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Doctoral Student Supervision (Jan 2008 - May 2019)
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 shiftwas 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 orderof 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.
Low-energy, beta-radiation-detected nuclear magnetic resonance (β-NMR) is applied to probe the magnetism of Au and Pd. The measurements were carried out at the TRIUMF β-NMR facility using optically spin-polarized ⁸Li⁺ as the probe. The behaviour of ⁸Li⁺+ in Au was investigated using samples in the form of a foil and a 100 nm film evaporated onto a MgO (100) substrate. The results are in overall agreement with those obtained previously in Ag, Cu, and Al. Narrow, temperature-independent resonances are observed and assigned to ions stopping in the octahedral interstitial and substitutional lattice sites; the latter appearing only for temperatures above 150 K which is attributed to a thermally-activated site change. The spin-lattice relaxation rate of substitutional site ions is less than half as fast at ambient temperature as that in the other simple metals. The rate is independent of external field for fields greater than 15 mT. A Korringa analysis for the substitutional ions indicates no significant electron enhancement over that of a free electron gas. For all four metals, the enhancements obtained are smaller than those for the host nuclei. No depth dependence was found for the resonances in Au.The highly exchange-enhanced metal Pd was studied using samples in the form of a foil and a 100 nm film evaporated onto a SrTiO₃ (100) substrate. Strongly temperature-dependent, negative shifts are observed that scale with the temperature dependence of the host susceptibility between room temperature and 110 K. The resonances appear as two partially resolved lines that exhibit similar behaviour with temperature. The linewidths are broad and double upon cooling. The data are indicative of ions stopping in a site of cubic symmetry. The spin-lattice relaxation rate increases linearly with increasing temperature and eventually saturates at higher temperatures, consistent with the prediction from spin fluctuation theory. In contrast to the simple metals, large Korringa enhancements are obtained in this host. Ferromagnetic dynamical scaling is observed to hold above 110 K. Features below this temperature indicate that the Li ions locally induce a further enhancement of the static susceptibility. The temperature dependence of the modified susceptibility is in keeping with the prediction for a weak itinerant ferromagnet just above the Curie temperature; however, there is no evidence of static order.
Beta-detected Nuclear Magnetic Resonance (β-NMR) uses highly spin polarized β-emitting nuclei as a probe. Besides its use in nuclear physics, it hasalso become a powerful and sensitive tool in condensed matter physics and materials science. At TRIUMF, β-NMR of ⁸Li+ has been developed to study materials in a depth-resolved manner, where the implantation depth of ⁸Li+ is controlled via electrostatic deceleration. In this thesis, β-NMR of ⁸Li+ has been used to study high-Tc cuprate superconductors (HTSC). The objectiveof this work is to search for spontaneous magnetic fields generated by a possible time-reversal symmetry breaking (TRSB) superconducting statenear the surface of hole-doped YBa₂Cu₃O₇−d (YBCO), and study the nature of the vortex lattice (VL) in YBCO and electron-doped Pr₂−xCexCuO₄−d(PCCO). For several advantages, our measurements were carried out by implanting ⁸Li+ in thin silver films evaporated on the superconductors.In our TRSB studies, the magnetic field distribution p(B) is measured 8 nm away from the Ag/YBCO interface in magnetic fields B₀ = 5 to 100 G, applied parallel to the interface. p(B) showed significant broadening below the Tc of ab- and c-axis oriented YBCO films. The broadening signals the existence of weak disordered magnetic fields near the surface of YBCO. From the broadening’s temperature and field dependence we draw an upper limit of 0.2 G on the magnitude of spontaneous magnetic fields associated with TRSB.To study the VL, p(B) is measured at average implantation depths ranging from 20 to 90 nm away from the Ag/YBCO or Ag/PCCO interface in B₀ = 0.1 to 33 kG, applied perpendicular to the surface. p(B) showed a dramatic broadening below Tc as expected from the emerging field lines ofthe VL in the superconductor. In YBCO, p(B) is symmetric and the dependence on B0 is much weaker than expected from an ideal VL, indicatingthat the vortex density varies across the face of the sample on a long length scale, likely due to vortex pinning at twin boundaries. In PCCO, a 2D VLis established due to the high anisotropy of the superconductor leading to a nearly symmetric p(B).