Orthorhombic distortion and orbital order in the vanadium spinel FeV2 O4

Under the terms of the Creative Commons Attribution license.-- et al.Using synchrotron and neutron diffraction measurements, we find a low-temperature orthorhombic phase in vanadium spinel FeV2O4. The orbital order of V3+ ions with tetragonal normal modes occurs at 68 K, and this leads to an appearance of the pseudotetragonal phase at a noncollinear ferrimagnetic transition temperature. Below the magnetic transition temperature, unconventional behavior of the orbital state of Fe2+ ions accompanied by the emergence of the orthorhombic phase was observed by using the normal mode analysis. We have also studied the structural properties of orbitally diluted materials. The orthorhombic phase, which is significantly affected by the other ions, is intrinsic in FeV2O4. We suggest the orthorhombic phase is strongly related with the double orbital states of Fe2+ and V3+ ions.This study was supported by the Ministry of Education, Culture, Sports, Science, and Technology of Japan through a Grant-in Aid for Scientific Research and by the Sasagawa Scientific Research Grant from The Japan Science Society.Peer Reviewe

The inverse perovskite BaLiF3: single-crystal neutron diffraction and analyses of potential ion pathways

Doped barium lithium trifluoride has attracted attention as component for scintillators, luminescent materials and electrodes. With lithium and fluoride, it contains two possibly mobile species, which may account for its ionic conductivity. In this study, neutron diffraction on oxide-containing BaLiF3 single-crystals is performed at up to 636.2°C. Unfortunately, ion-migration pathways could not be mapped by modelling anharmonic ion displacement or by inspecting the scattering-length density that was reconstructed via maximum-entropy methods. However, analyses of the topology and bond-valence site energies derived from the high-temperature structure reveal that the anions can migrate roughly along the edges of the LiF6 coordination octahedra with an estimated migration barrier of ∼0.64 eV (if a vacancy permits), whereas the lithium ions are confined to their crystallographic positions. This finding is not only valid for the title compound but for ion migration in all perovskites with Goldschmidt tolerance factors near unity

Magnetically-induced ferroelectricity in the (ND4)2[FeCl5(D2O)] molecular compound

The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH4)2[FeCl5(H2O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroicity. From the crystal structure analysis, we observed an order-disorder phase transition related to the ordering of the ammonium counterion. We have determined the magnetic structure below TN, at 2K and zero magnetic field, which corresponds to a cycloidal spin arrangement with magnetic moments contained in the ac-plane, propagating parallel to the c-axis. The observed ferroelectricity can be explained, from the obtained magnetic structure, via the inverse Dzyaloshinskii-Moriya mechanism

Magnetic inversion symmetry breaking and spin reorientation in Tb2MnNiO6: A polar strong ferromagnet

We report a description and comprehensive study on four successive magnetic transitions in ferromagnetic Tb2MnNiO6 double perovskite. In the ground state (P2(1)''), the moments of magnetic A and B sites order according to different nonpolar magnetic modes, but the coupling between them generates an overall polar symmetry which makes this oxide potentially multiferroic due to magnetic trilinear coupling, and therefore ferromagnetic and ferroelectric in its ground state. Its macroscopic magnetization is large (5 mu B/f.u) and not related to a weak ferromagnetic component induced by Dzyaloshinskii-Moriya interaction. However, a sharp and severe spin reorientation of the ferromagnetic transition-metal moments has been observed which opens the door to the magnetic switching of the ferroelectric state in this perovskite, and conversely to the control of the magnetization direction by electrical fields applied parallel to b. We also anticipate that in this material the direction of the magnetization (towards c/a) could be used as the key to switch the polar/nonpolar (ferroelectric/antiferroelectric) transformation

Magnetically-induced ferroelectricity in the (ND4)2[FeCl5(D2O)] molecular compound

The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH 4) 2 [FeCl 5 (H 2 O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroicity. From the crystal structure analysis, we observed an order-disorder phase transition related to the ordering of the ammonium counterion. We have determined the magnetic structure below T N, at 2 €‰K and zero magnetic field, which corresponds to a cycloidal spin arrangement with magnetic moments contained in the ac-plane, propagating parallel to the c-axis. The observed ferroelectricity can be explained, from the obtained magnetic structure, via the inverse Dzyaloshinskii-Moriya mechanism.Partial funding for this work is provided by the Ministerio Español de Ciencia e Innovación through projects MAT2010-16981, MAT2011-27233-C02-02. JARV acknowledges CSIC for a JAEdoc contract.Peer Reviewe

Magnetic order in a metal thiocyanate perovskite-analogue

Metal thiocyanate perovskite-analogues are a growing class of materials, but although they contain paramagnetic cations there have been no reports of their magnetic properties. Due to the large separations between the paramagnetic cations, with a shortest through-bond distance of 15.1 Å, these materials might be expected to be good examples of paramagnets. In this communication we investigate the magnetic properties of a metal thiocyanate framework Cr[Bi(SCN)6]·xH2O. We find that Cr[Bi(SCN)6]·xH2O undergoes long-range magnetic order at TN = 4.0(2) K. We use neutron powder diffraction to determine that Cr[Bi(SCN)6]·xH2O has a MnO-type {111}cubic-ordering as its ground state, consistent with frustrated nearest- and next-nearest-neighbour antiferromagnetic interactions. This suggests that appropriate design of metal thiocyanate perovskite-analogue structures may reveal a rich vein of frustrated magnetism

Helimagnets by disorder: Its role on the high-temperature magnetic spiral in the YBaCuFeO5 perovskite

Most of the spiral magnetoelectric multiferroics investigated in recent years are geometrically or exchangefrustrated magnets, where the presence of triangular or other frustrated spin networks produce low magnetic transition temperatures. This critically limits their potential uses. The exceptional stability of the spiral magnetic order (at TS) in the layered structure of the YBaCuFeO5 double perovskite involves a nonconventional mechanism: spiral order by disorder. The model has been theoretically developed by Scaramucci et al. [Phys.Rev.Res. 2, 013273 (2020)] after the discovery of a huge impact of cation disorder on TS [M. Morin et al., Nat. Commun. 7, 13758 (2016)]. In this work the influence of disorder (and only disorder) on the magnetic phase diagram is studied on a quantitative basis extending the range of previous studies. We thoroughly investigate the impact of frustration due to B-site disorder (nd) on the magnetic spirals in the reference composition YBaCuFeO5.The interplay between disorder, stability, and the detailed features of the incommensurate spiral magnetic orders were systematic, quantitative, and methodically investigated in samples of identical composition, spanning a wide range of nd values. Three different regimes are distinguished in the YBaCuFeO5 phase diagram versus disorder. A triple point is found in YBaCuFeO5 driven by Fe/Cu disorder that sets limits to TS and the cycloidal component of the helicoidal order. These layered materials appear as a very efficient realization of the avenue “spiral order by disorder” to supply functional helimagnets at normal working temperatures

The new powder diffractometer D1B of the Institut Laue Langevin

D1B is a medium resolution high flux powder diffractometer located at the Institut Laue Langevin, ILL. D1B a suitable instrument for studying a large variety of polycrystalline materials. D1B runs since 1998 as a CRG (collaborating research group) instrument, being exploited by the CNRS (Centre National de la Recherche Scientifique, France) and CSIC (Consejo Superior de Investigaciones Cientificas, Spain). In 2008 the Spanish CRG started an updating program which included a new detector and a radial oscillating collimator (ROC). The detector, which has a sensitive height of 100mm, covers an angular range of 128°. Its 1280 gold wires provide a neutron detection point every 0.1°. The ROC is made of 198 gadolinium- based absorbing collimation blades, regular placed every 0.67°. Here the present characteristics of D1B are reviewed and the different experimental performances will be presented