Phase transitions and slow spin dynamics of slightly inverted A-site spinel CoAl2−xGaxO4

We report the properties of an A-site spinel magnet, CoAl2−xGaxO4, and analyze its anomalous, low-temperature magnetic behavior, which is derived from inherent, magnetically frustrated interactions. Rietveld analysis of the x-ray diffraction profile for CoAl2−xGaxO4 revealed that the metallic ions were randomly distributed in the tetrahedral (A-) and octahedral (B-) sites in the cubic spinel structure. The inversion parameter eta could be controlled by varying the gallium (Ga) composition in the range 0.055 ≤ eta ≤ 0.664. The composition-induced Néel-to-spin-glass (NSG) transition occurred between 0.05 ≤ eta ≤ 0.08 and was verified by measurements of DC-AC susceptibilities chi and thermoremanent magnetization (TRM) below the Néel transition temperature TN. The relaxation rate and derivative with respect to temperature of TRM increased at both TN and the spin glass (SG) transition temperature TSG. The TRM decayed rapidly above and below these transitions. TRM was highly sensitive to macroscopic magnetic transitions that occurred in both the Néel and SG phases of CoAl2−xGaxO4. In the vicinity of the NSG boundary, there was a maximum of the TRM relaxation rate at Tmax < TN. With increasing inversion eta(x), the anomaly at Tmax merged with that of the Néel transition at a tricritical point (etatc, Ttc) = (0.08, 4.0 K), where the paramagnetic, Néel, and SG states met. We successfully extracted the relaxation time tau and other characteristic parameters from the TRM isothermal temporal evolution based on the Weron function derived for a purely stochastic process. To distinguish the magnetic states, we compared our results with previously studied inversion-free A-site spinel, CoRh2O4, and CoGa2O4 cluster glass.