Phase transitions and slow spin dynamics of slightly inverted A-site spinel CoAl_2-xGa_xO₄

We report the properties of an A-site spinel magnet, CoAl_2-xGa_xO₄, 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 CoAl_2-xGa_xO₄revealed that the metallic ions were randomly distributed in the tetrahedral (A-) and octahedral (B-) sites in the cubic spinel structure. The inversion parameterηcould be controlled by varying the gallium (Ga) composition in the range 0.055 ⩽η⩽ 0.664. The composition-induced Néel-to-spin-glass (NSG) transition occurred between 0.05 ⩽η⩽ 0.08 and was verified by measurements of DC-AC susceptibilitiesχand thermoremanent magnetization (TRM) below the Néel transition temperatureT_N. The relaxation rate and derivative with respect to temperature of TRM increased at bothT_Nand the spin glass (SG) transition temperatureT_SG. 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 CoAl_2-xGa_xO₄. In the vicinity of the NSG boundary, there was a maximum of the TRM relaxation rate atT_max<T_N. With increasing inversionη(x), the anomaly atT_maxmerged with that of the Néel transition at a tricritical point (η_tc,T_tc) = (0.08, 4.0 K), where the paramagnetic, Néel, and SG states met. We successfully extracted the relaxation timeτand 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 CoGa₂O₄cluster glass. We generated an inversion-temperature phase diagram based on the comprehensive measurements of DC and AC susceptibilities, TRM, and specific heat in the range 0.055 ⩽η⩽ 0.664 for CoAl_2-xGa_xO₄. Based on this phase diagram, we speculate that a NSG quantum critical phase transition occurred atη= 0.050(6). Our findings are consistent with suppression of the long-range order antiferromagnetic state in CoAl₂O₄revealed through neutron diffraction studies, even atT<<T_N.