Spin transport in two-layer-CVD-hBN/graphene/hBN heterostructures

TL;DR

This study investigates room temperature spin transport in graphene devices with a novel two-layer CVD-grown hBN tunnel barrier, revealing comparable mobility and relaxation times to traditional substrates, and unique spin injection behaviors modulated by bias.

Contribution

It introduces a two-layer CVD-hBN as a tunnel barrier in graphene, demonstrating its distinctive spin injection properties and bias-dependent polarization modulation.

Findings

Mobility and spin relaxation times similar to SiO2-based graphene devices.

Spin polarization can be positive or negative depending on contact resistance.

Differential spin injection polarization is tunable by DC bias without sign change.

Abstract

We study room temperature spin transport in graphene devices encapsulated between a layer-by-layer-stacked two-layer-thick chemical vapour deposition (CVD) grown hexagonal boron nitride (hBN) tunnel barrier, and a few-layer-thick exfoliated-hBN substrate. We find mobilities and spin-relaxation times comparable to that of SiO$_2$ substrate based graphene devices, and obtain a similar order of magnitude of spin relaxation rates for both the Elliott-Yafet and D'Yakonov-Perel' mechanisms. The behaviour of ferromagnet/two-layer-CVD-hBN/graphene/hBN contacts ranges from transparent to tunneling due to inhomogeneities in the CVD-hBN barriers. Surprisingly, we find both positive and negative spin polarizations for high-resistance two-layer-CVD-hBN barrier contacts with respect to the low-resistance contacts. Furthermore, we find that the differential spin injection polarization of the high-resistance contacts can be modulated by DC bias from -0.3 V to +0.3 V with no change in its sign, while its magnitude increases at higher negative bias. These features mark a distinctive spin injection nature of the two-layer-CVD-hBN compared to the bilayer-exfoliated-hBN tunnel barriers.