The popular explanation for the antiferromagnetic ordering transitions in these high- materials emphasizes the very small coupling, , between the two-dimensional layers, , and is estimated to be about . However, all these systems exhibit some kind of in-plane anisotropies, which is of order . An interesting case is the spin-one crystal, , discovered twenty years ago [Birgeneau:71a]. The magnetic behavior of exhibits very strong two-dimensional characters with an exchange coupling . It has a Néel ordering transition at , induced by an Ising-like anisotropy, .
Our simulation provides clear evidence to support the picture that the in-plane anisotropy is also quite important in bringing about the observed antiferromagnetic transition at the most interesting spin- case. Adding an anisotropy energy as small as will induce an ordering transition at . This striking effect and related results agree well with a wide class of experiments, and provide some insights into these types of materials.