MODIFICATION OF THE LANDAU-LIFSHITZ EQUATION IN THE PRESENCE OF A SPIN-POLARIZED CURRENT IN COLOSSAL-MAGNETORESISTIVE AND GIANT-MAGNETORESISTIVE MATERIALS

We derive a continuum equation for the magnetization of a conducting f erromagnet in the presence of a spin-polarized current. Current effect s enter in the form of a topological term in the Landau-Lifshitz equat ion. In the stationary situation the problem maps onto the motion of a classical charged particle in the field of a magnetic monopole. The s patial dependence of the magnetization is calculated for a one-dimensi onal geometry and suggestions for experimental observation are made. W e also consider time-dependent solutions and predict a spin-wave insta bility for large currents.