Ultrathin films on semiconductor substrates : growth, magneto-optical characteristics and spin injection
Ferromagnetic metal films on semiconductors are considered to be one of the mostlikely candidates to achieve an efficient spin injection at room temperature, which isone of the essential requirements for spintronics devices.The work presented focused on a study of the ferromagnetic film-semiconductorheterostructures, especially their magneto-optical properties and spin injection.Magnetic ultrathin films were grown on different GaAs substrates using an e-beamevaporation technique. The MBE system which is used to fabricate and analyseultrathin films is home designed and assembled. The design strategy and capabilitiesof this system in surface science studies are described in details in this thesis, as wellas the sample preparation and surface characterisation techniques.The magneto-optical effects were introduced, together with the representations of thepolarisation states including the Stokes parameters. The technique of using a dual-PEM (photo-elastic modulator) setup to simultaneously measure Stokes parameterswas developed and presented, including a novel calibration procedure and erroranalysis. The technique is readily applicable to the magneto-optical Kerr effect(MOKE) measurement. MOKE was utilised to study the magnetic properties of theseultrathin Ni, Fe films on various GaAs substrates.An investigation on the spin injection processes in ferromagnetic metalsemiconductorheterostructures was carried out using circularly polarisedphotoexcitation. The magnetisation effect on the chirality-dependent photocurrentprovides a convenient way to measure the magnetic asymmetry of the spinpolarisation states after the optical excitation. The influences of photon energy (aboveor below the band gap of GaAs), different heterostructures have been investigated,providing a better understanding of the spin injection process.
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