Nanoscale spin-dependent transport of electrons and holes in Si-ferromagnet structures

Given the rapid development of magnetic data storage and spin-electronics into the realm of nanotechnology, the understanding of the spin-dependent electronic transport and switching behavior of magnetic structures at the nanoscale is an important issue. We have developed spin-sensitive techniques with nanoscale resolution, referred to as ballistic electron magnetic microscopy (BEMM) and ballistic hole magnetic microscopy (BHMM). The techniques are based on the local tunnel injection of hot carriers from an STM tip, the subsequent spin-dependent transmission through the ferromagnetic thin film stack, and collection across a Schottky barrier. In this thesis, we have used BEMM and BHMM to study local spin-dependent transport of hot carriers (electrons or holes) as well as magnetic imaging of semiconductor-ferromagnet hybrid structures. First, hot electron transport is investigated in various n-Si/metal hybrid structures. It is observed that hot electron transmission in n-Si/Au //Ni81Fe19/Au /Co/Au structures is spin-dependent with a factor of ≈ 4 change in the BEEM signal as the relative magnetization of two ferromagnetic layers is changed from parallel to anti-parallel. The spin-dependent transmission of hot electrons is used to obtain nano-scale magnetic hysteresis curves. Furthermore, spin filtering of hot electrons is also used to study the magnetic reversal process of n-Si/Au/ NiFe/spacer/Co/Au structures. Apart from studying local spin-dependent hot electron transport, we have also explored the spin-dependent transport of hot holes with energy below EF. The hot hole attenuation lengths in Co in the energy range of 0.8-2 eV are determined and found to be on the order of 6-10Å showing an increase with the energy of injected holes. In p-Si/Ni81Fe19/Au/Co/Au system, it is observed for the first time that the hole current induced into the p-Si is dependent on the applied magnetic field, resulting in a large magnetocurrent of 130%. The origin of the spin filtering of holes is discussed using the electronic structure of ferromagnetic materials below the Fermi-level. Alternatively, we have also utilized the reverse mode of the technique (R-BHMM) to study spin-dependent hot hole excitations created by inelastic scattering of hot electrons. In this case it is also observed that the hole current depends sensitively on the magnetic field. A positive magnetocurrent of 180% at 1V is observed. To understand the large positive magnetocurrent due to collection of excited holes, a phenomenological model has been developed. Using the spin-filtering effect of hot holes, high-resolution magnetic imaging is demonstrated with a magnetic resolution better than 200Å. The discovery of spin-dependent hole transport opens up a new route for fundamental studies of spin dynamics of non-equilibrium carriers in ferromagnets. Together with its electron counterpart, hole spin-transport provides new avenues to design and explore novel nanoscale spintronics devices based on hot electron as well as hole transport.