Dynamic Nuclear Polarization by Electrical Spin Injection in Ferromagnet-Semiconductor Heterostructures

Electrical spin injection from Fe into ${\mathrm{A}\mathrm{l}}_x{\mathrm{G}\mathrm{a}}_{1-x}\mathrm{A}\mathrm{s}$ quantum well heterostructures is demonstrated in small ($<500\mathrm{O}\mathrm{e}$) in-plane magnetic fields. The measurement is sensitive only to the component of the spin that precesses about the internal magnetic field in the semiconductor. This field is much larger than the applied field and depends strongly on the injection current density. Details of the observed hysteresis in the spin injection signal are reproduced in a model that incorporates the magnetocrystalline anisotropy of the epitaxial Fe film, spin relaxation in the semiconductor, and the dynamic polarization of nuclei by the injected spins.

Figure 1

In-plane magnetization of the epitaxial Fe film (sample A) parallel to fields applied along [011] and $[01\overline{1}]$. Left inset: schematic of the experimental setup. The direction of light propagation $\widehat{\mathbf{k}}$ is along the sample normal, ${\mathbf{B}}_a$ is the applied field, and $\mathbf{S}$ is the precessing spin in the semiconductor. Right inset: cross-sectional schematic of the device.

Figure 2

(a),(b) Electroluminescence polarization as a function of field applied at $4°\pm 2°$ to $[01\overline{1}]$ for samples A and B, respectively, at several current densities. Each curve is labeled by the current density in units of $\mathrm{A}/{\mathrm{c}\mathrm{m}}^2$. (c) Sample A ELP (points) as a function of field, with a $-2.15\%$ offset removed, and model ELP signal (solid curve) scaled by a factor of 20. (d),(e) Model ELP signal, including the effective nuclear field, as a function of applied field for samples A and B, respectively. The curves are labeled by the effective field $b_N$ in kG [see Eq. (2)]. (f) Hanle curves measured for samples A and B. Curves in (a), (b), (d), and (e) are offset for clarity.

Figure 3

(a) Sample B electroluminescence polarization data as a function of magnetic field for directions close to the [011] axis. The field is applied in the (100) plane (middle), and $\pm 5°$ out of the (100) plane (top and bottom curves). There is an in-plane rotation of $5°$ in all three cases. The inset shows an expansion of the region near zero field. (b) Modeling results for the sample orientations used in (a) in the presence of a nuclear field $b_N=8\mathrm{k}\mathrm{G}$.