Evolution of a demagnetized spin state toward equilibrium

The temporal evolution of the nonequilibrium nearly homogeneous ferromagnetic state is analyzed within the general phenomenological spin-dynamics theory proposed by Baryakhtar, which employs the concepts of both local (relativistic) and nonlocal (exchange) spin relaxation. We demonstrate that strongly inhomogeneous spin dynamics can emerge on nanometer length scales. The mechanism responsible for this inhomogeneous dynamics is related to instabilities of the demagnetized spin state. These instabilities qualitatively modify the relaxation process toward equilibrium and may result in a dramatic underestimation of the characteristic time and amplitude of the spin dynamics. This finding can be exploited for the development of a new type of source for coherent spin-wave generation.