Spin polarized tunneling in ferromagnet/unconventional superconductor junctions

We study tunneling in ferromagnet/unconventional superconductor (F/S) junctions. We include the effects of spin polarization, interfacial resistance, and Fermi wavevector mismatch (FWM) between the F and S regions. Andreev reflection (AR) at the F/S interface, governing tunneling at low bias voltage, is strongly modified by these parameters. The conductance exhibits a very wide variety of features as a function of applied voltage.Comment: Revision includes new figures with angular averages and correction of minor error

Spin-Polarized Transport Across an La0.7_{0.7}Sr0.3_{0.3}MnO3_{3}/YBa2_{2}Cu3_{3}O7_{7} Interface: Role of Andreev Bound States

Transport across an La$_{0.7}$Sr$_{0.3}MnO$_{3}/YBa$_{2}Cu$_{3}$O$_{7}$(LSMO/YBCO), interface is studied as a function of temperature and surface morphology. For comparison, control measurements are performed in non-magnetic heterostructures of LaNiO$_{3}$/YBCO and Ag/YBCO. In all cases, YBCO is used as bottom layer to eliminate the channel resistance and to minimize thermal effects. The observed differential conductance re ects the role of Andreev bound states in a-b planes, and brings out for the first time the suppression of such states by the spin-polarized transport across the interface. The theoretical analysis of the measured data reveals decay of the spin polarization near the LSMO surface with temperature, consistent with the reported photoemission data.Comment: 5 pages LaTeX, 3 eps figures included, accepted by Physical Review

Andreev conductance of a domain wall

At low temperatures, the transport through a superconductor-ferromagnet tunnel interface is due to tunneling of electrons in pairs. Exchange field of a monodomain ferromagnet aligns electron spins and suppresses the two electron tunneling. The presence of the domain walls at the SF interface strongly enhances the subgap current. The Andreev conductance is proven to be proportional to the total length of domain walls at the SF interface.Comment: 4 pages and 1 figur

Current and Spin-Torque in Double Tunnel Barrier Ferromagnet - Superconductor - Ferromagnet Systems

We calculate the current and the spin-torque in small symmetric double tunnel barrier ferromagnet - superconductor - ferromagnet (F-S-F) systems. Spin-accumulation on the superconductor governs the transport properties when the spin-flip relaxation time is longer than the transport dwell time. In the elastic transport regime, it is demonstrated that the relative change in the current (spin-torque) for F-S-F systems equals the relative change in the current (spin-torque) for F-N-F systems upon changing the relative magnetization direction of the two ferromagnets. This differs from the results in the inelastic transport regime where spin-accumulation suppresses the superconducting gap and dramatically changes the magnetoresistance [S. Takahashi, H. Imamura, and S. Maekawa, Phys. Rev. Lett. 82, 3911 (1999)]. The experimental relevance of the elastic and inelastic transport regimes, respectively, as well as the reasons for the change in the transport properties are discussed.Comment: 7 page

Andreev Reflection in Ferromagnet/Superconductor/Ferromagnet Double Junction Systems

We present a theory of Andreev reflection in a ferromagnet/superconductor/ferromagnet double junction system. The spin polarized quasiparticles penetrate to the superconductor in the range of penetration depth from the interface by the Andreev reflection. When the thickness of the superconductor is comparable to or smaller than the penetration depth, the spin polarized quasiparticles pass through the superconductor and therefore the electric current depends on the relative orientation of magnetizations of the ferromagnets. The dependences of the magnetoresistance on the thickness of the superconductor, temperature, the exchange field of the ferromagnets and the height of the interfacial barriers are analyzed. Our theory explains recent experimental results well.Comment: 8 pages, 9 figures, submitted to Phys. Rev.

Spin Injection and Detection in Magnetic Nanostructures

We study theoretically the spin transport in a nonmagnetic metal connected to ferromagnetic injector and detector electrodes. We derive a general expression for the spin accumulation signal which covers from the metallic to the tunneling regime. This enables us to discuss recent controversy on spin injection and detection experiments. Extending the result to a superconducting device, we find that the spin accumulation signal is strongly enhanced by opening of the superconducting gap since a gapped superconductor is a low carrier system for spin transport but not for charge. The enhancement is also expected in semiconductor devices.Comment: 4 pages, 3 figure

Spin-polarized transport and Andreev reflection in semiconductor/superconductor hybrid structures

We show that spin-polarized electron transmission across semiconductor/superconductor (Sm/S) hybrid structures depends sensitively on the degree of spin polarization as well as the strengths of potential and spin-flip scattering at the interface. We demonstrate that increasing the Fermi velocity mismatch in the Sm and S regions can lead to enhanced junction transparency in the presence of spin polarization. We find that the Andreev reflection amplitude at the superconducting gap energy is a robust measure of the spin polarization magnitude, being independent of the strengths of potential and spin-flip scattering and the Fermi velocity of the superconductor.Comment: 4 pages, 2 figure

Spin current in ferromagnet/insulator/superconductor junctions

A theory of spin polarized tunneling spectroscopy based on a scattering theory is given for tunneling junctions between ferromagnets and d-wave superconductors. The spin filtering effect of an exchange field in the insulator is also treated. We clarify that the properties of the Andreev reflection are largely modified due to a presence of an exchange field in the ferromagnets, and consequently the Andreev reflected quasiparticle shows an evanescent-wave behavior depending on the injection angle of the quasiparticle. Conductance formulas for the spin current as well as the charge current are given as a function of the applied voltage and the spin-polarization in the ferromagnet for arbitrary barrier heights. It is shown that the surface bound states do not contribute to the spin current and that the zero-bias conductance peak expected for a d-wave superconductor splits into two peaks under the influence of the exchange interaction in the insulator.Comment: 14 pages, 11 figure