  Magnetization plateaus, visible as anomalies in magnetic susceptibility at
low temperatures, are one of the hallmarks of frustrated magnetism. We show how
an extremely robust half-magnetization plateau can arise from coupling between
spin and lattice degrees of freedom in a pyrochlore antiferromagnet, and
develop a detailed symmetry of analysis of the simplest possible scenario for
such a plateau state. The application of this theory to the spinel oxides
CdCr2O4 and HgCr2O4, where a robust half magnetization plateau has been
observed, is discussed.
