Antiferro-Quadrupolar Ordering in CeB₆

Abstract

The existence of an antiferro-quadrupolar ordering in the intermediate temperature region in CeB₆ is suggested by a model calculation. CeB₆ has two transition temperatures, T_m=3.3 K and T_N=2.3 K, enabling us to define three phases I, II and III in the sequence of decreasing temperature. The phase II shows unusual properties: A weak antiferromagnetism is induced and enhanced by the external magnetic field in the phase II, making the I–II phase boundary shift to the high-temperature side with increasing field. These properties can be explained by the present model based on the combined effect of the crystal field and the antiferro-quadrupolar interaction among 4f electrons on Ce³⁺ ions. The molecular-field approximation with the two-sublattice model is used. Detailed comparison is made with experiment for various thermal and magnetic quantities.