the $xy$–peak is shifted to lower temperatures when the axial field strength is increased.

Figure 7 shows the influence of the single-ion anisotropy $D$ on the specific heat for $J_{xz}=0$. Decreasing $D$, the $T_c$ peak and the anomaly are moved to lower temperatures, whereas $T_{xy}$ is slightly increased. A similar behavior is found for $J_{xz} \neq 0$, as shown in Figure 8 for $J_{xz}= 16.2 J'$ and $D = -8.1 J'$. For this choice of the parameters, the noncritical spin fluctuations appear at lower temperatures than the disordering of the planar spin components. The resulting specific heat has a peculiar shape consisting of a broad shoulder, the anomaly, and a superposed peak, the $T_{xy}$ peak. This shape is reminiscent of the magnetic specific heat of FeBr$_2$ and will be discussed in more details below.

Applying an additional planar field component, $H_x$, leads to a spin–flop phase in the $xy$–components at $T = 0$: the $y$–components of the layer magnetization have opposite signs in the different sublattices but the same absolute value, whereas the $x$–components are the same in every layer. As the axial field tends to stabilize the odd or “+” layers and to destabilize the even or “$-$” layers, 