Heisenberg model in a random field: phase diagram and tricritical   behavior
  By using the differential operator technique and the effective field theory scheme we study the tricritical behavior of Heisenberg classical model of spin-1/2 in a random field. The phase diagram in the T-h plane on a square and simple cubic lattice for a cluster with two spins is obtained when the random field is bimodal distributed. 
Douglas F. de Albuquerque and A. S. de Arruda