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.