Spin-dependent photoelectron tunneling from GaAs into magnetic cobalt

The spin dependence of the photoelectron tunnel current from free-standing GaAs films into out-of-plane magnetized cobalt films is demonstrated. The measured spin asymmetry ($A$), resulting from a change in light helicity, reaches $\pm 6\%$ around zero applied tunnel bias and drops to $\pm 2\%$ at a bias of $-1.6\hspace{0.5em}\mathrm{V}$ applied to the GaAs. This decrease is a result of the drop in the photoelectron-spin polarization that results from a reduction in the GaAs surface-recombination velocity. The sign of $A$ changes with that of the cobalt magnetization. In contrast, $A$ is negligible on nonmagnetic gold films.

Figure 1

(a) A top view of the GaAs cantilever. (b) The principle of the experiment showing optical excitation at the rear face of the GaAs cantilever. (c) Curves a and b show the spectra of the ${\sigma }^{\pm }$-polarized components of the cantilever luminescence under circularly polarized excitation. Curve c shows the polarization of the spectrum, about 8% for band-to-band emission. (d) The magnetic-field dependence of the magnetization perpendicular to the surface of the Co film as measured using the polar magneto-optical Kerr effect.

Figure 2

Curves a and b show the measured bias dependence of the spin asymmetry of the tunnel photocurrent into positively and negatively magnetized Co, respectively. Curve c is the asymmetry measured on a nonmagnetic gold surface. The bias dependence of $\delta {\rho }_m/{\rho }_m$ and of $\delta n_s/n_s$, normalized by factors of 10 and 1.3, respectively, are shown in curves d and e. The asymmetry, calculated using Eq. (1), is shown in curve f, and is in excellent agreement with the measured asymmetry.

Figure 3

Curves a and b, respectively, correspond to the unpolarized dark and photocurrents for tunneling into Co. The solid lines correspond to the calculations of the tunnel currents using a model[21] describing unpolarized tunneling of photoelectrons into metals. Curve c shows the resulting bias variation of the normalized recombination velocity $S/S_0$.

Figure 4

The tunnel junction and semiconductor-band structure versus position at applied biases of 0 V (top) and $-1\hspace{0.5em}\mathrm{V}$ (bottom). The insets show the semiconductor surface density of states in both cases. To the left of the tunnel junction is the metallic density of states plotted against energy. Spin-polarized conduction electrons are injected into the top half of the $3d$ minority spin band of Co. The injection energy $E_g-(1-f){\phi }_b$ is shown along with realistic representations of the densities of states of the majority (black) and minority (red) spins.