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

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 H-E(T)as well as the magnetization shift M-E(T), vanish around the Neel temperature T-N similar or equal to 45 K. We show that the magnetization shift M-E(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 M-E(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.