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Millimeter-sized PdAs2O6 single crystals are grown using the vapor transport technique. The magnetic order at [Formula: see text] K is studied by measuring magnetic properties, specific heat, and neutron single crystal diffraction. The anisotropic magnetic susceptibility and a metamagnetic transition observed in magnetic fields above 20 kOe suggest that the magnetic moment lies in the ab plane, consistent with the magnetic structure determined by neutron single crystal diffraction. Below 140 K, Pd2+ ions order ferromagnetically in the ab plane but antiferromagnetically along the crystallographic c axis. The ordered moment is refined to be 2.09(2) [Formula: see text]/Pd2+ using the fitted magnetic form factor of Pd2+ . A weak λ-type anomaly around T N was observed in specific heat and the magnetic entropy change across T N is 1.72 J mol-1 K.This small entropy change and the temperature dependence of the magnetic susceptibility support the presence of short range correlations in a wide temperature range [Formula: see text] 250 K. The comparison with SrRu2O6 suggests that the magnetic interactions in PdAs2O6 are dominated by Pd-(O-[Formula: see text]-O)-Pd super-superexchange and three dimensional despite the quasi-two-dimensional arrangement of magnetic ions. The comparison with NiAs2O6 suggests that increasing covalency of isostructural compounds is an effective approach to design and to discover new materials with higher magnetic order temperatures in the localized regime.

Structural and physical properties determined by measurements on large single crystals of the anisotropic ferromagnet MnBi are reported. The findings support the importance of magneto-elastic effects in this material. X-ray diffraction reveals a structural phase transition at the spin reorientation temperature $T_{SR}$ = 90 K. The distortion is driven by magneto-elastic coupling, and upon cooling transforms the structure from hexagonal to orthorhombic. Heat capacity measurements show a thermal anomaly at the crystallographic transition, which is suppressed rapidly by applied magnetic fields. Effects on the transport and anisotropic magnetic properties of the single crystals are also presented. Increasing anisotropy of the atomic displacement parameters for Bi with increasing temperature above $T_{SR}$ is revealed by neutron diffraction measurements. It is likely that this is directly related to the anisotropic thermal expansion in MnBi, which plays a key role in the spin reorientation and magnetocrystalline anisotropy. The identification of the true ground state crystal structure reported here may be important for future experimental and theoretical studies of this permanent magnet material, which have to date been performed and interpreted using only the high temperature structure., 9 pages, 6 figures, accepted for publication in Physical Review B

A flop of electric polarization from P$$\|$$c (Pc) to P$$\|$$ a (Pa) is observed in MnTiO3 as a spin flop transtion is triggered by a c-axis magnetic field, H$$\|$$c=7 T. The critical magnetic field for Pa is significantly reduced in Mn1-xNixTiO3 (x=0.33). Neutron diffraction measurements revealed similar magnetic arrangements for the two compositions where the ordered spins couple antiferromagnetically with their nearest intra- and inter-planar neighbors. In the x=0.33 system, the single ion anisotropies of Mn2+ and Ni2+ compete and give rise to an additional spin reorientation transition at TR. A magnetic field, Hc, aligns the spins along c for TRN. The rotation of the collinear spins away from the c-axis for TR alters the magnetic point symmetry and gives rise to new ME susceptibility tensor form. Such linear ME response provides satisfactory explanation for behavior of field-induced electric polarization in both compositions. As the Ni content increases to x=0.5 and 0.68, the ME effect disappears as a new magnetic phase emerges.

Controlling the ferroelectricity and magnetism in multiferroic materials has been an important research topic. We report the formation of a highly polarized state in multiferroic DyMnO3 single crystals which develops well above the magnetic transition temperatures, and we attribute it to the thermally stimulated depolarization current effect of excess holes forming Mn4+ ions in the system. We also show that this high temperature polarized state intimately correlates with the lower temperature ferroelectric state that is induced by the incommensurate spiral magnetic order of Mn spins. This study demonstrates an efficient approach to tune the multiferroicity in the manganite system.

The title compounds are prepared by reaction of silicon and carbon in either a Pr/Co or a Nd/Co eutectic flux (alumina crucible, 950 °C, 12 h and 850 °C for 48 h).

The crossover from localized- to itinerant-electron behavior is associated with many intriguing phenomena in condensed-matter physics. In this paper, we investigate the crossover from localized to itinerant regimes in the spinel system Mn$_{1-x}$Co$_x$V$_2$O$_4$. At low Co doping, orbital order (OO) of the localized electrons on the V3+ ions suppresses magnetic frustration by triggering a tetragonal distortion. With Co doping, electronic itinerancy melts the OO and suppresses the structural phase transition while the reduced spin-lattice coupling produces magnetic frustration. Neutron scattering measurements and first-principles-guided spin models reveal that the non-collinear state at high Co doping is produced by weakened local anisotropy and enhanced Co-V spin interactions., 5 pages, 4 figures

We present specific-heat and neutron-scattering results for the $S=1/2$ quantum antiferromagnet (dimethylammonium)(3,5-dimethylpyridinium)${\mathrm{CuBr}}_{4}$. The material orders magnetically at ${\mathit{T}}_{N}=1.99(2)$ K, and magnetic excitations are accompanied by an energy gap of 0.30(2) meV due to spin anisotropy. The system is best described as coupled two-leg spin-1/2 ladders with the leg exchange ${J}_{\mathrm{leg}}=0.60(2)$ meV, rung exchange ${J}_{\mathrm{rung}}=0.64(9)$ meV, interladder exchange ${J}_{\mathrm{int}}=0.19(2)$ meV, and an interaction-anisotropy parameter $\ensuremath{\lambda}=0.93(2)$, according to inelastic neutron-scattering measurements. In contrast to most spin ladders reported to date, the material is a rare example in which the interladder coupling is very near the critical value required to drive the system to a N\'eel-ordered phase without the assistance of a magnetic field.

We present a single crystal neutron scattering study of BaMn2Bi2, a recently synthesized material with the same ThCr2Si2-type structure found in several Fe-based unconventional superconducting materials. We show long range magnetic order, in the form of a G-type antiferromagnetic structure, to exist up to 390 K with an indication of a structural transition at 100 K. Utilizing inelastic neutron scattering we observe a spin-gap of 16meV, with spin-waves extending up to 55 meV. We find these magnetic excitations to be well fit to a J1-J2-Jc Heisenberg model and present values for the exchange interactions. The spin wave spectrum appears to be unchanged by the 100 K structural phase transition.

Most unconventional superconductors, including cuprates and iron-based superconductors, are derived from chemical doping or application of pressure on their collinearly magnetic-ordered parent compounds[1-5]. The recently discovered pressure-induced superconductor CrAs, as a rare example of a non-collinear helimagnetic superconductor, has therefore generated great interest in understanding microscopic magnetic properties and their interplay with superconductivity [6-8]. Unlike cuprates and iron based superconductors where the magnetic moment direction barely changes upon doping, here we show that CrAs exhibits a spin reorientation from the ab plane to the ac plane, along with an abrupt drop of the magnetic propagation vector at a critical pressure (Pc~0.6 GPa). This magnetic phase transition coincides with the emergence of bulk superconductivity, indicating a direct connection between magnetism and superconductivity. With further increasing pressure, the magnetic order completely disappears near the optimal Tc regime (P~0.94 GPa). Moreover, the Cr magnetic moments between nearest neighbors tend to be aligned antiparallel with increasing pressure toward the optimal superconductivity regime. Our findings suggest that the non-collinear helimagnetic order is strongly coupled to structural and electronic degrees of freedom, and that antiferromagnetic correlations associated with the low magnetic vector phase are crucial for superconductivity., Comment: Supplementary Information included

The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase-space for three-phonon scattering processes, rather than just the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optical ferroelectric mode.

We have studied the field-induced magnetic structures of Tb${}_{2}$Ti${}_{2}$O${}_{7}$ pyrochlore by single-crystal neutron diffraction under a field applied along the [111] axis, up to $H=12$ T and down to $T=40$ mK. We refined the magnetic structures with $\mathbit{k}=\mathbf{0}$ propagation vector by performing a symmetry analysis in the space group $R\overline{3}m$, reducing the number of free parameters to three only. The Tb moments gradually reorient towards the field direction, keeping close to a ``3-in, 1-out / 1-in, 3-out'' spin structure (magnetic space group $R\overline{3}{m}^{\ensuremath{'}}$) in the whole measured field range 0.05--12 T. Our results rule out the ``all-in/all-out'' structure previously proposed and do not support the existence of a magnetization plateau. We perform a quantitative comparison with mean-field calculations and we propose the presence of a low-temperature dynamic symmetry breaking of the local trigonal symmetry, akin to a dynamic Jahn-Teller effect, i.e., preserving the overall cubic symmetry. We discuss the possible origin of this off-diagonal mixing term in the crystal field Hamiltonian in terms of quadrupole-quadrupole interaction or magnetoelastic effects.

experiments indicate that part of the doped TM ions (TM =Fe, Co, and Ni) substitute for Ir, and the rest intercalate into the octahedral interstitial sites located in between IrTe2 layers. Due to the lattice mismatch between MnTe2 and IrTe2, Mn has limited solubility in IrTe2 lattice. The trigonal structure is stable in the whole temperature range 1.80 K T 300 K for all doped compositions. No long range magnetic order or superconductivity was observed in any doped compositions above 1.80 K. A spin glass behavior below 10 K was observed in Fe-doped IrTe2 from the temperature dependence of magnetization, electrical resistivity, and specic heat. The low temperature specic heat data suggest the electron density of states is enhanced in Feand Co-doped compositions but reduced in Ni-doped IrTe2. With the 3d transition metal doping the trigonal a-lattice parameters increases but the c-lattice parameter decreases. Detailed analysis of the single crystal x-ray diraction data shows that interlayer Te-Te distance increases despite a reduced c-lattice. The importance of the Te-Te, Te-Ir, and Ir-Ir bonding is discussed.

We construct a phase diagram of the parent compound Fe1+xTe as a function of interstitial iron x in terms of the electronic, structural, and magnetic properties. For a concentration of x < 10%, Fe1+xTe undergoes a "semimetal" to metal transition at approximately 70 K that is also first-order and coincident with a structural transition from a tetragonal to a monoclinic unit cell. For x ~ 14%, Fe1+xTe undergoes a second-order phase transition at approximately 58 K corresponding to a "semimetal" to "semimetal" transition along with a structural orthorhombic distortion. At a critical concentration of x ~ 11%, Fe1+xTe undergoes two transitions: the higher temperature one is a second-order transition to an orthorhombic phase with incommensurate magnetic ordering and temperature-dependent propagation vector, while the lower temperature one corresponds to nucleation of a monoclinic phase with a nearly commensurate magnetic wavevector. While both structural and magnetic transitions display similar critical behavior for x < 10% and near the critical concentration of x ~ 11%, samples with large interstitial iron concentrations show a marked deviation between the critical response indicating a decoupling of the order parameters. Analysis of temperature dependent inelastic neutron data reveals incommensurate magnetic fluctuations throughout the Fe1+xTe phase diagram are directly connected to the "semiconductor"-like resistivity above T_N and implicates scattering from spin fluctuations as the primary reason for the semiconducting or poor metallic properties. The results suggest that doping driven Fermi surface nesting maybe the origin of the gapless and incommensurate spin response at large interstitial concentrations., (17 pages, 17 figures, to be published in Physical Review B)

Yb2.77FeAl3.72Mg0.28Si2, Dy3-δFeAl4-xMgxSi2, Y3-δFeAl4-xMgxGe2, Er3-δFeAl4-xMgxGe2, and Yb5Fe4Al17Si6 are synthesized from the elements using an equimolar Mg/Al reactive flux (950 °C, 5 h).

The spin and orbital ordering in Dy${}_{1\ensuremath{-}x}$Tb${}_{x}$VO${}_{3}$ ($x=0$ and 0.2) was studied by measuring x-ray powder diffraction, magnetization, specific heat, and neutron single-crystal diffraction. The results show that G-OO/C-AF and C-OO/G-AF phases coexist in Dy${}_{0.8}$Tb${}_{0.20}$VO${}_{3}$ in the temperature range 2--60 K, and the volume fraction of each phase is temperature and field dependent. The ordering of Dy moments at $T$* = 12 K induces a transition from G-OO/C-AF to a C-OO/G-AF phase. Magnetic fields suppress the long-range order of Dy moments and thus the C-OO/G-AF phase below $T$*. The polarized moments induced at the Dy sublattice by external magnetic fields couple to the V 3d moments, and this coupling favors the G-OO/C-AF state. Also discussed is the effect of the Dy-V magnetic interaction and local structure distortion on the spin and orbital ordering in Dy${}_{1\ensuremath{-}x}$Tb${}_{x}$VO${}_{3}$.

Direct evidence of controlling the population of magnetic domains in BiFeO${}_{3}$ thin films through electric field is reported using neutron diffraction. By fabricating BiFeO${}_{3}$ thin films on vicinal SrTiO${}_{3}$ substrates, we have achieved ferroelectric monodomains as confirmed by piezoresponse force microscopy. The application of an electric field between the bottom SrRuO${}_{3}$ and the top electrode switches the ferroelectric domain state with concomitant changes in magnetic reflections observed with neutron diffraction, indicating changes in the antiferromagnetic domain populations. The observed magnetoelectric switching behavior by neutron diffraction is compared with the electric-field effect on the magneto-optical Kerr effect measurement on patterned pads of exchange coupled Co film deposited on top of the BiFeO${}_{3}$ films. The present result shows possible new directions for the realization of magnetoelectric devices.

Millimeter sized single crystals of Mo${}_{3}$Sb${}_{7}$ are grown using the self-flux technique and a thorough characterization of their structural, magnetic, thermal, and transport properties is reported. The structure parameters for the high-temperature cubic phase and the low-temperature tetragonal phase were determined with neutron single crystal diffraction. Both x-ray powder diffraction and neutron single crystal diffraction at room temperature confirmed that Mo${}_{3}$Sb${}_{7}$ crystallizes in Ir${}_{3}$Ge${}_{7}$-type cubic structure with space group $\mathit{Im}$ $\overline{3}$$m$. The cubic-tetragonal structure transition at 53 K is verified by the peak splitting of (4 0 0) reflection observed by x-ray single crystal diffraction and the dramatic intensity change of the (12 0 0) peak observed by neutron single crystal diffraction. The structural transition is accompanied by a sharp drop in magnetic susceptibility, electrical resistivity, and thermopower while cooling. A weak lambda anomaly was also observed around 53 K in the temperature dependence of specific heat, and the entropy change across the transition is estimated to be 1.80 J/mol Mo K. The temperature dependence of magnetic susceptibility was measured up to 750 K, and it follows a Curie-Weiss behavior above room temperature. Analysis of the low-temperature magnetic susceptibility suggests a spin gap of 110 K around 53 K. A typical phonon thermal conductivity was observed in the low temperature tetragonal phase. A glassy phonon thermal conductivity above 53 K suggests a structural instability in a wide temperature range. Superconductivity was observed at 2.35 K in the as-grown crystals, and the dimensionless specific heat jump $▵C(T)/{\ensuremath{\gamma}}_{n}{T}_{c}$ was determined to be 1.49, which is slightly larger than the BCS value of 1.43 for the weak-coupling limit.

Pressure-dependent transport measurements of Ir${}_{1\ensuremath{-}x}$Pt${}_{x}$Te${}_{2}$ are reported. With increasing pressure, the structural phase transition at high temperatures is enhanced while its superconducting transition at low temperatures is suppressed. These pressure effects make Ir${}_{1\ensuremath{-}x}$Pt${}_{x}$Te${}_{2}$ distinct from other studied $\mathit{TX}$${}_{2}$ systems exhibiting a charge density wave (CDW) state, in which pressure usually suppresses the CDW state and enhances the superconducting state. The results reveal that the emergence of superconductivity competes with the stabilization of the low-temperature monoclinic phase in Ir${}_{1\ensuremath{-}x}$Pt${}_{x}$Te${}_{2}$.

We used X-ray/neutron diffraction to determine the low temperature (LT) structure of IrTe2. A structural modulation was observed with a wavevector of k =(1/5, 0, 1/5) below Ts?285 K, accompanied by a structural transition from a trigonal to a triclinic lattice. We also performed the first principles calculations for high temperature (HT) and LT structures, which elucidate the nature of the phase transition and the LT structure. A local bonding instability associated with the Te 5p states is likely the origin of the structural phase transition in IrTe2., Comment: 6 pages, 6 figures

The recently discovered K-Fe-Se high-temperature superconductor has caused heated debate regarding the nature of its parent compound. Transport, angle-resolved photoemission spectroscopy, and STM measurements have suggested that its parent compound could be insulating, semiconducting, or even metallic [M. H. Fang, H.-D. Wang, C.-H. Dong, Z.-J. Li, C.-M. Feng, J. Chen, and H. Q. Yuan, Europhys. Lett. 94, 27009 (2011); F. Chen et al., Phys. Rev. X 1, 021020 (2011); and W. Li et al., Phys. Rev. Lett. 109, 057003 (2012)]. Because the magnetic ground states associated with these different phases have not yet been identified and the relationship between magnetism and superconductivity is not fully understood, the real parent compound of this system remains elusive. Here, we report neutron-diffraction experiments that reveal a semiconducting antiferromagnetic (AFM) phase with rhombus iron vacancy order. The magnetic order of the semiconducting phase is the same as the stripe AFM order of the iron pnictide parent compounds. Moreover, while the $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ block AFM phase coexists with superconductivity, the stripe AFM order is suppressed by it. This leads us to conjecture that the new semiconducting magnetic ordered phase is the true parent phase of this superconductor.

We present a comprehensive single crystal neutron diffraction investigation of the $$\rm Mn_{1-x}Co_{x}WO_4$$ with $$0.02\leq x \leq0.30$$. At lower concentration $$x \leq 0.05$$, the system is quickly driven into the multiferroic phase with spin structure forming an elliptical spiral order similar to the parent compound. The reduction of electric polarization is ascribed to the tilting of the spiral plane. For $$x\sim 0.075$$, the magnetic structure undergoes a spin flop transition that is characterized by a sudden rotation of the spin helix envelope into the $ac$ plane. This spin structure persists for concentration up to $x=0.15$, where additional competing magnetic orders appear at low temperature. For $$0.17 \leq x \leq 0.30$$, the system experiences another spin flop transition and recovers the low-$x$ spiral spin configuration. A simple commensurate spin structure with $$\vec{q}=(0.5,0,0)$$ is found to coexist with the incommensurate spiral order. The complex evolution of magnetic structure in Co doped $$\rm MnWO_4$$ contrasts sharply with other transition metal ion doped $$\rm Mn_{1-x}A_xWO_4$$ (A=Zn, Mg, Fe) where the chemical substitutions stabilize only one type of magnetic structure. The rich phase diagram of $$\rm Mn_{1-x}Co_{x}WO_4 $$ results from the interplay between magnetic frustration and spin anisotropy of the Co ions.

We have performed elastic and inelastic neutron scattering experiments on single crystal samples of the coordination polymer compound CuF${}_{2}$(H${}_{2}$O)${}_{2}$(pyz) (pyz = pyrazine) to study the magnetic structure and excitations. The elastic neutron diffraction measurements indicate a collinear antiferromagnetic structure with moments oriented along the [0.7 0 1] real-space direction and an ordered moment of 0.60 $\ifmmode\pm\else\textpm\fi{}$ 0.03 ${\ensuremath{\mu}}_{B}/$Cu. This value is significantly smaller than the single-ion magnetic moment, reflecting the presence of strong quantum fluctuations. The spin wave dispersion from magnetic zone center to the zone boundary points (0.5 1.5 0) and (0.5 0 1.5) can be described by a two-dimensional Heisenberg model with a nearest-neighbor magnetic exchange constant ${J}_{2D}=0.934$ $\ifmmode\pm\else\textpm\fi{}$ 0.0025 meV. The interlayer interaction ${J}_{\mathrm{perp}}$ in this compound is less than 1.5$%$ of ${J}_{2D}$. The spin excitation energy at the (0.5 0.5 0.5) zone boundary point is reduced when compared to the (0.5 1 0.5) zone boundary point by $\ensuremath{\sim}$10.3$%$ $\ifmmode\pm\else\textpm\fi{}$ 1.4$%$. This zone boundary dispersion is consistent with quantum Monte Carlo and series expansion calculations for the $S=\frac{1}{2}$ Heisenberg square lattice antiferromagnet, which include corrections for quantum fluctuations to linear spin wave theory.

The relationship between vacancy ordering and magnetism in ${\mathrm{TlFe}}_{1.6}{\mathrm{Se}}_{2}$ has been investigated via single crystal neutron diffraction, nuclear forward scattering, and transmission electron microscopy. The examination of chemically and structurally homogeneous crystals allows the true ground state to be revealed, which is characterized by Fe moments lying in the ab plane below 100 K. This is in sharp contrast to crystals containing regions of order and disorder, where a competition between $c$ axis and ab plane orientations of the moments is observed. The properties of partially disordered ${\mathrm{TlFe}}_{1.6}{\mathrm{Se}}_{2}$ are, therefore, not associated with solely the ordered or disordered regions. This contrasts the viewpoint that phase separation results in independent physical properties in intercalated iron selenides, suggesting a coupling between ordered and disordered regions may play an important role in the superconducting analogues.

The recently discovered K-Fe-Se high-temperature superconductor has caused heated debate regarding the nature of its parent compound. Transport, angle-resolved photoemission spectroscopy, and STM measurements have suggested that its parent compound could be insulating, semiconducting, or even metallic [M. H. Fang, H.-D. Wang, C.-H. Dong, Z.-J. Li, C.-M. Feng, J. Chen, and H. Q. Yuan, Europhys. Lett. 94, 27009 (2011); F. Chen et al., Phys. Rev. X 1, 021020 (2011); and W. Li et al., Phys. Rev. Lett. 109, 057003 (2012)]. Because the magnetic ground states associated with these different phases have not yet been identified and the relationship between magnetism and superconductivity is not fully understood, the real parent compound of this system remains elusive. Here, we report neutron-diffraction experiments that reveal a semiconducting antiferromagnetic (AFM) phase with rhombus iron vacancy order. The magnetic order of the semiconducting phase is the same as the stripe AFM order of the iron pnictide parent compounds. Moreover, while the sqrt[5]×sqrt[5] block AFM phase coexists with superconductivity, the stripe AFM order is suppressed by it. This leads us to conjecture that the new semiconducting magnetic ordered phase is the true parent phase of this superconductor.

We report the magnetic structure of TaFe${}_{1+y}$Te${}_{3}$ single crystals by means of neutron diffraction measurements. TaFe${}_{1+y}$Te${}_{3}$ possesses a layered structure with a formation of two-leg zigzag ladders along the $b$ axis. We find that TaFe${}_{1+y}$Te${}_{3}$ undergoes an antiferromagnetic transition at 178 K with Fe1 spins of the intraladders ferromagnetically aligned and with Fe1 spins of the interladders antiferromagnetically coupled. Furthermore, spins of the neighboring interstitial Fe2 ($y$) ions align parallel to the Fe1 spins of each ladder. These findings are distinct from the magnetic structure of the recently discovered spin-ladder compound BaFe${}_{2}$Se${}_{3}$. TaFe${}_{1+y}$Te${}_{3}$ may serve as a model system for investigating the interesting physics of quasi-one-dimensional ferromagnetic systems.

We have combined single-crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice Na${}_{2}$IrO${}_{3}$. The system orders magnetically below $18.1(2)$ K with Ir${}^{4+}$ ions forming zigzag spin chains within the layered honeycomb network with an ordered moment of $0.22(1){\ensuremath{\mu}}_{\text{B}}/\text{Ir}$ site. Such a configuration sharply contrasts with the N\'eel or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form a nearly ideal two-dimensional honeycomb lattice while the IrO${}_{6}$ octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much needed experimental insights into the magnetic and crystal structure that are crucial to the understanding of the exotic magnetic order and possible topological characteristics in the $5d$-electron-based honeycomb lattice.

The relationship between vacancy ordering and magnetism in TlFe(1.6)Se(2) has been investigated via single crystal neutron diffraction, nuclear forward scattering, and transmission electron microscopy. The examination of chemically and structurally homogeneous crystals allows the true ground state to be revealed, which is characterized by Fe moments lying in the ab plane below 100 K. This is in sharp contrast to crystals containing regions of order and disorder, where a competition between c axis and ab plane orientations of the moments is observed. The properties of partially disordered TlFe(1.6)Se(2) are, therefore, not associated with solely the ordered or disordered regions. This contrasts the viewpoint that phase separation results in independent physical properties in intercalated iron selenides, suggesting a coupling between ordered and disordered regions may play an important role in the superconducting analogues.

The evolution of the nuclear and magnetic structures of TlFe${}_{1.6}$Se${}_{2}$ was determined in the temperature range 5--450 K using single-crystal neutron diffraction. The Fe layers in these materials develop a corrugation in the magnetically ordered state. A canting away from the block checkerboard magnetic structure is observed in the narrow temperature range between approximately 100 and 150 K. In this same temperature range, an increase in the corrugation of the Fe layers is observed. At lower temperatures, the block checkerboard magnetic structure is recovered with a suppressed magnetic moment and abrupt changes in the lattice parameters. Microstructure analysis at 300 K using atomic-resolution Z-contrast scanning transmission electron microscopy reveals regions with ordered and disordered Fe vacancies, and the iron content is found to be uniform across the crystal. These findings highlight the differences between the alkali-metal and thallium materials, and indicate competition between magnetic ground states and a strong coupling of magnetism to the lattice in TlFe${}_{1.6}$Se${}_{2}$.

We investigate the physical properties and electronic structure of BaFe2-{\delta}Se3 crystals, which were grown out of tellurium flux. The crystal structure of the compound, an iron-deficient derivative of the ThCr2Si2-type, is built upon edge-shared FeSe4 tetrahedra fused into double chains. The semiconducting BaFe2-{\delta}Se3 with {\delta} \approx 0.2 ({\rho}295K = 0.18 {\Omega}\cdotcm and Eg = 0.30 eV) does not order magnetically, however there is evidence for short-range magnetic correlations of spin glass type (Tf \approx 50 K) in magnetization, heat capacity and neutron diffraction results. A one-third substitution of selenium with sulfur leads to a slightly higher electrical conductivity ({\rho}295K = 0.11 {\Omega}\cdotcm and Eg = 0.22 eV) and a lower spin glass freezing temperature (Tf \approx 15 K), corroborating with higher electrical conductivity reported for BaFe2S3. According to the electronic structure calculations, BaFe2Se3 can be considered as a one-dimensional ladder structure with a weak interchain coupling.

Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$. For the insulating Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$, neutron polarization analysis and single-crystal neutron diffraction unambiguously confirm the earlier proposed $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ block antiferromagnetic structure. For superconducting samples (${T}_{c}=30$ K), we find that in addition to the tetragonal $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superlattice structure transition at 513 K, the material develops a separate $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ superlattice structure at a lower temperature of 480 K. These results suggest that superconducting Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$ is phase separated with coexisting $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ and $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superlattice structures.

Structural, magnetic, electrical transport, and heat capacity data are reported for single crystals of TlFe{sub 1.6}Se{sub 2}. This compound crystallizes in a tetragonal structure similar to the ThCr{sub 2}Si{sub 2} structure, but with vacancies in the Fe layer. The vacancies can be ordered or disordered depending on temperature and thermal history. If the vacancies are ordered, the basal plane lattice constant increases from a to {radical}5a. Antiferromagnetic order with the Fe spins along the c axis occurs below T{sub N} {approx} 430 K, as shown by single-crystal neutron diffraction, and the magnetic structure is reported. In addition, for the vacancy-ordered crystal, two other phase transitions are found at T{sub 1} {approx} 140 K and T{sub 2} {approx} 100 K. The phase transitions at T{sub 1} and T{sub 2} are evident in heat capacity, magnetic susceptibility, resistivity data, a and c lattice parameters, and the unusual temperature dependence of the magnetic order parameter determined from neutron scattering. The phase transitions at T{sub 1} and T{sub 2} result in significant changes in the magnetic moment per iron, with 1.72(6) {micro}B observed at 300 K, 2.07(9) {micro}B at 140 K, 1.90(9) {micro}B at 115 K, and 1.31(8) {micro}B for 5 K if themore » same 'block checkerboard' magnetic structure is used at all temperatures. The phase transitions appear to be driven by small changes in the c lattice constant, large magnetoelastic coupling, and the localization of carriers with decreasing temperature.« less

We present neutron diffraction measurements on single-crystal samples of nonsuperconducting Ba(${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}$)${}_{2}$${\mathrm{As}}_{2}$ as a function of Cr doping for $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.47$. The average spin-density-wave moment is independent of concentration for $x\ensuremath{\leqslant}0.2$ and decreases rapidly for $x\ensuremath{\geqslant}0.3$. For concentrations in excess of 30% chromium, we find a new competing magnetic phase consistent with G-type antiferromagnetism which rapidly becomes the dominant magnetic ground state. Strong magnetism is observed for all concentrations measured, naturally explaining the absence of superconductivity in the Cr-doped materials.

This paper presents results of a recent study of multiferroic \CCO\ by means of single crystal neutron diffraction. This system has two close magnetic phase transitions at $T\nsub{N1}=24.2$ K and $T\nsub{N2}=23.6$ K. The low temperature magnetic structure below $T\nsub{N2}$ is unambiguously determined to be a fully 3-dimensional proper screw. Between $T\nsub{N1}$ and $T\nsub{N2}$ antiferromagnetic order is found that is essentially 2-dimensional. In this narrow temperature range, magnetic near neighbor correlations are still long range in the ($H,K$) plane, whereas nearest neighbors along the $L$-direction are uncorrelated. Thus, the multiferroic state is realized only in the low-temperature 3-dimensional state and not in the 2-dimensional state.

We studied the field-induced magnetic structures of the ${\text{Tb}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ pyrochlore by single-crystal neutron diffraction with a magnetic field applied along a [110] axis, focusing on the influence of a small misalignment. Both induced magnetic structures with $\mathbit{k}=\mathbf{0}$ and $\mathbit{k}=(0,0,1)$ propagation vectors are found to be sensitive to the misalignment, which controls the magnitude and orientation of the Tb moments involved in the $\ensuremath{\beta}$ chains, with local [111] anisotropy axis perpendicular to the field. For $\mathbit{k}=\mathbf{0}$, spin-icelike structures are observed for a misalignment of a few degrees whereas other structures, where the $\text{Tb-}\ensuremath{\beta}$ moments flip by ``melting'' on the field axis, occur when the field is perfectly aligned. The field evolution of the $\mathbit{k}=\mathbf{0}$ structure is well reproduced by a molecular field model with anisotropic exchange. We give a complete symmetry analysis of the $\mathbit{k}=\mathbf{0}$ and $\mathbit{k}=(0,0,1)$ magnetic structures, both being described by the basis functions of single irreducible representations.

We have measured neutron-diffraction patterns in a single-crystal sample of the pyrochlore compound ${\text{Er}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ in the antiferromagnetic phase $(T=0.3\text{ }\text{K})$, as a function of the magnetic field, up to 6 T, applied along the [110] direction. We determine all the characteristics of the magnetic structure throughout the critical point at ${H}_{c}=2\text{ }\text{T}$. As a main result, all Er moments align along the field at ${H}_{c}$ and their values reach a minimum. Using a four-sublattice self-consistent calculation, we show that the evolution of the magnetic structure and the value of the critical field are rather well reproduced using the same anisotropic exchange tensor as that accounting for the local paramagnetic susceptibility. In contrast, an isotropic exchange tensor does not yield the correct moment variations through the critical point. The model also accounts semiquantitatively for other experimental data obtained in previous works, such as the field dependence of the heat capacity, energy of the dispersionless inelastic modes and transition temperature.

We have studied the ${\text{Tb}}_{2}{\text{Sn}}_{2}{\text{O}}_{7}$ ordered spin ice by neutron diffraction under an isotropic pressure of 4.6 GPa, combined with a stress of 0.3(1) GPa. Measurements down to a temperature of 0.06 K and up to 100 K probe the effect of pressure both on ground state and spin fluctuation in the paramagnetic region. In the pressure-induced ground state, the ordered spin ice structure with a ${\mathbf{k}}_{0}=0$ propagation vector persists, but it coexists with a structure with ${\mathbf{k}}_{1}=(0,0,1)$. The ordered moment at 0.06 K is reduced, suggesting that pressure also enhances the spin liquid fluctuations at $T\ensuremath{\sim}0$. In the paramagnetic region, applying pressure changes the short-range spin correlations and suppresses the ferromagnetic correlations. The influence of pressure is discussed considering both isotropic pressure and stress effects.

We studied the field induced magnetic order in ${R}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ pyrochlore compounds with either uniaxial ($R=\mathrm{Ho}$, Tb) or planar ($R=\mathrm{Er}$, Yb) anisotropy, by polarized neutron diffraction. The determination of the local susceptibility tensor ${{\ensuremath{\chi}}_{\ensuremath{\Vert}},{\ensuremath{\chi}}_{\ensuremath{\perp}}}$ provides a universal description of the field induced structures in the paramagnetic phase (2--270 K), whatever the field value (1--7 T) and direction. Comparison of the thermal variations of ${\ensuremath{\chi}}_{\ensuremath{\parallel}}$ and ${\ensuremath{\chi}}_{\ensuremath{\perp}}$ with calculations using the rare earth crystal field shows that exchange and dipolar interactions must be taken into account. We determine the molecular field tensor in each case and show that it can be strongly anisotropic.

We used a local susceptibility approach in extensive polarized neutron diffraction studies of the spin liquid \tbti. For a magnetic field applied along the [110] and [111] directions, we found that, at high temperature, all Tb moments are collinear and parallel to the field. With decreasing temperature, the Tb moments reorient from the field direction to their local anisotropy axes. For the [110] field direction, the field induced magnetic structure at 10 K is spin ice-like, but with two types of Tb moments of very different magnitudes. For a field along [111], the magnetic structure resembles the so-called "one in-three out" found in spin ices, with the difference that all Tb moments have an additional component along the [111] direction due to the magnetic field. The temperature evolution of the local susceptibilities clearly demonstrates a progressive change from Heisenberg to Ising behavior of the Tb moments when lowering the temperature, which appears to be a crystal field effect., Conference Proceeding of Polarized Neutrons in Condensed Matter Investigations (Tokai, Japan, 2008)

We have studied the field-induced magnetic structures in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, in a wide temperature ($0.3lTl270\text{ }\text{ }\mathrm{K}$) and field ($0lHl7\text{ }\text{ }\mathrm{T}$) range, by single crystal polarized and unpolarized neutron diffraction, with $\mathbf{H}\ensuremath{\parallel}[110]$ axis. A ferromagneticlike structure with $\mathbf{k}=\mathbf{0}$ propagation vector is induced, whose local order at low field and low temperature is akin to spin ice. The four Tb ions separate in $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ chains having different values of the magnetic moments, which is quantitatively explained by taking the crystal field anisotropy into account. Above 2 T and below 2 K, an antiferromagneticlike structure with $\mathbf{k}=(0,0,1)$ is induced besides the $\mathbf{k}=\mathbf{0}$ structure. It shows a reentrant behavior and extends over a finite length scale. It occurs together with a broadening of the nuclear peaks, which suggests a field-induced distortion and magnetostriction effect.

Four types of isotropic exchange interactions of Mn(12)-Ac are obtained by using the classical Monte Carlo simulations. The equilibrium susceptibilities are well reproduced in the temperature range between 10 and 100 K. The calculated effective spin at 0.1 K coincides with the ground-state spin. Our results show that J(1) and J(2) are strong antiferromagnetic, but the magnitude of J(2) is much smaller than that of J(1). Both J(3) and J(4) favor weakly antiferromagnetic couplings. The effects of the exchange couplings on the magnetic properties and ground-state spin are investigated too. The magnetic susceptibilities below 100 K depend more on J(2) rather than on the stronger J(1). The weak exchange couplings J(3) and J(4) have significant frustration effects on the ground-state configuration.

The pyroelectric current effects in multiferroic TbMnO3 single crystal are reinvestigated in a wide temperature range. We report a pyroelectric current peak occurring around T = 107 K, which is much higher than the magnetic phase transition temperatures, after poling the sample from T ≥ 60 K to low temperatures, and we attribute it to the thermal release of trapped-charges in TbMnO3. We also show that the trapped-charges form a strong internal electric field that remarkably controls the ferroelectric polarization state induced by the cycloidal Mn3+ spins at lower temperature.

Incommensurate helical (or cycloidal) magnetic structure may have left- and right-wound states (helicity), which are in principle equally populated in a magnet with inversion symmetry. In addition, for a Heisenberg triangular antiferromagnet, clockwise and counter-clockwise rotations of the 120 degree spin structure provide another intriguing degree of freedom. Hence, a triangular magnet that has incommensurate helical ordering along the stacking direction will show intriguing interplay of the helicity (of the helical structure) and chirality (in the triangular plane). Such phenomenon is, however, rarely studied in the past since only one example, the Ba3NbFe3Si2O14 langathite, has been known to date [1]. In this work, we study MnSb2O6, which consists of distorted triangular lattice stacking along the c-axis [2,3]. MnSb2O6 belongs to the space group P321, and hence lacks inversion symmetry. Due to this fact, unique selection of the helicity and chirality may be expected. However, the earlier studies were carried out using unpolarized neutron diffraction with mostly the powder sample, and thus helicity and chirality selection cannot be concluded. Here, we have performed single-crystal diffraction experiment using polarized neutrons in addition to the unpolarized ones, and have succeeded in determination of the magnetic structure of MnSb2O6. The resulting magnetic structure is nearly cycloidal with the magnetic modulation vector q = (0, 0, 0.182) (see figure below). The spin rotation plane is, however, inclined from the ac-plane toward the b-axis for approximately 30 degrees. Polarization analysis indicates that both the helicity of the (nearly-) cycloidal structure and chirality of the in-plane 120 degree structure are uniquely selected. The 30 degree inclination from the ac-plane is a key finding of this work, allowing new kind of multiferroicity in this material.