An Investigation of the Frustrated Quantum Magnets CeCl3 and CeBr3

Ce3+-based magnetic systems are useful platforms for studying quantum magnetism, as the 4f1 electron configuration and strong spin-orbit coupling result in a well-defined effective spin 1/2 state. Consequently, modeling and studying these systems is theoretically straightforward. More broadly, frustrated magnetic systems are of significant theoretical interest due to the exotic phases that can result when conventional order is prevented. From the beginning of tying superconductivity to spin-liquid states to more recent potential applications in e.g., quantum computing and information, the promised fruits of frustrated magnetism have spurred many studies and improvements in experimental techniques. Cerium trihalides CeCl3 and CeBr3 are isostructural triangular-lattice compounds that were studied starting in the late 1960s. Despite significant effort at the time, no definitive solution for the magnetic ground state was ever made. Subsequent improvements in sample growth and greater interest in Ce3+ systems have motivated us to reopen the case. We report those results here, including solutions for the low-temperature magnetic structure and magnetic phase diagrams, showing that both compounds are highly frustrated but do adopt antiferromagnetic order around 100 mK with k = (1/3, 1/3, 1/2). This both completes the picture for these compounds and informs future studies of Ce3+-based magnetic materials, shedding light on the possibility of synthesizing other useful compounds.