Theoretical Analysis of the "Double-q" Magnetic Structure of CeAl2

TL;DR

This paper presents a theoretical model explaining the double-q magnetic structure of CeAl2 through competing interactions and Landau expansion analysis, aligning well with experimental magnetic structure data.

Contribution

It introduces a Landau expansion-based theoretical framework for the double-q magnetic structure in CeAl2, incorporating specific interaction parameters and extending insights to similar systems.

Findings

Double-q magnetic structure explained by Landau theory.

Estimated interaction parameters: ferromagnetic K_0 and antiferromagnetic J.

Agreement with experimental magnetic structure determinations.

Abstract

A model involving competing short-range isotropic Heisenberg interactions is developed to explain the "double-q" magnetic structure of CeAl$_2$. For suitably chosen interactions, terms in the Landau expansion quadratic in the order parameters explain the condensation of incommensurate order at wavevectors 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 wavevectors, in striking agreement with the magnetic structure determinations. Based on the value of the ordering temperature and of the Curie-Weiss $\Theta$ of the susceptibility, we estimate that the nearest neighbor interaction $K_0$ is ferromagnetic, with $K_0/k=-11\pm 1$K and the next-nearest neighbor interaction $J$ is antiferromagnetic with $J/k=6 \pm 1$K. We also briefly comment on the analogous phenomenon seen in the similar system TmS.