Sin(2φ) component in the current-phase relation of SFS Josephson junctions near the 0-π transition

We directly determined the Josephson current-phase relation (CPR) of superconductor-ferromagnet-superconductor (SFS) junctions by rf-SQUID interferometry, and corroborated it with measurements of the critical current as a function of temperature and magnetic field and rf-induced Shapiro steps in the current-voltage characteristics. Our Nb-Cu₄₇Ni₅₃-Nb trilayer junctions, with 2x2μm^2 area and 7nm CuNi thickness, show a transition with temperature from the usual Josephson 0-junction state to a π-junction state, defined by a phase difference of π in the ground state, at temperatures between 1.5K and 3.5K. Near the transition, we observe second harmonics in the CPR, deviations from the usual Fraunhofer diffraction pattern and half-integer Shapiro steps, all consistent with a sin(2φ) component in the CPR.