Tunable exchange bias in Y$_3$Fe$_5$O$_{12}$ film on Gd$_3$Ga$_5$O$_{12}$

Ferrimagnetic Y$_3$Fe$_5$O$_{12}$ grown on the (001) surface of paramagnetic Gd$_3$Ga$_5$O$_{12}$ experiences an exchange bias field, which has been attributed to the magnetism of an interface layer between the two materials. We report here that when grown using sputtering and with lower post-annealing temperatures than in previous works, the blocking temperature of the interface magnetic layer is lowered to about 7 K, while still displaying a strong exchange bias. This exchange bias is then found to be tunable between its two extreme values by carefully varying the field cooling protocol. This is attributed to a slow and complex dynamics of the spins of the interface-layer when it is warmed up close to its blocking (or melting) temperature, which is reminiscent of a spin glass.