Revealing the origin of the vertical hysteresis loop shifts in an exchange biased Co/YMnO$_3$ bilayer

TL;DR

This study investigates the origin of vertical hysteresis loop shifts in exchange-biased Co/O-YMnO3 bilayers, revealing that the shifts originate at the ferromagnetic Co interface, supported by magnetometry and transport measurements.

Contribution

It demonstrates that the magnetization shift in exchange bias is due to interfacial effects within the ferromagnetic Co layer, confirmed across different bilayer systems.

Findings

Magnetization and magnetotransport show asymmetries in hysteresis loops.

Exchange bias parameters vanish near the Néel temperature (~45 K).

Interface effects in Co layer cause the observed shifts.

Abstract

We have investigated exchange bias effects in bilayers composed by the antiferromagnetic o-YMnO$_3$ and ferromagnetic Co thin film by means of SQUID magnetometry, magnetoresistance, anisotropic magnetoresistance and planar Hall effect. The magnetization and magnetotransport 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 N\'eel temperature $T_N \simeq 45$ 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 those a similar temperature dependence as the one obtained from magnetization measurements. Because the o-YMnO$_3$ 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-YMO$_3$ 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.