CRITERIA FOR THE OCCURRENCE OF FERROMAGNETISM AND WEAK MAGNETIC ORDER IN NARROW-BAND METALS

Phenomenological correlations between the occurrence of different types of magnetic order, the value of the electronic specific heat divided by temperature gamma*, and the shortest distance d between Ce and U atoms in a number of metallic compounds are presented. Ferromagnetism is found to occur over restricted ranges of d and gamma*, and the magnetic ordering temperature T(m) and the ordered moment mu-or are maximized for d almost-equal-to 4.0-4.2 angstrom. "Weak magnetic order", for which mu-or << 0.5-mu-B, is found to occur in two distinct regimes: Large values of T(m) occur for d less than the "Hill limits" d(H) almost-equal-to 3.4 angstrom (Ce) and 3.5 angstrom (U), with gamma* almost-equal-to 3.5 X 10(4) erg cm-3K-2, while very low T(m) are found in several heavy fermion compounds near d almost-equal-to 4.15 angstrom and gamma* almost-equal-to 1 x 10(5) erg cm-3K-2. A plot of T(m) vs. volume-scaled mu-or implies that metallic 4f, 5f and 3d transition metal materials can be systematically described within a unified picture. The small exchange enhancement of the magnetic susceptibility of heavy fermion compounds and the surprising trend that ferromagnetism occurs in relatively strongly hybridized systems may be explained by the wavevector dependence of the electronic self-energy that is responsible for large effective mass.