Imaging spin diffusion in germanium at room temperature

We report on the nonlocal detection of optically oriented spins in lightly $n$-doped germanium at room temperature. Localized spin generation is achieved by scanning a circularly polarized laser beam ($\lambda =1550$ nm) on an array of lithographically defined Pt microstructures. The in-plane oriented spin generated at the edges of such microstructures, placed at different distances from a spin-detection element, allows for a direct imaging of spin diffusion in the semiconductor, leading to a measured spin diffusion length of about $10\mu \mathrm{m}$. Two different spin-detection blocks are employed, consisting of either a magnetic tunnel junction or a platinum stripe where the spin current is converted in an electrical signal by the inverse spin-Hall effect. The second solution represents the realization of a nonlocal spin-injection/detection scheme that is completely free from ferromagnetic functional blocks.

Figure 1

Lateral devices for optical spin generation and nonlocal electrical spin detection. Scanning electron microscopy images (a), (b) and corresponding sketches (c) ,(d) of the MTJ and ISHE devices, respectively. For the MTJ device, the spin-induced electrical signal is measured between the top layer and an Ohmic contact made of Au(250 nm)/Ti(10 nm) directly grown on Ge, while the ISHE signal is measured across the Pt stripe.

Figure 2

(a) Optical apparatus: BS denotes beam splitter, PEM denotes photoelastic modulator, and Pol denotes polarizer. (b) Optical generation of in-plane spin-polarized electrons. When the light beam is focused at the edges of a Pt pad, the $x$ component (red arrow) of the circularly polarized illuminating field induces antiphase oscillating charges that, in turn, generate a staticlike near field (red field lines). Such a quasistatic field has a strong $z$ component that, combining with the $\pi /2$ dephased $y$ component of the illuminating field (blue arrows), produces a circularly polarized electric field able to photoexcite in-plane spin-polarized electrons (green arrows). A complementary spin polarization is generated at the opposite edge.

Figure 3

(a), (b) Reflectivity ($R$) optical images recorded on the MTJ and ISHE devices, respectively. (c), (d) Corresponding simultaneously recorded MTJ and ISHE voltage signals. The incident power is $60\mu \mathrm{W}$ for the MTJ sensor and $900\mu \mathrm{W}$ for the ISHE detector. (e), (f) Voltage profiles along $x$ across the centers of the Pt pads. In both cases, the detector is located at $x=0$. The dots are the experimental data while the solid lines correspond to fits (see the text for the fitting function). The signals have been normalized to the laser power $W$. As illustrated by the dotted vertical line, corresponding to the maxima of the reflectivity $R$ profiles in (g) and (h), the signal is zero at the center of each Pt pad and positive (negative) on the left (right) edge.

Figure 4

Dependence of the normalized $\mathrm{\Delta }{V}_{\mathrm{ISHE}}$ signal on the degree of circular polarization (a) and power (b) of the incident light. $\mathrm{\Delta }{V}_{\mathrm{ISHE}}$ is obtained from the amplitude coefficient $A$ of the fitting shown in Fig. 3, and it has been normalized to unity for a 100% circularly polarized beam with a power equal to 2.9 mW.