  Magnetization plateaux, visible as anomalies in magnetic susceptibility at
low temperatures, are one of the hallmarks of frustrated magnetism. An
extremely robust half-magnetization plateau is observed in the spinel oxides
CdCr2O4 and HgCr2O4, where it is accompanied by a substantial lattice
distortion. We give an overview of the present state experiment for CdCr2O4 and
HgCr2O4, and show how such a half-magnetization plateau arises quite naturally
in a simple model of these systems, once coupling to the lattice is taken into
account.
