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 ($t_{\mathrm{IrMn}}=1.5$ to 20 nm)/FeGa (10 nm) with $K_u\perp K_{eb}$ and IrMn ($t_{\mathrm{IrMn}}\le 2$ nm)/FeGa (10 nm) with $K_u\left|\right|K_{eb}$. 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 $|K_u|>{\varepsilon }_{{90}^{\circ }}+K_{eb}$, where ${\varepsilon }_{{90}^{\circ }}$ is the 90° domain wall nucleation energy, and the EB leads to the appearance of the unusual asymmetrically shaped hysteresis loops.

Figure 1

X-ray diffraction measurements (a) θ-2θ scan and (b) in-plane Φ scans, for the rotational sample of IrMn (20 nm)/FeGa (10 nm) with $K_u\left|\right|K_{eb}$. (c) Atomic force microscopy image ($2\times 2\mu {\mathrm{m}}^2$) of the IrMn (20 nm)/FeGa (10 nm) film with $K_u\left|\right|K_{eb}$ grown by oblique deposition. The color contrast (dark to bright) for the height scale corresponds to 10 nm.

Figure 2

The geometries of anisotropies for the epitaxial IrMn/FeGa EB bilayers. A uniaxial anisotropy along or perpendicular to the FeGa[010] axis induced by oblique deposition and the EB induced unidirectional anisotropy along the FeGa[010] axis are superimposed on the FeGa cubic anisotropy. The coercive fields for the magnetic switching between different FeGa easy axes are defined. The external field is applied at an angle of ϕ with respect to the EB.

Figure 3

Typical longitudinal (‖) and transverse (⊥) MOKE loops measured at various field orientations. (a) Square loop, (b) asymmetrically shaped loop, and (c) one-side two-step loop are obtained at ϕ = 0°, 20°, and 90°, respectively, for the reference IrMn (20 nm)/FeGa (10 nm) bilayer with $K_u\left|\right|K_{eb}$. (d) Two-side two-step loop, (e) unusual asymmetrically shaped loop, and (f) biased three-step loop are obtained at ϕ = 5°, 20°, and 25°, respectively, for the IrMn (20 nm)/FeGa (10 nm) bilayer with $K_u\perp K_{eb}$. The red and blue curves correspond to the descending and ascending branches of the hysteresis loops, respectively. The arrows enclosed by a square represent the orientation of FeGa spins. The coercive fields based on the magnetic switching routes are presented as well.

Figure 4

Typical field orientation dependence of the experimentally observed switching fields (symbols) and the corresponding theoretical fitting results (curves) for (a) the reference IrMn (20 nm)/FeGa (10 nm) bilayer with $K_u\left|\right|K_{eb}$ grown by rotating substrates, the obliquely deposited films of (b) IrMn (20 nm)/FeGa (10 nm) with $K_u\left|\right|K_{eb}$, (c) IrMn (20 nm)/FeGa (10 nm) with $K_u\perp K_{eb}$, and (d) IrMn (1.5 nm)/FeGa (10 nm) with $K_u\left|\right|K_{eb}$.