Nucleation-type magnetization reversal by spin-polarized current in perpendicularly magnetized FePt layers

The magnetic field dependence of current-induced magnetization reversal was investigated for current-perpendicular-to-plane giant magnetoresistance pillars with perpendicularly magnetized FePt layers. Current-induced magnetization reversal under the tilted external magnetic field showed that a reversed domain is nucleated by spin-transfer torque and then propagates to complete magnetization reversal, although the formation of a multiple-domain state is not observed by static measurement. This nucleation-type current-induced magnetization reversal is characteristic of a system having large magnetic anisotropy and a small number of pinning sites for the domain wall, and explains the large magnetic field dependence of the critical current density for magnetization reversal (Jc). As the magnetic anisotropy of the FePt free layer is reduced, the magnetic field dependence of Jc becomes close to the values theoretically calculated for the coherent rotation mode, implying that the mode of the current-induced magnetization reversal changes from the nucleation of a reversed domain to the rotation of magnetization.