Revisiting the measurement of the spin relaxation time in graphene-based spintronic devices

Spin transport in graphene has strongly drawn attention because of the expected long spin relaxation time $\tau $sf, but, $\tau $sf derived from experiments is rarely above 1 ns, shorter than theoretically expected and also largely dispersed [1]. Here we reanalyze Hanle curves in graphene-based lateral spin valves with various contacts by using the recently established model of Hanle effect taking into account the spin absorption by contacts [2]. We found the reanalysis of Hanle curves of four samples from transparent contacts to tunnel contacts, reported in ref.3, gives longer and much less dispersed $\tau $sf ranging around 500 ps compared to the original values ranging from 84 ps to 495 ps. Extending our analysis to typical recent experiments with long spin diffusion length, $\lambda \sim $10 $\mu $m, we find the spin absorption is even more pronounced because the contact resistance R needed to suppress the spin absorption scales with the spin resistance $\propto \quad \lambda $. Thus R in the range of 100 kOhm is generally not large enough to suppress the spin absorption, and taking into account the contacts is more important to characterize the real $\tau $sf in graphene with $\lambda $ in this range. \\[4pt] [1] D. Pesin and A. H. MacDonald, Nmat 11, 409 (2012). [2] H. Idzuchi et al., PRB 89, 081308(R) (2014). [3] W. Han et al., PRL 105, 167202 (2010).