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+ ions. Using the multidetector D1B at the Institut Laue-Langevin high flux reactor, neutron diffraction diagrams have been measured above and below the transition temperature of 3.8 K. At 1.9 K very weak magnetic reflections have been found corresponding to an antiferromagnetic structure sinusoidally 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 a negative exchange between the 4f electron of Ce3+ and conduction electrons. (author)