In this work, we report a spin triplet supercurrent through a strong but half-metallic ferromagnet CrO2, as schematically shown in Fig. 1(a). The supercurrent can be switched with the direction of the magnetization, in analogy to spin valve transistors, and therefore could enable Josephson junctions controlled by magnetization.

In our experiment we patterned a sample (Fig. 1b) in which two s-wave superconductors (NbTiN) are coupled by the half-metallic ferromagnet CrO2, a material previously shown by this MRSEC to have 100% spin polarization. Using e-beam lithography, two "T"-shaped NbTiN electrodes with a separation of 0.3-1 µm are patterned on top of the CrO2 (Fig.1c). On cooling the sample to temperatures between 1 and 10K, we find that the current between the two electrodes, which can only pass through the ferromagnetic CrO2 film, is a supercurrent. Remarkably, we have observed a supercurrent through a half-metallic ferromagnet over a distance of 300 nm.

Fig. 1: (a), Simplified spin dependent density-of-states (DOS) of CrO2 with a gap for the spin-down DOS, hence no spin-flip scattering in the transport. (b) Schematic illustration and (c) scanning electron micrograph of the final device