Quantum spin model of a cubic ferromagnet in a magnetic field

The zero temperature phase diagram of a one-dimensional ferromagnet with cu bic single ion anisotropy in an external magnetic field is studied. The mea n-field approximation and the density-matrix renormalization group method a re applied. Two phases at finite magnetic fields are identified: a canted p hase with spontaneously broken symmetry and a phase with magnetization alon g the magnetic field. Both methods predict that the canted phase exists eve n for the single-ion anisotropy strong enough to destroy the magnetic order at zero magnetic field. In contrast to the mean-field theory, the density- matrix renormalization group predicts a reentrant behavior for the model. T he character of the phase transition at finite magnetic field has also been considered and the critical index beta (c) = 1/8 has been found.