  Motivated by recent experiments on TmB4 (arXiv:0712.1537), we examine the
phase diagram of the Shastry-Sutherland antiferromagnet in an applied magnetic
field in the limit of strong Ising anisotropy. In classical Ising limit, we
demonstrate that the only fractional magnetization plateau is at 1/3 of the
saturated magnetization. We study the perturbative influence of transverse
quantum spin fluctuations, and present evidence that they can stabilize a
narrow 1/2 magnetization plateau.
