Growth of half-metallic CrO2 nanostructures for superconducting spintronic applications

Superconductor-ferromagnet (S-F) hybrids based on half-metallic ferromagnets, such as CrO2, are excellent candidates for superconducting spintronic applications. This is primarily due to their fully spin polarized nature, which produces significantly enhanced long-range triplet proximity effects.. However, reliable production of CrO2-based Josephson junctions (JJs) is challenging, mainly because of the difficulty to control the transparency of the S/F interface. We have grown CrO2 nanowires by chemical vapor deposition on TiO2 substrates combined with selective area growth in trenches defined with amorphous SiOx. This allows us to create lateral JJs, with the nanowire as the weak link. We show that the nature of the growth is highly anisotropic, and that the morphology of the CrO2 nanostructructures changes systematically during the growth process, depending on the width of the trench. The detailed growth study enables us to synthesise multifaceted and highly homogeneous CrO2 wires. These are utilized to fabricate JJs with high S/F interface transparency, leading to large supercurrents. The well-defined geometry of the device allows us to reliably estimate an exceptionally high critical current density Jc = 10^9~Am^{-2} over a distance of 600~nm.