Brownian Motion of Spins Revisited

A phenomenological theory to describe the spin relaxation is presented. We consider one classical spin surrounded by a medium which serves as a heat bath. A Langevin equation and associated Fokker-Planck equation for the spin and the fluctuating magnetic field are derived. They guarantee the thermal equilibrium state as stationary solutions. In the narrowing and high temperature limits, the equation for the spin reduces to the well-known Landau-Lifshits equation and Bloch equation.