MAGNETIZATION DENSITY IN CE3AL11

Ce3Al11 is an orthorhombic compound in which the cerium atoms occupy t wo different sites. Due to the crystal structure, the presence of twin s cannot be avoided. Kondo effects are present, leading to unusual mag netic properties. Below the ordering temperature, 6.2 K, neutron diffr action experiments performed on powder, and more recently on single cr ystals, evidenced two different magnetic phases: the compound is ferro magnetic between 6.2 K and 3.2 K, and becomes modulated with an incomm ensurate propagation vector k = (0, 0, kz) at lower temperatures. In b oth phases, the cerium moments are very different on the two sites, on e of them being strongly reduced. However, from the integrated magneti c intensities alone, one cannot determine on which of the two sites th e moment is reduced. Polarized neutron measurements were undertaken to remove this ambiguity and to obtain the magnetization density maps. T he easy magnetization direction being along the b-direction, the exper iment was performed with b parallel to the applied field. However, in the projection onto the plane perpendicular to b, the two cerium sites are superimposed and cannot be separated. It was then necessary to me asure the magnetic density in three dimensions. To overcome the twin p roblem, measurements were performed for two orientations of the crysta l and a new procedure for data treatment was used. The magnetization d ensity maps obtained from Fourier inversion or by the three-dimensiona l maximum entropy method clearly show that the reduced cerium moment i s on the less symmetric crystallographic site.