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

We report thermomagnetic properties in magnetically frustrated A-site spinel magnets of CoAl2O4, CoRh2O4, and cluster glass CoGa2O4 to comprehensively show the proximity effects of the Neel-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 CoAl2O4 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 directcurrent and alternating-current susceptibilities and also thermoremanent magnetization (TRM) developing below the magnetic transitions at the Neel 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 tau 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 CoRh2O4 normal spinel (inversion-free A site) antiferromagnet and the CoGa2O4 random spinel spin-glass, the temperature variations of these parameters can distinguish the magnetic states. In contrast to the cases for CoRh2O4 and CoGa2O4, 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.

Automated summary

We report the thermomagnetic properties of magnetically frustrated A-site spinel magnets, such as CoAl2O4, CoRh2O4, and CoGa2O4, to demonstrate the proximity effects of the Neel-to-spin-spiral (NSS) transition. The magnetic state of CoAl2O4 remains controversial due to its proximity to the NSS transition and sensitivity to crystallographic disorder. The relaxation rate and temperature derivative of thermoremanent magnetization (TRM) were enhanced at the Neel point (T-N) and spin-glass transition temperature (T-SG), and they decayed rapidly above and below the transitions. The relaxation time tau and other characteristic parameters were successfully extracted from the isothermal relaxation of TRM, which was well fitted with a non-exponential relaxation form. The temperature variations of these parameters distinguished the magnetic states for CoRh2O4 and CoGa2O4, while an enhancement of the relaxation rate of TRM was indicated for CoAl2O4 at low temperatures, possibly due to the suppression of long-range antiferromagnetic order.