The specific heats and magnetic susceptibilities of alloys of cerium with lanthanum at low temperatures

Abstract

Specific heat measurements between 1.5°k and 20°k, and magnetic susceptibility measurements between 1.5°k and 290°k have been made on four alloys containing respectively 78.9%, 37.6%, 27.0% and 5.03% by weight of cerium in lanthanum. Comparison is made with similar previous measurements on pure cerium. The single anomaly in the specific heat of pure cerium at 12.3°k. splits into two in the alloys, and as the dilution is increased these anomalies move to lower temperatures. The lower of the two only is accompanied by a change in the magnetic properties, a maximum occurring in the susceptibility of the most concentrated alloy and temperature-independent paramagnetism setting in suddenly in the next two. In the most dilute alloy the anomalies in both the specific heat and in the susceptibility appear to have moved to temperatures below 1.5°k. Above 80°K the magnetic susceptibilities of all the alloys obeyed a Curie-Weiss law, σ = C/(T–θ). The Curie constant C varied smoothly from 56–1 × 10⁻⁴ e.m.u./g Ce for the most concentrated alloy (cf. 55–3 × 10⁻⁴ e.m.u./g for pure cerium) to about 140 × 10⁻⁴e.m.u./gCe for the most dilute alloy. As in pure cerium, anti-ferromagnetism is believed to be the explanation of the anomalous magnetic behaviour of the alloys. However, the double specific heat anomalies in the latter suggest that the ordering of the 4f electronic orbitals takes place in two stages, while the temperature-independent paramagnetism of the more dilute alloys may be the consequence of freedom of the direction of magnetic ordering to orient itself perpendicular to the applied field. The increase in the Curie constant suggests that the population of the 4f shell increases, with dilution of the alloys, to 2 or 3 electrons per atom of cerium.