Bloch-Redfield theory of high-temperature magnetic fluctuations in   interacting spin systems
  We study magnetic fluctuations in a system of interacting spins on a lattice at high temperatures and in the presence of a spatially varying magnetic field. Starting from a microscopic Hamiltonian we derive effective equations of motion for the spins and solve these equations self-consistently. We find that the spin fluctuations can be described by an effective diffusion equation with a diffusion coefficient which strongly depends on the ratio of the magnetic field gradient to the strength of spin-spin interactions. We also extend our studies to account for external noise and find that the relaxation times and the diffusion coefficient are mutually dependent. 
Andrew Sykes, Dmitry Solenov, and Dmitry Mozyrsky
cond-mat.stat-mech cond-mat.str-el