Cluster glass transition and relaxation in the random spinel CoGa₂ O₄

We report magnetic properties in the random spinel magnet CoGa₂ O₄. Rietveld analysis of the x-ray diffraction profile for CoGa₂ O₄ reveals that the Co and Ga ions are distributed randomly in the tetrahedral A sites and octahedral B sites in the cubic spinel structure. CoGa₂ O₄ exhibits a spin-glass transition at T_SG = 8.2K that is confirmed by measurements of the dc and ac susceptibilities and thermoremanent magnetization (TRM) that develops below T_SG. From the frequency dependence of the freezing temperature Tf for CoGa₂ O₄, it is indicated that the relaxation time τ(T) follows a Vogel-Fulcher law τ = τ₀ exp [-E_a / k_B (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 T_SG and decays rapidly above and below T_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.