Magnetism in the triangular lattice materials CeCl₃ and CeBr₃

There is currently great interest in systems with J = 1/2 ground state and Ising-like interactions that may be geometrically frustrated, as in the triangular lattice, or frustrated by competing interactions, like the honeycomb Kitaev model. This interest has spurred our investigation of the triangular lattice rare earth materials CeCl₃ and CeBr₃, where the Ce³⁺ ion is expected to have an effective J = 1/2 ground state. Some magnetic properties of these materials were studied many years ago but prior to this investigation there had been no direct measurements of the magnetic order or excitations using neutron scattering. Here we report neutron scattering investigations of crystal field excitations in CeBr₃, confirming the J=1/2 ground state, and neutron diffraction measurements of the magnetic order in CeCl₃, showing that the system orders antiferromagnetically at low temperature with an ordering wave-vector (1/3 1/3 1/2).