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Non-trivial phonon dynamics and significant electron-phonon coupling of the high frequency modes in a Dirac semimetal
Authors:
Debasmita Swain,
Akash Dey,
Anushree Roy,
Kush Saha,
Sitikantha D. Das
Abstract:
Using finite temperature Raman spectroscopy, we investigate the electron-phonon interactions
(EPI) and phonon-phonon scattering dynamics in the Dirac semimetal Cd3As2 in different fre quency regimes. Strong softening of the Raman shifts below 200 K is observed for almost all the
phonon modes with a marked deviation from the standard anharmonic behavior. The experimen tally observed Raman linew…
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Using finite temperature Raman spectroscopy, we investigate the electron-phonon interactions
(EPI) and phonon-phonon scattering dynamics in the Dirac semimetal Cd3As2 in different fre quency regimes. Strong softening of the Raman shifts below 200 K is observed for almost all the
phonon modes with a marked deviation from the standard anharmonic behavior. The experimen tally observed Raman linewidth seems to be captured well by a combination of EPI, relevant at
low temperature (LT) and phonon-phonon scattering, which is predominant at high temperatures
(HT), leading to an observable minima in the thermal evolution of the linewidth. While this fea ture is most prominently observed in the highest-frequency Raman mode (196 cm-1), its intensity
gradually diminishes as the Raman frequency decreases. Computation of the electronic contribution
to the phonon linewidth, for both the high and low frequency modes, from the phonon self-energy
shows that it qualitatively mimics the experimental observations. It is found that phonon-induced
interband scattering results in the presence of a maxima in phonon linewidth that crucially depends
on the finiteness of the chemical potential.
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Submitted 3 March, 2025;
originally announced March 2025.
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Evidences for local non-centrosymmetricity and strong phonon anomaly in EuCu2As2: A Raman spectroscopy and lattice dynamics study
Authors:
Debasmita Swain,
Mainak Dey Sarkar,
Andrzej Ptok,
G. Vaitheeswaran,
Anushree Roy,
Sitikantha D Das
Abstract:
Phonon modes and their association with the electronic states have been investigated for the metallic EuCu$_{2}$As$_{2}$ system. In this work, we present the Raman spectra of this pnictide system which clearly shows the presence of seven well defined peaks above $100$~cm$^{-1}$ that is consistent with the locally non-centrosymmetric {\it P4/nmm} crystal structure, contrary to that what is expected…
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Phonon modes and their association with the electronic states have been investigated for the metallic EuCu$_{2}$As$_{2}$ system. In this work, we present the Raman spectra of this pnictide system which clearly shows the presence of seven well defined peaks above $100$~cm$^{-1}$ that is consistent with the locally non-centrosymmetric {\it P4/nmm} crystal structure, contrary to that what is expected from the accepted symmorphic {\it I4/mmm} structure. Lattice dynamics calculations using the {\it P4/nmm} symmetry attest that there is a commendable agreement between the calculated phonon spectra at the $Γ$ point and the observed Raman mode frequencies, with the most intense peak at $\sim 232$~cm$^{-1}$ being ascribed to the A$_{1g}$ mode. Temperature dependent Raman measurements show that there is a significant deviation from the expected anharmonic behaviour around $165$~K for the A$_{1g}$ mode, with anomalies being observed for several other modes as well, although to a lesser extent. Attempts are made to rationalize the observed anomalous behavior related to the hardening of the phonon modes, with parallels being drawn from metal dichalcogenide and allied systems. Similarities in the evolution of the Raman peak frequencies with temperature seem to suggest a strong signature of a subtle electronic density wave instability below $165$~K in this compound.
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Submitted 9 January, 2025;
originally announced January 2025.
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Possible signature of broken symmetry state near the quantum critical point in P doped BaFe$_2$As$_2$: A Raman spectroscopy study
Authors:
Debasmita Swain,
Soumyadeep Ghosh,
K. Bera,
Sven Friedemann,
Haranath Ghosh,
Anushree Roy,
Sitikantha D Das
Abstract:
We study the iron-pnictide compound BaFe$_2$(As$_{1-x}$P$_x$)$_2$ for x $\sim$0.23, with a doping concentration near quantum criticality and enhanced nematic fluctuating state in the doping-temperature phase diagram. Transport measurements confirm the presence of a magneto-structural transition at 60K from the tetragonal to the orthorhombic phase, followed by a superconducting transition below 16K…
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We study the iron-pnictide compound BaFe$_2$(As$_{1-x}$P$_x$)$_2$ for x $\sim$0.23, with a doping concentration near quantum criticality and enhanced nematic fluctuating state in the doping-temperature phase diagram. Transport measurements confirm the presence of a magneto-structural transition at 60K from the tetragonal to the orthorhombic phase, followed by a superconducting transition below 16K. The temperature and polarisation dependent Raman spectra reveal that there is a phonon mode at 211 cm$^{-1}$, followed by two broad modes (BM) between 400 and 700 cm$^{-1}$, having an energy difference of 15 meV, in the temperature range between 300K and 80K. In the non-superconducting state, the phonon mode exhibits expected polarization dependence as well as temperature evolution due to anharmonicity, strong anisotropic and thermally inert behaviour are observed for the BM. Electronic structure calculations for doped and undoped BaFe$_2$As$_2$ show that while Fe $d_{xz}$ and $d_{yz}$ orbitals do not split in the tetragonal phase, the splitting energy is 13.5 meV in the orthorhombic phase of the doped system, which is reasonably close to the experimentally observed value of the energy separation of the BM. We believe that reported BM possibly are the signature of electronic Raman scattering involving the crystal field levels of $d$-orbitals of Fe$^{2+}$ due to local breaking of the C$_4$ symmetry of the parent compound in the doped system.
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Submitted 7 April, 2022;
originally announced April 2022.
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Magnetic anisotropy of the alkali iridate Na$_{2}$IrO$_{3}$ at high magnetic fields: evidence for strong ferromagnetic Kitaev correlations
Authors:
Sitikantha. D Das,
Sarbajaya Kundu,
Zengwei Zhu,
Eundeok Mun,
Ross D. McDonald,
Gang Li,
Luis Balicas,
Alix McCollam,
Gang Cao,
Jeffrey. G. Rau,
Hae-Young Kee,
Vikram Tripathi,
Suchitra E. Sebastian
Abstract:
The magnetic field response of the Mott-insulating honeycomb iridate Na$_{2}$IrO$_{3}$ is investigated using torque magnetometry measurements in magnetic fields up to 60 tesla. A peak-dip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering ($\approx 15~K$) temperature. Using exact diagonalization calculations, we show that s…
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The magnetic field response of the Mott-insulating honeycomb iridate Na$_{2}$IrO$_{3}$ is investigated using torque magnetometry measurements in magnetic fields up to 60 tesla. A peak-dip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering ($\approx 15~K$) temperature. Using exact diagonalization calculations, we show that such a distinctive signature in the torque response constrains the effective spin models for these classes of Kitaev materials to ones with dominant ferromagnetic Kitaev interactions, while alternative models with dominant antiferromagnetic Kitaev interactions are excluded. We further show that at high magnetic fields, long range spin correlation functions decay rapidly, signaling a transition to a long-sought-after field-induced quantum spin liquid beyond the peak-dip structure. Kitaev systems are thus revealed to be excellent candidates for field-induced quantum spin liquids, similar physics having been suggested in another Kitaev material $α-$RuCl$_{3}$.
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Submitted 24 October, 2018; v1 submitted 10 August, 2017;
originally announced August 2017.
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A Kondo route to spin inhomogeneities in the honeycomb Kitaev model
Authors:
S. D. Das,
K. Dhochak,
V. Tripathi
Abstract:
Paramagnetic impurities in a quantum spin-liquid can result in Kondo effects with highly unusual properties. We have studied the effect of locally exchange-coupling a paramagnetic impurity with the spin-1/2 honeycomb Kitaev model in its gapless spin-liquid phase. The (impurity) scaling equations are found to be insensitive to the sign of the coupling. The weak and strong coupling fixed points are…
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Paramagnetic impurities in a quantum spin-liquid can result in Kondo effects with highly unusual properties. We have studied the effect of locally exchange-coupling a paramagnetic impurity with the spin-1/2 honeycomb Kitaev model in its gapless spin-liquid phase. The (impurity) scaling equations are found to be insensitive to the sign of the coupling. The weak and strong coupling fixed points are stable, with the latter corresponding to a noninteracting vacancy and an interacting, spin-1 defect for the antiferromagnetic and ferromagnetic cases respectively. The ground state in the strong coupling limit in both cases has a nontrivial topology associated with a finite Z2 flux at the impurity site. For the antiferromagnetic case, this result can be obtained straightforwardly owing to the integrability of the Kitaev model with a vacancy. The strong-coupling limit of the ferromagnetic case is however nonintegrable, and we address this problem through exact-diagonalization calculations with finite Kitaev fragments. Our exact diagonalization calculations indicate that that the weak to strong coupling transition and the topological phase transition occur rather close to each other and are possibly coincident. We also find an intriguing similarity between the magnetic response of the defect and the impurity susceptibility in the two-channel Kondo problem.
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Submitted 18 September, 2015;
originally announced September 2015.
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Quantum Criticality in the 122 Iron Pnictide Superconductors Emerging from Orbital-Selective Mottness
Authors:
S. D. Das,
M. S. Laad,
L. Craco,
J. Gillett,
V. Tripathi,
S. E. Sebastian
Abstract:
The twin issues of the nature of the normal state and competing order(s) in the iron arsenides are central to understanding their unconventional, high-Tc superconductivity. We use a combination of transport anisotropy measurements on detwinned Sr(Fe(1-x)Co(x))2As2 single crystals and local density approximation plus dynamical mean field theory (LDA + DMFT) calculations to revisit these issues. The…
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The twin issues of the nature of the normal state and competing order(s) in the iron arsenides are central to understanding their unconventional, high-Tc superconductivity. We use a combination of transport anisotropy measurements on detwinned Sr(Fe(1-x)Co(x))2As2 single crystals and local density approximation plus dynamical mean field theory (LDA + DMFT) calculations to revisit these issues. The peculiar resistivity anisotropy and its evolution with x are naturally interpreted in terms of an underlying orbital-selective Mott transition (OSMT) that gaps out the dxz or dyz states. Further, we use a Landau-Ginzburg approach using LDA + DMFT input to rationalize a wide range of anomalies seen up to optimal doping, providing strong evidence for secondary electronic nematic order. These findings suggest that strong dynamical fluctuations linked to a marginal quantum-critical point associated with this OSMT and a secondary electronic nematic order constitute an intrinsically electronic pairing mechanism for superconductivity in Fe arsenides.
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Submitted 14 October, 2015; v1 submitted 2 December, 2014;
originally announced December 2014.
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Andreev Reflection Like Enhancement Above Bulk $T_c$ in Electron Underdoped Iron Arsenides
Authors:
H. Z. Arham,
C. R. Hunt,
J. Gillett,
S. D. Das,
S. E. Sebastian,
D. Y. Chung,
M. G. Kanatzidis,
L. H. Greene
Abstract:
We use point contact spectroscopy (PCS) to probe the superconducting properties of electron doped $\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\rm{x = 0.05, 0.055, 0.07, 0.08}$) and hole doped $\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$. PCS directly probes the low energy density of states via Andreev reflection, revealing two distinct superconducting gaps in both compound families. Apart from the electron underdoped…
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We use point contact spectroscopy (PCS) to probe the superconducting properties of electron doped $\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\rm{x = 0.05, 0.055, 0.07, 0.08}$) and hole doped $\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$. PCS directly probes the low energy density of states via Andreev reflection, revealing two distinct superconducting gaps in both compound families. Apart from the electron underdoped $\rm{Ba(Fe_{1-x}Co_{x})_2As_2}$, the excess current due to Andreev reflection for the compounds follows the typical BCS temperature dependence. For underdoped $\rm{Ba(Fe_{1-x}Co_{x})_2As_2}$, the temperature dependence of the excess current deviates from that of BCS, developing a tail at higher temperatures and surviving above bulk $T_c$. Possible explanations for this anomalous behavior are explored.
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Submitted 7 July, 2013;
originally announced July 2013.
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Detection of Orbital Fluctuations Above the Structural Transition Temperature in the Iron-Pnictides and Chalcogenides
Authors:
H. Z. Arham,
C. R. Hunt,
W. K. Park,
J. Gillett,
S. D. Das,
S. E. Sebastian,
Z. J. Xu,
J. S. Wen,
Z. W. Lin,
Q. Li,
G. Gu,
A. Thaler,
S. Ran,
S. L. Bud'ko,
P. C. Canfield,
D. Y. Chung,
M. G. Kanatzidis,
L. H. Greene
Abstract:
We use point contact spectroscopy to probe $\rm{AEFe_2As_2}$ ($\rm{AE=Ca, Sr, Ba}$) and $\rm{Fe_{1+y}Te}$. For $\rm{AE=Sr, Ba}$ we detect orbital fluctuations above $T_S$ while for AE=Ca these fluctuations start below $T_S$. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temperatures where an in-plane resistive anisotr…
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We use point contact spectroscopy to probe $\rm{AEFe_2As_2}$ ($\rm{AE=Ca, Sr, Ba}$) and $\rm{Fe_{1+y}Te}$. For $\rm{AE=Sr, Ba}$ we detect orbital fluctuations above $T_S$ while for AE=Ca these fluctuations start below $T_S$. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temperatures where an in-plane resistive anisotropy is known to exist. We predict an in-plane resistive anisotropy of $\rm{Fe_{1+y}Te}$ above $T_S$. Our data are examined in light of the recent work by W.-C. Lee and P. Phillips (arXiv:1110.5917v2). We also study how joule heating in the PCS junctions impacts the spectra. Spectroscopic information is only obtained from those PCS junctions that are free of heating effects while those PCS junctions that are in the thermal regime display bulk resistivity phenomenon.
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Submitted 4 June, 2012; v1 submitted 12 January, 2012;
originally announced January 2012.
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Isothermal magnetic entropy behavior in Tb5Si3: Sign reversal and non-monotonic variation with temperature, and implications
Authors:
Niharika Mohapatraa,
Sitikantha D. Das,
K. Mukherjee,
E. V. Sampathkumaran
Abstract:
The magnetic entropy change (\DeltaS), a measure of the magnetocaloric effect, in Tb5Si3, a compound exhibiting unusual positive magnetoresistance following a magnetic-field-induced transition below magnetic transition temperature (~ 69 K), has been investigated. We found that \DeltaS is negative in the paramagnetic state. At the magnetic transition temperature, \DeltaS shows sign reversal from ne…
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The magnetic entropy change (\DeltaS), a measure of the magnetocaloric effect, in Tb5Si3, a compound exhibiting unusual positive magnetoresistance following a magnetic-field-induced transition below magnetic transition temperature (~ 69 K), has been investigated. We found that \DeltaS is negative in the paramagnetic state. At the magnetic transition temperature, \DeltaS shows sign reversal from negative (in the paramagnetic state) to positive value in the magnetically ordered state. The high-field state which is interestingly the high resistive state is found to be associated with higher entropy i.e. large positive \DeltaS, behaving like a paramagnet. On the basis of this observation, we conclude that the magnetic field induces magnetic fluctuations in the system resulting in positive magnetoresistance, thereby rendering support to the idea of 'inverse metamagnetism' in this compound. In addition, we note that Arrott plots present an interesting scenario.
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Submitted 28 November, 2011;
originally announced November 2011.
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Gap-like feature in the normal state of $\bf{X(Fe_{1-x}Co_x)_2As_2}$, $\bf{X=Ba,Sr}$ and $\bf{Fe_{1+y}Te}$ revealed by Point Contact Spectroscopy
Authors:
H. Z. Arham,
C. R. Hunt,
W. K. Park,
J. Gillett,
S. D. Das,
S. E. Sebastian,
Z. J. Xu,
J. S. Wen,
Z. W. Lin,
Q. Li,
G. Gu,
A. Thaler,
S. L. Budko,
P. C. Canfield,
L. H. Greene
Abstract:
Point contact spectroscopy reveals a gap-like feature above the magnetic and structural transition temperatures for underdoped $Ba(Fe_{1-x}Co_x)_2As_2$, $SrFe_2As_2$ and $Fe_{1+y}Te$. The conductance spectrum starts showing an enhancement at temperatures as high as 177 K for $BaFe_2As_2$ ($T_N$ $\sim$ 132 K) and 250 K for $SrFe_2As_2$ ($T_N$ $\sim$ 192 K). Possible origins for this enhancement are…
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Point contact spectroscopy reveals a gap-like feature above the magnetic and structural transition temperatures for underdoped $Ba(Fe_{1-x}Co_x)_2As_2$, $SrFe_2As_2$ and $Fe_{1+y}Te$. The conductance spectrum starts showing an enhancement at temperatures as high as 177 K for $BaFe_2As_2$ ($T_N$ $\sim$ 132 K) and 250 K for $SrFe_2As_2$ ($T_N$ $\sim$ 192 K). Possible origins for this enhancement are discussed in light of recent experimental claims of nematicity in these materials. We construct a modified phase diagram for Co-doped Ba122 showing a gap-like feature existing above $T_N$ and $T_S$ for the underdoped regime.
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Submitted 12 August, 2011;
originally announced August 2011.
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Magnetism of fine particles of Kondo lattices, obtained by high-energy ball-milling
Authors:
E. V. Sampathkumaran,
K. Mukherjee,
Kartik K Iyer,
Niharika Mohapatra,
Sitikantha D Das
Abstract:
Despite intense research in the field of strongly correlated electron behavior for the past few decades, there has been very little effort to understand this phenomenon in nano particles of the Kondo lattices. In this article, we review the results of our investigation on the fine particles (less than 1 micron) of some of the alloys obtained by high-energy ball-milling to bring out that this synth…
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Despite intense research in the field of strongly correlated electron behavior for the past few decades, there has been very little effort to understand this phenomenon in nano particles of the Kondo lattices. In this article, we review the results of our investigation on the fine particles (less than 1 micron) of some of the alloys obtained by high-energy ball-milling to bring out that this synthetic method paves a way to study strong electron correlations in nanocrystals of such alloys. We primarily focus on the alloys of the series, CeRu(2-x)Rh(x)Si2, lying at different positions in Doniach's magnetic phase diagram. While CeRu2Si2, a bulk paramagnet, appears to become magnetic (of a glassy type) below about 8 K in fine particle form, in CeRh2Si2, an antiferromagnet (T_N= 36 K) in bulk form, magnetism is destroyed (at least down to 0.5 K) in fine particles. In the alloy, CeRu(0.8)Rh(1.2)Si2, at the quantum critical point, no long range magnetic ordering is found
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Submitted 10 January, 2011;
originally announced January 2011.
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Itinerant Spin Excitations in SrFe2As2 Measured by Inelastic Neutron Scattering
Authors:
R. A. Ewings,
T. G. Perring,
J. Gillett,
S. D. Das,
S. E. Sebastian,
A. E. Taylor,
T. Guidi,
A. T. Boothroyd
Abstract:
We report inelastic neutron scattering measurements of the magnetic excitations in SrFe2As2, the parent of a family of iron-based superconductors. The data extend throughout the Brillouin zone and up to energies of ~260meV. An analysis with the local-moment J_1-J2 model implies very different in-plane nearest-neighbor exchange parameters along the $a$ and $b$ directions, both in the orthorhombic a…
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We report inelastic neutron scattering measurements of the magnetic excitations in SrFe2As2, the parent of a family of iron-based superconductors. The data extend throughout the Brillouin zone and up to energies of ~260meV. An analysis with the local-moment J_1-J2 model implies very different in-plane nearest-neighbor exchange parameters along the $a$ and $b$ directions, both in the orthorhombic and tetragonal phases. However, the spectrum calculated from the J1-J2 model deviates significantly from our data. We show that the qualitative features that cannot be described by the J1-J2 model are readily explained by calculations from a 5-band itinerant mean-field model.
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Submitted 16 June, 2011; v1 submitted 16 November, 2010;
originally announced November 2010.
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Dimensional Tuning of the Magnetic-Structural Transition in A(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (A=Sr,Ba)
Authors:
Jack Gillett,
Sitikantha D. Das,
Paul Syers,
Alison K. T. Ming,
Jose I. Espeso,
Chiara M. Petrone,
Suchitra E. Sebastian
Abstract:
A phase diagram of superconducting Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ as a function of doping (x) is determined by a series of thermodynamic and transport measurements on single crystals. On comparison with a similar phase diagram for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (Co-doped Ba122), we find that the increased dimensionality of Co-doped Sr122 results in a single first-order-like transition where the ma…
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A phase diagram of superconducting Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ as a function of doping (x) is determined by a series of thermodynamic and transport measurements on single crystals. On comparison with a similar phase diagram for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (Co-doped Ba122), we find that the increased dimensionality of Co-doped Sr122 results in a single first-order-like transition where the magnetic and structural transitions coincide, unlike the case of Co-doped Ba122 that exhibits split quasicontinuous magnetic and structural transitions. We relate this dimensionally-tuned splitting in the magnetic and structural transitions to the relative size of superconducting temperatures in these materials.
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Submitted 8 May, 2010;
originally announced May 2010.
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An anomalous butterfly-shaped magnetoresistance loop in an alloy, Tb4LuSi3
Authors:
K. Mukherjee,
Sitikantha D Das,
Niharika Mohapatra,
Kartik K Iyer,
E. V. Sampathkumaran
Abstract:
Magnetic-field (H) induced first-order magnetic transition and the assiciated electronic phase-separation phenomena are active topics of research in magnetism. Magnetoresistance (MR) is a key property to probe these phenomena and, in literature, a butterfly-shaped MR loop has been noted while cycling the field, with the envelope curve lying below the virgin curve in MR versus H plots of such mat…
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Magnetic-field (H) induced first-order magnetic transition and the assiciated electronic phase-separation phenomena are active topics of research in magnetism. Magnetoresistance (MR) is a key property to probe these phenomena and, in literature, a butterfly-shaped MR loop has been noted while cycling the field, with the envelope curve lying below the virgin curve in MR versus H plots of such materials. Here, we report an opposite behavior of MR loop for an alloy, Tb4LuSi3, at low temperatures (<<20 K) in the magnetically ordered state. Such an anomalous curve reveals unexpected domination of higher resistive high-field phase in electronic conduction, unlike in other materials where conducion is naturally by low-resistive high-field phase that follows first-order transition. The observed features reveal an unusual electronic phase separation, namely involving high-resistive high-field phase and low-resistive virgin phase.
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Submitted 27 January, 2010;
originally announced January 2010.
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Influence of pressure on the magnetic behavior and the anomalous magnetoresistance in Tb5Si3
Authors:
Niharika Mohapatra,
Sitikantha D Das,
K. Mukherjee,
Kartik K Iyer,
E. V. Sampathkumaran
Abstract:
The compound, Tb5Si3, crystallizing in Mn5Si3-type hexagonal structure, was recently reported by us to exhibit a sudden and huge enhancement in electrical resistivity (rho) at a critical magnetic field (H_cr) in the magnetically ordered state (<70 K) tracking isothermal magnetization (M) behavior. We have investigated the influence of external pressure (<15 kbar) and negative chemical pressure i…
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The compound, Tb5Si3, crystallizing in Mn5Si3-type hexagonal structure, was recently reported by us to exhibit a sudden and huge enhancement in electrical resistivity (rho) at a critical magnetic field (H_cr) in the magnetically ordered state (<70 K) tracking isothermal magnetization (M) behavior. We have investigated the influence of external pressure (<15 kbar) and negative chemical pressure induced by Ge substitution for Si on M and rho as a function of temperature (5-300 K) and magnetic field (<120 kOe), with the primary aim of understanding the field-induced anomalies. Focussing on isothermal M and magnetoresistance (MR) at two temperatures, 5 and 20K, we find that this rho anomaly persists under external as well as negative chemical pressures, however with a large change in the H_cr. The pressure-derivative of H_cr is negative and this trend and the MR behavior at the H_cr are comparable to that observed in some Laves phase itinerant magnetic systems. On the basis of this observation, we speculate that the magnetic fluctuations induced at this critical field could be responsible for the MR anomal.ies
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Submitted 11 December, 2009;
originally announced December 2009.
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Magnetic behavior of nanocrystalline ErCo2
Authors:
Sitikantha D Das,
Niharika Mohapatra,
Kartik K Iyer,
R. D. Bapat,
E. V. Sampathkumaran
Abstract:
We have investigated the magnetic behavior of the nanocrystalline form of a well-known Laves phase compound, ErCo2 - the bulk form of which has been known to undergo an interesting first-order ferrimagnetic ordering near 32 K - synthesized by high-energy ball-milling. It is found that, in these nanocrystallites, Co exhibits ferromagnetic order at room temperature as inferred from the magnetizati…
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We have investigated the magnetic behavior of the nanocrystalline form of a well-known Laves phase compound, ErCo2 - the bulk form of which has been known to undergo an interesting first-order ferrimagnetic ordering near 32 K - synthesized by high-energy ball-milling. It is found that, in these nanocrystallites, Co exhibits ferromagnetic order at room temperature as inferred from the magnetization data. However, the magnetic transition temperature for Er sublattice remains essentially unaffected as though the (Er)4f-Co(3d) coupling is weak on Er magnetism. The net magnetic moment as measured at high fields, sat at 120 kOe, is significantly reduced with respect to that for the bulk in the ferrimagnetically ordered state and possible reasons are outlined. We have also compared the magnetocaloric behavior for the bulk and the nano particles.
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Submitted 26 June, 2009;
originally announced June 2009.
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Magnetic ordering in the fine particles of some bulk Pauli-paramgnets
Authors:
Sitikantha D. Das,
S. Narayana Jammalamadaka,
Kartik K Iyer,
E. V. Sampathkumaran
Abstract:
Fine particles of some of the extensively studied 'enhanced' Pauli paramagnets - LuCo2, ScCo2, ZrCo2 and CeRu2Si2, have been prepared by high-energy ball-milling and investigated for their magnetic behavior. In the case of Co systems, these particles show ferromagnetic features even at room temperature. In the case of CeRu2Si2, a well-known Kondo lattice which is non-magnetic in the bulk form, f…
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Fine particles of some of the extensively studied 'enhanced' Pauli paramagnets - LuCo2, ScCo2, ZrCo2 and CeRu2Si2, have been prepared by high-energy ball-milling and investigated for their magnetic behavior. In the case of Co systems, these particles show ferromagnetic features even at room temperature. In the case of CeRu2Si2, a well-known Kondo lattice which is non-magnetic in the bulk form, features attributable to the onset of magnetic order below about 8 K could be seen in the fine particles. The magnitudes of the magnetic moment of these small particles at high fields are sufficiently large to conclude that the observed magnetic ordering is not just a surface effect. This transformation to magnetic ordering in small particles could be widespread among enhanced Pauli paramagnets.
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Submitted 26 June, 2009;
originally announced June 2009.
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Magnetic behavior of nano crystals of a spin-chain system, Ca3Co2O6: Absence of multiple steps in the low temperature isothermal magnetization
Authors:
Niharika Mohapatra,
Kartik K Iyer,
Sitikantha D Das,
B. A. Chalke,
S. C. Purandare,
E. V. Sampathkumaran
Abstract:
We report that the major features in the temperature dependence of dc and ac magnetization of a well-known spin-chain compound, Ca3Co2O6, which has been known to exhibit two complex magnetic transitions due to geometrical frustration (one near 24 K and the other near 10 K), are found to be qualitatively unaffected in its nano form synthesized by high-energy ball-milling. However, the multiple st…
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We report that the major features in the temperature dependence of dc and ac magnetization of a well-known spin-chain compound, Ca3Co2O6, which has been known to exhibit two complex magnetic transitions due to geometrical frustration (one near 24 K and the other near 10 K), are found to be qualitatively unaffected in its nano form synthesized by high-energy ball-milling. However, the multiple steps in isothermal magnetization - a topic of current interest in low-dimensional systems - known for the bulk form well below 10 K is absent in the nano particles. We believe that this finding will be useful to the understanding of the 'step' magnetization behavior of such spin-chain systems.
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Submitted 11 April, 2009;
originally announced April 2009.
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Enhancement of positive magnetoresistance following a magnetic-field-induced ferromagnetic transition in an intermetallic compound, Tb5Si3
Authors:
S. Narayana Jammalamadaka,
Niharika Mohapatra,
Sitikantha D Das,
E. V. Sampathkumaran
Abstract:
We report the existence of a field-induced ferromagnetic transition in the magnetically ordered state (<69 K) of an intermetallic compound, Tb5Si3, and this transition is distinctly first-order at 1.8 K (near 60 kOe), whereas it appears to become second order near 20 K. The finding we stress is that the electrical resistivity becomes suddenly large in the high-field state after this transition a…
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We report the existence of a field-induced ferromagnetic transition in the magnetically ordered state (<69 K) of an intermetallic compound, Tb5Si3, and this transition is distinctly first-order at 1.8 K (near 60 kOe), whereas it appears to become second order near 20 K. The finding we stress is that the electrical resistivity becomes suddenly large in the high-field state after this transition and this is observed in the entire temperature range in the magnetically ordered state. Such an enhancement of 'positive' magnetoresistance (below 100 kOe) at the metamagnetic transition field is unexpected on the basis that the application of magnetic field should favor a low-resistive state due to alignment of spins.
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Submitted 1 February, 2009;
originally announced February 2009.
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Magnetic anomalies in Gd6Co1.67Si3 and Tb6Co1.67Si3
Authors:
S. Narayana Jammalamadaka,
Niharika Mohapatra,
Sitikantha D Das,
Kartik K Iyer,
E. V. Sampathkumaran
Abstract:
The compounds, Gd6Co1.67Si3 and Tb6Co1.67Si3, recently reported to form in a Ce6Ni2Si3-derived hexagonal structure (space group: P6_3 / m) and to order magnetically below 295 and 190 K respectively, have been investigated by detailed magnetization (M) studies in the temperature interval 1.8-330 K as a function of magnetic field (H). The points of emphasis are: We observe multiple steps in the M(…
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The compounds, Gd6Co1.67Si3 and Tb6Co1.67Si3, recently reported to form in a Ce6Ni2Si3-derived hexagonal structure (space group: P6_3 / m) and to order magnetically below 295 and 190 K respectively, have been investigated by detailed magnetization (M) studies in the temperature interval 1.8-330 K as a function of magnetic field (H). The points of emphasis are: We observe multiple steps in the M(H) curve for the Tb compound at 1.8 K while increasing H, but these steps do not appear in the reverse cycle of H. At higher temperatures, such steps are absent. However, this 'staircase' behavior of M(H) is not observed for the Gd compound at any temperature and the isothermal magnetization is not hysteretic unlike in Tb compound. From the M(H) data measured at close intervals of temperature, we have derived isothermal entropy change (Delta S) and it is found that Delta S follows a theoretically predicted H^2/3-dependence.
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Submitted 1 February, 2009;
originally announced February 2009.
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Magnetic anomalies in Nd6Co(1.67)Si3: Surprising first order transitions in the low-temperature isothermal magnetization
Authors:
Niharika Mohapatra,
S. Narayana Jammalamadaka,
Sitikantha D. Das,
E. V. Sampathkumaran
Abstract:
We present the results of magnetic measurements on Nd6Co(1.67)Si3, a compound recently reported to crystallize in a hexagonal structure (space group P6_3/m) and to undergo long range magnetic ordering below 84 K. The results reveal that the magnetism of this compound is quite complex with additional magnetic anomalies near 50 and 20 K. There are qualitative changes in the isothermal magnetizatio…
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We present the results of magnetic measurements on Nd6Co(1.67)Si3, a compound recently reported to crystallize in a hexagonal structure (space group P6_3/m) and to undergo long range magnetic ordering below 84 K. The results reveal that the magnetism of this compound is quite complex with additional magnetic anomalies near 50 and 20 K. There are qualitative changes in the isothermal magnetization behavior with the variation of temperature. Notably, there is a field-induced spin reorientation as the temperature is lowered below 20 K. A finding we stress is that this transition is discontinuous for 1.8K in the virgin curve, but the first order character appears only after a field-cycling for a narrow higher temperature range near 5 K. Thus, this compound serves as an example for the stabilisation of first-order transition induced by magnetic-field-cycling. The issues of 'Phase co-existence' and 'meta-stability' after a field-cycling at low temperatures in this compound are also addressed.
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Submitted 1 February, 2009;
originally announced February 2009.
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Magnetic behavior of nanocrystalline LaMn2Ge2
Authors:
S. Narayana Jammalamadaka,
Sitikantha D Das,
B. A. Chalke,
E. V. Sampathkumaran
Abstract:
The compound, LaMn2Ge2, crystallizing in ThCr2Si2-type tetragonal crystal structure, has been known to undergo ferromagnetic order below (T_C=) 326 K. In this article, we report the magnetic behavior of nanocrystalline form of this compound, obtained by high-energy ball-milling. T_C of this compound is reduced maginally for the nanoform, whereas there is a significant reduction of the magnitude…
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The compound, LaMn2Ge2, crystallizing in ThCr2Si2-type tetragonal crystal structure, has been known to undergo ferromagnetic order below (T_C=) 326 K. In this article, we report the magnetic behavior of nanocrystalline form of this compound, obtained by high-energy ball-milling. T_C of this compound is reduced maginally for the nanoform, whereas there is a significant reduction of the magnitude of the saturation magnetic moment with increasing milling time. The coercive field however increases with decreasing particle size. Thus, this work provides a route to tune these parameters by reducing the particle size in this ternary family.
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Submitted 12 June, 2008;
originally announced June 2008.