E. M. Rudenko

Inelastic electron tunneling across magnetically active interfaces in cuprate and manganite heterostructures modified by electromigration processes

We report a study of the electron tunneling transport in point-contact junctions formed by a sharp Ag tip and two different highly correlated oxides, namely, a magnetoresistive manganite La0.66Ca0.34MnO3 and a superconducting cuprate LaBa2Cu3O7−x. Strong chemical modifications of the oxide surface (supposedly, oxygen ion displacements) caused by applying high voltages to the junctions have been observed. This effect is believed to be responsible for an enormous growth of inelastic tunneling processes across a transition region that reveals itself in an overall V-shaped conductance background, with a strong temperature impact. The mechanism of the inelastic scattering is ascribed to charge transmission across magnetically active interfaces between two electrodes forming the junction. To support the latter statement, we have fabricated planar junctions between Cr and Ag films with an antiferromagnetic chromium oxide Cr2O3 as a potential barrier and at high-bias voltages have found an identical conductance t...

Spin injection and giant tunnel-current blocking in ferromagnet-superconductor heterostructures

Giant growth of the differential resistance has been observed in a tunnel junction consisting of superconducting lead with Heusler’s ferromagnetic alloy Co2CrAl for low bias voltages. This effect is attributed to the appearance of a nonequilibrium state in the lead film as a result of spin injection into the superconductor.

Tunnel spin injection and the conductivity of ferromagnet-superconductor heterostructures with zero bias

The giant spin blocking of tunnel currents discovered by us (Low. Temp. Phys. 36, 186 (2010)) is investigated theoretically and experimentally in ferromagnet F (Co2CrAl)- insulator I- superconductor S (Pb) heterostructures with a wide range of specific resistances (10−7–10−4 Ω · cm2). The magnitude of this effect is found to depend on the specific resistance of the junction in the normal state and on recombination spin depolarization. A theoretical model which provides an adequate description of the tunnelling of spin-polarized electrons in F-I-S junctions is proposed. It is found that the normalized conductivity σFS of an F-I-S tunnel junction can be considerably lower than the fundamental normalized conductivity σNS of an N-I-S junction (where N is a normal metal). The proposed model is used to estimate the degree of spin polarization p of films of the ferromagnetic semimetal Co2CrAl (Heusler alloy) with B2- and L21-type crystal structures, which is close to 1 (p ≈ 0.97). The temperature dependence σFS(...

Current-voltage characteristics of the double tunnel junctions Co2CrAl–I–Pb/Sn–I–Pb under spin-polarized tunnel injection

Current-voltage characteristics and existence features of a superconducting state under the action of tunneling injection of the spin-polarized current Iinj with a high spin polarization degree (p ≈ 0.97) were studied in double tunneling junctions ferromagnet-insulator-superconductor-insulator-superconductor Co2CrAl–I–Pb/Sn–I–Pb. An empirical formula was proposed for describing a dependence of the energy gap Δ on Iinj. A new spin-polarized inhomogeneous superconducting state consisting of superconducting and resistive regions was found. An existence of the proximity effect through a tunnel barrier between ferromagnetic and superconducting films was established. An effective recombination time of spin-polarized quasiparticles in the nonequilibrium superconductor Pb was estimated.

Tunnel injection of spin-polarized current in Co2CrxFe1−xAl (x = 1, 0.6) insulator-superconductor heterostructures

F-I-S tunnel junctions of Co2CrAl-I-Pb and Co2Cr0.6Fe0.4Al-I-Pb based on films of semimetal ferromagnetic Heusler alloys Co2CrAl (Curie temperature TC ≈ 334 K) have been fabricated and the features of spin-polarized current in them studied. A theoretical model of spin blocking of the tunnel current shows that the degree of spin polarization of the conduction electrons in quasi-single crystal films of Co2Cr0.6Fe0.4Al and Co2CrAl with B2- and L21-type order is 0.91–0.97.

Current-voltage characteristics of low-resistance tunnel structures ferromagnet-insulator-superconductor Co2CrAl–I–Pb

A physical model of tunneling processes leading to a nonequilibrium superconducting state in ferromagnet-superconductor junctions is proposed and confirmed experimentally. The model is based on the analysis of experimental current-voltage characteristics and the determination of junction voltage dependences of the energy gap, the effective chemical potential and the effective temperature of quasiparticles in the modified energy distribution of quasiparticles as well as on taking into account the change in the density of electron states due to destruction of the Cooper pairs, caused by the proximity effect.

Proximity phenomena in double-barrier structure NbZr/NbOx/Al/AlOy/NbZr

A tunneling structure NbZr/NbOx/Al/AlOy/NbZr with a thin barrier in the NbZr/NbOx/Al junction and 4 to 6-nm-thick Al interlayer was prepared and studied experimentally. A proximity effect between NbZr and Al through NbOx barrier has been observed. An electrical voltage was generated in the NbOx barrier and a coexistence of the proximity effect and applied voltage in the junction NbZr/NbOx/Al has been observed. This experiment could be described on the basis of a model for coherent charge transport in superconducting/normal proximity structures.

Mutual conversion of singlet and triplet ordering in singlet superconductor junctions with ferromagnetic nickel

A study of the density of quasiparticle states of a lead film, which is a conventional superconductor with spin-singlet electron pairing, as a function of the nanoscale ferromagnetic nickel layer thickness that is in direct contact with the lead (inverse proximity effect). It is found that the penetration depth of superconducting correlations into the ferromagnetic nickel is of the same order of magnitude as in contacts involving lead and a normal metal. This behavior can be explained by the appearance of an inhomogeneous exchange field at the interface, which leads to the effective conversion of spin-singlet (rapidly decaying in a ferromagnet) Cooper pairs into spin-triplet pairs, which are stable with respect to exchange interaction.

Josephson properties of transparent tunnel junctions

With allowance for possible deviations of the current–phase relation from the usual sinusoidal dependence, a modified Ferrell–Prange equation is used to examine the steady-state electrodynamics of transparent Josephson junctions in an external magnetic field. It is shown that in this case the penetration of a weak magnetic field into a transparent Josephson junction is generally nonexponential, and the field Hc1 at which vortices arise in transparent junctions and the dependence of the critical current on the magnetic field in them are calculated.