Probing Tunneling Spin Injection into Graphene via Bias Dependence

TL;DR

This study systematically investigates how bias voltage influences spin injection efficiency in graphene spin valves with different tunnel barriers, revealing sign reversal linked to bias voltage and attributing it to the electronic structure of ferromagnetic electrodes.

Contribution

It provides a detailed analysis of bias dependence in tunneling spin injection into graphene, highlighting the role of electrode electronic structure over other mechanisms.

Findings

Bias voltage causes nonlinear and sign-reversing spin signals.

Sign reversal correlates with bias voltage, not current.

Electronic structure of electrodes explains bias dependence.

Abstract

The bias dependence of spin injection in graphene lateral spin valves is systematically studied to determine the factors affecting the tunneling spin injection efficiency. Three types of junctions are investigated, including MgO and hexagonal boron nitride (hBN) tunnel barriers and direct contacts. A DC bias current applied to the injector electrode induces a strong nonlinear bias dependence of the nonlocal spin signal for both MgO and hBN tunnel barriers. Furthermore, this signal reverses its sign at a negative DC bias for both kinds of tunnel barriers. The analysis of the bias dependence for injector electrodes with a wide range of contact resistances suggests that the sign reversal correlates with bias voltage rather than current. We consider different mechanisms for nonlinear bias dependence and conclude that the energy-dependent spin-polarized electronic structure of the ferromagnetic electrodes, rather than the electrical field-induced spin drift effect or spin filtering effect of the tunnel barrier, is the most likely explanation of the experimental observations.