Determination of spin relaxation times in heavy metals via second-harmonic spin injection magnetoresistance

In tunnel junctions between ferromagnets and heavy elements with strong spin orbit coupling the magnetoresistance is usually dominated by tunneling anisotropic magnetoresistance (TAMR). This makes conventional DC spin injection techniques impractical for determining the spin relaxation time τₛ. Here, we show that this obstacle for measurements of τₛ can be overcome by second-harmonic spin-injection magnetoresistance (SIMR). In the second-harmonic signal the SIMR is comparable in magnitude to TAMR, thus enabling Hanle-induced SIMR as a powerful tool to directly determine τₛ. Using this approach we determined the spin relaxation time of Pt and Ta and their temperature dependences. The spin relaxation in Pt seems to be governed by the Elliott-Yafet mechanism due to a constant resistivity ×0.16em0exspin relaxation time product over a wide temperature range.