Key points are not available for this paper at this time.
The reorientation process in a magnetic field in two-dimensional isotropic and XY quantum Heisenberg antiferromagnets is shown to occur through the intermediate phase with unbroken continuous symmetry and constant magnetization equal to one third of the saturation value. The same reorientation process is also found in the more complicated classical models.
Chubukov et al. (Mon,) studied this question.