Magnetic and Transport Properties of Mechanosynthesized FeCr<sub>2</sub>S<sub>4</sub> Sulfospinel

Abstract

The FeCr2S4 sulfospinel has been synthesized from the elemental components, by highenergy ball milling. The magnetic properties of the resulting crystallites (mean diameter d = 120±10 nm) have been studied by means of ac and dc susceptibility, Mossbauer and transport measurements. Magnetization curves showed a parato ferrimagnetic transition of FeCr2S4 phase at TC = 165(3) K, and a cusp at TL ≈ 70 K. The evolution of Mossbauer spectra at different temperatures indicates the existence of a disordered surface layer, and its thickness was estimated to be t = 8(2) nm. Transport measurements show that the onset of the insulator-metal (IM) transition at T =241 K, a ~30% higher temperature than previous reports. In contrast, the 'colossal' magnetoresistance (CMR) effect observed in bulk FeCr2S4 is greatly reduced. Introduction Sulfospinels have attracted attention from decades due to their complex magnetic and transport properties [1-3]. For some transition metals like Cu, Cr, Cd, the corresponding sulfospinel compound have revealed several unique properties such as metal-to-insulator transition at high pressure, and the ability to form both metallic and insulating spin glasses [4,5]. As found in most spinels, the crystal structure of FeCr2S4 is cubic, space group Fd3m, with a normal ionic distribution, i.e., with Fe and Cr ions occupying the tetrahedral 8a and octahedral 16d sites respectively. However, Mossbauer data on different samples has shown that Fe can be also present, yielding partially inverted configurations [6]. The system is a p-type semiconductor, showing a parato ferrimagnetic transition at TC ≈ 170 K, and also spin-glass behavior has been reported below 60 K [7] Recently, FeCr2S4 spinel has been revisited since it presents colossal magnetoresistance near TC from different origin than the double-exchange (DE) mechanism previously found in manganites [8]. This compound has been described as a non-DE system because, even for samples with small amounts of Fe, no evidence of site exchanging has been reported so far. There is still no consensus on which are the microscopic mechanisms that govern the CMR effect in manganites, pyrochlores and sulfospinels, nor even on whether the CMR effect observed in these compounds have the same origin. Since the magnetic properties are intimately associated to their transport properties, we have attempted to study the influence of magnetic disorder on the transport properties. We present a study on the magnetic and transport properties of the obtained FeCr2S4 phase, by means of a.c. and d.c. magnetization, resistivity and Mossbauer spectroscopy measurements. † E-mail address: goya@macbeth.if.usp.br 2 Journal of Metastable and Nanocrystalline Materials 10 20 30 40 50 60 70 80 90 3.5 39 3.0 15 2.5 00 2.0 41 1.9 25 1.7 70 1.5 80 1.5 24 1.4 39 1.3 15 1.2 51 1 .17 6 1.1 19 1.1 02 1.1 19 1.1 76 1.2 51 1.3 15 1.4 39 1.5 24 1.5 80 1.7 70 1.9 25 2.0 41 2.5 00 3.0 15 3.5 39 + + + + + ∗ + FeCr2S4 (CrFe)3S4 Int en sit y ( a.u .)