Mesoscopic magnetic states in metallic alloys with strong electronic correlations: a percolative scenario for CeNi 1-x Cux.

We present evidence for the existence of magnetic clusters of approximately 20 A in the strongly correlated alloy system CeNi 1-x Cux (0.7<or=x<or=0.2) based on small angle neutron scattering experiments as well as the occurrence of staircaselike hysteresis cycles at very low temperature (100 mK). An unusual feature is the observation of long-range ferromagnetic order below the cluster-glass transition without any indication of a sharp transition at a Curie temperature. These observations strongly support a phenomenological model where a percolative process connects both magnetic states. The model can account for all the puzzling data previously obtained in this system, providing a new perspective with regard to the magnetic ground state of other alloyed compounds with small magnetic moments or weak ferromagnetism with intrinsic disorder effects.