Early Li-8+beta-NMR investigations in GaAs and Ge
ABSTRACT In this paper, we describe initial studies of the structure and dynamics associated with Li+8 in bulk crystalline GaAs and Ge. At low temperatures in GaAs, the amplitude of the Li+8 resonance signal at ≈3T indicates that a large fraction (at least 70%) of the Li end up in locations with cubic symmetry (i.e. the tetrahedral interstitial and substitutional sites). The linewidth of the β-NMR Li+8 resonance increases dramatically above 150K, reaches a maximum at about 290K, and decreases again. This suggests that the Li starts to change its location, probably from an interstitial to a substitutional site, at ≈150K. Experiments in Ge are also described. In this sample, a narrow resonance is seen at low temperatures that is likely due to Li located at an interstitial site. Near room temperature, it appears that Li is converting to another site.
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ABSTRACT: Low-energy β-detected nuclear magnetic resonance was used to investigate the spatial dependence of the hyperfine magnetic fields induced by Fe in the nonmagnetic Ag of an Au(40 Å)/Ag(200 Å)/Fe(140 Å) (001) magnetic multilayer grown on GaAs. The resonance line shape in the Ag layer shows dramatic broadening compared to that of intrinsic Ag. This broadening is attributed to large magnetic fields induced in this layer by the magnetic Fe layer. We find that the induced hyperfine field in the Ag follows a power law decay away from the Ag/Fe interface with power −1.93(8), and a field extrapolated to 0.23(5) T at the interface.Physical review. B, Condensed matter 04/2008; 77(14). DOI:10.1103/PhysRevB.77.144429
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ABSTRACT: A beta-detected NMR experiment is carried out on a semi-insulating GaAs crystal using a low energy spin polarized Li-8(+) beam to investigate the magnetic properties as a function of temperature and depth. The range of depths probed, from similar to 137 nm down to similar to 17 nm, coincides with the region of electronic band-bending due to the surface. We find a resonance broadened by the host nuclear dipoles, consistent with an earlier report, but using a pulsed radio frequency scheme, we are able to make higher resolution measurements. We find a small negative shift of the resonance (relative to an MgO reference) at room temperature that is independent of depth. Remarkably, as the temperature is reduced below about 150 K, the resonance shifts negatively and becomes depth dependent, with a larger negative shift deeper into the sample. This shift saturates below 10 K, i.e. it does not follow a Curie's law that might be expected for dilute magnetic impurities. Possible origins are discussed.Physics Procedia 01/2012; 30:227-230. DOI:10.1016/j.phpro.2012.04.079
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ABSTRACT: To test for the microscopic magnetic phase separation in the dilute magnetic semiconductor Ga1-xMnxAs suggested by low energy muon spin rotation measurements, we present a detailed analysis of the amplitudes of the Li-8 beta-detected nuclear magnetic resonance in an epitaxially grown thin film of x = 5.4% Mn doped GaAs on a semi-insulating GaAs substrate with magnetic transition temperature T-C=72 K. The spectrum at 100 K corresponds to 73% of the full room temperature amplitude, and at 60 K to about 62%. The 11% loss of signal through the magnetic transition is much smaller than that similar to 50% found by low energy mu SR, and may be entirely due to an amplitude change intrinsic to GaAs. This lack of evidence for phase separation is, however, consistent with the full volume fraction magnetism found by a second low energy mu SR measurement on a different sample using weak transverse field.Physics Procedia 01/2012; 30:174-177. DOI:10.1016/j.phpro.2012.04.066