Abstract
In this paper, the magnetic and transport properties of ${\mathrm{FeCr}}_{2\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{S}}_{4}$ $(0<x<~0.4)$ are studied. M\"ossbauer spectra reveal that this system belongs to an inverse spinel type, where the Ga ion has ``preferes'' to occupy the tetrahedral site. With increasing Ga content, the transport behavior is found to transit into variable-range hopping process gradually both at temperatures far below ${T}_{c}$ and at $T>{T}_{c}.$ It is believed that the random distribution of cations in inverse spinel material may induce a large Coulomb potential fluctuation, which causes carrier localization and transforms the conduction from narrow band semiconducting behavior into a variable-range hopping process at temperatures far below ${T}_{c}.$ When $T>{T}_{c},$ since the carriers are further localized by the presence of magnetic disorder, variable-range hopping again dominates the conduction behavior. Accordingly, by taking account of the relative weight of the magnetic and chemical disorder, we give an explanation for the variation of the colossal magnetoresistance effect.
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