Two-dimensional Heisenberg antiferromagnet in a transverse field

We investigate the magnetic reorientation in a two-dimensional anisotropic antiferromagnet due to a transverse magnetic field. Using a many-body Green's function approach, we show that the magnetization component perpendicular to the applied field (and along the easy-axis of the anti- ferromagnet) initially increases with increasing field strength. We show that this unexpected result arises from the suppression of quantum and thermal fluctuations in the antiferromagnet. Above the Neel temperature, this effect leads to a reappearance of a magnetic moment along the easy-axis. In this communication we study the properties of a 2D anisotropic AFM with spin S = 1/2 on a square lat- tice in a transverse magnetic field perpendicular to the easy axis of the anisotropy. To this end, we develop a many-body Green's function method 16 that is based on the equation-of-motion formalism. Our results are two- fold. First, the staggered AFM magnetization along the easy axis increases with increasing strength of the trans- verse magnetic field. This effect is quite unexpected since the field is directed perpendicular to the magnetization component. We find, however, that the presence of a weak transverse field leads to the suppression of quan- tum and thermal fluctuations in the AFM, the former being responsible for the decrease of the zero tempera- ture sublattice magnetization from its saturation value, S = 1/2. While a similar behavior has been predicted