Superparamagnetic relaxation in Co0.7Cu0.15Fe0.15Cr2S4

Abstract

${\mathrm{Co}}_{0.7}$ ${\mathrm{Cu}}_{0.15}$ ${\mathrm{Fe}}_{0.15}$ ${\mathrm{Cr}}_2$ ${\mathrm{S}}_4$ has been studied by Mössbauer spectroscopy, x-ray diffraction, and magnetic measurements. The crystal structure is found to be a cubic spinel with the lattice parameter $a_0=9.917\mathrm{}$ Å. Absence of quadrupole splitting suggests that iron ions occupy only tetrahedral sites. It is notable that as the temperature increases toward the Néel temperature, $T_N=252\mathrm{}$ K, line broadening and a pronounced central peak appear, suggesting superparamagnetic relaxation. The temperature dependence of the effective anisotropy energy is also found to decrease rapidly with increasing temperature, thereby supporting the observed rapid increase of the superparamagnetic relaxation rate below $T_N$.