The T=0 RFIM on a Bethe lattice: correlation functions along the   hysteresis loop
  We consider the Gaussian random field Ising model (RFIM) on the Bethe lattice at zero temperature in the presence of a uniform external field and derive the exact expressions of the two-point spin-spin and spin-random field correlation functions along the saturation hysteresis loop. To complete the analytical description and suggest possible approximations for the RFIM on Euclidian lattices we also compute the corresponding direct correlation functions (or proper vertices) and show that they decay rapidly with the distance in the weak-coupling/large disorder regime; their range, however, is not limited to the nearest-neighbor distance. 
Xavier Illa and M.L. Rosinberg
cond-mat.dis-nn