Ferromagnetic films of fcc (001) Co, epitaxially grown on a NiMn antiferrom
agnetic underlayer, exhibit double-shifted magnetization curves. Exchange c
oupling not only provides a unidirectional bias field but also induces a un
iaxial 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.