Cluster glass transition and relaxation in the random spinel CoGa2 O4

Abstract

We report magnetic properties in the random spinel magnet CoGa2O4. Rietveld analysis of the x-ray diffraction profile for CoGa2O4 reveals that the Co and Ga ions are distributed randomly in the tetrahedral A sites and octahedral B sites in the cubic spinel structure. CoGa2O4 exhibits a spin-glass transition at TSG=8.2K that is confirmed by measurements of the dc and ac susceptibilities and thermoremanent magnetization (TRM) that develops below TSG. From the frequency dependence of the freezing temperature Tf for CoGa2O4, it is indicated that the relaxation time τ(T) follows a Vogel-Fulcher law τ=τ0exp[-Ea/kB(T-T0)]. An analysis of specific heat suggested that a doublet ground state of the octahedrally coordinated Co2+ was stabilized by spin-orbit and crystal field couplings. The relaxation rate of TRM is considerably enhanced at TSG and decays rapidly above and below TSG. 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.