Spin accumulation in ferromagnets
  Using a density matrix formulation for the effective action, we obtain a set of macroscopic equations that describe spin accumulation in a non-homogeneous ferromagnet. We give a new expression for the spin current which extends previous work by taking into account the symmetry of the ferromagnetic state through a careful treatment of the exchange term between the conduction electrons and the magnetization, i.e, d-electrons. We consider a simple application which has been discussed previously and show that in this case spin accumulation is an interface effect confirming earlier results arrived at by different methods. 
W.N.G. Hitchon, R.W. Chantrell, A. Rebei