Phase transitions and slow spin dynamics of slightly inverted A-site spinel CoAl2−x Ga x O4

Abstract

We report the properties of an A-site spinel magnet, CoAl2−x Ga x O4, 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−x Ga x O4 revealed that the metallic ions were randomly distributed in the tetrahedral (A-) and octahedral (B-) sites in the cubic spinel structure. The inversion parameter η could 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 susceptibilities χ and thermoremanent magnetization (TRM) below the Néel transition temperature T N. The relaxation rate and derivative with respect to temperature of TRM increased at both T N and the spin glass (SG) transition temperature T 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 CoAl2−x Ga x O4. In the vicinity of the NSG boundary, there was a maximum of the TRM relaxation rate at T max < T N. With increasing inversion η(x), the anomaly at T max merged 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 time τ 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. 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 CoAl2−x Ga x O4. Based on this phase diagram, we speculate that a NSG quantum critical phase transition occurred at η = 0.050(6). Our findings are consistent with suppression of the long-range order antiferromagnetic state in CoAl2O4 revealed through neutron diffraction studies, even at T << T N.