f-Electron delocalization/localization and the abrupt disappearance of uranium magnetic ordering with dilution alloying

We have applied the model and technique previously applied to the change of Curie temperature with pressure for correlated-electron uranium systems to predict the change in Curie temperature and ordered moment with dilution a lloying. The theory is remarkably successful in its predictions, and this s uccess has important implications for the overall understanding of magnetic ordering in correlated-electron systems including heavy fermion systems. F or US, the dilution alloying behavior found experimentally is dramatic. In UxLa1-xS, the magnetic ordering abruptly disappears at about 55% uranium. O ur ab initio-based theory quantitatively predicts this abrupt disappearance while also quantitatively predicting the monotonic decrease of Curie tempe rature with pressure for undiluted US. In addition, in agreement with exper iment, the theory predicts the correct trend for the magnetic ordering to d isappear with dilution, while at the same time absolutely and quantitativel y predicting the nonmonotonic variation of Curie temperature with pressure, for USe and UTe. The ab initio-based model gives absolute material-specifi c predictions using input from the local density approximation paramagnetic uranium f-electron-projected density of states plus ab initio calculated v alues of the correlation energy U. The key physics of the model is the reco gnition and quantification of the concept that the f spectral density in th e vicinity of a specific uranium nucleus (so-to-speak in the muffin-tin sph ere) can either be in a stable f(3) configuration for a long enough period of time that, through coupling to other such stable f(3) sites, it can magn etically order, or can be in a situation such that the configuration fluctu ates rapidly between f3 and f(2), and for purposes of magnetic ordering act s Like a hole in the f-electron lattice in the same way that substitution o f lanthanum for uranium creates a hole. Both pressure and dilution alloying , by causing an increase in f-delocalization, increase the fraction of uran ium sites in the rapidly fluctuating, magnetically ineffective, condition. For dilution alloying, at a certain point this increase in fluctuations cau ses the stable f(3) component to fall abruptly below a critical value neces sary to sustain any magnetic ordering, and hence brings about a catastrophi c collapse in magnetic ordering. (C) 1998 American Institute of Physics. [S 0021-8979(98)21111-X].