Magnetism in intermediate-valence YbAl3:: a polarized neutron diffraction study

Polarized neutron diffraction experiments on YbAl3 have revealed the following: (i) a field-induced magnetic moment on the ytterbium site only, which is well described by a Yb3+ free-ion form factor; (ii) a second maximum at T approximate to 15 K in the temperature dependence of the localized magnetic moment in addition to the well-known maximum at T = 130 K; and (iii) a small temperature-independent positive conduction electron polarization The possible microscopic origin of those observations is discussed. A comparison with the isostructural intermediate-valence compound CeSn3 reveals important differences which increase our understanding of the ground state in those compounds.