Superconducting exchange coupling between ferromagnets

This paper shows for the first time that it is possible to use superconductivity to directly control the magnetic state of a device. The figure shows the resistance (a) and magnetisation (b) versus magnetic field for a GdN/Nb/GdN superconducting spin valve. In both figures it can be seen that the two GdN ferromagnetic insulator layers switch at different magnetic fields and that between 1.5 and 2 K the resistance shows a complete switch from resistive to superconducting states. The key element of the results is that the upper switching field (at around 15 mT) increases rapidly as the temperature is decreased demonstrating that the superconductivity increases the stability of the antiparallel state. This superconducting exchange coupling had been predicted in the 1960s but never previously observed.