Colossal Proximity Effect in a Superconducting Triplet Spin Valve Based on the Half-Metallic Ferromagnet CrO2

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, CrO2. 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.