The van Hemmen–Kondo model for disordered cerium systems

Abstract

The interplay between disorder and strong correlations has been observedexperimentally in disordered cerium alloys such as Ce(Ni, Cu) orCe(Pd, Rh). In the case of Ce(Ni, Cu) alloys with a Cu concentrationx between 0.6 and 0.3, the first studies have shown a smooth transition withdecreasing temperature from a spin glass phase to ferromagnetism; forx smaller than 0.2, a Kondo phase has been observed. The situation is more complicated nowdue 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 studiedtheoretically. The Kondo effect is described by the usual mean-field approximation; we havetreated the SG behavior successively by the Sherrington–Kirkpatrick model, then by theMattis model and finally by the van Hemmen model, which takes both a ferromagnetic partand a site-disorder random part for the intersite exchange interaction. We present here theresults obtained by the van Hemmen–Kondo model: for a large Kondo exchangeJK, a Kondo phase is obtained while, for smallerJK, the succession of an SG phase, a mixed SG–FE one and finally an FE one has beenobtained with decreasing temperature. This model improves the theoretical description ofdisordered Kondo systems by providing a simpler approach for further calculations ofmagnetic clusters and can, therefore, account for recent experimental data on disorderedcerium systems.