New Fluoride-based Magnetoelectrics

Lead Research Organisation: University of St Andrews
Department Name: Chemistry

Abstract

Magnetoelectrics are a special class of solid-state material which simultaneously possess both magnetic and electrical ordering properties. Currently, many information storage technologies are based on either magnetic or electrical ordering and switchability. If a materials possesses both these properties then, in principle, it should be possible to exploit this advantageously, for example by storing data electrically, but reading it magnetically. There has therefore been a recent resurgence of interest worldwide in such magnetoelectric materials, driven largely by the materials science and physics communities. Although this has led to considerable fundamental understanding of exisiting materials it has also flagged up a serious lack of new materials being discovered and developed. This 'discovery' aspect is the realm of the solid state chemist, and in this project we aim to address this problem from a solid state chemistry perspective. We shall explore new mixed metal fluoride materials as potential magnetoelectrics, basing our search on a sound understanding of the structural and compositional chemistry of this family of compounds. We shall characterise our new compounds using a variety of crystallographic and physical (ie. magnetic and electrical) techniques in order to pin down the key structure-property-composition relations of these materials. Ultimately, we aim to provide a range of new materials exhibiting magnetoelectric effects, on which the materials science and physics communities will be able to base more applied and developmental work.
 
Description Many information storage technologies are based on eithermagnetic or electrical ordering and switchability. If a material possesses boththese properties then, in principle, it should be possible to exploit thisadvantageously, for example by storing data electrically, but reading itmagnetically. Most of the previous work on these systems had focussed onmixed metal oxides. Our aim was to prepare and study specifically fluoride-based rather than oxide-basedmultiferroics. Several exploratory synthetic approaches were taken, including solid-state,solvothermal, and later, ionothermal methods. Several new compounds ofpotential interest as multiferroics were prepared. Also, several previouslyknown, but poorly characterised, multiferroic fluorides were studied in orderto understand their behaviour in greater detail. Brief highlights of thepublished work include:1.The previously know multiferroic K0.6FeF3 was characterised by high-resolutionsynchrotron powder diffraction. Contrary to previous work it was discoveredthat these materials display a complex and intrinsic phase separation andphase transition phenomenon as a function of temperature. This could onlyhave been discovered using this type of difffraction experiment, and mayhave important consequences for the interpretation of the reportedmultiferroic properties of this family of compounds.2. The cryolite-type compound (NH4)3FeF6, previously reported to bemultiferroic, was shown to be magnetically inactive, and therefore no longera candidate for multiferroic behaviour.3. Magnetic ordering in the kagome compound Cs2ZrCu3F12 was shown tobe associated with a structural phase transition at sub-ambienttemperatures, driven by a crystal-chemical coordination preference of theZr4+ cation. A comparison has been made with the related 'valence-bond solid'phase Rb2SnCu3F12, which shows no magnetic ordering.4. Various explorations in ionothermal fluoride chemistry led to the partiallyserendipitous discovery of another kagome compound based on V4+. This isthe first of its type, and shows the exciting property of 'quantum spin liquid'behaviour.