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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.

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.

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.

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

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.

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.

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.

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

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 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.

Crystal and magnetic structures of bilayer (n = 2) Ruddlesden-Popper phase La1-2x Sr1+2x Mn2O7 have been reviewed and their relationship with the colossal magnetoresistive behavior discussed. Experimental results of inelastic and quasielastic neutron scattering on the 40% hole-doped (x = 0.4) bilayer man-ganite La1.2Sr1.8Mn2O7 have been described. A fully quantum theory of the spin dynamics of ferromagnetic manganites is developed, starting from the minimal double exchange model. This is compared with experimental results for a ferromagnetic bilayer system. A mean-field type theory of doping dependence of the exchange interactions in bilayer manganites has also been developed and compared is compared with the experimental results.

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., Comment: 11 pages, 8 figures

The recent surge of interest in phase-change materials GeTe, ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$, and related compounds motivated us to revisit the structural phase transition in GeTe in more detail than was done before. The rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied using 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. The 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.

Room-temperature ferromagnetism in Mn-doped chalcopyrites is a desire aspect when applying those materials to spin electronics. However, dominance of high Curie-temperatures due to cluster formation or inhomogeneities limited their consideration. Here we report how an external perturbation such as applied hydrostatic pressure in CdGeP₂:Mn induces a two serial magnetic transitions from ferromagnet to non-magnet state at room temperature. This effect is related to the unconventional properties of created MnP magnetic clusters within the host material. Such behavior is also discussed in connection with ab initio density functional calculations, where the structural properties of MnP indicate magnetic transitions as function of pressure as observed experimentally. Our results point out new ways to obtain controlled response of embedded magnetic clusters.

We construct a theory of spin wave excitations in the bilayer manganite La(1.2)Sr(1.8)Mn2O7 based on the simplest possible double-exchange model, but including leading quantum corrections to the spin wave dispersion and damping. Comparison is made with recent inelastic neutron scattering experiments. We find that quantum effects account for some part of the measured damping of spin waves, but cannot by themselves explain the observed softening of spin waves at the zone boundary. Furthermore a doping dependence of the total spin wave dispersion and the optical spin wave gap is predicted.

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 3He polarizer and analyzer. This development is very important for extending the capabilities of the imaging instrument at ORNL providing a polarized beam with a large field-of-view, which can be further used in combination with optical devices like Wolter optics, focusing guides, or other lenses for the development of microscope arrangement. Such a setup can be of advantage for the existing and future imaging beamlines at the pulsed neutron sources. The first proof-of-concept experiment is performed to study the ferromagnetic phase transition in the Fe3Pt sample. We also demonstrate that the polychromatic neutron beam in combination with in situ 3He cells can be used as the initial step for the rapid measurement and qualitative analysis of radiographs.

The magnetic structure of the Eu2+ moments in the superconducting EuFe2(As1-xPx)2 sample with x = 0.19 has been determined using neutron scattering. We conclude that the Eu2+ moments are aligned along the c direction below T_C = 19.0(1) K with an ordered moment of 6.6(2) mu_B in the superconducting state. An impurity phase similar to the underdoped phase exists within the bulk sample which orders antiferromagnetically below T_N = 17.0(2) K. We found no indication of iron magnetic order, nor any incommensurate magnetic order of the Eu2+ moments in the sample.

Neutron scattering investigations of the magnetic ordering and spin waves have been carried out on the bilayer manganite La1.2Sr1.8Mn2O7 which shows colossal magnetoresistance (CMR) effect close to the ferromagnetic phase transition TC≈128 K. The dispersions of both the acoustic and the optic modes along [100] and [001] have been measured and by using a effective localised spin model the complete set of exchange parameters has been determined. The strong anisotropy in the exchange interactions suggests that La1.2Sr1.8Mn2O7 can be considered as a quasi two-dimensional ferromagnet consisting of weakly coupled ferromagnetic bilayers. Although an effective localised spin model can explain the spin wave dispersion for small momentum and energy transfers close to the zone centre, it is no longer valid for larger momentum and energy transfers close to the zone boundary. The energy widths of the spin waves are found to be much larger than the instrument resolution showing the dual localised–itinerant character of the electronic system.

The magnetic excitations in the weak itinerant ferromagnet ${\mathrm{Ni}}_{3}\mathrm{Al}$ have been investigated in the ordered phase as well as in the paramagnetic phase by means of inelastic neutron scattering, in order to search for critical fluctuations. The magnetic excitations have been interpreted within the framework of both itinerant and mode-mode coupling theory (MMT) for an isotropic ferromagnet. The parametrization of the linewidth for the critical scattering shows that the dynamical critical exponent $z\ensuremath{\simeq}2.47\ifmmode\pm\else\textpm\fi{}0.16$ is close to the value expected for an isotropic Heisenberg model, proving that critical fluctuations are also important in weak-itinerant ferromagnets. Moreover, the temperature dependence of the spin waves compares well with the predictions of MMT. The correlation length in weak itinerant magnets is very large leading to pronounced short-range order and to critical fluctuations far away from ${T}_{C}.$ The results show that it is necessary to include critical fluctuations in the theory for itinerant magnetism.

Antisite disorder is observed to have significant impact on the magnetic properties of the double perovskite Y2CoMnO6 which has been recently identified as a multiferroic. A paramagnetic-ferromagnetic phase transition occurs in this material at T-c approximate to 75 K. At 2K, it displays a strong ferromagnetic hysteresis with a significant coercive field of H-c approximate to 15 kOe. Sharp steps are observed in the hysteresis curves recorded below 8K. In the temperature range 2K

Inelastic neutron scattering experiments were performed to investigate the crystalline electric field (CEF) excitations of Nd3+ (J = 9/2) in the iron pnictide NdFeAsO. The crystal field level structures for both the high-temperature paramagnetic phase and the low-temperature antiferromagnetic phase of NdFeAsO are constructed. The variation of CEF excitations of Nd3+ reflects not only the change of local symmetry but also the change of magnetic ordered state of the Fe sublattice. By analyzing the crystal field interaction with a crystal field Hamiltonian, the crystal field parameters are obtained. It was found that the sign of the fourth and sixth-order crystal field parameters change upon the magnetic phase transition at 140 K, which may be due to the variation of exchange interactions between the 4f and conduction electrons.

We have used polarized and unpolarized neutron diffraction to determine the spatial distribution of the magnetization density induced by a magnetic field of 9 T in the tetragonal phase of K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$. The maximum entropy reconstruction shows clearly that most of the magnetization is confined to the region around the iron atoms whereas there is no significant magnetization associated with either Se or K atoms. The distribution of magnetization around the Fe atom is slightly nonspherical with a shape which is extended along the $\ensuremath{\langle}0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}1\ensuremath{\rangle}$ direction in the projection. Multipolar refinement results show that the electrons which give rise to the paramagnetic susceptibility are confined to the Fe atoms and their distribution suggests that they occupy 3$d$ ${t}_{2g}$-type orbitals with around 66% in those of xz/yz symmetry. Detail modeling of the magnetic form factor indicates the presence of an orbital moment to the total paramagnetic moment of Fe${}^{2+}$.

The possibility of coupling through the hyperfine interaction of nuclear spins with the electronic spin system has given rise to hope for potential novel applications in spintronics and quantum computations. We investigated the dispersion of nuclear spin waves in such a coupled system, Nd2CuO4, by using neutron-spin-echo spectroscopy at millikelvin temperatures. Our results show the existence of dispersion of nuclear spin waves in Nd2CuO4 at T D 40 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 A° , which is much smaller than that expected theoretically.

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., Published version, minor changes

Neutron scattering experiments were performed to investigate magnetic order and magnetic excitations in ternary iron chalcogenide K${}_{2}$Fe${}_{4}$Se${}_{5}$. The formation of a superlattice structure below 580 K together with the decoupling between the Fe-vacancy order-disorder transition and the antiferromagnetic order transition appears to be a common feature in the $A$${}_{2}$Fe${}_{4}$Se${}_{5}$ family. The study of spin dynamics of K${}_{2}$Fe${}_{4}$Se${}_{5}$ reveals two distinct energy gaps at the magnetic Brillouin zone center, which indicates the presence of magnetic anisotropy and the decrease of local symmetry due to electronic and orbital anisotropy. The low-energy spin wave excitations of K${}_{2}$Fe${}_{4}$Se${}_{5}$ can be properly described by linear spin wave theory within a Heisenberg model. Compared to iron pnictides, K${}_{2}$Fe${}_{4}$Se${}_{5}$ exhibits a more two-dimensional magnetism as characterized by large differences not only between out-of-plane and in-plane spin wave velocities, but also between out-of-plane and in-plane exchange interactions.

Spin waves in the type-III ordered antiferromagnetic state of the frustrated t- t ′ Hubbard model on the face-centred-cubic (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 the 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 MnS2 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.

Using high resolution powder neutron diffraction data, we show that there is a distinct anomaly in the structural noncentrosymmetry around the magnetic transition point TN for bulk and nanoscale BiFeO3. It appears that the structural noncentrosymmetry - which gives rise to the ferroelectric polarization - is suppressed anomalously by ~1% (of the average noncentrosymmetry at above the magnetic transition) in the bulk sample and by ~12% in the nanoscale sample as the magnetic transition is approached from higher temperature. This observation shows that the multiferroic coupling improves in the nanoscale sample which is expected to brighten the application prospects of nanoscale BiFeO3 in nanospintronics-based sensor devices.PACS Nos. 75.70.Cn, 75.75.-c

The effect of hydrostatic pressure on resistivity and magnetic ac susceptibility has been studied in Mn-doped CdGeAs2 room-temperature (RT) ferromagnetic chalcopyrite with two types of MnAs micro-clusters. The slight increase of temperature by about 30 K in the region between RT and Curie temperature TC causes a significant change in the positions of pressure-induced semiconductor-metal transition and magnetic phase transitions in low pressure area. By conducting measurements of the anomalous Hall resistance in the field H \leq 5 kOe, we present experimental evidence for pressure-induced metamagnetic-like state during the paramagnetic phase at pressure P = 5 GPa.

We have investigated low energy excitations in metallic HoAl2 by high resolution neutron spectroscopy. At T=3 K we found clear inelastic peaks in the energy loss and energy gain sides along with the central elastic peak. We interpret these inelastic peaks to be due to the transitions from hyperfine-split nuclear levels. The energy which is E = 25.03 ± 0.02 μ eV at T=3 K, decreases continuously and becomes zero at T N ≈ 30 K . The intensity of the inelastic peak remains more or less constant as a function temperature before the it merges with the central elastic peak at TN. The energy of nuclear spin excitations in HoAl2 seems to follow the order parameter of the ferromagnetic phase transition.

The charge ordered La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3\ensuremath{-}\ensuremath{\delta}}$ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, M\"ossbauer, and polarized neutron studies. A complex scenario of short-range charge and magnetic ordering is realized from the polarized neutron studies in nanocrystalline specimen. This short-range ordering does not involve any change in spin state and modification in the charge disproportion between Fe${}^{3+}$ and Fe${}^{5+}$ compared to bulk counterpart as evident in the M\"ossbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe${}^{3+}$ and Fe${}^{5+}$ are about 3.15 ${\ensuremath{\mu}}_{B}$ and 1.57 ${\ensuremath{\mu}}_{B}$ for bulk, and 2.7 ${\ensuremath{\mu}}_{B}$ and 0.53 ${\ensuremath{\mu}}_{B}$ for nanocrystalline specimen, respectively. The destabilization of charge ordering leads to magnetic phase separation, giving rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K attains a value as high as 4.4 kOe for average size $\ensuremath{\sim}$70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass-like transition around $\ensuremath{\sim}$65 K, below which EB appears. Overall results propose that finite-size effect directs the complex glassy magnetic behavior driven by unconventional short-range charge and magnetic ordering, and magnetic phase separation appears in nanocrystalline LSFO.