Spin triplet supercurrents in thin films of ferromagnetic CrO2

Abstract

Superconducting correlations can penetrate in a ferromagnet via proximity effect but over a very short length of the order of a few nanometers because ferromagnetic exchange field tries to align the antiparallel spins of electrons in a Cooper pair. Theoretically, it was suggested that rather a long-range proximity effect can occur if spin-triplet (with parallel spins) Cooper pairs are generated at the interface that requires a magnetic inhomogeneity (spin-active interface). Experimentally, it was first observed at the Delft University using CrO2 (a half metallic ferromagnet). In these devices the induced supercurrent was not both well controlled and well understood. We have provided new evidence that supercurrents can flow through CrO2 films, deposited on sapphire substrates, over a length of 1_m. The analysis shows that these films exhibit six-fold magnetic-anisotropy that might provide the required spin-active interface. Still, low reproducibility suggests a weak control over the spin-active interface. Such a control appears better with an artificially created spin-active interface, using an additional ferromagnetic layer of Ni (2nm) with CrO2. From investigations, it suggests that there are other scattering phenomena (different spin dependent magnetic scattering from different magnetic layers) or magnetic moment disorder at the interface involved to generate the odd-frequency spin-triplet supercurrent.