Supercurrents in ferromagnetic CrO2 nanowires
Jan Aarts
Huygens - Kamerlingh Onnes Laboratory,
Leiden Institute of Physics, Leiden University
After a brief introduction to the Leiden Institute of Physics and its research themes, I will give an overview of recent developments in the field of superconductor / ferromagnet hybrids. In particular, after earlier work on the phenomenon of so-called π-junctions (Josephson junctions which shift the phase of the superconducting order parameter by π), there is now much interest in long-range proximity effects in ferromagnets as a result of odd-frequency triplet Cooper pairs. Triplet supercurrents can be made to flow through ferromagnets over lengths which are similar to those in normal metals. Moreover, the supercurrent in the ferromagnet is supposed to be spin-polarized, which could lead to a number of novel applications in the domain of spintronics (‘superspintronics‘). The mechanism to generate such triplets is to engineer a spin-active interface, for instance by inserting an F*/N sandwich (N a normal metal and F* a different ferromagnet) between the S- and the F-layer.
Of particular interest in this respect are halfmetallic ferromagnets such as CrO2. In our recent work, both on Josephson junctions [1], and on pseudo-spinvalves [2] we showed that the effects of triplets induced in CrO2 are significantly larger than those in conventional ferromagnets such as Co, presumably due to the strong reduction of spin flip scattering. Very recently we succeeded in growing nanowires of CrO2 in which supercurrents of the order of 109 A/m2 were made to flow over a distance of 600 nm. Magnetic domains can also be induced in such wires, and the wires can be used to generate spin currents, making them a very promising platform for superspintronics.
[1] M. S. Anwar, M. Veldhorst, A. Brinkman and J. Aarts, ‘Long-range supercurrents in ferromagnetic CrO2 using a multilayer contact structure‘, Appl. Phys. Lett. 100, 052602 (2012).
[2] A. Singh, S. Voltan, K. Lahabi and J. Aarts, ‘Colossal proximity effect in a superconducting triplet spin valve based on the half-metallic ferromagnet CrO2‘, PR X 5, 021019 (2015).
