Spin-filter Josephson junctions

Abstract

Josephson junctions with ferromagnetic barriers have been intensively investigated in recent years1. Of particular interest has been the realization of so called π-junctions with a built-in phase difference2, and induced triplet pairing3,4. Such experiments have so far been limited to systems containing metallic ferromagnets. Although junctions incorporating a ferromagnetic insulator (IF) have been predicted to show a range of unique properties including π-shifts with intrinsically low dissipation5,6 and an unconventional temperature dependence7 of the critical current Ic, difficulties with the few known IF materials have prevented experimental tests. Here we report supercurrents through magnetic GdN barriers and show that the field and temperature dependence of Icis strongly modified by the IF. In particular we show that the strong suppression of Cooper pair tunnelling by the spin filtering of the IF barrier can be modified by magnetic inhomogeneity in the barrier.