Revealing the origin of the vertical hysteresis loop shifts in an exchange biased Co/YMnO3 bilayer

Abstract

We have investigated exchange bias effects in bilayers composed of the antiferromagnetic o-YMnO3 and ferromagnetic Co thin film by means of SQUID magnetometry, magnetoresistance, anisotropic magnetoresistance and the planar Hall effect. The magnetization and magneto-transport properties show pronounced asymmetries in the field and magnetization axes of the field hysteresis loops. Both exchange bias parameters, the exchange bias field HE(T) as well as the magnetization shift ME(T), vanish around the Néel temperature TN ≃ 45 K. We show that the magnetization shift ME(T) is also measured by a shift in the anisotropic magnetoresistance and planar Hall resistance having a similar temperature dependence as the one obtained from magnetization measurements. Because the o-YMnO3 film is highly insulating, our results demonstrate that the ME(T) shift originates at the interface within the ferromagnetic Co layer. To show that the main results obtained are general and not because of some special characteristics of the o-YMO3 layer, similar measurements were done in Co/CoO micro-wires. The transport and magnetization characterization of the micro-wires supports the main conclusion that these effects are related to the response of the ferromagnetic Co layer at the interface.