Event Details

Why does a frustrated magnet expand upon cooling? Structural transitions, magnetization plateaus, and negative thermal expansion in magnetic spinels

  • 2022-04-20
  • Prof. Karlo Penc, Wigner Research Centre for Physics, Budapest.

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.