Measurement of the current-phase relation of superconductor/ferromagnet/superconductor<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>π</mml:mi></mml:math>Josephson junctions

Abstract

We present measurements of the current-phase relation (CPR) of superconductor-ferromagnet-superconductor (SFS) Josephson junctions as a function of temperature. The CPR is determined by incorporating the junction into a superconducting loop coupled to a dc SQUID, allowing measurement of the junction phase difference. We find that the critical current of $\mathrm{Nb}\text{\ensuremath{-}}{\mathrm{Cu}}_{0.47}{\mathrm{Ni}}_{0.53}\text{\ensuremath{-}}\mathrm{Nb}$ Josephson junctions with barrier thickness $\ensuremath{\sim}22\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ changes sign at $T&lt;{T}_{\ensuremath{\pi}}\ensuremath{\sim}2--4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, indicating that the junction becomes a $\ensuremath{\pi}$ Josephson junction. We find no evidence for second-order Josephson tunneling near ${T}_{\ensuremath{\pi}}$ in the CPR predicted by several theories.