy, controlled by the sample thickness, as compared to usual skyrmionic samples, so that the uniform state corresponds to the zero field ground state and the skyrmions to metastable excitations. Although skyrmion stabilization at zero field is demonstrated over a broad range of thicknesses, electrical control appeared to be more demanding to avoid skyrmion deformation. In the thinnest samples, the large magnetic anisotropy prevents skyrmion deformations and we show that they can be nucleated progressively by current pulses, which underlines that the only possible transition occurs between uniform and skyrmion states. The solitonic skyrmions in zero applied magnetic field have the same properties as compared to field stabilized ones, with a long-term stability and high mobility when excited by a spin-orbit torque. △ Less

Submitted 5 December, 2022; originally announced December 2022.

Comments: 8 pages, 4 figures

Skyrmion inertia in synthetic antiferromagnets

Authors: Sujit Panigrahy, Sougata Mallick, Joao Sampaio, Stanislas Rohart

Abstract: We describe the dynamics of magnetic skyrmions in synthetic antiferromagnets (SAF), with a finite interlayer coupling. Due to the opposite gyrovector of the skyrmions in the two SAF layers, the coupled skyrmions reach a stationary regime with a spatial separation, in a direction orthogonal to their velocity. As a consequence, and contrary to the ferromagnetic situation, a transient regime necessar… ▽ More We describe the dynamics of magnetic skyrmions in synthetic antiferromagnets (SAF), with a finite interlayer coupling. Due to the opposite gyrovector of the skyrmions in the two SAF layers, the coupled skyrmions reach a stationary regime with a spatial separation, in a direction orthogonal to their velocity. As a consequence, and contrary to the ferromagnetic situation, a transient regime necessarily occurs, with a finite acceleration, related to an inertia, and that limits its time response. The formalism developed here, based on two coupled Thiele equations, allows a quantitative description of this phenomenon. The time constant associated to the transient regime scales inversely with the antiferromagnetic coupling constant. We also show that the coupling force reaches a maximal value at a finite skyrmion separation. This sets a maximum velocity limit, beyond which the the coupling force cannot stabilize the bound state. △ Less

Submitted 5 December, 2022; originally announced December 2022.

Comments: 19 pages, 5 figures

Journal ref: Phys. Rev. B 106, 144405 (2022)

Size-dependent mobility of skyrmions beyond pinning in ferrimagnetic GdCo thin films

Authors: Léo Berges, Eloi Haltz, Sujit Panigrahy, Sougata Mallick, Raphäel Weil, Stanislas Rohart, Alexandra Mougin, João Sampaio

Abstract: Magnetic skyrmions are swirling magnetic textures that can be efficiently driven with spin-orbit torques with a deflected trajectory. However, pinning slows skyrmions down and alters their trajectory, which prevents a quantitative comparison to analytical models. Here, we study skyrmions driven by spin-orbit torques at room temperature in ferrimagnetic GdCo thin films, an amorphous material with l… ▽ More Magnetic skyrmions are swirling magnetic textures that can be efficiently driven with spin-orbit torques with a deflected trajectory. However, pinning slows skyrmions down and alters their trajectory, which prevents a quantitative comparison to analytical models. Here, we study skyrmions driven by spin-orbit torques at room temperature in ferrimagnetic GdCo thin films, an amorphous material with low pinning. Above a sharp current depinning threshold, we observe a clearly linear velocity increase with current that extrapolates to zero and a constant deflection angle, reaching high velocities up to 200 m/s. The mobility increases and the depinning threshold current decreases with the skyrmion diameter, which we vary using an external magnetic field. An analytical model based on the Thiele equation quantitatively reproduces these findings with a single fitting parameter. This validates the linear flow regime description and shows, in particular, the important role of skyrmion size in its dynamics. △ Less

Submitted 21 September, 2022; v1 submitted 8 August, 2022; originally announced August 2022.

Comments: Supplemental information available from the final journal

Journal ref: Phys. Rev. B 106, 144408 (2022)

Organic charged polaritons in the ultrastrong coupling regime

Authors: Mao Wang, Suman Mallick, Anton Frisk Kockum, Karl Börjesson

Abstract: We embedded an all-hydrocarbon-based carbocation in a metallic microcavity that was tuned to resonance with an electronic transition of the carbocation. The measured Rabi splitting was 41% of the excitation energy, putting the system well into the ultrastrong coupling regime. Importantly, due to the intrinsic charge on the carbocation, the polaritons that form carry a significant charge fraction… ▽ More We embedded an all-hydrocarbon-based carbocation in a metallic microcavity that was tuned to resonance with an electronic transition of the carbocation. The measured Rabi splitting was 41% of the excitation energy, putting the system well into the ultrastrong coupling regime. Importantly, due to the intrinsic charge on the carbocation, the polaritons that form carry a significant charge fraction $0.55 e_0$ and a large charge-to-mass ratio ($\sim 2400 e_0 / m_0$). Moreover, the ground state of the ultrastrongly coupled system is calculated to carry about one percent of one elementary charge, implying a potentially enhanced charge transport across the molecules. These unique properties of our system, together with its convenient preparation, provide a practical platform to study charged polaritons in the ultrastrong coupling regime. △ Less

Submitted 31 August, 2021; originally announced August 2021.

Comments: 6+4 pages, 4+3 figures

Journal ref: Physical Review Research 4, 023016 (2022)

Driving skyrmions with low threshold current density in Pt/CoFeB thin film

Authors: Brindaban Ojha, Sougata Mallick, Sujit Kumar Panigrahy, Minaxi Sharma, André Thiaville, Stanislas Rohart, Subhankar Bedanta

Abstract: Magnetic skyrmions are topologically stable spin swirling particle like entities which are appealing for next generation spintronic devices. The expected low critical current density for the motion of skyrmions makes them potential candidates for future energy efficient electronic devices. Several heavy metal/ferromagnetic (HM/FM) systems have been explored in the past decade to achieve faster sky… ▽ More Magnetic skyrmions are topologically stable spin swirling particle like entities which are appealing for next generation spintronic devices. The expected low critical current density for the motion of skyrmions makes them potential candidates for future energy efficient electronic devices. Several heavy metal/ferromagnetic (HM/FM) systems have been explored in the past decade to achieve faster skyrmion velocity at low current densities. In this context, we have studied Pt/CoFeB/MgO heterostructures in which skyrmions have been stabilized at room temperature (RT). It has been observed that the shape of the skyrmions are perturbed even by the small stray field arising from low moment magnetic tips while performing the magnetic force microscopy (MFM), indicating presence of low pinning landscape in the samples. This hypothesis is indeed confirmed by the low threshold current density to drive the skyrmions in our sample, at velocities of few 10m/s. △ Less

Submitted 23 January, 2023; v1 submitted 4 June, 2021; originally announced June 2021.

Degenerate skyrmionic states in synthetic antiferromagnets

Authors: Mona Minakshee Manjaree Bhukta, Braj Bhusan Singh, Sougata Mallick, Stanislas Rohart, Subhankar Bedanta

Abstract: Topological magnetic textures, characterized by integer topological charge $S$, are potential candidates in future magnetic logic and memory devices, due to their smaller size and expected low threshold current density for their motion. An essential requirement to stabilize them is the Dzyaloshinskii-Moriya interaction (DMI) which promotes a particular chirality, leading to a unique value of $S$ i… ▽ More Topological magnetic textures, characterized by integer topological charge $S$, are potential candidates in future magnetic logic and memory devices, due to their smaller size and expected low threshold current density for their motion. An essential requirement to stabilize them is the Dzyaloshinskii-Moriya interaction (DMI) which promotes a particular chirality, leading to a unique value of $S$ in a given material. However, recently coexistence of skyrmions and antiskyrmions, with opposite topological charge, in frustrated ferromagnets has been predicted using $J_1$--$J_2$--$J_3$ classical Heisenberg model, which opens new perspectives, to use the topological charge as an additional degree of freedom. In this work, we propose another approach of using a synthetic antiferromagnetic (SAF) system, where one of the ferromagnetic (FM) layer has isotropic and the other FM layer has anisotropic DMI to promote the existence of skyrmions and antiskyrmions, respectively. A frustrated interaction arises due to the coupling between the magnetic textures in the FM layers, which enables the stabilization and coexistence of 6 novel elliptical topological textures. △ Less

Submitted 29 October, 2022; v1 submitted 14 October, 2020; originally announced October 2020.

Comments: 11 pages

Skyrmion Racetrack memory with an antidot

Authors: Aroop Kumar Behera, Chandrasekhar Murapaka, Sougata Mallick, Braj Bhusan Singh, Subhankar Bedanta

Abstract: Skyrmion racetrack memory has a lots of potential in future non-volatile solid state devices. In general such devices require current to nucleate skyrmions via spin transfer torque (STT) effect. Further the current is also required to drive the skyrmions in the nanowire device. However the current applied during nucleation of successive skyrmions may have unwanted perturbation \emph{viz.} Joule he… ▽ More Skyrmion racetrack memory has a lots of potential in future non-volatile solid state devices. In general such devices require current to nucleate skyrmions via spin transfer torque (STT) effect. Further the current is also required to drive the skyrmions in the nanowire device. However the current applied during nucleation of successive skyrmions may have unwanted perturbation \emph{viz.} Joule heating and skyrmion Hall effect, on the propagation of previously generated skyrmions. Therefore new methodology is required to decouple the generation and propagation of skyrmions. Here we present a novel route via micromagnetic simulation for generation of skyrmions from triangular antidot structure in a ferromagnetic nanotrack using local oersted field. Antidots are holes in a magnetic nanoelement. Controlled skyrmion injection can be achieved by tuning the dimensions of the antidots that are placed at either end of the nanotrack. Multiple skyrmions can be simultaneously generated by incorporating more number of antidots. Here we propose a novel design to realise skyrmionic racetrcak memory where one can individually generate and manipulate the skyrmions within the nanotrack. △ Less

Submitted 26 August, 2020; v1 submitted 8 July, 2019; originally announced July 2019.

Tunability of domain structure and magnonic spectra in antidot arrays of Heusler alloy

Authors: Sougata Mallick, Sucheta Mondal, Takeshi Seki, Sourav Sahoo, Thomas Forrest, Francesco Maccherozzi, Zhenchao Wen, Saswati Barman, Anjan Barman, Koki Takanashi, Subhankar Bedanta

Abstract: Materials suitable for magnonic crystals demand low magnetic damping and long spin wave (SW) propagation distance. In this context Co based Heusler compounds are ideal candidates for magnonic based applications. In this work, antidot arrays (with different shapes) of epitaxial $\mathrm{Co}_2\mathrm{Fe}_{0.4}\mathrm{Mn}_{0.6}\mathrm{Si}$ (CFMS) Heusler alloy thin films have been prepared using e-be… ▽ More Materials suitable for magnonic crystals demand low magnetic damping and long spin wave (SW) propagation distance. In this context Co based Heusler compounds are ideal candidates for magnonic based applications. In this work, antidot arrays (with different shapes) of epitaxial $\mathrm{Co}_2\mathrm{Fe}_{0.4}\mathrm{Mn}_{0.6}\mathrm{Si}$ (CFMS) Heusler alloy thin films have been prepared using e-beam lithography and sputtering technique. Magneto-optic Kerr effect and ferromagnetic resonance analysis have confirmed the presence of dominant cubic and moderate uniaxial magnetic anisotropies in the thin films. Domain imaging via x-ray photoemission electron microscopy on the antidot arrays reveals chain like switching or correlated bigger domains for different shape of the antidots. Time-resolved MOKE microscopy has been performed to study the precessional dynamics and magnonic modes of the antidots with different shapes. We show that the optically induced spin-wave spectra in such antidot arrays can be tuned by changing the shape of the holes. The variation in internal field profiles, pinning energy barrier, and anisotropy modifies the spin-wave spectra dramatically within the antidot arrays with different shapes. We further show that by combining the magnetocrystalline anisotropy with the shape anisotropy, an extra degree of freedom can be achieved to control the magnonic modes in such antidot lattices. △ Less

Submitted 5 July, 2019; originally announced July 2019.

Journal ref: Phys. Rev. Applied (2019)

A novel chiral spin texture: Antiferromagnetic Skyrmionium

Authors: Mona Minakshee Manjaree Bhukta, Abhilash Mishra, Gajanan Pradhan, Sougata Mallick, Braj Bhusan Singh, Subhankar Bedanta

Abstract: Exotic spin textures viz. chiral domain wall, vortices, skyrmion, skyrmionium, etc. have recently emerged as active field of research because of their potential applications in high density data storage technology and logic gate computing. Magnetic skyrmionium is a skyrmion like soliton, which carries zero topological quantum number. Skyrmioniums are superior to conventional skyrmions in ferromagn… ▽ More Exotic spin textures viz. chiral domain wall, vortices, skyrmion, skyrmionium, etc. have recently emerged as active field of research because of their potential applications in high density data storage technology and logic gate computing. Magnetic skyrmionium is a skyrmion like soliton, which carries zero topological quantum number. Skyrmioniums are superior to conventional skyrmions in ferromagnets due to their negligible skyrmion hall effect and higher velocity. The physical properties of both skyrmion and skyrmionium have been investigated rigorously in ferromagnetic systems. Recent observations hint that such chiral spin structures in antiferromagnetic (AFM) systems are more promising in comparison to the ferromagnetic ones because of their robustness towards external perturbation, absence of Skyrmion hall effect, etc. However skyrmionium in AFM materials are not reported in literature so far. In this work, we demonstrate that skyrmionium can be created and stabilized in AFM materials by application of spin polarized current in an experimentally feasible geometry. We have further studied the dynamics of AFM skyrmionium by applying spin polarized current. △ Less

Submitted 3 November, 2018; v1 submitted 18 October, 2018; originally announced October 2018.

Comments: 9 pages, 7 figures

Size and shape of skyrmions for variable Dzyaloshinskii-Moriya interaction and uniaxial anisotropy

Authors: Aroop Kumar Behera, Swapna Sindhu Mishra, Sougata Mallick, Braj Bhusan Singh, Subhankar Bedanta

Abstract: We have performed micromagnetic simulations to study the formation of skyrmions in ferromagnetic elements with different shapes having perpendicular anisotropy. The strength of Dzyaloshinskii-Moriya interaction (D) and uniaxial anisotropy (K) are varied to elucidate the regime in which skyrmion formation can take place. It is found that for a certain combination of D and K skyrmion formation does… ▽ More We have performed micromagnetic simulations to study the formation of skyrmions in ferromagnetic elements with different shapes having perpendicular anisotropy. The strength of Dzyaloshinskii-Moriya interaction (D) and uniaxial anisotropy (K) are varied to elucidate the regime in which skyrmion formation can take place. It is found that for a certain combination of D and K skyrmion formation does not happen. Further we also observed that for large D and small K values, finite size effect dominates which in turn hinders formation of typical N'eel (spherical) skyrmions. However the resulting magnetic phase is skyrmionic in nature and has different shape. We also have found that the shape of the magnetic nano element has a significant role in determining the final magnetic state in addition to the competing D and K values. △ Less

Submitted 27 April, 2018; originally announced April 2018.

Comments: 7 pages, 7 figures

Journal ref: J. Phys. D: Appl. Phys. 51 285001 (2018)

Exact solution of an exclusion process with three classes of particles and vacancies

Authors: K. Mallick, S. Mallick, N. Rajewsky

Abstract: We present an exact solution for an asymmetric exclusion process on a ring with three classes of particles and vacancies. Using a matrix product Ansatz, we find explicit expressions for the weights of the configurations in the stationary state. The solution involves tensor products of quadratic algebras. We present an exact solution for an asymmetric exclusion process on a ring with three classes of particles and vacancies. Using a matrix product Ansatz, we find explicit expressions for the weights of the configurations in the stationary state. The solution involves tensor products of quadratic algebras. △ Less

Submitted 16 March, 1999; originally announced March 1999.

Comments: 18 pages, no figures, LaTeX

Report number: RU-3-16-99