Magnetocaloric effect in the triangulated Kagome lattice Cu9Cl2(cpa)6

Abstract

The spin frustrated magnetism of the 2-D molecular magnet material Cu9Cl2(cpa)6 (cpa = anion of 2-carboxypentonic acid), abbreviated as CPA, has been the subject of experimental and theoretical studies that suggest this Heisenberg lattice may be among the most frustrated of materials, along with other Kagome, garnet and pyrochlore systems. The CPA framework is a triangles-in-triangles, or a triangulated-Kagome-lattice (TKL) for which M(T,H) phase diagrams rich in topologically induced spin-liquid states should result from deliberate chemical manipulations. While the spin frustrated topology of CPA makes it of interest for the fundamental physics of quantum spin liquids (QSLs), we report here that the low temperature magnetothermodynamic properties also make it of interest for the study of the magnetocaloric effect (MCE). Highly frustrated materials that do not have clearly distinctive first- or second-order phase transitions can have MCEs due to the persistent entropy of low-lying eigenstates with large degrees of degeneracy. We present field-dependent data up to H = 1T that allow estimates for the MCE of CPA to be calculated from magnetization and demonstrate that a H-T phase boundary exists for temperatures above T = 2K in applied fields below H = 1T. When taken in combination with the phase boundary discovered in the heat capacity data below T = 2K, as well as synthetic results that demonstrate CPA can be taken as a broad materials class, the presence of this second phase boundary suggest chemical variations should present tremendous opportunity to design additional materials. The synthetic challenge will be to produce high quality crystals with consistent, well-understood chemical compositions.