Exchange Biased Vortices

Soft ferromagnetic discs with submicrometer diameter can reverse their magnetization via nucleation and annihilation of a vortex state. We prepared 400 nm diameter discs of Ni$_{80}$Fe$_{20}$/IrMn bilayers, where the exchange coupling between the ferromagnetic Ni$_{80}$Fe$_{20}$ and the antiferromagnetic IrMn modifies the magnetization reversal. Annealing of the Ni$_{80}$Fe$_{20}$/IrMn discs in an applied field establishes an exchange bias along the direction of the magnetic field during annealing. Magneto-optic Kerr effect measurements with the field applied along the exchange bias direction reveal a typical vortex hysteresis loop, which is now shifted with respect to zero field. Magnetic force microscopy in applied fields confirm that the reversal is via a vortex state. When the applied field is rotated with respect to the exchange bias direction the nucleation and annihilation fields reduce slightly, until at a critical angle of $\approx 80^{\circ}$, beyond which no vortex nucleation is observed. Micromagnetic simulations show that beyond the critical angle the magnetization reversal occurs via rotation of a C-state.