Triplet current Josephson junctions of ferromagnetic halfmetallic (La,Sr)MnO₃ and low-T_c NbTi

Superconducting junctions with a ferromagnet as weak link, where the supercurrent is carried by triplet correlations, have been much studied in the last decade. Mostly, such junctions consist of simple metals or alloys such as Nb and Co. Generating a long range triplet current requires some form of inhomogeneous magnetization at, or close to, the ferromagnet / superconductor interface, which is brought about by using a second and different ferromagnet in the stack of layers. Junctions made with oxide ferromagnetic metals such as perovskite manganites have been much less studied, and those only in all-oxide combination with perovskite superconductors, in particular YBa₂Cu₃O₇. Here we demonstrate clear supercurrents in lateral Josephson junctions consisting of ferromagnetic and fully spin-polarized La_0.7Sr_0.3MnO₃ (LSMO), superconducting NbTi, and a trench made by Focused Ion Beam milling. The trench width, and therefore the junction length, was of the order 20 nm, larger than what was studied in all-oxide systems. When applying an out-of-plane magnetic field, bar shaped junctions show a Fraunhofer-type interference pattern, while disk-shaped junctions show stronger currents along the edge of the disk. We find Josephson couplings (I_cR_N products) around 50 μV, which is large compared to all-metal junctions. The half metal only allows a triplet current, and thereofore it is surprising that no extra ferromagnet is needed to generate the supercurrent. We surmise that sufficient magnetic inhomogeneity exists in the the LSMO layer close to the interface with NbTi to work as triplet generator. Combinations of oxide magnets with alloy superconductors appear a promising new route to realizing superconducting spintronics.