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Beta-detected NMR of ⁸Li⁺ in spintronic materials

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Title: Beta-detected NMR of ⁸Li⁺ in spintronic materials
Author: Song, Qun
Degree Doctor of Philosophy - PhD
Program Physics
Copyright Date: 2012
Publicly Available in cIRcle 2012-11-30
Abstract: β-detected Nuclear Magnetic Resonance (βNMR) employs radioactive ⁸Li⁺ , which is optically spin-polarized, as a local probe to study magnetism in materials via β decay. In this thesis, βNMR is applied to spintronic materials, including GaAs, Ga₁₋xMnx Asand Fe/GaAs heterostructures in a depth-controlled manner at TRIUMF. High resolution β-NMR measurements were carried out on GaAs crystals (semi-insulating (SI-GaAs) and heavily doped n-type (n-GaAs)) as a control experiment for β-NMR on Fe/GaAs heterostructures. A small resonance shift was observed and found to be dependent on depth, temperature and doping. The depth dependence is only observed in SI-GaAs and not in n-GaAs. The resonance shift below 150 K in both GaAs is ∼ 100 ppm, on the same order of some Knight shifts of ⁸Li⁺ in noble metals. Ga₁₋xMnxAs is the first βNMR study on a ferromagnetic material through the ferromagnetic transition. Both spin lattice relaxation (SLR) and resonance of ⁸Li⁺ were measured. Two resonances were clearly resolved from the nonmagnetic GaAs substrate and the magnetic Ga₁₋xMnxAs film. The latter one negatively shifts and is linearly proportional to the applied field. The hyperfine coupling constant AHF of ⁸Li⁺ in Ga₁₋xMnxAs is found to be negative. The SLR rate λ does not follow Korringa’s Law and its amplitude shows a weak temperature dependence through TC. The behaviours of AHF and λ suggest that the delocalized holes originate from a Mn derived impurity band. No evidence of magnetic phase separation is found. ⁸Li⁺ provides a new depth-dependent local probe to detect injected spin polarization. We measured the ⁸Li⁺ resonance in Fe/GaAs heterostructures with semi-insulating and heavily doped n-type substrates, with and without injected current. With zero current, no spin polarization at thermal equilibrium is found. A new current injection system was designed and setup to conduct current injection from the thin Fe layer into the n-GaAs substrate. We found effects of local Joule heating and a very small stray field caused by the injected current but no convincing evidence of injected spin polarization.
URI: http://hdl.handle.net/2429/43638
Scholarly Level: Graduate

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