Superconducting triplet pairings and anisotropic magnetoresistance effects in ferromagnet/superconductor/ferromagnet double-barrier junctions

Andreas Costa and Jaroslav Fabian

Phys. Rev. B 104, 174504 – Published 8 November 2021

Abstract

Ferromagnetic spin valves offer the key building blocks to integrate giant- and tunneling-magnetoresistance effects into spintronics devices. Starting from a generalized Blonder-Tinkham-Klapwijk approach, we theoretically investigate the impact of interfacial Rashba and Dresselhaus spin-orbit couplings on the tunneling conductance, and thereby the magnetoresistance characteristics, of ferromagnet/superconductor/ferromagnet spin-valve junctions embedding thin superconducting spacers between the either parallel or antiparallel magnetized ferromagnets. We focus on the unique interplay between usual electron tunnelings—that fully determine the magnetoresistance in the normal-conducting state—and the peculiar Andreev reflections in the superconducting state. In the presence of interfacial spin-orbit couplings, special attention needs to be paid to the spin-flip (“unconventional”) Andreev-reflection process that is expected to induce superconducting triplet correlations in proximitized regions. As a transport signature of these triplet pairings, we detect conductance double peaks around the singlet-gap energy, reflecting the competition between the singlet and an additionally emerging triplet gap; the latter is an effective superconducting gap that can be ascribed to the formation of triplet Cooper pairs through interfacial spin-flip scatterings (i.e., to the generation of an effective triplet-pairing term in the order parameter). We thoroughly analyze the Andreev reflections' role in connection with superconducting magnetoresistance phenomena, and eventually unravel huge conductance and magnetoresistance magnetoanisotropies—easily exceeding their normal-state counterparts by several orders of magnitude—as another experimentally accessible fingerprint of unconventional Andreev reflections. Our results provide an important contribution to establish superconducting magnetic spin valves as an essential ingredient for future superconducting-spintronics concepts.