Exchange bias model for Fe/FeF ₂ : Role of domains in the ferromagnet

Exchange bias (EB) is a shift of the hysteresis loop from its normal position, symmetric around H = 0, to HE ≠ 0. It occurs when thin ferromagnetic (F) films are deposited on a variety of antiferromagnetic (AF) materials. EB is also associated with several additional remarkable features: i) the bulk magnetizations of the F is orthogonal to the AF easy axis; ii) HE is of similar magnitude for compensated and uncompensated AF interface layers; iii) the sign of HE can assume both positive and negative values; and, iv) the magnetization |M(H ≪ − Hc)| ≠ |M(H ≫ + Hc)| , where Hc is the coercive field. Here we propose a model that describes the EB phenomenon for a compensated interface. Based on the experimental evidence, and extensive computer simulations, we suggest that close to the Néel temperature a canted spin configuration in the AF interface freezes into a metastable state. As a consequence, the EB energy is reversibly stored in a spring-like magnet, or incomplete domain wall (IDW), in the F slab. The results we extract from our model, both analytically and through simulations, are qualitatively and quantitatively compatible with the available experimental information.