arXiv subjects

Tapan Chatterji

Publications and source records attributed to Tapan Chatterji.

At least 19 recordsLinked to original sources

arXiv2023

Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe

Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the $<$100$>$c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model that couples polarization fluctuations through long-range elastic interactions. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes, and show that spontaneous anisotropy is ubiquitous in cubic IV-VI materials.

cond-mat.mtrl-sci↗

arXiv2018

A neutron tomography study: probing the spontaneous crystallization of randomly packed granular assemblies

We study the spontaneous crystallization of an assembly of highly monodisperse steel spheres under shaking, as it evolves from localized icosahedral ordering towards a packing reaching crystalline ordering. Towards this end, real space neutron tomography measurements on the granular assembly are carried out, as it is systematically subjected to a variation of frequency and amplitude. As expected, we see a presence of localized icosahedral ordering in the disordered initial state (packing fraction around 0.62). As the frequency is increased for both the shaking amplitudes (0.2 and 0.6 mm) studied here, there is a rise in packing fraction, accompanied by an evolution to crystallinity. The extent of crystallinity is found to depend on both the amplitude and frequency of shaking. We find that the icosahedral ordering remains localized and its extent does not grow significantly, while the crystalline ordering grows rapidly as an ordering transition point is approached. In the ordered state, crystalline clusters of both face centered cubic (FCC) and hexagonal close packed (HCP) types are identified, the latter of which grows from stacking faults. Our study shows that an earlier domination of FCC gives way to HCP ordering at higher shaking frequencies, suggesting that despite their coexistence, there is a subtle dynamical competition at play. This competition depends on both shaking amplitude and frequency, as our results as well as those of earlier theoretical simulations demonstrate. It is likely that this involves the very small free energy difference between the two structures.

cond-mat.soft↗

arXiv2018

Giant Exchange Bias in the Single-layered Ruddlesden-Popper Perovskite SrLaCo0.5Mn0.5O4

Exchange bias (EB) as large as ~5.5 kOe is observed in SrLaCo0.5Mn0.5O4 which is the highest ever found in any layered transition metal oxides including Ruddlesden-Popper series. Neutron diffraction measurement rules out long-range magnetic ordering and together with dc magnetic measurements suggest formation of short-range magnetic domains. AC magnetic susceptibility, magnetic memory effect and magnetic training effect confirm the system to be a cluster spin glass. By carrying out density functional calculations on several model configurations, we propose that EB is originated at the boundary between Mn-rich antiferromagnetic and Co-rich ferromagnetic domains at the sub-nanoscale. Reversal of magnetization axis on the Co-side alters the magnetic coupling between the interfacial Mn and Co spins which leads to EB. Our analysis infers that the presence of competing magnetic interactions is sufficient to induce exchange bias and thereby a wide range of materials exhibiting giant EB can be engineered for designing novel magnetic memory devices.

cond-mat.mtrl-sci↗

arXiv2018

Hyperfine interaction in cobalt by high-resolution neutron spectroscopy

We have investigated the ferromagnetic phase transition of elemental Co by high-resolution neutron backscattering spectroscopy. We monitored the splitting of the nuclear levels by the hyperfine field at the Co nucleus. The energy of this hyperfine splitting is identified as the order parameter of the ferromagnetic phase transition. By measuring the temperature dependence of the energy we determined the critical exponent $β= 0.350 \pm 0.002$ and the ferromagnetic Curie temperature of $T_{\text{C}} = 1400$~K. The present result of the critical exponent agrees better with the predicted value (0.367) of the 3-dimensional Heisenberg model than that determined previously by NMR.

cond-mat.str-el↗

arXiv2016

Spin waves in the AF state of the $t$-$t'$ Hubbard model on the fcc lattice: competing interactions, frustration, and instabilities

Spin waves in the type-III ordered antiferromagnetic state of the frustrated $t$-$t'$ Hubbard model on the fcc lattice are calculated to investigate finite-$U$-induced competing interaction and frustration effects on magnetic excitations and instabilities. Particularly strong competing interactions generated due to interplay of fcc lattice geometry and magnetic order result in significant spin wave softening. The calculated spin wave dispersion is found to be in qualitative agreement with the measured spin wave dispersion in the pyrite mineral $\rm Mn S_2$ obtained from inelastic neutron scattering experiments. Instabilities to other magnetic orders (type I, type II, spiral, non-collinear), as signalled by spin wave energies turning negative, are also discussed.

cond-mat.str-el↗

arXiv2016

Magnetic structures and magnetic phase transitions in the Mn-doped orthoferrite TbFeO$_3$ studied by neutron powder diffraction

The magnetic structures and the magnetic phase transitions in the Mn-doped orthoferrite TbFeO$_3$ studied using neutron powder diffraction are reported. Magnetic phase transitions are identified at $T^\mathrm{Fe/Mn}_N \approx$ 295~K where a paramagnetic-to-antiferromagnetic transition occurs in the Fe/Mn sublattice, $T^\mathrm{Fe/Mn}_{SR} \approx$ 26~K where a spin-reorientation transition occurs in the Fe/Mn sublattice and $T^\mathrm{R}_N \approx$ 2~K where Tb-ordering starts to manifest. At 295~K, the magnetic structure of the Fe/Mn sublattice in TbFe$_{0.5}$Mn$_{0.5}$O$_3$ belongs to the irreducible representation $Γ_4$ ($G_xA_yF_z$ or $Pb'n'm$). A mixed-domain structure of ($Γ_1 + Γ_4$) is found at 250~K which remains stable down to the spin re-orientation transition at $T^\mathrm{Fe/Mn}_{SR}\approx$ 26~K. Below 26~K and above 250~K, the majority phase ($> 80\%$) is that of $Γ_4$. Below 10~K the high-temperature phase $Γ_4$ remains stable till 2~K. At 2~K, Tb develops a magnetic moment value of 0.6(2)~$μ_\mathrm{B}/$f.u. and orders long-range in $F_z$ compatible with the $Γ_4$ representation. Our study confirms the magnetic phase transitions reported already in a single crystal of TbFe$_{0.5}$Mn$_{0.5}$O$_3$ and, in addition, reveals the presence of mixed magnetic domains. The ratio of these magnetic domains as a function of temperature is estimated from Rietveld refinement of neutron diffraction data. Indications of short-range magnetic correlations are present in the low-$Q$ region of the neutron diffraction patterns at $T < T^\mathrm{Fe/Mn}_{SR}$. These results should motivate further experimental work devoted to measure electric polarization and magnetocapacitance of TbFe$_{0.5}$Mn$_{0.5}$O$_3$.

cond-mat.str-el↗

arXiv2015

Spin-Lattice Coupling and Frustrated Magnetism in Fe-doped Hexagonal LuMnO3

Strong spin-lattice coupling and prominent frustration effects observed in the 50$\%$ Fe-doped frustrated hexagonal ($h$)LuMnO$_3$ are reported. A Néel transition at $T_{\mathrm N} \approx$ 112~K and a possible spin re-orientation transition at $T_{\mathrm {SR}} \approx$ 55~K are observed in the magnetization data. From neutron powder diffraction data, the nuclear structure at and below 300~K was refined in polar $P6_3cm$ space group. While the magnetic structure of LuMnO$_3$ belongs to the $Γ_4$ ($P6'_3c'm$) representation, that of LuFe$_{0.5}$Mn$_{0.5}$O$_3$ belongs to $Γ_1$ ($P6_3cm$) which is supported by the strong intensity for the $\mathbf{(100)}$ reflection and also judging by the presence of spin-lattice coupling. The refined atomic positions for Lu and Mn/Fe indicate significant atomic displacements at $T_{\mathrm N}$ and $T_{\mathrm {SR}}$ which confirms strong spin-lattice coupling. Our results complement the discovery of room temperature multiferroicity in thin films of $h$LuFeO$_3$ and would give impetus to study LuFe$_{1-x}$Mn$_x$O$_3$ systems as potential multiferroics where electric polarization is linked to giant atomic displacements.

cond-mat.str-el↗

arXiv2015

Unifying local and average structure in the phase change material GeTe

The prototypical phase change material GeTe shows an enigmatic phase transition at Tc ca. 650 K from rhombohedral (R3m) to cubic (Fm-3m) symmetry. While local probes see little change in bonding, in contrast, average structure probes imply a displacive transition. Here we use high energy X-ray scattering to develop a model consistent with both the local and average structure pictures. We detect a correlation length for domains of the R3m structure which shows power law decay upon heating. Unlike a classical soft mode, it saturates at ca. 20 Å above Tc. These nanoclusters are too small to be observed by standard diffraction techniques, yet contain the same local motif as the room temperature structure, explaining previous discrepancies. Finally, a careful analysis of the pair distribution functions implies that the 0.6 % negative thermal expansion (NTE) at the R3m -Fm-3m transition is associated with the loss of coherence between these domains.

cond-mat.mtrl-sci↗

arXiv2015

Origin of anomalous breakdown of Bloch's rule in the Mott-Hubbard insulator MnTe$_2$

We reinvestigate the pressure dependence of the crystal structure and antiferromagnetic phase transition in MnTe$_2$ by the rigorous and reliable tool of high pressure neutron powder diffraction. First-principles density functional theory calculations are carried out in order to gain microscopic insight. The measured Néel temperature of MnTe$_2$ is found to show unusually large pressure dependence of $12$ K GPa$^{-1}$. This gives rise to large violation of Bloch's rule given by $α=\frac{d\log T_N}{d\log V}=-\frac{10}{3} \approx -3.3$, to a $α$ value of -6.0 $\pm$ 0.1 for MnTe$_2$. The ab-initio calculation of the electronic structure and the magnetic exchange interactions in MnTe$_2$, for the measured crystal structures at different pressures, gives the pressure dependence of the Neél temperature, $α$ to be -5.61, in close agreement with experimental finding. The microscopic origin of this behavior turns to be dictated by the distance dependence of the cation-anion hopping interaction strength.

cond-mat.str-el↗

arXiv2015

Anomalous temperature-induced volume contraction in GeTe

The recent surge of interest in phase change materials GeTe, Ge$_2$Sb$_2$Te$_5$, and related compounds motivated us to revisit the structural phase transition in GeTe in more details than was done before. Rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied by high resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. Results of our studies clearly show an anomalous volume contraction of 0.6\% at the phase transition from the rhombohedral to cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.

cond-mat.mtrl-sci↗

arXiv2015

Spin-driven symmetry breaking in the frustrated fcc magnet MnS2

We report the characterisation of natural samples of the cubic pyrite mineral MnS2 using very high resolution synchrotron X-ray diffraction techniques. At low temperatures we find a new low temperature polymorph, which results from coupling between magnetic and lattice degrees of freedom. Below the magnetic ordering temperature T_N= 48 K, we detect a pseudo-tetragonal distortion with a tiny c/a ratio of 1.0006. The structure can be refined in the space group Pbca. The symmetry lowering reduces magnetic frustration in the fcc Mn2+ lattice and is likely responsible for the previously reported lock-in of the magnetic propagation vector. This behaviour is similar to the frustration driven symmetry breaking reported in other three-dimensional Heisenberg magnets like the chromate spinels

cond-mat.str-el↗

arXiv2015

Antisite Disorder-induced Exchange Bias Effect in Multiferroic Y2CoMnO6

Exchange bias effect in the ferromagnetic double perovskite compound Y$_2$CoMnO$_6$, which is also a multiferroic, is reported. The exchange bias, observed below 8~K, is explained as arising due to the interface effect between the ferromagnetic and antiferromagnetic clusters created by {\it antisite} disorder in this material. Below 8~K, prominent ferromagnetic hysteresis with metamagnetic "steps" and significant coercive field, $H_c \approx$ 10~kOe are observed in this compound which has a $T_c \approx$ 75~K. A model based on growth of ferromagnetic domains overcoming the elastic energy of structurally pinned magnetic interfaces, which closely resembles martensitic-like transitions, is adapted to explain the observed effects. The role of {\it antisite} disorder in creating the domain structure leading to exchange bias effect is highlighted in the present work.

cond-mat.dis-nn↗

arXiv2014

Giant pressure-induced volume collapse in the pyrite mineral MnS2

Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spin-state transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S=5/2 mineral Hauerite undergoes an unprecedented 22 % collapse driven by a conceptually different magnetic mechanism. Using synchrotron x-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic groundstate using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S=1/2 moments are quenched by dimerisation. Our results show how the emergence of metal-metal bonding can stabilise giant spin-lattice coupling in Earth's minerals.

cond-mat.str-el↗

arXiv2013

Direct evidence of nuclear spin waves in Nd$_2$CuO$_4$ by high-resolution neutron-spin-echo spectroscopy

We investigated the dispersion of nuclear spin waves in Nd$_2$CuO$_4$ by using neutron spin-echo spectroscopy at millikelvin temperatures. Our results show unambiguously the existence of dispersion of nuclear spin waves in Nd$_2$CuO$_4$ at T = 30 mK. A fit of the dispersion data with the spin wave dispersion formula gave the Suhl-Nakamura interaction range to be of the order of 10 Å.

cond-mat.str-el↗

arXiv2013

Phonon-Magnon coupling in CoF$_2$ investigated by time-of-flight neutron spectroscopy

We report the results of inelastic neutron scattering investigation on the model antiferromagnet CoF$_2$ by time-of-flight neutron spectroscopy. We measured the details of the scattering function $S(Q,ω)$ as a function of temperature with two different incident neutron wavelengths. The temperature and Q dependence of the measured scattering function suggests the presence of magnon-phonon coupling in almost all branches. The present results are in agreement with the strong magnetoelastic effects observed previously.

cond-mat.mtrl-sci↗

arXiv2013

Hyperfine interaction in CoCl$_2$ investigated by high resolution neutron spectroscopy

We investigated low energy nuclear spin excitations in the layered compound CoCl$_2$ by high resolution back-scattering neutron spectroscopy. We detected inelastic peaks at $E = 1.34 \pm 0.03$ $μ$eV on both energy loss and energy sides of the central elastic peak at $T = 2$ K. The energy of the inelastic peaks decrease with temperature continuously and become zero at $T_N \approx 25$ K at which the two ielastic peaks merge with the central elastic peak. We interpret the low energy excitations to be due to the transition between hyperfine field split nuclear levels. The present data together with the data on other Co compounds show that the energy of the nuclear spin excitations of a number of compounds follow a linear relationship with the electronic magnetic moment of the Co ion whereas that of other compounds deviate appreciably from this linear behaviour. We ascribe this anomalous behaviour to the presence of unquenched orbital moments of the Co ions.

cond-mat.mtrl-sci↗

arXiv2012

Magnetoelastic effects in multiferroic HoMnO$_3$

We have investigated magnetoelastic effects in multiferroic HoMnO$_3$ below the antiferromagnetic phase transition $T_N \approx 75$ K by neutron powder diffraction. The lattice parameter $a$ of the hexagonal unit cell of HoMnO$_3$ decrease in the usual way at lower temperatures and then shows abrupt contraction below $T_N$ whereas the lattice parameter $c$ increases continuously with decreasing temperature and shows an anomalous increase below $T_N$. The unit cell volume decreases continuously with decreasing temperature and undergoes abrupt contraction below $T_N$. By fitting the background thermal expansion for a nonmagnetic lattice with the Einstein-Grüneisen equation we determined the lattice strains $Δa$, $Δc$ and $ΔV$ due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models.

cond-mat.str-el↗

arXiv2012

Magnetoelastic effects in multiferroic YMnO$_3$

We have investigated magnetoelastic effects in multiferroic YMnO$_3$ below the antiferromagnetic phase transition, $T_N \approx 70$ K, using neutron powder diffraction. The $a$ lattice parameter of the hexagonal unit cell of YMnO$_3$ decreases normally above $T_N$, but decreases anomalously below T$_N$, whereas the $c$ lattice parameter increases with decreasing temperature and then increases anomalously below T$_N$. The unit cell volume also undergoes an anomalous contraction below $T_N$. By fitting the background thermal expansion for a non-magnetic lattice with the Einstein-Grüneisen equation, we determined the lattice strains $Δa$, $Δc$ and $ΔV$ due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of the Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models and have established that the lattice strain due to the magnetoelastic effect in YMnO$_3$ couples with the square of the ordered magnetic moment or the square of the order parameter of the antiferromagnetic phase transition.

cond-mat.str-el↗