Ferromagnet-insulator-superconductor Research Articles

Transport properties of silicene-based ferromagnetic-insulator-superconductor junction

We study the tunneling conductance of a silicene-based ferromagnet/insulator/superconductor (FIS) junction by the use of the spin-dependent Dirac-Bogoliubov de-Gennes equation. We demonstrate that the conductance spectra are strongly affected by exchange energy h, Fermi energy EF, and external perpendicular electric field Ez. In the thin barrier limit of insulator silicene IS, the zero-bias charge conductance of the FIS silicene junction oscillates as a function of barrier strength χG. It is shown that the period of oscillations changes from π/2 to π corresponding to undoped and doped silicene. Remarkably, in contrast to that of the graphene FIS junction where the conductance only vanishes at the exchange energy h=EF, here due to the buckled structure of silicene, there is a transport gap region for the range of h values and the magnitude of such a gap region can be controlled by Ez. Moreover, it is found that by appropriate choice of h and Ez, it is possible to achieve a fully spin and valley-polarized charge conductance through the FIS silicene junction. This property suggests experimentally measuring the Fermi energy of silicene.

Fabrication of superconductor–ferromagnet–insulator–superconductor Josephson junctions with critical current uniformity applicable to integrated circuits

Nb Josephson junctions (JJs) were fabricated with a Pd89Ni11 ferromagnetic interlayer and an AlOx tunnel barrier layer for use in large-scale superconducting integrated circuits. The junctions had a small critical current (Ic) spread, where the standard deviation 1σ was less than 2% at 4.2 K for junctions with the same designed size. It was observed that the electrical behavior of the junctions could be controlled by manipulating the film thickness of the PdNi interlayer. The junctions behaved as a π-JJ for thicknesses of 9 and 11 nm, showing 1σ in the Ic spread of 1.2% for 9 nm.

Nonequilibrium Effect in Ferromagnet-Insulator-Superconductor Tunneling Junction Currents

Nonequilibrium effect due to the imbalance in the number of the ? and ? spin electrons has been studied for the tunneling currents in the ferromagnet-insulator-superconductor (FIS) tunneling junctions within a phenomenological manner. It has been stated how the nonequilibrium effect should be observed in the spin-polarized quasiparticle tunneling currents, and pointed out that the detectable nonequilibrium effect could be found in the FIS tunneling junction at 77 K using HgBa2Ca2Cu3O8+? (Hg-1223) high-Tc superconductor rather than Bi2Sr2CaCu2O8+? (Bi-2212) one.

Superconductivity-induced magnetization depletion in a ferromagnet through an insulator in a ferromagnet-insulator-superconductor hybrid oxide heterostructure.

Coupling between superconducting and ferromagnetic states in hybrid oxide heterostructures is presently a topic of intense research. Such a coupling is due to the leakage of the Cooper pairs into the ferromagnet. However, tunneling of the Cooper pairs though an insulator was never considered plausible. Using depth sensitive polarized neutron reflectivity we demonstrate the coupling between superconductor and magnetic layers in epitaxial La2/3Ca1/3MnO3 (LCMO)/SrTiO3/YBa2Cu3O7-δ (YBCO) hybrid heterostructures, with SrTiO3 as an intervening oxide insulator layer between the ferromagnet and the superconductor. Measurements above and below the superconducting transition temperature (TSC) of YBCO demonstrate a large modulation of magnetization in the ferromagnetic layer below the TSC of YBCO in these heterostructures. This work highlights a unique tunneling phenomenon between the epitaxial layers of an oxide superconductor (YBCO) and a magnetic layer (LCMO) through an insulating layer. Our work would inspire further investigations on the fundamental aspect of a long range order of the triplet spin-pairing in hybrid structures.

Detection of small exchange fields in S/F structures

Ferromagnetic materials with exchange fields Eex smaller or of the order of the superconducting gap Δ are important for applications of corresponding (s-wave) superconductor/ferromagnet/superconductor (SFS) junctions. Presently such materials are not known but there are several proposals how to create them. Small exchange fields are in principle difficult to detect. Based on our results we propose reliable detection methods of such small Eex. For exchange fields smaller than the superconducting gap the subgap differential conductance of the normal metal–ferromagnet–insulator–superconductor (NFIS) junction shows a peak at the voltage bias equal to the exchange field of the ferromagnetic layer, eV=Eex. Thus measuring the subgap conductance one can reliably determine small Eex<Δ. In the opposite case Eex>Δ one can determine the exchange field in scanning tunneling microscopy (STM) experiment. The density of states of the FS bilayer measured at the outer border of the ferromagnet shows a peak at the energy equal to the exchange field, E=Eex. This peak can be only visible for small enough exchange fields of the order of few Δ.

Virtual Andreev Reflection in Topological Insulator-Based Ferromagnet/Insulator/Superconductor

The property of quasiparticle tunneling on the surface of topological insulator (TI) ferromagnet-insulator-superconductor (FM-I-SC) junction is investigated. The virtual Andreev reflection (VAR) takes place when the wave vectors of electron and hole-like quasiparticles in FM are different due to gauge filed. We find that the VAR exhibits anisotropy in contrast to those in conventional and graphene-based junctions, because of gauge field on the surface of TI. The current in the junction can be carried by pure VAR, by tuning in-plane magnetic field. We also find that, the perfect linear slope of the conductance under varying in-plane magnetic fields is predicted, applicable for TI-based spintronic devices.

Heat Transport in Graphene Ferromagnet-Insulator-Superconductor Junctions

We study heat transport in a graphene ferromagnet-insulator-superconducting junction. It is found that the thermal conductance of the graphene ferromagnet-insulator-superconductor (FIS) junction is an oscillatory function of the barrier strength χ in the thin-barrier limit. The gate potential U0 decreases the amplitude of thermal conductance oscillation. Both the amplitude and phase of the thermal conductance oscillation varies with the exchange energy Eh. The thermal conductance of a graphene FIS junction displays the usual exponential dependence on temperature, reflecting the s-wave symmetry of superconducting graphene.

Tunneling conductance of graphene ferromagnet-insulator-superconductor junctions

We study the transport properties of a graphene ferromagnet-insulator superconductor (FIS) junction within the Blonder-Tinkham-Klapwijk formalism by solving spin-polarized Dirac-Bogoliubov-de-Gennes equation. We find that the retro and specular Andreev reflections in the graphene FIS junction are drastically modified in the presence of exchange interaction and that the spin-polarization ($P_T$) of tunneling current can be tuned from the positive to negative value by bias voltage ($V$). In the thin-barrier limit, the conductance $G$ of a graphene FIS junction oscillates as a function of barrier strength $\chi$. Both the amplitude and phase of the conductance oscillation varies with the exchange energy $E_{ex}$. For $E_{ex}<E_F$ (Fermi energy), the amplitude of oscillation decreases with $E_{ex}$. For $E_{ex}^{c}>E_{ex}>E_F$, the amplitude of oscillation increases with $E_{ex}$, where $E_{ex}^{c}=2E_{F}+U_{0}$ ($U_{0}$ is the applied electrostatic potential on the superconducting segment of the junction). For $E_{ex} > E_{ex}^{c}$, the amplitude of oscillation decreases with $E_{ex}$ again. Interestingly, a universal phase difference of $\pi/2$ in $\chi$ exists between the $G-\chi$ curves for $E_{ex}>E_F$ and $E_{ex}<E_F$. Finally, we find that the transitions between retro and specular Andreev reflections occur at $eV=|E_{F}-E_{ex}|$ and $eV=E_{ex}+E_{F}$, and hence the singular behavior of the conductance near these bias voltages results from the difference in transport properties between specular and retro Andreev reflections.

Spin-polarized quasiparticle transport in cuprate superconductors

The effects of spin-polarized quasiparticle transport in superconducting ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. These effects are compared with the injection of simple quasiparticles into control samples of perovskite nonmagnetic metal-insulator-superconductor (N-I-S). Systematic studies of the critical current density ${(J}_{c})$ as a function of the injection current density ${(J}_{\mathrm{inj}}),$ temperature $(T),$ and the thickness (d) of the superconductor reveal drastic differences between the F-I-S and N-I-S heterostructures, with strong suppression of ${J}_{c}$ and a rapidly increasing characteristic transport length near the superconducting transition temperature ${T}_{c}$ only in the F-I-S samples. The temperature dependence of the efficiency $(\ensuremath{\eta}\ensuremath{\equiv}\ensuremath{\Delta}{J}_{c}{/J}_{\mathrm{inj}};$ $\ensuremath{\Delta}{J}_{c}:$ the suppression of critical current due to finite ${J}_{\mathrm{inj}})$ in the F-I-S samples is also in sharp contrast to that in the N-I-S samples, suggesting significant redistribution of quasiparticles in F-I-S due to the longer lifetime of spin-polarized quasiparticles. Application of conventional theory for nonequilibrium superconductivity to these data further reveal that a substantial chemical potential shift ${\ensuremath{\mu}}^{*}$ in F-I-S samples must be invoked to account for the experimental observation, whereas no discernible chemical potential shift exists in the N-I-S samples, suggesting strong effects of spin-polarized quasiparticles on cuprate superconductivity. The characteristic times estimated from our studies are suggestive of anisotropic spin relaxation processes, possibly with spin-orbit interaction dominating the c-axis spin transport and exchange interaction prevailing within the ${\mathrm{CuO}}_{2}$ planes. Several alternative scenarios attempted to account for the suppression of critical currents in F-I-S samples are also critically examined, and are found to be neither compatible with experimental data nor with the established theory of nonequilibrium superconductivity.

Do ballistic channels contribute to the magnetoresistance in magnetic tunnel junctions?

The barrier quality and conduction mechanisms in ferromagnet–insulator–ferromagnet (FIF) junctions are studied by fabricating adjacent ferromagnet–insulator–superconductor (FIS) junctions, using the same barrier and top electrode in both junctions. Transport measurements on the FIS junctions provide information about the transport processes in the FIF junctions. The FIS junctions have a large zero-bias conductance (ZBC) that varies with barrier thickness, in part due to ballistic channels in parallel with the tunnel barrier. As the ZBC increases, the magnetoresistance (MR) in the FIF junctions decreases, implying that these ballistic channels are not magnetoresistive and that their presence reduces the total MR in the FIF junctions.

Spin Injection: A Survey and Review

The study of the transport and relaxation of spin-polarized carriers in the solid state began about 30 years ago. Tunneling spectroscopy was applied to ferromagnet–insulator–superconductor junctions to demonstrate the polarization of interfacial currents. The use of a ferromagnetic material as an injector and/or detector of polarized carriers has since become a valuable tool, and spin injection has been applied to nonmagnetic metals, superconductors, and semiconductors. The spin injection phenomenology is reviewed in the context of two topics of continuing importance for basic and applied research: (i) the transmission of polarized carriers across ferromagnet/nonmagnetic material interfaces and (ii) carrier spin relaxation inside the nonmagnetic material.

Dynamic pair breaking in cuprate superconductors via injection of spin-polarized quasiparticles in perovskite F–I–S heterostructures

We report experimental evidence of dynamic Cooper pair breaking induced by spin-polarized quasiparticles in cuprate superconductors by studying the critical current density and quasiparticle density of states of ferromagnet–insulator–superconductor (F–I–S) heterostructures. The spin diffusion length and relaxation time are also estimated.

Nonequilibrium superconductivity under spin-polarized quasiparticle currents in perovskite ferromagnet-insulator-superconductor heterostructures

We report phenomena manifesting nonequilibrium superconductivity induced by spin-polarized quasiparticles in perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. Measurements of the critical current ${J}_{c},$ using a pulsed current technique to minimize Joule heating, reveal a monotonic increase with increasing insulator thickness; and for F-I-S with thin insulating barriers, a slight increase in ${J}_{c}$ is observed under small injection currents ${I}_{m}$ from the ferromagnet, followed by a strong suppression of ${J}_{c}$ under large ${I}_{m}.$ In contrast, no effect of injection on ${J}_{c}$ can be detected in the N-I-S control sample (N: nonmagnetic metal). The spin diffusion time and length in cuprate superconductors are also estimated and compared with our experimental results.