Dynamic equations for individual spins in materials with magnetic order: The ideal ferromagnet

The time and temperature dependence of individual spins is studied in an id eal ferromagnet. Starting from the Heisenberg Hamiltonian in a magnetic fie ld and, building on linear-response theory, we derive the linearized micros copic analogy to the macroscopic Landau-Lifshitz equations of motion for ma gnetization. The dynamical equations take into account the influence of spi n correlations in addition to the molecular field. In order to illustrate t he validity of our approach, we calculate the temperature-dependent spin-wa ve spectrum and Landau damping by employing a finite-temperature perturbati on theory for spin operators. The latter is adapted to suit higher-order sp in-correlation functions which can be described by simple quasiparticle int eractions with the spin system. Our results agree with those established on the thermodynamics of the Heisenberg model. In addition, we provide a meth od of how to deduce macroscopic equations of the Landau-Lifshitz type from first principles, including temperature dependence and damping. [S0163-1829 (99)06141-X].