Room-temperature spin-lifetime anisotropy exceeding 60 in bilayer graphene spin valves proximity coupled to WSe$_2$

TL;DR

The paper addresses achieving large spin lifetime anisotropy in graphene spin valves by proximity coupling BLG to WSe2, enabling control over spin relaxation via spin–orbit proximity. The authors employ non-local spin-valve, z-Hanle, oblique Hanle, and x-Hanle measurements, supported by 3D Bloch–Torrey simulations and Stoner–Wohlfarth modeling to extract anisotropy parameters. They report a room-temperature anisotropy of at least $\xi\ge60$, with out-of-plane spin lifetime $\tau_\perp$ around $250$–$300$ ps and in-plane lifetime $\tau_\parallel<4$ ps in the WSe2 region, while reference regions show near-isotropic behavior and long in-plane lifetimes. Mobility remains high in the presence of WSe2, highlighting the practical potential of BLG/WSe2 heterostructures for spintronic devices and paving the way toward even larger anisotropies with optimized substrates and twist angles.

Abstract

A spin lifetime anisotropy between in-plane and out-of-plane spins in bilayer graphene (BLG) can be achieved by spin-orbit proximity coupling of graphene to transition metal dichalcogenides. This coupling reduces the in-plane spin lifetime due to proximity-induced spin scattering, while the out-of-plane spin lifetime remains largely unaffected. We show that at room temperature spin lifetime anisotropy exceeds 60 in a bilayer graphene lateral spin valve proximity coupled to WSe$_2$. The out-of-plane spin lifetime of about 250 ps closely matches that of a BLG reference region not in contact with WSe$_2$. In contrast, the estimated in-plane spin lifetime of less than 4 ps leads to a complete suppression of the in-plane spin signal at the ferromagnetic Co/MgO spin detector. The proximity coupling of WSe$_2$ to BLG is particularly promising, as it does not compromise the charge carrier mobility within the graphene channel.

Figures (4)

• Figure 1: (a) Schematic cross-section of the device illustrating the stacking order and the wiring in case of a non-local spin transport measurement. (b) Optical image of the device with the reference regions A and B and the WSe$_2$ (TMD) proximity coupled region C. (c) In case of the oblique Hanle measurement, the external magnetic field is oriented at an angle $\beta$ between the easy-axis of the ferromagnetic electrodes (blue arrows) ($\beta$ = 0$^\circ$) and the out-of-plane field direction of a standard Hanle spin precession measurements ($\beta$ = 90$^\circ$). (d) In the x-Hanle configuration the external magnetic field points in the x-direction, which is defined as the direction perpendicular to the ferromagnetic contacts and within the plane of the graphene channel.
• Figure 2: Results for BLG reference regions. (a)z-Hanle (90$^\circ$) and spin-valve measurements (0$^\circ$) of reference region B. (b) oblique Hanle in reference region A at selected angles. (c)x-Hanle measurement with fit (black dotted curve), and (d) the evaluation of the anisotropy from the oblique Hanle measurements for reference region A. The colored dots relate to the curves shown in (b), the black straight line corresponds to $\xi=1$, and the red line is the fit to the data.
• Figure 3: x-Hanle measurement and its numerical simulations in the WSe$_2$ region. (a)x-Hanle measurement and best simulation result (green lines) with $\tau_\perp$=[300]ps and $\xi=60$. (b) Variation of the spin-lifetime anisotropy $\xi$ for a constant $\tau_\perp$=[300]ps. (c) Variation of $\tau_\perp$ for a constant $\xi=60$. The spin diffusion coefficient is set to 0.025 m$^2$/s for the simulations.
• Figure 4: Symmetrized and normalized oblique Hanle measurements of the WSe$_2$-region and simulation curves. (a) Measurement set at different angles $\beta$ with corresponding simulations using the best match parameters from (b) and (c), i.e. $\tau_\perp$=[250]ps and $\xi=60$. (b) Measurement at $\beta=40^\circ$ and simulated curves for which the spin-lifetime anisotropy $\xi=60$ was kept constant while varying the out-of-plane spin lifetime $\tau_{\perp}$. (c) Similarly to (b) $\tau_{\perp}$=[250]ps is kept constant while varying $\xi$. The spin diffusion coefficient is set to 0.025 m$^2$/s for the simulations.