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 CoOx 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 (HAF) and coupled to YIG antiparallelly. The PEB increases with increasing HAF and saturates at HAF = 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.