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

Abstract

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(T) of a Co2CrAl-I-Pb F-I-S tunnel junction is studied experimentally. A theoretical model is proposed which provides an adequate description of the temperature behavior of the normalized conductivity σFS(T) with features of spin-polarized tunnelling taken into account.

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

“…1 it is seen that CVC of the junctions Co 2 CrAl-I-Pb and Sn-I-Pb are significantly different: in particular, the CVC of the F-I-S-junction are below those of the N-I-S-junction, the differential conductivity of the F-I-S-junction G FS < G NS at voltages V ( D/e (D % 1.34 meV is the energy gap of the superconductor Pb). 3,10 A variation of the differential conductance G NS and G FS3,10,11 can occur due to deviation of the electron density of elementary excitations in a superconductor, N S (E), in a near-barrier contact area from the density of states, according to the BCS theory N T (E k ), and due to change of the Fermi-Dirac distribution function f(E k ) of elementary excitations (see Eq. (1)).…”