Title: Negative thermal expansion in the plateau state of the magnetically-frustrated spinel CdCr2O4

Speaker: Dr. Karlo Penc, Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest, Hungary

Abstract: The CdCr2O4 is a frustrated magnetic insulator that shows a large one-half magnetization plateau. In this seminar, I will first explain the origin of the plateau. Then, I will discuss the negative thermal expansion observed at the onset of the magnetization plateau [Rossi et al., Phys. Rev. Lett. 123, 027205 (2019)]. On the experimental side, dilatometry provides a precise phase diagram in a magnetic field of up to 30 Tesla. These measurements revealed a large negative thermal expansion associated with the collinear half-magnetization plateau for magnetic fields above 27 Tesla. The resulting phase diagram is compared with a microscopic theory for spin-lattice coupling, and the origin of the negative thermal expansion is identified as a large negative change in magnetization with temperature, coming from a nearly localized band of spin excitations in the plateau phase.