Cluster glass transition and relaxation in the random spinel CoGa2O4

Abstract

We report magnetic properties in the random spinel magnet ${\mathrm{CoGa}}_{2}{\mathrm{O}}_{4}$. Rietveld analysis of the x-ray diffraction profile for ${\mathrm{CoGa}}_{2}{\mathrm{O}}_{4}$ reveals that the Co and Ga ions are distributed randomly in the tetrahedral $A$ sites and octahedral $B$ sites in the cubic spinel structure. ${\mathrm{CoGa}}_{2}{\mathrm{O}}_{4}$ exhibits a spin-glass transition at ${T}_{\mathrm{SG}}=8.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ that is confirmed by measurements of the dc and ac susceptibilities and thermoremanent magnetization (TRM) that develops below ${T}_{\mathrm{SG}}$. From the frequency dependence of the freezing temperature ${T}_{\mathrm{f}}$ for ${\mathrm{CoGa}}_{2}{\mathrm{O}}_{4}$, it is indicated that the relaxation time $\ensuremath{\tau}(T)$ follows a Vogel-Fulcher law $\ensuremath{\tau}={\ensuremath{\tau}}_{0}\mathrm{exp}[\ensuremath{-}{E}_{a}/{k}_{\mathrm{B}}(T\ensuremath{-}{T}_{0})]$. An analysis of specific heat suggested that a doublet ground state of the octahedrally coordinated ${\mathrm{Co}}^{2+}$ was stabilized by spin-orbit and crystal field couplings. The relaxation rate of TRM is considerably enhanced at ${T}_{\mathrm{SG}}$ and decays rapidly above and below ${T}_{\mathrm{SG}}$. The time course of TRM is reproduced by nonexponential relaxation forms, such as a stretched exponential (Kohlrausch) as well as Ogielski and Weron relaxation forms. This behavior is displayed universally in glass systems, and the characteristic parameters associated with these functions were reasonable.