Low-Temperature Magnetic Transitions in Some Rare-Earth Trichlorides

Abstract

The magnetic susceptibilities of Ce${\mathrm{Cl}}_3$, Pr${\mathrm{Cl}}_3$, and Nd${\mathrm{Cl}}_3$ single crystals and of polycrystalline Sm${\mathrm{Cl}}_3$ have been measured in the temperature region between 0.2 and 4.2°K. Each of these compounds exhibits anomalous behavior. Nd${\mathrm{Cl}}_3$ has two extremely sharp spikes in its susceptibility-versus-$T$ curve at 1.035 and 1.745°K. These spikes are strongly affected by a weak external field and also depend on the amplitude and frequency of the ac measuring field. Ce${\mathrm{Cl}}_3$ has a single sharp spike in its susceptibility at 0.345°K. Pr${\mathrm{Cl}}_3$ and Sm${\mathrm{Cl}}_3$ have rather broad maxima in their susceptibilities at about 0.7 and 0.4°K, respectively. It is shown, on the basis of a simple molecular field model, that the superexchange coupling between nearest neighbors is ferromagnetic, while that between next-nearest neighbors is antiferromagnetic. The two spikes in the Nd${\mathrm{Cl}}_3$ susceptibility curve are ascribed to two successive cooperative magnetic transitions. An attempt is made to correlate in a quantitative way the temperatures of the transitions in Ce${\mathrm{Cl}}_3$ and Nd${\mathrm{Cl}}_3$, and to explain why no similar transitions were observed in Pr${\mathrm{Cl}}_3$.