Spin-filtering of non-equilibrium holes in a semiconductor-ferromagnet hybrid structure

Although spin-dependent transmission of hot electrons has been well addressed, the complementary spin-transport of non-equilibrium holes (below the Fermi level) has never been studied. Using a semiconductor/ferromagnet hybrid structure, it is shown here that a thin ferromagnetic film acts as an efficient spin-filter for holes. This has important implications not only in understanding several non-equilibrium phenomena, but also in realizing complementary building blocks for use in spintronics. To investigate hole spin transport, Ballistic Hole Magnetic Microscopy (BHMM) has been developed. Here, the tip of an STM is positively biased such that unpolarized hot holes are injected into a ferromagnetic metal stack grown on top of a p-type Si semiconductor. A Schottky contact between Au and p-Si acts as the collector energy barrier for the transmitted holes. Hot hole transport with energies of 0.3 to 2 eV below the Fermi level has been studied for a p-Si/Au/Co stack with varying Co thickness. The hole attenuation length has been found to be short and increases from 6-10 Å in the energy range 0.8-2 eV. For a NiFe/Au/Co trilayer, the hole transmission is clearly spin dependent with a large magnetocurrent (MC) of 130%.