Determination of the spin-lifetime anisotropy in graphene using oblique spin precession

TL;DR

This paper introduces a reliable method to measure spin-lifetime anisotropy in graphene without large magnetic fields, revealing isotropic spin relaxation likely caused by magnetic impurities or spin-orbit effects.

Contribution

A novel measurement approach for spin-lifetime anisotropy in graphene that works at various carrier densities and temperatures, avoiding large out-of-plane magnetic fields.

Findings

Spin-lifetime anisotropy is independent of carrier density and temperature.

Spin relaxation in graphene is isotropic within experimental uncertainty.

Magnetic impurities or spin-orbit fields likely dominate spin relaxation.

Abstract

We determine the spin-lifetime anisotropy of spin-polarized carriers in graphene. In contrast to prior approaches, our method does not require large out-of-plane magnetic fields and thus it is reliable for both low- and high-carrier densities. We first determine the in-plane spin lifetime by conventional spin precession measurements with magnetic fields perpendicular to the graphene plane. Then, to evaluate the out-of-plane spin lifetime, we implement spin precession measurements under oblique magnetic fields that generate an out-of-plane spin population. We find that the spin-lifetime anisotropy of graphene on silicon oxide is independent of carrier density and temperature down to 150 K, and much weaker than previously reported. Indeed, within the experimental uncertainty, the spin relaxation is isotropic. Altogether with the gate dependence of the spin lifetime, this indicates that the spin relaxation is driven by magnetic impurities or random spin-orbit or gauge fields.