Studies on ferromagnetic metal/gallium arsenide heterostructures and spin electronic devices
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The ferromagnetic metals MnAs and Fe were each grown by molecular beam epitaxy on the semiconductor GaAs. The magnetic properties of these materials were measured by vibrating sample magnetometer and through the magneto-optical Kerr effect. The ferromagnet Fe, when grown on GaAs, shows an anisotropy which is a superposition of the bulk anisotropy and an in plane uniaxial anisotropy. The ferromagnet MnAs, when grown on GaAs, shows a strong in plane uniaxial anisotropy with a hard direction along the MnAs hexagonal c-axis. The domain reversal of MnAs is directly imaged by Kerr effect microscopy. The process is observed as a large scale domain wall propagating across the sample. The configuration of the domain structure is described in terms of a balance between competing micromagnetic energies. The two materials systems are applied in spintronic device studies. The electronic properties of the Fe/GaAs interface are characterized with Schottky diodes. It is found that the conductance with applied voltage conforms to the Brinkman Dynes and Rowell model for electron tunneling through the Schottky barrier, a necessary requirement for spin injection applications. The effect of shape anisotropy on patterned MnAs/GaAs samples is studied in preparation for control of the coercive field value of micron scale ferromagnetic contacts made from this material. Spin light emitting diodes are fabricated from MnAs/GaAs heterostructures. The spin injection from the MnAs contact is measured as a maximum of 60% at 7K. The results represent the first direct measurement of efficient spin injection from this material. The improvement over similar devices reported in literature likely results from the non-standard processing techniques and an optimized tunneling contact. The Fe/GaAs system is applied in the fabrication of three terminal spin valve devices which failed to show any spin related effects. The device failure is related to the absence of shape anisotropic control in the Fe-based contacts and it is suggested that future work should continue with MnAs.