Spin and valence fluctuations in CeMn4Al8 and CeMn6Al6

The magnetic susceptibility and X-ray photoelectron spectra of CeMn4Al8 and CeMn6Al6 and of the reference compounds YMn4Al8, CeCu4Al8, YMn6Al6 and LaMn6Al6 are reported. The occurrence of a Mn moment in the investigated compounds is strongly correlated with the critical value d similar or equal to 2.6 Angstrom of the Mn-Mn distance below which the Mn moment collapses. It is revealed that the magnetic character of the Mn moment in the crystallographic sites for which d(Mn-Mn)less than or equal to 2.6 Angstrom changes from an itinerant electron type to a local moment type with increasing temperature. This behavior is in agreement with the prediction of spin fluctuation theory. Both the magnetic susceptibility and Ce 3d and 4d XPS spectra confirmed the mixed valence state of Ce ions in CeMn4Al8 and CeMn6Al6. The f occupancy, n(f), and coupling, Delta, between the f level and the conduction states are derived to be about 0.83 and 80 meV, respectively. The Mn 3s XPS splitting is discussed in terms of the time scale for the spin fluctuation and photoemission processes. (C) 1998 Elsevier Science S.A. All rights reserved.