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 σ^FS of an F-I-S tunnel junction can be considerably lower than the fundamental normalized conductivity σ^NS 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 σ^FS(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 σ^FS(T) with features of spin-polarized tunnelling taken into account.