Abstract:
The quantum description of Heisenberg cubic ferromagnets with single-ion anisotropy
is considered. The spin operators are represented in terms of the Hubbard operators,
and this makes it possible to take into account exactly the effects of single-ion anisotropy
of arbitrary symmetry, and also an arbitrary orientation of the external magnetic field.
In conjunction with solution of the single-ion problem, this makes it possible to investigate
the influence of anisotropy on the magnetization of a cubic ferromagnet in the ground
state and the spectrum of single-ion energies. It is shown that for a cubic ferromagnet
with $S=2$ there is a change in the kind of phase transition, from the second to the first,
at $K/I_0=1.912$, and for $K>4I_0$ the ferromagnetic ordering disappears. The Curie
temperature of cubic ferromagnets with $S=2,\ 5/2,\ 3,\ 7/2$ is studied as a function of
the anisotropy.