Tunnel effect in ferromagnetic half-metal Co$_2$CrAl-superconductor heterostructures

Ferromagnetic half-metal Co$_2$CrAl films and tunnel contacts Co$_2$CrAl - insulator (I) - Pb are fabricated and investigated. It is found that the normalized differential conductivity $\sigma ^{\rm FS}$ of such tunnel junctions with low resistance is larger than the normalized differential conductivity $\sigma ^{\rm NS}$ of known normal metal - I - superconductor type tunnel junctions. It is shown that the observed increase in $\sigma ^{\rm FS}$ is caused by the accumulation of spin polarized electrons in a superconductor and can be used for estimating the spin polarization degree $P$ in ferromagnets. This method shows that $P$ of L2$_1$-type ordered Co$_2$CrAl Heusler alloy films at $T = 4.2{\rm K}$ is close to 1.