Magnetic and crystal structures of the one-dimensional ferromagnetic chain pyroxene NaCrGe2O6

International audienceWe investigated the magnetic and structural properties of the ferromagnetic pyroxene NaCrGe2O6 by superconducting quantum interference device magnetometry and powder neutron diffraction. This material is the only ferromagnetic member of the recently reported multiferroic pyroxene family AMX2O6 A=Li,Na; M =Fe, Cr; X=Si,Ge . Below TC=6 K, the magnetic structure is characterized by one-dimensional magnetic chains with spins aligned along the c axis of the monoclinic cell. The magnetic moment of Cr3+ is significantly reduced by about 25%. We show that this is likely the result of the low dimensionality of the system. The associated magnetic space group is C2 /c . This symmetry does not allow a linear magnetoelectric effect. No structural phase transition was observed down to 1.8 K

Magnetic and crystal structures of the magnetoelectric pyroxene LiCrSi2O6

International audienceWe investigated the magnetic and crystal structures of the recent reported magnetoelectric system LiCrSi2O6 by powder neutron diffraction. Below TN=11.5 K, an antiferromagnetic order appears. It is characterized by an antiferromagnetic coupling within the CrO6 octahedra chains and a ferromagnetic coupling between the chains. The magnetic order is commensurate with the lattice with k=0. The associated magnetic space group is P21 /c. This symmetry is in agreement with the reported magnetoelectric effect. We show that the magnetic frustration in this system is small. Finally, we discuss our results using a Landau phenomenological model and in the light of the literature

High-pressure Raman study of Fe(IO3)3: Soft-mode behavior driven by coordination changes of iodine atoms

This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.jpcc.0c06541.[EN] We report high-pressure Raman spectroscopy studies of Fe(IO3)(3) up to nearly 21 GPa that have been interpreted with the help of density functional theory calculations, which include the calculation of phonon dispersion curves and elastic constants at different pressures. Zero-pressure Raman-active mode frequencies and their pressure dependences have been determined. Modes have been assigned and correlated to atomic movements with the help of calculations. Interestingly, in the high-frequency region, there are several modes that soften under compression. These modes have been identified as internal vibrations of the IO3 coordination polyhedron. Their unusual behavior is a consequence of the changes induced by pressure in the coordination sphere of iodine, which gradually change from a threefold coordination to an almost sixfold coordination under compression. The coordination change is favored by the decrease of the stereoactivity of the iodine lone electron pair so that likely a real sixfold coordination is attained after a first-order phase transition previously reported to occur above 21 GPa. The strong nonlinear behavior found in Raman-active modes as well as in theoretically calculated elastic constants has been discovered to be related to the occurrence of two previously unreported isostructural phase transitions at 1.5-2.0 and 5.7-6.0 GPa as shown by dynamic instabilities close to the Brillouin zone center.This work was supported by the Spanish Ministry of Science, Innovation and Universities, the Spanish Research Agency (AEI), the European Fund for Regional Development (ERDF, FEDER) under grants MAT2016-75586-C4-1/2/3-P, PID2019-106383GB-C41/42/43, and RED2018-102612-T (MALTA Consolider-Team Network), and the Generalitat Valenciana under grant Prometeo/2018/123 (EFIMAT). A.L. and D.E. would like to thank the Generalitat Valenciana for the Ph.D. fellowship GRISOLIAP/2019/025).Liang, A.; Rahman, S.; Rodriguez-Hernandez, P.; Muñoz, A.; Manjón, F.; Nenert, G.; Errandonea, D. (2020). High-pressure Raman study of Fe(IO3)3: Soft-mode behavior driven by coordination changes of iodine atoms. The Journal of Physical Chemistry C. 124(39):21329-21337. https://doi.org/10.1021/acs.jpcc.0c06541S21329213371243

Gradual localization of 5f states in orthorhombic UTX ferromagnets - polarized neutron diffraction study of Ru substituted UCoGe

We report on a microscopic study of the evolution of ferromagnetism in the Ru substituted ferromagnetic superconductor (FM SC) UCoGe crystallizing in the orthorhombic TiNiSi-type structure. For that purpose, two single crystals with composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge have been prepared and characterized by magnetization, AC susceptibility, specific heat and electrical resistivity measurements. Both compounds have been found to order ferromagnetically below TC = 6.5 K and 7.5 K, respectively, which is considerably higher than the TC = 3 K of the parent compound UCoGe. The higher values of TC are accompanied by enhanced values of the spontaneous moment mspont. = 0.11 mB/f.u. and mspont. = 0.21 mB/f.u., respectively in comparison to the tiny spontaneous moment of UCoGe (about 0.07mB/f.u.). No sign of superconductivity was detected in either compound. The magnetic moments of the samples were investigated on the microscopic scale using polarized neutron diffraction (PND) and for UCo0.88Ru0.12Ge also by soft X-ray magnetic circular dichroism (XMCD). The analysis of the PND results indicates that the observed enhancement of ferromagnetism is mainly due to the growth of the orbital part of the uranium 5f moment mL(U), reflecting a gradual localization of the 5f electrons with Ru substitution. In addition, the parallel orientation of the U and Co moments has been established in both substituted compounds. The results are discussed and compared with related isostructural ferromagnetic UTX compounds (T - transition metals, X - Si, Ge) in the context of a varying degree of the 5f-ligand hybridization

Orbital ordering and multiferroics

Transition metal oxides can exhibit a wide variety of properties which can be tuned by changing factors such as composition, magnetic field and temperature. The complex physical phenomena involved can give rise to functional materials such as magnetic field sensors or ferroelectrics. The discovery of new functional materials is a challenge for the development of smart systems. To guide this search, a clear understanding of the relationship between the physical properties and the atomic scale structure of the materials is needed. Of fundamental importance is the crystal chemistry of a given compound. Crystal chemistry refers to two aspects: symmetry and the distribution of atoms in the unit cell. The crystal chemistry can be investigated by means of synchrotron X-ray and neutron diffraction. These techniques provide information that can be used as a starting point for a better understanding of the physical properties. The physical properties are directly related to the symmetry exhibited by the system. Thus the symmetry is an important tool to investigate materials of interest.

Structural and magnetic properties of the low-dimensional fluoride β-FeF3(H2O)2·H2O

International audienc

Experimental evidence for an intermediate phase in the multiferroic YMnO3

6 pages 5 figures. to be submitted to PRBInternational audienceWe have carried out high temperature synchrotron x-ray experiments, differential scanning calorimetry and thermomechanical analysis on a single-crystal of YMnO3 grown by the Floating Zone Technique. These experiments show two anomalies at about 1100K and 1350K demonstrating the existence of an intermediate phase between the ferroelectric P63cm structure and the high temperature centrosymmetric phase with P63/mmc symmetry. This work identifies for the first time the different high temperature phase transitions in YMnO3