  We investigate Josephson coupling through a ferromagnetic thin film using
Superconductor-Insulator-Ferromagnet-Superconductor planar junctions. Damped
oscillations of the critical current are observed as a function of the
ferromagnetic layer thickness. We show that they result from the exchange
energy gained or lost by a quasiparticle Andreev-reflected at the
ferromagnet-superconductor interface. The critical current cancels out at the
transition from positive ("0") to negative ("$\pi$") coupling, in agreement
with theoretical calculations.
