The anisotropic Heisenberg ferromagnet in a magnetic field

The one- and two-dimensional spin S=12 anisotropic Heisenberg ferromagnet model is studied by the double-time Green's function method within the random phase approximation. The magnetization, susceptibility and transverse correlation functions are found to be dependent on the anisotropy. Our result shows that the position and height of maximum of the susceptibility yields the power-law behaviors with respect to the magnetic field. It does not support the 23 power law which is obtained from the mean-field Landau's theory. For the isotropic case, our results are in agreement with the results obtained by other theoretic techniques. For S=12 ferromagnetic chain system (C6H11NH3)CuBr3, the crossover of the anisotropic system to XY-kind behavior is expected at low temperature T≈5.5K, which agrees with experiment results and theoretical calculations.