Magnetic properties in spinel Fe 1+ xCr 2− xS 4 with CMR effect

Abstract

The spinel compounds Fe 1+ x Cr 2− x S 4 ( x=0, 0.1) with CMR effects but without Jahn–Taller ions are investigated. Mössbauer spectra identify that, in the FeCr 2S 4, Fe ions occupy tetrahedral sites in the Fe 2+ valence state forming an Fe sub-lattice, the Cr ions occupy octahedral sites with a +3 valence forming an Cr sub-lattice; in Fe 1.1Cr 1.9S 4, the excessive Fe ions substitute for Cr ions in the +3 valence. ESR measurements show that only one ESR signal, with a g-factor near 2, appears at temperatures above T c (the PM state) for each sample; at T< T 1 ( T 1 corresponding to the minimum resistivity in the ρ–T curve below T c), the original PM peak in the ESR spectra splits into a high-field ESR peak and a lower-field ESR peak. Therefore, the micro-magnetism in this system can be illustrated as below: for FeCr 2S 4, the spins of the Fe and Cr sub-lattices both enter ferromagnetic states. At the same time, the magnetic moments of these two sub-lattices are anti-parallel to each other; for Fe 1.1Cr 1.9S 4, the spins of the excess Fe ions are anti-parallel to those of the Cr ions, a fact which weakens not only the internal field of the Cr sub-lattice but also the coupling between these two sub-lattices. From this, the abnormal transport behavior revealed by ρ–T curves for Fe 1+ x Cr 2− x S 4 can be well explained.