Theoretical analysis of the double-q magnetic structure of CeAl2

A model involving competing short-range isotropic Heisenberg interactions is developed to explain the double-q magnetic structure of CeAl2 Theta. For suitably chosen interactions, terms in the Landau expansion quadratic in the order parameters explain the condensation of incommensurate order at wave vectors in the star of (1/2-delta,1/2+delta,1/2)(2 pi/a), where a is the cubic lattice constant. We show that the fourth-order terms in the Landau expansion lead to the formation of the so-called double-q magnetic structure in which long-range order develops simultaneously at two symmetry-related wave vectors, in striking agreement with the magnetic structure determinations. Based on the value of the ordering temperature and of the Curie-Weiss temperature Theta of the susceptibility, we estimate that the nearest-neighbor interaction K-0 is ferromagnetic with K-0/k=-11 +/- 1 K and the next-nearest neighbor interaction J is antiferromagnetic with J/k=6 +/- 2 K. We also briefly comment on the analogous phenomena seen in the similar system TmS.