High temperature spin cluster glass behavior in Co- and Si-substituted garnet ferrite thin films

In this study, lutetium iron garnet (Lu3Fe5O12; LuIG) thin films have been grown on Y3Al5O12 (1 1 1) substrates by a pulsed laser deposition and are co-substituted with strong magnetically anisotropic ions of Co2+ and nonmagnetic ones of Si4+. LuIG is a garnet-structured ferrimagnet with an exceptionally low Gilbert damping constant as 10(-5). The substitution of Co2+ and Si4+ induces a chemical disorder and competing anti-ferromagnetic and ferromagnetic interactions, which induce a spin glass behavior. When the Co-Si substitution ratio is increased, a spin-freezing behavior is observed below the spin-freezing temperature T-G = 190-220 K of the Lu3Fe4Co0.5Si0.5O12 film; this indicates the presence of a cluster glass phase. The film demonstrates a characteristic memory effects in the temperature range 120-180 K, which are below T-G; this reveals the presence of a metastable magnetic state in the mull-valley energy landscape with shallow activation energy barriers. Ferromagnetic resonance measurements confirm increased perpendicular anisotropy energy induced by Co substitution and spin-wave characteristics including the inverse spin Hall effect voltage (V-ISHE). Lu3Fe5-2xCoxSixO12 films show an increase in damping and a decrease in V-ISHE at the cryogenic temperature, which are attributed to the impurity relaxation mechanism. Therefore, the Co- and Si-substituted films exhibit strong spin-wave damping at a low temperature, which reflects the spin-freezing dynamics of cluster glass behaviors.