Interplay between exchange bias and uniaxial anisotropy in a ferromagnetic/antiferromagnetic exchange-coupled system

The effect of the relative orientation and magnitudes of the exchange bias and the uniaxial shape anisotropy has been systematically investigated in nanometer-sized lines of Ni80Fe20/FeMn bilayers using the magneto-optic Kerr effect for different orientations of the applied magnetic field. The measurements exhibit peculiar magnetic behavior when the exchange bias, the uniaxial anisotropy, and the applied magnetic field are not collinear. When the applied magnetic field is perpendicular to the exchange bias, the magnitude and the orientation of the uniaxial anisotropy determine the magnitude and sign of the loop shift. Furthermore, when the exchange bias and the uniaxial anisotropy are parallel, the shift of the hysteresis loop changes nonmonotonically with the orientation of the applied magnetic field and exhibits a maximum loop shift that exceeds the value that would be expected from the interface coupling alone. A simple modified coherent rotation model provides a good description of the behavior of the hysteresis loops in these patterned exchange bias systems. These results show clearly that, in addition to the loop shift, it is necessary to characterize all other magnetic anisotropies in order to extract meaningful conclusions about the interfacial coupling.