Modeling the oblique spin precession in lateral spin valves for accurate determination of the spin lifetime anisotropy: Effect of finite contact resistance and channel length

Abstract

The spin lifetime anisotropy is an important quantity for investigating the spin relaxation mechanisms in graphene and in heterostructures of two-dimensional materials. We generalize the diffusive spin transport equations of oblique spin precession in a lateral spin valve with finite contact resistance. This yields a method to determine the spin lifetime anisotropy ratio = ! / ∥ , which is the ratio between lifetimes of spin polarized perpendicular and parallel to the graphene surface. By solving the steady-state Bloch equations, we show that the line-shape of the oblique spin precession signal can be described with six dimensionless parameters, which can be solved analytically. We demonstrate that the anisotropic spin precession characteristics can be strongly suppressed by contact induced spin relaxation originating from conductance mismatch between the channel material and electrodes. To extract the spin lifetime anisotropy ratio accurately, we develop a closed form equation that includes the effect of finite contact resistance. Furthermore, we demonstrate that in the high contact resistance regime, the minimum channel length required for accurately determining the spin lifetime anisotropy for a sufficiently low external magnetic field is only determined by the diffusion coefficient of the channel material, as opposed to the spin diffusion length. Our work provides an accurate model to extract the spin lifetime anisotropy ratio from the oblique spin precession measurement, and can be used to guide the device design for such measurements. 2 I. INTRODUCTIONSpintronics aims to utilize the spin degree freedom of charge carriers for logic operation and information storage [1]. In recent years, graphene has emerged as one of the most efficient spin channel materials [2], exhibiting gate tunable spin transport, long spin lifetimes and long spin diffusion lengths at room temperature [3][4][5][6][7][8][9][10][11][12]. These make graphene a promising material for spintronics applications [13][14][15][16][17][18][19]. What makes graphene even more special is the high tunability of its properties. Due to the atomically thin nature of graphene, its properties are strongly subject to the environment, such as surface flatness [20][21][22][23][24][25], adatom adsorption [26][27][28][29][30][31][32][33], or in proximity with other materials [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. This allows manipulation of graphene's spin transport and magnetic properties, which further enriches the possibilities of graphene for spintronics.Among all the properties in graphene, spin-orbit coupling is of particular interest. The intrinsic spin-orbit coupling in graphene is predicted to be very weak, with a magnitude of only ~ 30 [51-53]. However, this value can be enhanced by several orders of magnitude by modifying graphene surface with adatoms, hybridizing with metal, or in proximity with strong spin-orbit coupling material [35,37-40,47,54-57]. Such strong spin-orbit coupling interaction is essential for new phe...

Heterostructures of graphene and hBN: Electronic, spin-orbit, and spin relaxation properties from first principles

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