Ultrathin films on semiconductor substrates : growth, magneto-optical characteristics and spin injection

Ferromagnetic metal films on semiconductors are considered to be one of the most likely candidates to achieve an efficient spin injection at room temperature, which is one of the essential requirements for spintronics devices. The work presented focused on a study of the ferromagnetic film-semiconductor heterostructures, especially their magneto-optical properties and spin injection. Magnetic ultrathin films were grown on different GaAs substrates using an e-beam evaporation technique. The MBE system which is used to fabricate and analyse ultrathin films is home designed and assembled. The design strategy and capabilities of this system in surface science studies are described in details in this thesis, as well as the sample preparation and surface characterisation techniques. The magneto-optical effects were introduced, together with the representations of the polarisation states including the Stokes parameters. The technique of using a dual- PEM (photo-elastic modulator) setup to simultaneously measure Stokes parameters was developed and presented, including a novel calibration procedure and error analysis. The technique is readily applicable to the magneto-optical Kerr effect (MOKE) measurement. MOKE was utilised to study the magnetic properties of these ultrathin Ni, Fe films on various GaAs substrates. An investigation on the spin injection processes in ferromagnetic metalsemiconductor heterostructures was carried out using circularly polarised photoexcitation. The magnetisation effect on the chirality-dependent photocurrent provides a convenient way to measure the magnetic asymmetry of the spin polarisation states after the optical excitation. The influences of photon energy (above or below the band gap of GaAs), different heterostructures have been investigated, providing a better understanding of the spin injection process