Lateral spin injection and detection via electrodeposited Fe/GaAs contacts

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(Thesis) Ph.D.
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Experimental studies of lateral spin injection and detection through electrodeposited Fe/GaAs tunnel contacts are reported in this thesis. An enhanced spin valve voltage is demonstrated via non-local lateral spin transport measurements compared to their vacuum-deposited counterparts. We have proposed a simple theoretical model to explain this result. Combined with experimental evidence for interfacial oxygen from atom probe tomography, we speculate that the enhancements occur due to a magnetic iron oxide layer forming at the Fe/GaAs interface during the electrodeposition. This layer acts as a tunnel barrier with a spin-dependent height. This discovery of greatly enhanced spin injection into GaAs via electrodeposited contacts introduces a promising new direction for the development of practical semiconductor spintronic devices. This thesis addresses three major challenges: i) The electrodeposition of Fe onto an epitaxial n-GaAs layer on a semi-insulating substrate to fabricate the tunnel contacts and lower-doped channel required for lateral spin injection. ii) Demonstration of spin accumulation and transport using patterned contacts in lateral configurations. iii) Understanding magnetic in-homogeneities and defects in the thin Fe film and correlating these to the observed enhanced spin injection. Continuous Fe film coverage was achieved over a desired area of the epitaxial GaAs by creating a uniform potential at the back of the sample. Nucleation and growth of Fe was observed within a range of applied current densities from 0.05 to 0.20 mA/mm^2, with the best Fe epitaxy occurring at 0.15 mA/mm^2. Modelling via a micromagnetic simulator showed that magnetic hysteresis curves from the electrodeposited Fe did not follow the standard behavior of a thin Fe film (single or polycrystalline). Instead, these Fe films demonstrated inhomogeneous magnetization controlled by strong local uniaxial anisotropies along both the and crystallographic directions. The presence of defects and coalescence boundaries responsible for these in-homogeneities were detected by transmission electron microscopy. Spin valve and Hanle measurements showed evidence of a local magnetostatic field, possibly originating from magnetic impurities at the electrodeposited Fe/epitaxial GaAs interface. We suggest that these magnetic impurities enhanced the tunneling probability and the spin accumulation within the GaAs channel while reducing the spin lifetime.
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Thesis advisor: Kavanagh, Karen
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