On the magnetic ordering of CeAl2

The magnetic behaviour of CeAl₂ at low temperature is not apparent : contradictory experiments have assumed either an antiferromagnetic ordering or a moment reduction of the Ce³⁺ ion. Using the multidetector D1B at the I.L.L. high flux reactor, we have measured neutron diffraction diagrams, above and below the transition temperature of 3.8 K. At 1.9 K, we have found very weak magnetic reflections corresponding to an antiferromagnetic structure sinusoǐdally modulated, according to the propagation vector (0.612, 0.388, 0.5). The moment reduction in such a modulated structure at 1.9 K can hardly be explained by a thermal partial disordering of the moments. It is rather due to the existence of a singlet ground state resulting from the negative exchange between the 4f electron of Ce³⁺ and the conduction electrons.