Dynamics of ordering of isotropic magnets

We study the dynamics of ordering of the nonconserved and conserved Heisenberg magnet. The dynamics consists of two parts — an irreversible dissipation into a heat bath and a reversible precession induced by a torque due to the local molecular field. For quenches both to T=0 and Tc, we show that the torque is irrelevant when the dynamics is nonconserved but relevant when the dynamics is conserved and is governed by a new nontrivial fixed point.