Spin injection into superconductors and quasi-particle mediated spin Hall effects

Competition between superconductivity and magnetism often brings about novel phenomena. This certainly applies to spintronics. According to theoretical predictions, superconductivity may enhance the spin Hall effect due to the increase in the resistance of quasiparticles which mediate spin transport in superconductors. However there has been neither experimental observation of the spin Hall effect (SHE) nor the enhancement of the effect in spite of the great deal of interest. We have recently demonstrated non-local spin injection into superconducting Nb and NbN by employing a spin absorption technique in lateral spin valve structures consisting of two ferromagnetic Permalloy (Py: Ni₈₁Fe₁₉) wires and Nb or NbN wire in between bridged by a nonmagnetic Cu wire. This device enables us to determine t_sf of superconducting Nb free from both the magnetic proximity effect on a superconductor and the charging effect in a small tunnel junction. Compared with previous studies, Nb or NbN was employed as a superconductor because it is more intriguing in that it can bring about novel phenomena like the quasi-particle mediated spin Hall effect due to its strong spin-orbit (SO) interaction; in addition it has even higher superconducting critical temperature T_C than Al . Our non-local spin injection experiments revealed that the spin lifetime in Nb indeed increased 4 times compared with the normal state. Calculations based on the Usadel equation can also well reproduce the experimental results. Furthermore we have also succeeded for the first time in detecting quasi-particle mediated SHE in the superconducting NbN, that also exhibits surprising enhancement in the magnitude over 2000 times compared to that in the normal state with a decrease of the spin injection current. These findings may provide a new insight into superconducting spintronics.