Nonlinear Valley and Spin Valves in Bilayer Graphene

TL;DR

This paper investigates nonlinear transport phenomena in bilayer graphene, revealing spin and valley-dependent effects and demonstrating potential for novel device functionalities through enhanced control of valley and spin polarization.

Contribution

It introduces the observation of second-order nonlinear spin currents in bilayer graphene with ferromagnetic contacts and links valley polarization to nonlinear transport behaviors.

Findings

Observation of second-order nonlinear spin current with spin valve-like behavior

Enhanced critical magnetic field for in-plane magnetic moment rotation

Indication of out-of-plane valley polarization induced by ferromagnetic proximity

Abstract

Nonlinear transport plays a vital role in probing the quantum geometry of Bloch electrons, valley chirality, and carrier scattering mechanisms. The nonlinear Hall effect, characterized by a nonlinear scaling of Hall voltage with longitudinal current, has been explored to reveal the Berry curvature and quantum metric related physics. In this work, we extend the study of nonlinear transport to spin and valley degrees of freedom. Using bilayer graphene devices with Fe3GeTe2 contacts, we observe a second-order nonlinear spin current exhibiting spin valve-like behaviors. By tracking magnetic moment precession under an in-plane magnetic field, we identify a significantly enhanced critical magnetic field required for in-plane rotation, suggesting out-of-plane valley polarization induced by ferromagnetic proximity. These findings offer deep insights into the interplay of valley and spin in second-order nonlinear transport, opening avenues for promising device applications.