MAGNETIC-ORDERING AND ANOMALOUS GROUND-STATE IN CEAL2

The intermetallic Laves phase compound CeAl2 presents some unusual properties, which makes the exact nature of its ground state a matter of great interest. A first step towards an explanation would be the knowledge of any magnetic structure below 3.8 K (phase transition temperature). We have carried out very accurate neutron diffraction experiments on a polycrystalline sample, completed by experiments on a single crystal. Several magnetic reflections have been found which are characteristic of the propagation vector k→=1/2+τ, 1/2-τ, 1/2. The corresponding magnetic structure consists of antiferromagnetic (11̄0) planes in which the magnetic moments, lying along [111], are modulated according to a sine wave propagation along [11̄0]. No variation of k→ nor any tendency towards antiphase structure appear down to 0.4 K. This result is not coherent with the Kramers' degeneracy of the Ce3+ ion because it implies a non magnetic level as a ground state. On the other hand previous polarized neutron experiments have shown an antiparallel coupling between the spins of the cerium and the conduction electrons. Both results, as well as other physical properties, agree with the existence of a singlet ground state associated with a strong d-f coupling.