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 Condensed Matter Thursday, October 12, 2006 4:00 PM Physics Building, Room 204 Note special time. Note special room. Doron Bergman [Host: Seunghun Lee] UC Santa Barbara "Magnetic ordering in a half--polarized magnetization plateau of the pyrochlore antiferromagnet" ABSTRACT:The classical pyrochlore antiferromagnet (AFM) is considered the most'' geometrically frustrated system. Classically, this leads to the absence of any ordering transition at non-zero temperature, even in an applied magnetic field. Recent experiments on the spinel chromites, ACr2o4(A=Cd,Hg) show the existance of a very robust magnetization plateau in a strong magnetic field, and a simultaneous magnetic ordering. We describe a model of spin-lattice coupling that explains both the plateau formation and the observed ordering on the plateau. The predictions are confirmed by recent neutron scattering and x-ray scattering experiments (S. H. Lee et al.). The same model applied to zero magnetic field predicts a reduced but still large ground state degeneracy, including the states observed in both the Cd and Hg materials. This is consistent with the dominance of spin-lattice interactions, with weak additional effects determining the low field magnetic ordering.