  We examine the evolution of ion-beam Weibel instability at strong
collisionless shocks in weakly magnetized media. We find that a finite
background magnetic field substantially affects both linear and nonlinear
phases of the instability, depending on whether the background electrons behave
magnetized or not. Particle-in-cell simulations for magnetized electrons
identify a dynamo-like mechanism of magnetic field amplification, which
eventually leads to spontaneous magnetic reconnection. We conclude that this
scenario is applicable to typical young supernova remnant shocks.
