Slow spin dynamics in a CoM₂O₄ A-site spinel (M = Al, Ga, and Rh)

Abstract We report thermomagnetic properties in magnetically frustrated A-site spinel magnets of CoAl 2 O 4 , CoRh 2 O 4 , and cluster glass CoGa 2 O 4 to comprehensively show the proximity effects of the Néel-to-spin-spiral (NSS) transition, which is predicted theoretically to occur in a magnetically frustrated A-site spinel antiferromagnet. The low-temperature magnetic phase remains controversial since the magnetic state of CoAl 2 O 4 is in the vicinity of the NSS transition derived by the inherent magnetic frustrated interaction for an A-site spinel magnet and is also considerably sensitive to crystallographic disorder. The antiferromagnetic and spin-glass transitions were detected by measuring the direct-current and alternating-current susceptibilities and also thermoremanent magnetization (TRM) developing below the magnetic transitions at the Néel point ( T N ) and spin-glass transition temperature ( T SG ). The relaxation rate and the temperature derivative of TRM were both enhanced at T N and T SG , and they decayed rapidly above and below the transitions. We succeeded in extracting the relaxation time τ and other characteristic parameters from the isothermal relaxation of TRM, which was well fitted with a non-exponential relaxation form formulated by Weron based on purely stochastic theory in order to describe the originally dielectric relaxation. For the typical CoRh 2 O 4 normal spinel (inversion-free A site) antiferromagnet and the CoGa 2 O 4 random spinel spin-glass, the temperature variations of these parameters can distinguish the magnetic states. In contrast to the cases for CoRh 2 O 4 and CoGa 2 O 4 , an enhancement of the relaxation rate of TRM for CoAl2O4 is indicated at low temperatures, which is probably related to the suppression of long-range antiferromagnetic order revealed by neutron diffraction studies.