Half-Integer Shapiro Steps at the 0- Crossover of a Ferromagnetic Josephson Junction

257005-1 We investigate the current-phase relation of S=F=S junctions near the crossover between the 0 and the ground states. We use Nb=CuNi=Nb junctions where this crossover is driven both by thickness and temperature. For a certain thickness a nonzero minimum of critical current is observed at the crossover temperature. We analyze this residual supercurrent by applying a high frequency excitation and observe the formation of half-integer Shapiro steps. We attribute these fractional steps to a doubling of the Josephson frequency due to a sin 2 current-phase relation. This phase dependence is explained by the splitting of the energy levels in the ferromagnetic exchange field.