Triplet Josephson Effect with Magnetic Feedback in a Superconductor-Ferromagnet Heterostructure

Abstract

We study the ac Josephson effect in a superconductor-ferromagnet heterostructure with a variable magnetic configuration. The system supports triplet proximity correlations whose dynamics is coupled to the magnetic dynamics. This feedback dramatically modifies the behavior of the junction. The currentphase relation becomes double periodic at both very low and high Josephson frequencies ! J . At intermediate frequencies, the periodicity in ! J t may be lost. DOI: 10.1103/PhysRevLett.100.207001 PACS numbers: 74.78.Fk, 72.25.Ba, 74.50.+r Spin-dependent transport through hybrid structures combining ferromagnets (F) and normal metals has recently attracted a lot of interest, motivated by the prospect of potential technological applications in the field of spintronics [1]. Particular attention is given to two complementary effects involving mutual influence between electric current and magnetic configuration. The first is giant magnetoresistance [2] in which the conductance is much larger when different magnetic regions have their magnetic moments aligned than when they are antialigned. The opposite effect is the appearance of torques acting on magnetic moments when an electric current flows through the system [3]. These nonequilibrium current-induced torques appear due to nonconservation of spin currents accompanying a flow of charge through ferromagnetic regions. They allow manipulation of the magnetic configuration, including switching between the opposite directions or steady-state precession, without application of magnetic fields [4]. The two effects combined promise important practical applications in nonvolatile memory, programmable logic, and microwave oscillators.A conceptually different situation occurs when a ferromagnet is coupled to a superconductor (S), since the spin current through the superconducting part vanishes [5]. This additional constraint modifies the nonequilibrium torques, opening the possibility of perpendicular alignment of magnetic moments. Furthermore, if a magnetic structure is contacted by two superconductors, the proximity effect may be present, leading to a finite Josephson current through the structure at equilibrium. The torques generated by this current correspond to an equilibrium effective exchange interaction between the magnetic moments which can be controlled by the phase difference between the superconductors [6]. The same mechanism enables the reciprocal effect in which the supercurrent depends on the magnetic configuration.For a uniform ferromagnet, the observation of these effects requires a very thin magnetic layer, since the proximity effect is suppressed at short distances. However, in nonuniform ferromagnets, a long-range proximity effect can exist due to triplet superconducting correlations [7]. This triplet proximity effect (TPE) strongly enhances the associated Josephson current. It is important that TPE essentially depends on the magnetic configuration of the system [7]. Hence S/F multilayers exhibiting TPE are especially suitable for studying the Josephson-ind...

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