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 antiferromagnet) 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 N\'eel temperature, this effect leads to a reappearance of a magnetic moment along the easy-axis. 
P.J. Jensen, K.H. Bennemann, D.K. Morr, H. Dreysse