Knight shift and nuclear spin relaxation in Fe/n-GaAs heterostructures

TL;DR

This study explores nuclear spin polarization and relaxation in Fe/n-GaAs heterostructures, revealing how hyperfine and quadrupolar interactions influence NMR signals and electron spin polarization measurements.

Contribution

It provides a quantitative model of donor site occupation and nuclear relaxation, linking hyperfine interactions to measurable NMR shifts and spin polarization.

Findings

Quadrupolar relaxation limits nuclear spin polarization near donors.

Knight shift correlates with electron spin polarization.

NMR signal amplitude depends on isotope-specific relaxation rates.

Abstract

We investigate the dynamically polarized nuclear-spin system in Fe/\emph{n}-GaAs heterostructures using the response of the electron-spin system to nuclear magnetic resonance (NMR) in lateral spin-valve devices. The hyperfine interaction is known to act more strongly on donor-bound electron states than on those in the conduction band. We provide a quantitative model of the temperature dependence of the occupation of donor sites. With this model we calculate the ratios of the hyperfine and quadrupolar nuclear relaxation rates of each isotope. For all temperatures measured, quadrupolar relaxation limits the spatial extent of nuclear spin-polarization to within a Bohr radius of the donor sites and is directly responsible for the isotope dependence of the measured NMR signal amplitude. The hyperfine interaction is also responsible for the $2\text{ kHz}$ Knight shift of the nuclear resonance frequency that is measured as a function of the electron spin accumulation. The Knight shift is shown to provide a measurement of the electron spin-polarization that agrees qualitatively with standard spin transport measurements.