Thermal and magnetic stability of unidirectional anisotropy in spin-valve magnetoresistive heads

The temperature and time dependencies of magnetic anisotropies for NiFe layers pinned by two typical antiferromagnetic films, NiO and FeMn, were studied to understand the stability of unidirectional anisotropy. Torque curves were measured isothermally after application of a magnetic field from a direction perpendicular to the unidirectional anisotropy for certain time periods. Applying the field caused the torque curve to shift toward the field direction and its amplitude to decrease. The amount of shift and the normalized decrease in the torque amplitude were proportional to the logarithmic time of the applied perpendicular field. The changes observed in the NiO/NiFe film are twice as large as those in the NiFe/FeMn film at the same holding time and temperature. The external magnetic field caused rotation in the direction of the unidirectional anisotropy and a reduction in the exchange coupling strength. These changes may be explained by the thermal fluctuation aftereffect model. The unidirectional anisotropy in the NiO/NiFe film was less stable compared with that in the NiFe/FeMn film