Parker, Dinah Rosenwyn; (2004) Magnetic and Electronic Properties of Ternary Chalcogenide Spinels. Doctoral thesis (Ph.D), UCL (University College London).

Text Magnetic and electronic properties of ternary chalcogenide spinels.pdf Download (35MB)

Abstract

Compounds with the spinel structure have been extensively studied in the past as they exhibit a wide range of interesting electronic and magnetic properties. The structure, with the general formula AB2X4 (where A and B are metal ions and X is O, S, Se or Te), can undergo many different cation and anion substitutions. One example of a magnetically interesting spinel is CuCr2Se4, which is a high TC ferromagnet. Substituting copper with zinc on the A-site results in a range of magnetic behaviour that includes collinear ferromagnetism, conical ferromagnetism, canted antiferromagnetism and spin glass. The other end member of the Cu1-xZnxCr2Se4 series, ZnCr2Se4, is an antiferromagnet with a helical magnetic structure. The electronic and magnetic properties of these compounds are dependent on the delicate balance of near-neighbour and further-neighbour magnetic interactions taking place between the chromium ions, which leads to the highly diverse behaviour observed across the series. Compounds from the Cu1-xZnxCr2Se4 series have been synthesized and their properties investigated using a variety of techniques. Structural analysis has been carried out using X-ray powder diffraction methods. The magnetic properties have been studied using DC SQUID magnetometry, AC susceptibility and neutron diffraction. High pressure AC susceptibility has also been used to study the effect of changing the Cr-Cr bond distance on the magnetic properties, while the composition remains unchanged. Electrical transport measurements have revealed a change from metallic to semiconducting behaviour across the series with increasing x. A magnetoresistance effect is observed in compounds across the series. Where x > 0.1, a crossover between negative and positive magnetoresistance is seen from low to high temperature. Compounds in the Cu1-xZnxCr2Se4-ySy and CuCr2Se4-yBry series have also been prepared to study the effect of substitutions on the X-site on the magnetic properties.

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