  We study an anisotropic Heisenberg antiferromagnet with ferromagnetic
transverse spin exchange using exact quantum Monte Carlo methods. Such a model
is relevant to a class of rare earth tetraboride materials that display a range
of magnetization plateaus under applied magnetic field. The layered arrangement
of magnetic ions in these materials is topologically equivalent to the
Shastry-Sutherland lattice. In this frustrated geometry, we study the interplay
of next-nearest neighbor interactions in stabilizing a plateau at half the
saturation magnetization (or 1/2 plateau). We also show hysteresis-like
behavior at the onset of the 1/3 plateau.
