Field and temperature-controlled positive and negative exchange biases in CoO/YIG bilayers on GGG(111)

Yttrium iron garnet (Y₃Fe₅O₁₂, YIG) is an essential ferromagnetic insulator for enabling the transport of pure spin. The exchange coupling at the antiferromagnet/ferromagnet (AFM/FM) interface provides a means for engineering spintronic devices of YIG. In the previous results, the partially oxidized CoO_x layer near the CoO/YIG interface is parallelly-coupled with the YIG layer, inducing the room temperature negative exchange bias (RT-NEB). In this study, we fabricated further oxidized CoO on YIG with sufficient O₂-pressure, in which the room temperature positive exchange bias (RT-PEB) was observed. This RT-PEB can be explained if the CoO interface spins are aligned by the applied magnetic field (H_AF) and coupled to YIG antiparallelly. The PEB increases with increasing H_AF and saturates at H_AF = 500 Oe. When the temperature is decreased below the Néel temperature of CoO, the PEB is gradually changed to NEB due to the dominance of AFM-CoO/YIG coupling. These findings pave the way for reversible modulation of the interfacial magnetic properties of insulating AFM/FM heterostructures.