Field-induced magnetic reorientation in two-dimensional anisotropic quantum ferromagnet

Field-induced magnetic reorientation of the two-dimensional (2D) Heisenberg model with arbitrary spin is treated by the many-body Green's function theory. The orientation of the magnetization is determined by the expectation values less than S-alpha greater than (alpha=x,z) of the spin components of different spins from which the orientation angle 0 is obtained as a function of the temperature and of the transverse magnetic field for various parameters of the exchange anisotrophy in the (x,y)-plane. Results obtained within the Green's function theory are compared with the mean-field theory.