Studies of ferromagnetic semiconducting hybrid structures
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Ga 1-x Mn x Sb random and GaSb/Mn digital alloys at low growth temperatures by MBE have been fabricated and studied to investigate effect of Sb/Ga flux ratio on the magnetic and electronic properties. The magnetic and magnetotransport properties of random alloys are strongly dependent on Sb/Ga flux ratio. The coercive field and negative magnetoresistance increase with decreasing Sb/Ga flux ratio, while the Curie temperature remains constant at approximately 23 K , with no systematic dependence on the hole density. In contrast, the Curie temperatures for the GaSb:Mn digital alloys with different Mn surface coverages depend significantly on the Sb/Ga flux ratio, and it is also directly correlated with the hole density. Epitaxial growth of ferromagnetic thin films directly on semiconductors as well as ferromagnetic III-Mn-V semiconductors has attracted much interest of many researchers because hybrid semiconductor-ferromagnet structures are relevant to spintronic applications that rely on spin injection and tunneling from a ferromagnet into a semiconductor. Ferromagnetic metal MnAs has been one of promising magnetic materials because of its high ferromagnetic transition temperature (T C ~ 320 K ), the relatively small coercive field and its structural compatibility with commonly used III-V semiconductors. MnAs thin films with high structural quality were epitaxially grown on GaAs substrates using molecular beam epitaxy (MBE). MnAs films have been found that two structurally distinct phases, α- and β-MnAs coexist in a range near the bulk phase transition temperature T C instead of an abrupt transition. MFM experiments reveal that the stripes of -MnAs have complicated magnetic domain structures at room temperature. Also the magnetic domains are very different when the temperature is decreased. Magnetization studies of MnAs epilayers, mesas without and with a Cr cap layer were carried out to investigate finite-size weakening of ferromagnetism and exchange bias effect for spintronic devices. The exchange bias effect with antiferromagnetic Cr cap layers on small size mesas resulted in significant enhancement of ferromagnetism and the effect was more clear in samples where significant demagnetizing effect is expected. We have studied also several different spintronic device structures. We have grown epitaxial MnAs/GaAs/MnAs ferromagnet/semiconductor trilayer heterostructures, spin Bragg reflectors with a periodic magnetic bars and FRITD structures with different conditions and studied their properties.