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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 $$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., Published version, substantial changes, many references removed due to journal limits :-( Please see the supporting information online. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format (see conditions)

Using the new polarized neutron diffraction (PND) setup at MLZ the spin reorientation transition in the magnetocaloric orthoferrite HoFeO3 was studied at different wavelength. The various experiments provided reproducible results demonstrating high reliability of the used setup. We show that during the phase transition at TSR=53 K in an external magnetic field applied along crystal c-axis, the ordered magnetic moment of the Fe sublattice rotates from the crystallographic direction b to a not just in the ab plane, but through z axis. This means that the applied field breaks the orthorhombic symmetry allowing some magnetization parallel to z within a short temperature region. Interestingly, this is the same temperature region where large magnetocaloric effect for HoFeO3 was previously reported. A general model of the magnetic structure of HoFeO3, unconstrained by the orthorhombic symmetry, would allow the magnitudes and directions of the moments on each of the 8 magnetic sublattices in the unit cell to be independent of one-another, leading to 24 independent magnetic parameters. PND measurements were used to determine the absolute sign of the Dzyaloshinskii-Moriya interaction (DMI) in the ab plane for the Fe magnetic sublattice at 65 K. DMI plays an important role in the energy balance of the system.

Results of inelastic neutron scattering experiments on SnSe single crystals at high temperatures along with theoretical studies based on the density functional theory are reported. Our experiments reveal significant softening of the transverse acoustic branch along the [ 0 , ξ , 0 ] direction in the low-temperature α-SnSe of P b n m symmetry as temperature approaches T c = 807 K from below. This process is followed by a condensation of the zone-boundary Y-phonon of the high-temperature β-SnSe with C m c m symmetry at the onset of phase transition. The employed theoretical approach supports experimental observations and demonstrates that the phase change in SnSe is mediated by an unstable zone-boundary phonon with the Y 2 + irreducible representation within the C m c m symmetry space group of the high-temperature β-SnSe. The present work provides a detailed understanding of the soft-mode dynamics in SnSe and conclusively shows that the α ⇌ β structural transformation in this currently topical thermoelectric material is of displacive type.

The effect of high pressure on electron transport and on the field dependence of the transverse magnetoresistance has been studied in a hybrid nanocomposite based on the Zn0.1Cd0.9GeAs2 matrix and MnAs clusters. A record high negative magnetoresistance of ~74% is formed near a pressure-induced structural transition (P≈ 3.5 GPa). The considered scattering mechanisms include both the contribution from MnAs clusters at relatively low pressures (up to 0.7 GPa) and spin-dependent scattering by localized magnetic moments in the Mn-substituted structure of the matrix in the region of the structural transition. The presence of the positive magnetoresistance region associated with the two-band transport model in the high-pressure phase, as well as the large negative magnetoresistance, is described in the framework of the semiempirical Khosla–Fischer expression.

By performing high-pressure single-crystal neutron-diffraction measurements, the evolution of structure and magnetic ordering in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ under hydrostatic pressure were investigated. Both the tetragonal-to-orthorhombic structural transition and the Fe spin-density-wave transition are gradually suppressed and become decoupled with increasing pressure. The antiferromagnetic order of the Eu sublattice is, however, robust against the applied pressure up to 24.7 kbar, without showing any change of the ordering temperature. Under the pressure of 24.7 kbar, the lattice parameters of ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ display clear anomalies at 27(3) K, well consistent with the superconducting transition observed in previous high-pressure resistivity measurements. Such an anomalous thermal expansion around ${T}_{c}$ strongly suggests the appearance of bulk superconductivity and strong electron-lattice coupling in ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$ induced by the hydrostatic pressure. The coexistence of long-range ordered Eu antiferromagnetism and pressure-induced superconductivity is quite rare in the ${\mathrm{EuFe}}_{2}{\mathrm{As}}_{2}$-based iron pnictides.

The magnetism of the double perovskite compounds ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{FeCoO}}_{6}$ $(x=0,1,2)$ is contrasted using magnetization, neutron diffraction, and electron paramagnetic resonance with the support from density functional theory calculations. ${\mathrm{La}}_{2}{\mathrm{FeCoO}}_{6}$ is identified as a long-range ordered antiferromagnet displaying a near-room temperature transition at ${T}_{N}=270$ K, accompanied by a low temperature structural phase transition at ${T}_{S}=200$ K. The structural phase transformation at ${T}_{S}$ occurs from $R\overline{3}c$ at 300 K to $Pnma$ at 200 K. The density functional theory calculations support an insulating noncompensated antiferromagnetic structure. The long-range ordered magnetism of ${\mathrm{La}}_{2}{\mathrm{FeCoO}}_{6}$ transforms to short-range glassy magnetism as La is replaced with Sr in the other two compounds. The magnetism of ${\mathrm{La}}_{2}{\mathrm{FeCoO}}_{6}$ is differentiated from the nonequilibrium glassy features of ${\mathrm{Sr}}_{2}{\mathrm{FeCoO}}_{6}$ and ${\mathrm{SrLaFeCoO}}_{6}$ using the cooling-and-heating-in-unequal-fields magnetization protocols. This contrasting magnetism in the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{FeCoO}}_{6}$ series is evidenced in electron paramegnetic resonance studies. The electronic density of states estimated using the density functional theory calculations contrast the insulating feature of ${\mathrm{La}}_{2}{\mathrm{FeCoO}}_{6}$ from the metallic nature of ${\mathrm{Sr}}_{2}{\mathrm{FeCoO}}_{6}$. From the present suite of experimental and computational results on ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{FeCoO}}_{6}$, it emerges that the electronic degrees of freedom, along with antisite disorder, play an important role in controlling the magnetism observed in double perovskites.

Magnetic nanocomposites based on MnAs clusters embedded in a chalcopyrite host usually do not exhibit large magnetoresistance (MR) at room temperature, while pronounced effects are localized at very low temperatures. In the present work, we observed an appearance of large pressure-induced negative and positive MR at room temperature in the Zn0.1Cd0.9GeAs2 hybrid system containing 10% MnAs inclusions. With the applied pressure, a substantial modification of the electron transport from semimetallic to semiconducting type occurs, followed by a subsequent structural transition at P ≈ 3.5 GPa into almost metallic high-pressure phase. This picture is simultaneously supported by temperature-dependent and room temperature high-pressure transport measurements. Using a semiempirical expression, taking into account a spin-dependent scattering of charge carriers due to MnAs nanoclusters, as well as a two-band conductivity model, we have been able to partially describe the observed MR effects. The predominantly weak positive contribution at P = 1 GPa, which is well described in the framework of the proposed approach indicates the presence of spin-polarized charge carriers. Based on the two-band model calculations, a negative spin polarization was found at P ≥ 3 GPa that ascribed to a structural change of the matrix. As our results indicate, an emerging MR in the structural transition region is characterized by a complex behavior. In particular, the negative part of MR demonstrates a magnetic field dependence different than Δρ/ρ0 ∼ H2, suggesting the presence of unusual scattering mechanisms in magnetotransport. I. INTRODUCTION

The pyrite mineral MnS 2 was recently shown to undergo a giant pressure-induced volume collapse at ∼ 12 GPa, via a disordered intermediate phase. The high pressure arsenopyrite phase is stabilised by metal-metal bonding, a mechanism now shown to be ubiquitous for Mn 2 + chalcogenides. Here we report a spectroscopic investigation of this transition up to pressures of 22 GPa. Using XANES we show that the transition does not involve a change in oxidation state, consistent with the arsenopyrite crystal structure proposed at high pressure. Notably, the XANES spectrum is almost identical in the pressure-induced disordered phase, and after crystallisation induced by laser-heating. The former is therefore a ‘valence bond glass’, and is likely disordered due to kinetic hindrance of the phase transition. We also detect electronic changes in the compressed pyrite phase, and this is confirmed by Raman scattering which shows that the disulphide vibrations in the pyrite phase saturate before the volume collapse. Together with detailed DFT calculations, these results indicate that electronic changes precede valence bond formation between the Mn 2 + cations.

Physical review / B covering condensed matter and materials physics 100(21), 214432 (2019). doi:10.1103/PhysRevB.100.214432, Published by Inst., Woodbury, NY

Spin wave dispersion in the frustrated fcc type-III antiferromagnet MnS$_2$ has been determined by inelastic neutron scattering using a triple-axis spectrometer. Existence of multiple spin wave branches, with significant separation between high-energy and low-energy modes highlighting the intrinsic magnetic frustration effect on the fcc lattice, is explained in terms of a spin wave analysis carried out for the antiferromagnetic Heisenberg model for this $S=5/2$ system with nearest and next-nearest-neighbor exchange interactions. Comparison of the calculated dispersion with spin wave measurement also reveals small suppression of magnetic frustration resulting from reduced exchange interaction between frustrated spins, possibly arising from anisotropic deformation of the cubic structure.

The scope of magnetic neutron scattering has been expanded by the observation of electronic Dirac dipoles (anapoles) that are polar (parity-odd) and magnetic (time-odd). A zero-magnetization ferromagnet Sm0.976Gd0.024Al2 with a diamond-type structure presents Dirac multipoles at basis-forbidden reflections that include the standard (2, 2, 2) reflection. Magnetic amplitudes measured at four such reflections are in full accord with a structure factor calculated from the appropriate magnetic space group., Comment: 4 pages, 3 figures

Exchange bias (EB) as large as \ensuremath{\sim}5.5 kOe is observed in $\mathrm{SrLaC}{\mathrm{o}}_{0.5}\mathrm{M}{\mathrm{n}}_{0.5}{\mathrm{O}}_{4}$, 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 suggests 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 subnanoscale. 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.

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 [Formula: see text] and the ferromagnetic Curie temperature of [Formula: see text] K. The present result of the critical exponent agrees better with the predicted value (0.367) of the three-dimensional Heisenberg model than that determined previously by nuclear magnetic resonance.

Results of inelastic neutron scattering experiments and ab initio molecular dynamics simulations for GeTe–the parent compound of phase-change materials are reported. The inelastic neutron spectra of GeTe powder samples have been determined in the temperature range extending from 300 to 700 K. The phonon peaks undergo thermal shifts resulting from anharmonic effects being weaker for acoustic than optic modes. A small concentration of free charge carries arising from the presence of Ge-vacancies was found not to affect significantly the neutron weighted phonon densities of states of GeTe. The spectral pattern changes qualitatively across the structural phase transition, but the local structure of GeTe remains hardly affected, as confirmed by the analysis of temperature dependence of the pairdistribution function obtained from ab initio molecular dynamics investigations. The present theoretical studies support in a wide extent our experimental observations and also those provided by local probe methods.

The double perovskites form a class of materials with diverse tunable electronic and magnetic ground states and properties and promise to be excellent functional materials for the near future. Here, high-resolution neutron spectroscopic studies reveal their magnetic ground states and their dynamical response, which seem to be typical of their complex ground states. The studied double perovskite materials Sr${}_{2\ensuremath{-}x}$La${}_{x}$FeCoO${}_{6}$ ($x$=0, 1, 2), of which the first (Sr${}_{2}$FeCoO${}_{6}$ with $x$ = 0) shows a canonical spin-glass ground state, the second (SrLaFeCoO${}_{6}$ with $x$ = 1) shows a so-called magnetic glass state, and the third (La${}_{2}$FeCoO${}_{6}$ with $x$ = 2) has a magnetically ordered ground state. All these magnetic properties can be tuned and engineered by varying the proportion of Sr and La in these materials.

We have determined the temperature evolution of the spin and orbital moments in the zero magnetization ferromagnet Sm$_{1-x}$Gd$_x$Al$_2$ (x = 0.024) by combining polarized and unpolarized single crystal neutron diffraction data. The sensitivity of the polarized neutron technique has allowed the moment values to be determined with a precision of $\approx 0.1$~\mub. Our results clearly demonstrate that, when magnetised by a field of 8T, the spin and orbital moments in Sm$_{1-x}$Gd$_x$Al$_2$ are oppositely directed so that the net magnetization is very small. Below 60 K the contributions from spin and orbital motions are both about 2\mub\ with that due to orbital motion being slightly larger than that due to spin. Between 60 and 65 K the contributions of each to the magnetization fall rapidly and change sign at \Tcomp\ $\approx 67$K above which the aligned moments recover but with the orbital magnetization still slightly higher than the spin one. These results imply that above \Tcomp\ the small resultant magnetization of the \smion\ ion is oppositely directed to the magnetizing field. It is suggested that this anomaly is due to polarization of conduction electron spin associated with the doping Gd$^{3+}$ ions.

We elaborate here why the antiferromagnetically ordered GdCrO$_3$ responds in a diamagnetic way under certain conditions, by monitoring the evolution of the microscopic global and local magnetic phases. Using high energy $\sim$ 0.3 eV neutrons, the magnetic ordering is shown to adopt three distinct magnetic phases at different temperatures: G$_x^{Cr}$,A$_y^{Cr}$,F$_z^{Cr}$ below N\'eel temperature = 171 K; (F$_x^{Cr}$, C$_y^{Cr}$, G$_z^{Cr}$)$\bullet$( F$_x$$^{Gd}$,C$_y$$^{Gd}$) below 7 K and an intermediate phase for 7 K $ \le T \le$ 20 K in the vicinity of spin-reorientation phase transition. Although, bulk magnetometry reveals a huge negative magnetization (NM) in the terms of both magnitude and temperature range ( $M_{- max}$ ( 18 K)$\sim$ 35 $\times M_{+ max}$ (161 K), $\Delta T \sim 110$ K in presence of $\mu_0H$ = 0.01 T); the long-range magnetic structure and derived ordered moments are unable to explain the NM. Real-space analysis of the total (Bragg's + diffuse) scattering reveals significant magnetic correlations extending up to $\sim$ 9 $\AA$. Accounting for these short-range correlations with a spin model reveals spin frustration in the S= 3 ground state, comprising competing first, second and third next nearest exchange interactions with values J$_1$ = 2.3 K, J$_2$ = -1.66 K and J$_3$ = 2.19 K in presence of internal field, governs the observance of NM in GdCrO$_{3}$., Comment: 11 pages, 8 figures

The present study aims at clarifying the positions of hydrogen atoms in H2V3O8. Starting with the structure provided by Oka (J. Solid State Chem., 1990, 89, 372) a model was established using first-principles total energy calculations. Optimized structure data were obtained for two possible configurations within the orthorhombic symmetry representation of the V–O frame. The hydrogen atoms were located close to the oxygen atom labelled O(6) by Oka, in a water-like H–O–H moiety. The two different configurations are distinguished by either parallel or perpendicular orientation of the H–O–H plane with regards to the crystallographic b-axis. In addition, neutron diffraction measurements at 4 K and ambient temperature were performed. The hydrogen atom positions were found in proximity of O(6), in a similar arrangement as predicted theoretically. In particular, the structure obtained from the analysis of the 4 K data matches the model with H–O–H plane parallel to the b-axis.

The classical belief about the mechanism of spin reorientation phase transition (SRPT) and ground-state magnetic structure in ${\mathrm{SmCrO}}_{3}$ has become intriguing because of inconsistent bulk magnetization observations. The presence of highly neutron-absorbing Sm atom has so far evaded the determination of microscopic magnetic structure. In the present report, we have utilized very high-energy ``hot neutrons'' to overcome the Sm absorption and to determine the thermal evolution of magnetic configurations. Unambiguously, three distinct phases are observed: the uncompensated canted antiferromagnetic structure ${\mathrm{\ensuremath{\Gamma}}}_{4}({G}_{x},{A}_{y},{F}_{z};{F}_{z}^{R})$ occurring below the N\'eel temperature (${T}_{N}=191$ K), the collinear antiferromagnetic structure ${\mathrm{\ensuremath{\Gamma}}}_{1}({A}_{x},{G}_{y},{C}_{z};{C}_{z}^{R})$ occurring below 10 K, and a nonequilibrium configuration with cooccurring ${\mathrm{\ensuremath{\Gamma}}}_{1}$ and ${\mathrm{\ensuremath{\Gamma}}}_{4}$ phases in the neighborhood of the SRPT (10 K $\ensuremath{\le}T\ensuremath{\le}$ 40 K). In differing to the earlier predictions, we divulge the SRPT to be a discontinuous transition where chromium spins switch from the $a\ensuremath{-}b$ crystallographic plane to the $b\ensuremath{-}c$ crystallographic plane in a discrete manner with no allowed intermediate configuration. The canting angle of chromium ions in the $a\ensuremath{-}b$ plane is unusually not a thermal constant, rather it is empirically discerned to follow exponential behavior. The competition between magnetocrystalline anisotropy and free energy derived by isotropic and antisymmetric exchange interactions between different pairs of magnetic ions is observed to govern the mechanism of SRPT.

We demonstrate that small but finite ferroelectric polarization ($\sim$0.01 $\mu$C/cm$^2$) emerges in orthorhombic LuFeO$_3$ ($Pnma$) at $T_N$ ($\sim$600 K) because of commensurate (k = 0) and collinear magnetic structure. The synchrotron x-ray and neutron diffraction data suggest that the polarization could originate from enhanced bond covalency together with subtle contribution from lattice. The theoretical calculations indicate enhancement of bond covalency as well as the possibility of structural transition to the polar $Pna2_1$ phase below $T_N$. The $Pna2_1$ phase, in fact, is found to be energetically favorable below $T_N$ in orthorhombic LuFeO$_3$ ($albeit$ with very small energy difference) than in isostructural and nonferroelectric LaFeO$_3$ or NdFeO$_3$. Application of electric field induces finite piezostriction in LuFeO$_3$ via electrostriction resulting in clear domain contrast images in piezoresponse force microscopy., Comment: 12 pages, 8 figures

In the present study, we report a new setup for polarized neutron imaging at the ORNL High Flux Isotope Reactor CG-1D beamline using an in situ

Polarised neutron diffraction measurements have been made on HoFeO3 single crystals magnetised in both the [0 0 1] and [1 0 0] directions (Pbnm setting). The polarisation dependencies of Bragg reflection intensities were measured both with a high field of [Formula: see text] T parallel to [0 0 1] at [Formula: see text] K and with the lower field [Formula: see text] T parallel to [1 0 0] at [Formula: see text] K. A Fourier projection of magnetization induced parallel to [0 0 1], made using the hk0 reflections measured in 9 T, indicates that almost all of it is due to alignment of Ho moments. Further analysis of the asymmetries of general reflections in these data showed that although, at 70 K, 9 T applied parallel to [0 0 1] hardly perturbs the antiferromagnetic order of the Fe sublattices, it induces significant antiferromagnetic order of the Ho sublattices in the [Formula: see text] plane, with the antiferromagnetic components of moment having the same order of magnitude as the induced ferromagnetic ones. Strong intensity asymmetries measured in the low temperature [Formula: see text] structure with a lower field, 0.5 T [Formula: see text] [1 0 0] allowed the variation of the ordered components of the Ho and Fe moments to be followed. Their absolute orientations, in the [Formula: see text] domain stabilised by the field were determined relative to the distorted perovskite structure. This relationship fixes the sign of the Dzyalshinski-Moriya (D-M) interaction which leads to the weak ferromagnetism. Our results indicate that the combination of strong y-axis anisotropy of the Ho moments and Ho-Fe exchange interactions breaks the centrosymmetry of the structure and could lead to ferroelectric polarization.

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.

Unusual features in magnetization resembling the kinetic arrest of a magnetic glass state are observed in the La-doped double perovskite, SrLaFeCoO

AIP Advances 7(4), 045106 (2017). doi:10.1063/1.4979710, Published by American Inst. of Physics, New York, NY

In order to understand the origin of the huge quasielastic magnetic scattering observed previously with a back-scattering neutron spectrometer, we have re-investigated the low energy excitations in HoCrO3 by inelastic neutron scattering in a much wider energy range with time-of-flight neutron spectrometers. The inelastic signals are due to the excitations between the ground state doublet of the Ho ion. The quasielastic signal is due to the fluctuation of the disordered Ho moments. At low temperature the intensity of quasielastic scattering is small. It starts increasing as the temperature increases above 30 K. At the same temperature, the elastic intensity due to Ho moment ordering decreases in a similar way. This observation strengthens the hypothesis that the quasielastic scattering is due the fluctuations of the disordered Ho moments. The time scale of fluctuations has been determine from the quasielastic scattering and was found to vary from about 22 ps at [Formula: see text] K to about 2.5 ps at [Formula: see text] K. The stretched exponential line shape indicates a distribution of decay rates at low temperatures.

Hydrostatic pressure studies of the isothermal magnetization and volume changes up to 7 GPa of magnetic composite containing MnP clusters in an insulating $\mathrm{CdGe}{\mathrm{P}}_{2}$ matrix are presented. Instead of alleged superparamagnetic behavior, a pressure-induced magnetization process was found at zero magnetic field, showing gradual enhancement in a low-field regime up to $H\ensuremath{\geqslant}5$ kOe. The simultaneous application of pressure and magnetic field reconfigures the MnP clusters with antiferromagnetic alignment, followed by onset of a field-induced metamagnetic transition. An unusual hysteresis in magnetization after pressure cycling is observed, which is also enhanced by application of the magnetic field, and indicates reversible metamagnetism of MnP clusters. We relate these effects to the major contribution of structural changes in the composite, where limited volume reduction by 1.8% is observed at $P\ensuremath{\sim}5.2$ GPa.

We have investigated the magnetic ordering and spin dynamics of the solid solution series Ba1–xEuxSi with 0 < x ≤ 1 applying X-ray diffraction, electric conductivity measurements, Mossbauer spectroscopy, muon spin depolarization, and neutron diffraction. Our results suggest a spin glass-like behavior of Ba1–xEuxSi for concentrations close to x = 0.3 exist. Different spin ordering phenomena are found, which are dependent on the relative europium concentrations.

We have investigated nearly monodispersive MnO nanoparticles of 10 nm diameter by polarised neutron diffraction with XYZ-polarisation analysis. We found no long-range ordering down to 3.5 K. A broad magnetic peak appeared close to Q = (1/2,1/2,1/2) signifying short-range antiferromagnetic correlations. The correlation length was found to be about 2.4 nm at T =3.5 K. The correlation length decreases rapidly with increasing temperature and becomes about 0.7 nm at T =250 K.

We have investigated the hyperfine interaction in Co 2 SiO 4 by inelastic neutron scattering with a high resolution back-scattering neutron spectrometer. The energy spectrum measured from a Co 2 SiO 4 powder sample revealed inelastic peaks at E = 1.387 ± 0.006 μ eV at T =3.5 K on both energy gain and energy loss sides. The inelastic peaks move gradually towards lower energy with increasing temperature and finally merge with the elastic peak at the electronic magnetic ordering temperature T N ≈ 50 K . The inelastic peaks have been interpreted to be due to the transition between hyperfine-split nuclear level of the 59 Co isotopes with spin I = 7 2 . The temperature dependence of the energy of the inelastic peak in Co 2 SiO 4 showed that this energy can be considered to be the order parameter of the antiferromagnetic phase transition. The determined hyperfine splitting in Co 2 SiO 4 deviates from the linear relationship between the ordered electronic magnetic moment and the hyperfine splitting in Co, Co–P amorphous alloys and CoO presumably due to the presence of unquenched orbital moment. These results are very similar to those of CoF 2 recently reported by Chatterji and Schneider [7].

The discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

Multiferroic materials are the materials that show several cooperative ordering phenomena simultaneously. Here we consider only those materials that show both antiferromagnetism and ferroelectricity and in which the ordering parameters are coupled to each other. I shall review our recent neutron scattering investigations on multiferroic hexagonal manganites YMnO3. We have determined the spin wave dispersions in YMnO3 in symmetry directions by inelastic neutron scattering and have fitted the results with a simple nearest-neighbour anisotropic Heisenberg model. We have investigated the spin dynamics of YMnO3 above TN ∼ 70 K and have shown the existence of persistent albeit heavily damped spin waves.

The crystal structure of PtAl 4 was determined from synchrotron data ( λ = 0.5608 A) of a twinned crystal. PtAl 4 crystallizes in the trigonal space group P 3 c 1 (no. 158). The lattice constants are a = 13.077(1) A and b = 9.6342(7) A. The crystal structure represents a new structure type (Pearson symbol hP90) and consists of layers of edge and face sharing Al 10 -polyhedra, centered by Pt-atoms. The polyhedra layers are stacked along the crystallographic c -axis and are connected by common corners. Two out of 14 Al-positions show positional disorder and occupational deficiencies.

Spin-selected polarized X-ray absorption near-edge structures (SSXANES) at the Mn K-edge from a bilayer La1.2Sr1.65Ca0.15Mn2O7 single crystal have been studied with high resolution, both in the ferromagnetic (15 K) as well as paramagnetic phase (300 K). The orientation-dependent SSXANES spectra show unmistakable temperature dependence as the system makes the ferromagnetic to paramagnetic phase transition. The pre-edge structures are too intense to be ascribed to weak quadrupole transitions and are interpreted in terms of hybridization of Mn 3d orbitals with O2p and Mn 4p orbitals over and above similar onsite hybridization. The results also indicate possible existence of a small local (time-frozen) ferromagnetic ordering in the macroscopically disordered state. Need for further experimental and theoretical work on the SSXANES spectra from the bilayer system is emphasized.

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 $\Gamma_4$ ($G_xA_yF_z$ or $Pb'n'm$). A mixed-domain structure of ($\Gamma_1 + \Gamma_4$) 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 $\Gamma_4$. Below 10~K the high-temperature phase $\Gamma_4$ remains stable till 2~K. At 2~K, Tb develops a magnetic moment value of 0.6(2)~$\mu_\mathrm{B}/$f.u. and orders long-range in $F_z$ compatible with the $\Gamma_4$ 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$_3$., Comment: 16 pages, 7 figures, Accepted to J. Appl. Phys., 2016

We report a comprehensive specific heat and inelastic neutron scattering study to explore the possible origin of multiferroicity in HoCrO$_3$. We have performed specific heat measurements in the temperature range 100 mK - 290 K and inelastic neutron scattering measurements were performed in the temperature range 1.5 - 200 K. From the specific heat data we determined hyperfine splitting at 22.5(2) $\mu$eV and crystal field transitions at 1.379(5) meV, 10.37(4) meV, 15.49(9) meV and 23.44(9) meV, indicating the existence of strong hyperfine and crystal field interactions in HoCrO$_3$. Further, an effective hyperfine field is determined to be 600(3) T. The quasielastic scattering observed in the inelastic scattering data and a large linear term $\gamma=6.3(8)$ mJmol$^{-1}$K$^{-2}$ in the specific heat is attributed to the presence of short range exchange interactions, which is understood to be contributing to the observed ferroelectricity. Further the nuclear and magnetic entropies were computed to be, $\sim$$17.2$ Jmol$^{-1}$K$^{-1}$ and $\sim$34 Jmol$^{-1}$K$^{-1}$, respectively. The entropy values are in excellent agreement with the limiting theoretical values. An anomaly is observed in peak position of the temperature dependent crystal field spectra around 60 K, at the same temperature an anomaly in the pyroelectric current is reported. From this we could elucidate a direct correlation between the crystal electric field excitations of Ho$^{3+}$ and ferroelectricity in HoCrO$_3$. Our present study along with recent reports confirm that HoCrO$_3$, and $R$CrO$_3$ ($R=$ Rare earth) in general, possess more than one driving force for the ferroelectricity and multiferroicity., Comment: 17 pages, 6 figures, published in J. Phys.: Condens. Matter

Total scattering based atomic pair distribution function (PDF) analysis, with the advent of high data throughput neutron powder diffractometers, helps understanding the nature of the Jahn-Teller (JT) phase transition in La1-xCaxMnO3 colossal magnetoresistive (CMR) manganites. The JT distortion of the MnO6 octahedra, is long-range ordered in the orthorhombic (O) phase, but disappears in the pseudo-cubic (O') phase crystallographically. An anomalous unit cell volume contraction occurs at the transition. The PDF study indicates that the distortion persists locally deep in the O' phase, contrary to the crystallographic view. Simultaneously, local structural features observed in PDF at 10.3 A, sensitive to the oxygen sublattice changes, evolve dramatically across the transition. The same effect is observed irrespective of the way the O-O' phase boundary is crossed: it is seen both in the temperature series data for x=0, and in the doping series data at 310 K and at 550 K.

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 Mn(2+) lattice and is likely responsible for the previously reported lock-in of the magnetic propagation vector. This behaviour is similar to the spin-Peierls phase transitions reported in other three-dimensional Heisenberg magnets like the chromate spinels.

Strong spin-lattice coupling and prominent frustration effects observed in the 50 Fe-doped frustrated hexagonal IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn1.gif] (h)̑ext{LuMnO}_3} are reported. A Néel transition at IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn2.gif] {T_{\mathrm{N}}≈112\ ̑ext{K}} and a possible spin re-orientation transition at IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn3.gif] {T_{\mathrm{SR}}≈55\ ̑ext{K}} are observed in the magnetization data. From neutron powder diffraction data, the nuclear structure at and below 300 K was refined in polar P 6 3 cm space group. While the magnetic structure of LuMnO 3 belongs to the IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn4.gif] {Γ_4\ (P6'_3c'm)} representation, that of LuFe 0.5 Mn 0.5 O 3 belongs to IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn5.gif] {Γ_1\ (P6_3cm)} which is supported by the strong intensity for the (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 IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn6.gif] {T_{\mathrm{N}}} and IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn7.gif] {T_{\mathrm{SR}}} which confirms strong spin-lattice coupling. Our results complement the discovery of room temperature multiferroicity in thin films of IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn8.gif] {h̑ext{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.

We reinvestigate the pressure dependence of the crystal structure and antiferromagnetic phase transition in ${\mathrm{MnTe}}_{2}$ using 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 N\'eel temperature of ${\mathrm{MnTe}}_{2}$ is found to show unusually large pressure dependence of $12\phantom{\rule{4pt}{0ex}}\text{K}\phantom{\rule{4.pt}{0ex}}{\text{GPa}}^{\ensuremath{-}1}$. This gives rise to a large violation of Bloch's rule given by $\ensuremath{\alpha}=\frac{dlog{T}_{N}}{dlogV}=\ensuremath{-}\frac{10}{3}\ensuremath{\approx}\ensuremath{-}3.3$, to an $\ensuremath{\alpha}$ value of $\ensuremath{-}6.0\ifmmode\pm\else\textpm\fi{}0.1$ for ${\mathrm{MnTe}}_{2}$. The ab initio calculation of the electronic structure and the magnetic exchange interactions in ${\mathrm{MnTe}}_{2}$ for the measured crystal structures at different pressures indicates the pressure dependence of the Ne\'el temperature $\ensuremath{\alpha}$ is $\ensuremath{-}5.61$, in close agreement with experimental findings. The microscopic origin of this behavior turns out to be dictated by the distance dependence of the cation-anion hopping interaction strength.

We have investigated the spin dynamics of the quasi-two-dimensional bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7 , which shows colossal magnetoresistance (CMR) behaviour, by inelastic neutron scattering. We have measured the spin wave dispersion at T = 16 K along [ 1 0 0 ] , followed the temperature evolution of the spin wave dispersion close to T C ≈ 128 K and also measured the scattered neutron intensities in the ( Q , ω ) space above T C up to T = 200 K . The magnetic inelastic intensity above T C looks qualitatively very similar to those below T C and have a structure in the ( Q , ω ) space suggesting persistent spin waves.

We have undertaken a new study of the magnetic and crystalline structures of YMnO3 and HoMnO3 using neutron diffraction combined with neutron polarimetry. It is shown how the long-standing problem of distinguishing between magnetic structure models which give nearly identical diffraction intensities may be resolved. The experiments show that the magnetic structure of YMnO3 has space group P63' (or P63) rather than P63'cm' (or P63cm) with Mn moments of 3.14(3) μB inclined at 11(1)° to [10.0] (or [12.0] for P63) in the (00.1) plane. For HoMnO3 the experiments confirm the P63'c'm symmetry at 50 K and the spin-rotation transition leading to a low-temperature structure with magnetic space group P63'cm'. In this structure both the Ho1 and Ho2 moments can order with spins parallel to [00.1] in ferromagnetic layers, anti-ferromagnetically coupled. The Ho1 and Ho2 moments in neighbouring layers are found to be oppositely oriented. The ordered moments at 2 K are 3.32(8), 4.17(13) and 1.31(11) μB on the Mn, Ho1 and Ho2 atoms respectively. No evidence was found, in this zero-field study, for the magnetic transition at ≈5 K reported in the literature (Lottermoser et al 2004 Nature 30 541; Vajk et al 2005 Phys. Rev. Lett. 94 87601). Nuclear structure refinements show that in YMnO3 at 10 K and in HoMnO3 at all temperatures below 50 K any deviation of the manganese x parameter from 1/3 is less than 0.0008.

The lithium boride LiBx (0.82 < × ≤ 1.0) contains linear boron chains isolelectronic to carbyne. The negatively charged boron chains are embedded in and stabilized by a lithium matrix. At room temperature they are disordered with respect to each other and also not commensurate with the Li matrix. Below about 150 K a phase transition takes place, where the boron chains successively order with respect to each other, but remaining still incommensurate with the Li substructure. The crystal structure of LiBx at 2 K was determined from neutron powder diffraction data and refined in the (3+1)-dimensional superspace group (αβγ) The unit cell parameters are a = 4.012(1) A, b = 4.012(1) A, c = 2.794(1) A, α = 90°, β = 90°, γ = 120° for the lithium and a = 4.017(1) A, b = 4.019(1) A, c = 1.578(1) A, α=93.35(1), β = 92.83(1)°, γ = 119.71(1)° for the boron substructure, respectively. The q-vector is (0.1267 0.1482 1.7707). The boron atoms within the linear chains are equidistant, suggesting a cumulated double bond system, similar to a cumulenic chain in the carbon case. Band structure calculations and the electron localization function give more insight into the unusual physical properties of LiBx.

We have investigated the pressure-induced structural phase transition in ReO3 by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P–T) phase diagram. The critical pressure is Pc=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become Pc=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M3 phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO6 octahedra and the Re–O–Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.

We have performed muon spin relaxation ( μ SR ) experiments on undoped CaMnO3 and electron-doped Ca 1 - x Sm x MnO 3 for x = 0.1 , 0.15 and 0.3. Undoped CaMnO3 is a G-type antiferromagnet with a Neel temperature of T N ≈ 122 K . The μ SR signal shows three well-defined frequencies, which decrease continuously with temperature and becomes zero at T N ≈ 122 K . The electron-doped Ca0.85Sm0.15MnO3 which shows colossal magnetoresistive (CMR) behaviour, undergoes a structural and magnetic transition at T c ≈ 125 K . The μ SR signal shows heavily damped oscillating signal. The frequency decreases continuously with increasing temperature. Ca0.9Sm0.1MnO3 and Ca0.7 Sm0.3MnO3 show only relaxation and do not show any frequency. We have determined temperature variation of the muon relaxation rates below and above the magnetic and charge order transitions under zero and longitudinal magnetic fields, which give important information about the nature of these transitions.

The dependence of the magnetic structure of CuO on hydrostatic pressure up to 18 kbar has been studied by means of neutron diffraction at low temperatures. The pressure dependences of both the Neel temperature TN and the incommensurate–commensurate lock-in transition temperature TL have been determined. The Neel temperature, which is 230 K at ambient pressure, increases to TN = 235 K at P = 18 kbar, whereas the lock-in transition temperature, which is 213 K at ambient pressure, decreases continuously to TL = 207 K at P = 18 kbar. Since hydrostatic pressure increases the stability range of the incommensurate phase, it is likely that at very high pressures the incommensurate phase would be the only stable ordered magnetic phase of CuO and the commensurate antiferromagnetic phase would be completely suppressed.

We have investigated the magnetic ordering and the incommensurate–commensurate phase transition in EuAs3 by zero-field (ZF) and longitudinal-field μSR. In the commensurate phase, stable at temperatures below TL=10.3 K, the ZF muon signal exhibits oscillations corresponding to four muon precession frequencies the lowest of which behaves anomalously. The muon signal shows no oscillation but exponential decay in the incommensurate phase stable in temperature range from TL≈10.3 K up to TN≈11 K. The temperature dependence of the fitted relaxation rate shows divergence-like behaviour at the ordering temperature TN≈11 K and also at the lock-in transition TL≈10.3 K. The results are in qualitative agreement with those previously obtained by neutron and X-ray magnetic scattering investigations except for the anomalous temperature dependence of the lowest frequency in the commensurate phase. We propose a model for this anomalous behaviour.

We have investigated the magnetic ordering of Nd2-x Ce x CuO4 for x = 0, 0.09, 0.13, 0.15 and 0.18 by neutron diffraction at low temperatures down to 33 mK and under magnetic field up to 5 tesla applied along [1,-1,0] crystallographic direction. At zero applied magnetic field Cu magnetic sublattice orders at $T_{N} \approx 250$ , 210, 130 and 105 K for x = 0, 0.09, 0.13, 0.15, respectively. No long range magnetic order of the Cu could be detected for x = 0.18. The magnetic order of Nd was found in all samples, with a gradual increase of the polarized magnetic moment with decreasing temperature, saturating around 1 K. Hyperfine induced nuclear polarization of the Nd nuclear spin has been observed below about 400 mK for samples with x = 0, 0.13, 0.15 and 0.18. Field variation of the intensities of the principal and superstructure reflections of Nd2-x Ce x CuO4 at millikelvin temperatures shows a field-induced second-order double-k to single-k phase transition at H c = 0.75 and 0.56 tesla for samples with x = 0 and 0.15, respectively at T = 50 mK. We have also investigated the polarization of the Nd electronic sublattice due to the field of the Cu sublattice by the element specific X-ray resonant magnetic scattering investigation with synchrotron radiation.

We have investigated non-resonant high energy X-ray magnetic scattering from EuAs3 both in the antiferromagnetic and in the incommensurate phase by using an X-ray energy of 104 and 106 keV. In the antiferromagnetic phase, we obtained a signal to background ratio of about 10:1 for the magnetic Bragg peak at Q = ( − 1 , 0 , 1 / 2 ) and a maximum count rate of about 200 counts/s at T=3.1 K. To our knowledge this is the first reported observation of the non-resonant magnetic signal from a rare-earth ion at X-ray energy as high as 106 keV. The temperature dependence of the integrated intensity of the (−1,0,1/2); magnetic reflection has been measured and compared with that obtained previously by neutron diffraction. We measured the integrated intensities of several magnetic reflections from the antiferromagnetic phase and have compared them with those calculated from the magnetic structure model derived from neutron diffraction. The intensities of the magnetic satellite reflections from the incommensurate phase have been measured and have been found to be very weak. We also investigated the temperature variation of the lattice spacing close to the magnetic ordering transition and have found a large magnetoelastic anomaly at the lock-in phase transition.