Why does a frustrated magnet expand upon cooling? Structural transitions, magnetization plateaus, and negative thermal expansion in magnetic spinels
ABSTRACT :
Magnetization plateaus, visible as anomalies in magnetic susceptibility at low temperatures, are one of the hallmarks of frustrated magnetism. Usually, thermal and quantum fluctuations stabilize collinear spin structures in a degenerate manifold of a frustrating system. The collinear spins parallel with the magnetic field show up as magnetization plateaus, and their width is tiny. In this talk, I will describe a different mechanism, the coupling between spin and lattice degrees of freedom, that provides a robust half-magnetization plateau in the pyrochlore lattice [1]. A simple microscopic model based on this mechanism, complemented by symmetry analysis, explains the experimentally observed half-magnetization plateau in CdCr2O4 [2], ZnCr2O4, and HgCr2O4 spinel compounds. The magnetic Cr3+ ion with S=3/2 forms the highly frustrated pyrochlore lattice. Furthermore, recent dilatometry experiments on CdCr2O4 in high magnetic fields up to 30 T revealed another surprising effect, the negative thermal expansion: upon cooling the material was expanding and not shrinking, as most materials do when cooled [3]. We trace back the origin of this phenomenon to the temperature dependence of the magnetization, the strong spin-lattice coupling, and t he existence of a nearly localized band of spin excitation.