correspond to a sharp phase transition and does not show a significant size dependency, in contrast to the critical peak at higher temperatures.[@ple97] Both peaks are present when computing the specific heat of Ising metamagnets and involve solely $S^z$. The third peak, again, results from the disordering of the planar spin components, see Figure 5b.

The presence of the nondiagonal spin exchange moves both $T_c$ and the anomaly to lower temperatures, see Figure 6a, whereas $T_{xy}$ only decreases, moderately, for large values of $J_{xz}$. The anomaly in the specific heat approaches the $xy$–peak, its height being reduced, when $J_{xz}$ is increased, and finally merges with the $T_{xy}$ peak. Again, a threshold value exists for $\left| J_{xz} \right|$, at about 18 $J'$, beyond which the total magnetization in $z$–direction, $M_z = \left( m_z^1 + m_z^2 \right) /2$, changes discontinuously, see Figure 6b. The disordering of the planar components at $T_{xy}$ is then also clearly discontinuous, as can be seen in the jump of the order parameter $M_y^{op} = \left( m_y^1
- m_y^2 \right) /2$, see Figure 6c.

Changing the value of the axial field $H_z$ does not change $T_{xy}$ if $J_{xz} = 0$. For nonvanishing nondiagonal couplings, however, the position of 