Spin Control of Drifting Electrons Using Local Nuclear Polarization in Ferromagnet-Semiconductor Heterostructures

We demonstrate methods to locally control the spin rotation of moving electrons in a GaAs channel. The Larmor frequency of optically injected spins is modulated when the spins are dragged through a region of spin-polarized nuclei created at a $\mathrm{MnAs}/\mathrm{GaAs}$ interface. The effective field created by the nuclei is controlled either optically or electrically using the ferromagnetic proximity polarization effect. Spin rotation is also tuned by controlling the carrier traverse time through the polarized region. We demonstrate coherent spin rotations of $5\pi \mathrm{rad}$ during transport.

Figure 1

(a) Nuclear spins are polarized at the $\mathrm{MnAs}/\mathrm{GaAs}$ interface using an optical imprinting beam. (b) Nonlocal TRKR measurement geometry used to inject and detect drifting electron spins in the channel after imprinting the nuclear region. (c) Nuclear spins are also polarized by forward biasing the Schottky junction at the $\mathrm{MnAs}/\mathrm{GaAs}$ interface. During both optical and electrical imprinting $i_{\mathrm{channel}}$ is off.

Figure 2

(a) TRKR versus delay time in a control GaAs channel (with no MnAs) as the pump beam is rastered away from the probe beam along the channel with $B_{\mathrm{ext}}=0.2850\mathrm{T}$ at $T=8\mathrm{K}$. The drift velocity is extracted from line cuts taken along these data. (b) Drift velocity measured from rastered TRKR scans as a function of channel current.

Figure 3

(a) Nonlocal TRKR delay scans at fixed pump-probe separation of $35\mu \mathrm{m}$ for specified channel currents between 0.5 and 2.0 mA with no imprinting at $T=8\mathrm{K}$. These data are taken in a patterned $\mathrm{MnAs}/\mathrm{GaAs}$ device using the scheme shown in Fig. 1b. (b) Nonlocal TRKR versus time data at fixed delay indicated by arrow in (a) for $i_{\mathrm{channel}}=0.75\mathrm{mA}$ at $T=8\mathrm{K}$. Data are taken after optically polarizing the nuclei [Fig. 1a] with a 1.75 mW imprinting beam for 20 min. Solid line shows a fit to Eq. (2). Inset: Time dependence of the nuclear-induced spin rotation measured in (b). (c) Maximum electron spin rotation as a function of temperature at $\Delta t=5\mathrm{ns}$, $i_{\mathrm{channel}}=0.75\mathrm{mA}$ after applying a 1.75 mW imprinting beam for 20 min. For all data $B_{\mathrm{ext}}=0.2850\mathrm{T}$. Error bars are comparable to the size of the data points.

Figure 4

(a) Maximum electron spin rotation measured as a function of the imprinting beam power at $\Delta t=6\mathrm{ns}$, $i_{\mathrm{channel}}=0.75\mathrm{mA}$ after imprinting for 20 min. (b) Maximum electron spin rotation measured as a function of delay time for $i_{\mathrm{channel}}=0.75$, 1.25, and 1.75 mA after imprinting with a 1.75 mW beam for 20 min. (c) Schottky current and maximum electron spin rotation measured versus forward bias Schottky voltage at $\Delta t=4.5\mathrm{ns}$, $i_{\mathrm{channel}}=0.75\mathrm{mA}$ after imprinting for 20 min. For all data $B_{\mathrm{ext}}=0.2850\mathrm{T}$ and $T=8\mathrm{K}$. Error bars are comparable to the size of the data points.