Exchange-bias-induced double-shifted magnetization curves in Co biaxial films

Ferromagnetic films of fcc (001) Co, epitaxially grown on a NiMn antiferromagnetic underlayer, exhibit double-shifted magnetization curves. Exchange coupling not only provides a unidirectional bias field but also induces a uniaxial anisotropy, breaking the biaxial symmetry of Co. The double-shifted magnetization loops appear at certain range of Co thickness, in which both the shift field and exchange-bias field decreases as the Co film thickness increases. An interesting starlike astroid was found and it suggested that an interfacial biquadratic coupling results in double-shifted magnetization curves.