Combining superconductors and ferromagnets : from p-junctions to spin-triplets
Huygens - Kamerlingh Onnes Laboratory, Leiden University
The study of superconductor (S) / ferromagnet (F) hybrids is a by now a well-established field. One of the early interests was formed by the realization that a thin F layer in an SFS structure could lead to a phase change of π between the condensates in the two S contacts. Such π-junctions are still investigated for use in devices such as superconducting memory elements. More recently it became clear that also spin triplet pair correlations can be induced in ferromagnets and lead to long-range supercurrents. These triplets are the centrepiece of the newly emerging field of superconducting spintronics. They are generated by engineering magnetic inhomogeneities at the S/F interface, for instance by using a sandwich of different layers, in order to mix the two spin channels and rotate the quantization axis. Using ferromagnets also allows control over the supercurrents, and two examples will be discussed. One concerns CrO2, which is a fully spin-polarized ferromagnet, in which it is possible to induce supercurrents with very high current density (above 109 A/m2) over distances approaching a micron. The other example demonstrates a device which also allows control over the supercurrent distribution based on a disk-shaped Josephson junction with a Co layer containing a magnetic vortex. With a magnetic field we can change the position of the vortex, and thereby the supercurrent pathways. This device exemplifies a novel platform where adaptable supercurrent paths can be customized for a given application.
Professor Jan Aarts studied physics at the University of Amsterdam, specializing in Condensed Matter Physics. He received PhD from the same university on a thesis with the title ‘Unstable magnetic moments in Ce compounds’, in the field of what these days is called ‘Correlated electron systems’.Next he moved to the Philips Research Laboratories in Eindhoven where he worked on growth of semiconductors with molecular beam epitaxy, and characterization by photoemission spectroscopy.
Professor Jan Aarts came to Leiden in 1987 and started to work on metallic and superconducting multilayers and on high-Tc materials, with emphasis on superconducting vortex dynamics. This work led to research on superconductor-ferromagnetic hybrids on the one hand, and to investigating Colossal Magnetoresistance effects in perovskite manganites on the other. The current research lines in his group are focusing on inducing supercurrents in ferromagnets, and on the properties of electron gases at the interface of non-conducting oxides. A new line is to fabricate mesoscopic structures of the chiral p-wave superconductor Sr2RuO4using a Focused Ion Beam.
Apart from research, Professor Jan Aarts was Teaching Director of the Physics Department between 2008 and 2014, he is a member of the Leiden Teachers Academy, and since May 2018 he is Scientific Director of the Leiden Institute of Physics (LION).