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

Abstract

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 (${t}_{\mathrm{IrMn}}=1.5$ to 20 nm)/FeGa (10 nm) with ${K}_{u}\ensuremath{\perp}{K}_{eb}$ and IrMn (${t}_{\mathrm{IrMn}}\ensuremath{\le}2$ nm)/FeGa (10 nm) with ${K}_{u}||{K}_{eb}$. 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 $|{K}_{u}|>{\ensuremath{\varepsilon}}_{{90}^{\ensuremath{\circ}}}+{K}_{eb}$, where ${\ensuremath{\varepsilon}}_{{90}^{\ensuremath{\circ}}}$ is the 90\ifmmode^\circ\else\textdegree\fi{} domain wall nucleation energy, and the EB leads to the appearance of the unusual asymmetrically shaped hysteresis loops.