Efficient spin injection into graphene through trilayer hBN tunnel barriers

TL;DR

This study demonstrates enhanced spin injection efficiency into bilayer graphene using trilayer hBN tunnel barriers, achieving up to 60% polarization at room temperature, with insights into bias and gating effects.

Contribution

It provides the first detailed comparison of spin injection using 2L and 3L hBN barriers, showing improved polarization with trilayer hBN and analyzing bias and gating influences.

Findings

Spin injection polarization reaches -60% at -250 mV bias.

DC spin polarization of ~50% is achieved, 30% higher than with 2L-hBN.

Bias dependence is not due to exchange interaction or local gating.

Abstract

We characterize the spin injection into bilayer graphene fully encapsulated in hBN using trilayer (3L) hexagonal boron nitride (hBN) tunnel barriers. As a function of the DC bias, the differential spin injection polarization is found to rise up to -60% at -250 mV DC bias voltage. We measure a DC spin polarization of $\sim$ 50%, a 30% increase compared to 2L-hBN. The large polarization is confirmed by local, two terminal spin transport measurements up to room temperature. We observe comparable differential spin injection efficiencies from Co/2L-hBN and Co/3L-hBN into graphene and conclude that possible exchange interaction between cobalt and graphene is likely not the origin of the bias dependence. Furthermore, our results show that local gating, arising from the applied DC bias is not responsible for the DC bias dependence. Carrier density dependent measurements of the spin injection efficiency are discussed, where we find no significant modulation of the differential spin injection polarization. We also address the bias dependence of the injection of in-plane and out-of-plane spins and conclude that the spin injection polarization is isotropic and does not depend on the applied bias.