Size dependence of exchange bias in ferromagnetic/antiferromagnetic bilayers

When the lateral size of exchange-coupled ferromagnetic-antiferromagnetic bilayers is reduced to a deep submicron size (below 0.2 µm), both the exchange bias and the coercive field increase compared to those of macroscopic samples. We attribute this size dependence to the increasing importance of the magnetostatic interaction. By using the Landau-Lifshitz-Gilbert equation, the size dependence of the hysteresis loops of polycrystalline films is calculated and quantitative pictures are established to describe the crossover from small to large samples.