Combining superconductors (S) and ferromagnets (F) offers the opportunity to create a new class of superconducting spintronic devices. In particular, the S=F interface can be specifically engineered to convert singlet Cooper pairs to spin-polarized triplet Cooper pairs. The efficiency of this process can be studied using a so-called triplet spin valve (TSV), which is composed of two F layers and a S layer. When the magnetizations in the two F layers are not collinear, singlet pairs are drained from the S layer, and triplet generation is signaled by a decrease of the critical temperature T c . Here, we build highly efficient TSVs using a 100% spin-polarized half-metallic ferromagnet, CrO 2 . The application of out-of-plane magnetic fields results in an extremely strong suppression of T c , by well over a Kelvin. The observed effect is an order of magnitude larger than previous studies on TSVs with standard ferromagnets. Furthermore, we clearly demonstrate that this triplet proximity effect is strongly dependent on the transparency and spin activity of the interface. Our results are particularly important in view of the growing interest in generating long-range triplet supercurrents for dissipationless spintronics.

Superconductor-ferromagnet (S-F) hybrids based on half-metallic ferromagnets, such as CrO 2 , are ideal candidates for superconducting spintronic applications. This is primarily due to the fully spin-polarized nature of CrO 2 , which produces enhanced long-range triplet proximity effects. However, reliable production of CrO 2 -based Josephson junctions (JJs) has proved to be extremely challenging because of a poorly controlled interface transparency and an incomplete knowledge of the local magnetization of the CrO 2 films. To address these issues, we use a bottom-up approach to grow CrO 2 nanowires on prepatterned substrates via chemical-vapor deposition. A comprehensive study of the growth mechanism enables us to reliably synthesize faceted, homogeneous CrO 2 wires with a well-defined magnetization state. Combining these high-quality wires with a superconductor produces JJs with a high interface transparency, leading to exceptionally large 100% spin-polarized supercurrents, with critical current densities exceeding 10 9 Am −2 over distances as long as 600 nm. These CrO 2 -nanowire-based JJs thus provide a realistic route to creating a scalable device platform for dissipation-less spintronics.

Spin-triplet Cooper pairs induced in ferromagnets form the centrepiece of the emerging field of superconducting spintronics. Usually the focus is on the spin-polarization of the triplets, potentially enabling low-dissipation magnetization switching. However, the magnetic texture which provides the fundamental mechanism for generating triplets also permits control over the spatial distribution of supercurrent. Here we demonstrate the tailoring of distinct supercurrent pathways in the ferromagnetic barrier of a Josephson junction. We combine micromagnetic simulations with three-dimensional supercurrent calculations to design a disk-shaped structure with a ferromagnetic vortex which induces two transport channels across the junction. By using superconducting quantum interferometry, we show the existence of two channels. Moreover, we show how the supercurrent can be controlled by moving the vortex with a magnetic field. This approach paves the way for supercurrent paths to be dynamically reconfigured in order to switch between different functionalities in the same device.