Spin-polarized tunneling with GaAs tips in scanning tunneling microscopy

We describe a model as well as experiments on spin-polarized tunneling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. We present a transport model that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunneling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface. We present experimental data obtained in a scanning tunneling microscope with a GaAs tip and a Pt/Co multilayer sample under ambient conditions. A helicity asymmetry of tunnel current (i.e., a circular-polarization-dependent tunnel current) was found with a magnitude of 4 pA, which was verified not to be due to variations of the optical power. According to our model and estimations, this observation can be explained by spin-polarized tunneling, with a lower limit to the semiconductor surface spin splitting and spin lifetime of 4 mV and 0.4 ns, respectively.