Nematic versus ferromagnetic spin filtering of triplet Cooper pairs in superconducting spintronics

摘要

We consider two types of magnetic Josephson junctions (JJ). They are formed by two singlet superconductors S and magnetic layers between them so that the JJ is a heterostructure of the S_m/S_m type, where S_m includes two magnetic layers with noncollinear magnetization vectors. One layer is represented by a weak ferromagnet and another one-the spin filter-is either conducting strong ferromagnet (nematic or N-type JJ) or magnetic tunnel barrier with spin-dependent transparency (magnetic or M-type JJ). Due to spin filtering only a fully polarized triplet component penetrates the normal n wire and provides the Josephson coupling between the superconductors S. Although both filters let to pass triplet Cooper pairs with total spin S parallel to the filter axes, the behavior of nematic and magnetic JJs is completely different. Whereas in the nematic case the charge and spin currents, I_q and I_(sp), do not depend on mutual orientation of the filter axes, both currents vanish in magnetic JJ in the case of antiparallel filter axes, and change sign with reversal of the filter direction. The obtained expressions for I_q and I_(sp) clearly show a duality between the superconducting phase φ and the angle α between the exchange fields in the weak magnetic layers.