Inversion of Spin Signal and Spin Filtering in Ferromagnet|Hexagonal Boron Nitride-Graphene van der Waals Heterostructures.

M. Venkata Kamalakar,Saroj P. Dash,André Dankert,Paul J. Kelly

doi:10.1038/srep21168

Abstract

Two dimensional atomically thin crystals of graphene and its insulating isomorph hexagonal boron nitride (h-BN) are promising materials for spintronic applications. While graphene is an ideal medium for long distance spin transport, h-BN is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. Here, we demonstrate the spin filtering effect in cobalt|few layer h-BN|graphene junctions leading to a large negative spin polarization in graphene at room temperature. Through nonlocal pure spin transport and Hanle precession measurements performed on devices with different interface barrier conditions, we associate the negative spin polarization with high resistance few layer h-BN|ferromagnet contacts. Detailed bias and gate dependent measurements reinforce the robustness of the effect in our devices. These spintronic effects in two-dimensional van der Waals heterostructures hold promise for future spin based logic and memory applications.

Highlights

The CVD h-BN layer used as tunnel barrier in our experiment, was grown by chemical vapor deposition (CVD) on a copper foil and has high crystalline quality with ~90% coverage
The CVD h-BN is placed on graphene by wet chemical etching and transfer processes, respectively
The h-BN/PMMA layer is washed with 10% HCl and deionized water, and subsequently transferred onto the chip containing graphene flakes

Summary

Methods

The Graphene devices with Co|h-BN tunnel contacts were prepared on a highly doped Si substrate with 285 nm SiO2. Graphene flakes are exfoliated on a cleaned SiO2/Si substrate with predefined Ti/Au markers by repeated peeling of highly oriented pyrolytic graphite (HOPG) using the conventional scotch tape technique. The CVD h-BN is placed on graphene by wet chemical etching and transfer processes, respectively. The h-BN/PMMA layer is washed with 10% HCl and deionized water, and subsequently transferred onto the chip containing graphene flakes. The resulting chip containing a h-BN|graphene heterostructure is annealed at 400° C in Ar/H2 atmosphere. The annealing improves the adhesion between the h-BN and graphene and ensures the removal of possible residues of resist remaining from the transfer process of h-BN. The h-BN layer is patterned in regions covering graphene flakes by electron beam lithography and subsequent Ar-etching. The electrical and spin transport measurements were performed in a liquid helium cryostat with a superconducting magnet

Results

Discussion

Conclusion