shown in Figure 10. Indeed, the shoulder, resulting from the noncritical spin fluctuations, changes into a maximum when $H_z$ is increased, whereas the $xy$–peak becomes sharper, the anomaly in the axial spin components and the disordering of the planar spin components both moving to lower temperatures. Note, however, that in the present model the disordering of the $xy$–components also takes place for small fields, see above.

The magnetization data resulting from the Hamiltonian (1) also compare favorable to the experiments. For example, without an applied planar field, the transverse spin–ordering at $T_{xy}$ does not lead to a discontinuity of the axial magnetization (if $J_{xy}$ is not too large, see above), in accordance with the magnetometric measurements[@dea95], showing no jump in the axial magnetization despite a sharp peak in the specific heat. An additional planar field component yields a low-temperature spin–flop phase in the $xy$–components, which is compatible with the experiment.[@pet98] The change in the $y$-component in the magnetization per layer may then occur through a continuous or a discontinuous phase transition, depending on the values of the parameters. Experimentally, one observes, in a tilted field, a ’weakly first–order’ transition,[@pet98] thus being supposedly at the border between these two scenarios.

One 