anomaly and the low-temperature $xy$ peak are not intimately related. The modification of, for instance, the exchange $J_{xz}$ or the degree of Ising-like anisotropy $D$ changes their respective positions and may lead in an axial field to specific heat data having the peculiar shape of the magnetic specific heat of FeBr$_2$, see Figure 8. One should notice that, in order to obtain this peculiar shape with the considered set of diagonal coupling constants, the single–ion anisotropy has to be decreased considerably as compared to the values obtained from inelastic neutron scattering measurements. Nevertheless, one must keep in mind that both $D$ and the strengths of the diagonal couplings were derived from the experiments without taking a possible nondiagonal spin exchange into account.[@yel75; @pou93]

The evolution of the temperature dependent specific heat of FeBr$_2$ with increasing axial fields is also of interest.[@aru96] For small fields only one peak, the critical peak at $T_c$, has been observed. For larger fields, a shoulder with a superposed peak appears at lower temperatures. The shoulder evolves into a maximum with increasing fields whereas the superposed peak becomes sharper, the whole being shifted to lower temperatures. This large–field behavior is well rendered in the simulations, as 