  We have investigated the classical phase dynamics of underdamped
ferromagnetic Josephson junctions by measuring the switching probability in
both the stationary and nonstationary regimes down to 350 mK. We found the
escape temperature to be the bath temperature, with no evidence of additional
spin noise. In the nonstationary regime, we have performed a pump-probe
experiment on the Josephson phase by increasing the frequency of the junction
current bias. We show that an incomplete energy relaxation leads to dynamical
phase bifurcation. Bifurcation manifests itself as premature switching,
resulting in a bimodal switching distribution. We directly measure the phase
relaxation time by following the evolution of the bimodal switching
distribution when varying the bias frequency. Numerical simulations account for
the experimental values of the phase relaxation time.
