Low-Temperature Magnetic Properties of Some Hexagonal Rare-Earth Trihalides

We have made measurements of the low-temperature heat capacity, magnetic susceptibility, and chlorine nuclear quadrupole resonance (NQR) of Ce${\mathrm{Cl}}_3$, Pr${\mathrm{Cl}}_3$, Nd${\mathrm{Cl}}_3$, Sm${\mathrm{Cl}}_3$, and Ce${\mathrm{Br}}_3$. Some measurements have been made on Pr${\mathrm{Br}}_3$ and Nd${\mathrm{Br}}_3$. The experimental results can be closely correlated with antiferromagnetic linear chain models. The NQR shows that long-range order occurs in Pr${\mathrm{Cl}}_3$ at 0.428 K. In Nd${\mathrm{Cl}}_3$, the NQR line disappeared at 0.50 K and the resonances produced by an internal field appeared at 0.17 K. In Ce${\mathrm{Cl}}_3$, the NQR resonance disappeared at 0.110 K and no lines were found above 0.054 K. In Sm${\mathrm{Cl}}_3$, the NQR line disappeared at 0.21 K and no resonance was observed above 0.1 K. We have measured the internal field, which is proportional to the sublattice magnetization, at the chlorine nucleus in Pr${\mathrm{Cl}}_3$ and Nd${\mathrm{Cl}}_3$ below their Néel temperatures, and have determined the magnitude and the direction of the internal field at the chlorine nucleus relative to the principal axes of the electric field gradient.