Magnetization reversal in epitaxial exchange-biased IrMn/FeGa bilayers with anisotropy geometries controlled by oblique deposition

We fabricated epitaxial exchange biased (EB) IrMn/FeGa bilayers by oblique deposition and systematically investigated their magnetization reversal. Two different configurations with the uniaxial magnetic anisotropy K-u parallel and perpendicular to the unidirectional anisotropy K-eb were obtained by controlling the orientation of the incident FeGa beam during deposition. A large ratio of K-u/K-eb was obtained by obliquely depositing the FeGa layer to achieve a large K-u while reducing the IrMn thickness to obtain a small K-eb. Besides the previously reported square loops, conventional asymmetrically shaped loops, and one-sided and two-sided two-step loops, unusual asymmetrically shaped loops with a three-step magnetic transition for the descending branch and a two-step transition for the ascending branch and biased three-step loops were observed at various field orientations in the films of both IrMn (t(IrMn) = 1.5 to 20 nm)/FeGa (10 nm) with K-u perpendicular to K-eb and IrMn (t(IrMn) <= 2 nm)/FeGa (10 nm) with K-u parallel to K-eb. Considering the geometries of anisotropies, a model based on domain wall nucleation and propagation was employed to quantitatively describe the angular dependent behaviors of IrMn/FeGa bilayers. The biased three-step magnetic switching was predicted to take place when vertical bar K-u vertical bar > epsilon(90 degrees) + K-eb, where epsilon(90 degrees) is the 90 degrees domain wall nucleation energy, and the EB leads to the appearance of the unusual asymmetrically shaped hysteresis loops.