Vortex states in patterned exchange biased NiO/Ni samples

We investigated the magnetization reversal of arrays of exchange biased NiO/Ni squares with superconducting quantum interference device magnetometry and micromagnetic simulations. The edges of the squares were 0.5, 1.5, and 3.0 μm long. The NiO/Ni structures exhibit vortexlike hysteresis loops, and micromagnetic calculations show that this feature is due to several vortices nucleating in the islands. Furthermore, for the arrays with squares of 1.5 μm edge length, the sign of the exchange bias field changes, as compared to the same continuous NiO/Ni layer. We attribute the vortex nucleation and the change in the exchange bias field to the interplay between shape and unidirectional anisotropy.