Tunneling Anisotropic Magnetoresistance and Spin-Orbit Coupling in $\mathrm{Fe}/\mathrm{GaAs}/\mathrm{Au}$ Tunnel Junctions

We report the observation of tunneling anisotropic magnetoresistance effect in the epitaxial metal-semiconductor system $\mathrm{Fe}/\mathrm{GaAs}/\mathrm{Au}$. The observed twofold anisotropy of the resistance can be switched by reversing the bias voltage, suggesting that the effect originates from the interference of the spin-orbit coupling at the interfaces. Corresponding model calculations reproduce the experimental findings very well.

Figure 1

(a) Schematic picture of a $\mathrm{Fe}/\mathrm{GaAs}/\mathrm{Au}$ tunnel structure; the top Au contact has a diameter of $16\mu \mathrm{m}$; the square bottom Fe contact has a side length of $100\mu \mathrm{m}$ and is attached to Fe leads; (b) schematic top view on the iron layer; (c)–(h) tunneling resistance depending on $\mathbf{B}$ swept under different angles $\varphi$ measured at 4.2 K and $-90\mathrm{mV}$ bias (Au contact grounded). The double step switching mechanism is illustrated in (b) and (e) (see text). Data taken between $\varphi =90°$ and 180° show the same characteristics.

Figure 2

(a) $\varphi$ scan of the tunneling resistance at 4.2 K and $-90\mathrm{mV}$ bias in a saturation magnetic field $|\mathbf{B}|=0.5\mathrm{T}$ and a theoretical fit; (b)  $\varphi$ scan at $+90\mathrm{mV}$; (c) $\varphi$ scans for different bias voltages. Symbols correspond to experimental results for $-90$, $-50$, 50, 90, and 135 mV bias; solid lines correspond to theoretical results with ${\alpha }_l=42.3\mathrm{eV}{\AA }^2$, ${\alpha }_l=45.8\mathrm{eV}{\AA }^2$, ${\alpha }_l=-0.6\mathrm{eV}{\AA }^2$, ${\alpha }_l=-17.4\mathrm{eV}{\AA }^2$, and ${\alpha }_l=-25.1\mathrm{eV}{\AA }^2$, respectively. The absolute resistance values range between $750\Omega$ and $974\Omega$ in the investigated voltage regime.

Figure 3

(a) Schematics of the conduction band profile along the growth direction of the heterostructure; the tunneling barrier with the Schottky barrier height $V_S$ is gray shaded. Spin splitting of the bands in Fe is indicated. (b) A spacial view of the model system. The vector $\mathbf{n}$ determines the magnetization direction in Fe with respect to the [100] direction ($x$ axis). The SOI-induced spin precession of the electrons during tunneling is characterized by the vector $\mathbf{w}({\mathbf{k}}_{\parallel })$ (see text). Polar plots of $|\mathbf{w}({\mathbf{k}}_{\parallel })|$ for a fixed value of $|{\mathbf{k}}_{\parallel }|$ are schematically represented in (c) and (d) for cases in which $\tilde{\alpha }\tilde{\gamma }>0$ and $\tilde{\alpha }\tilde{\gamma }<0$, respectively. The anisotropy of the field $\mathbf{w}({\mathbf{k}}_{\parallel })$ in the ${\mathbf{k}}_{\parallel }$ space is transferred to the tunneling transmissivity (see text) and transformed into the observed anisotropy of the TAMR.