  The exact solution of the spin one-half Falicov-Kimball model, with random
hopping between the lattice sites, is used to explain the anomalous magnetic
response of Yb-based valence-fluctuating intermetallic compounds. The anomalous
behavior arises from an entropy-driven local-moment--nonmagnetic transition of
unhybridized Yb ions in these materials which can also be used to explain the
observed metamagnetism and resistivity anomalies.
