Seminários

case of Ce(Ni,Cu) alloys with a Cu concentration x between 0.6 and 0.3, the first studies

have shown a smooth transition with decreasing temperature from a spin glass phase

to ferromagnetism; for x smaller than 0.2, a Kondo phase has been observed. The

situation is more complicate now due to the recent observation of magnetic clusters.

The competition between the Kondo effect, the spin glass (SG) and the ferromagnetic

(FE) ordering has been extensively studied theoretically. The Kondo effect is described

by the usual mean field approximation; we have treated the SG behavior successively

by the Sherrington-Kirkpatrick model, then by the Mattis model and finally by the

van Hemmen model, which takes both a ferromagnetic part and a site-disorder random

part for the inter-site exchange interaction. We present here the results obtained by

the van Hemmen-Kondo model: for a large Kondo exchange JK, a Kondo phase is

obtained, while, for smaller JK, the succession of a spin glass phase, a mixed SG-FE

one and finally a FE one has been obtained with decreasing temperature. This model

improves the theoretical description of disordered Kondo systems by providing a simpler

approach for further calculations of magnetic clusters and can, therefore, account for

recent experimental data on disordered Cerium systems.