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

T Naka; J Valenta; T Nakane; S Ishii; M Nakayama; H Mamiya; K Takehana; N Tsujii; Y Imanaka; Y Matsushita; H Abe; T Uchikoshi; H Yusa

doi:10.1088/1361-648X/ad12fc

Phase transitions and slow spin dynamics of slightly inverted A-site spinel CoAl_{2- x}Ga _x O₄

J Phys Condens Matter. 2023 Dec 15;36(12). doi: 10.1088/1361-648X/ad12fc.

Authors

T Naka¹, J Valenta¹, T Nakane², S Ishii³, M Nakayama¹, H Mamiya⁴, K Takehana⁵, N Tsujii¹, Y Imanaka⁵, Y Matsushita⁶, H Abe⁷, T Uchikoshi², H Yusa¹

Affiliations

¹ Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan.
² Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan.
³ Department of Physics, Tokyo Denki University, Hatoyama, Saitama 350-0394, Japan.
⁴ Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan.
⁵ Center for Basic Research on Materials, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan.
⁶ Research Network and Facility Services Divisions, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan.
⁷ Joint and Welding Research Institution, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.

PMID: 38056003
DOI: 10.1088/1361-648X/ad12fc

Abstract

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.

Keywords: magnetic frustration; phase diagram; quantum critical point; thermoremanent magnetization.