Theory of fast time evolution of nonequilibrium spin states in magnetic heterostructures

The temporal evolution of highly nonequilibrium and spatially nonuniform spin states is analyzed on the basis of a general, phenomenological spin dynamics theory, employing the concept of both local (relativistic) and nonlocal (exchange) spin relaxations. The developed theory is applied here to describe the ultrafast spin evolution arising after the action of a femtosecond laser pulse on magnetic heterostructures containing layers of two different ferromagnets; specifically, we consider here both Ni-Fe and Ni-Ru-Fe heterostructures. As a consequence of the laser excitation, nonuniform spin distributions are created in the layered systems, which form the initial state for the spin dynamics calculations. The results obtained provide an explanation of recent experiments on the magnetization recovery in laser-pumped Ni-Ru-Fe heterostructures. The importance of the nonlocal character of the magnetization recovery for such systems is established. In particular, the experimentally observed strong dependence of the spin recovery on the relative orientation of the magnetic moments of the layers is explained theoretically. If the nonlocal spin relaxation is dominating, the evolution from an initial nonuniform magnetization profile is concurrent with the creation of a strong spin current flowing between the layers.