Magnetic polaron conduction in the colossal magnetoresistance materialFe1−xCdxCr2S4

Abstract

The magnetotransport properties of the spinel ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ have been studied. We found that the magnetic polarons dominate the conduction at temperatures above the Curie temperature ${\mathrm{T}}_{c}.$ Upon substitution of Fe by Cd, the carrier density decreases, and the magnetoresistance increases. As a result, the magnetic polarons become more stable, in agreement with the theoretical predictions of Majumdar and Littlewood for the magnetoresistance in low carrier density ferromagnets. The evolution of magnetic polarons in ${\mathrm{CdCr}}_{2}{\mathrm{S}}_{4}$ has been studied with electron spin resonance (ESR) measurements. As the temperature increases, the peak-to-peak linewidth $\ensuremath{\Delta}{H}_{\mathrm{pp}}$ decreases in a broad temperature range, providing direct evidence for the existence of magnetic polarons. The analysis of the ESR spectra reveals the existence of two temperature scales, which are related to the evolution of the magnetic polarons. The first temperature, denoted as ${T}_{p},$ corresponds to the formation of magnetic polarons. The second temperature, denoted as ${T}^{*},$ can be related to the temperature where correlations among the magnetic polarons occur.