Magnetic Properties of Rare-Earth Monopnictides in High Magnetic Fields

Abstract

Since more than two decades, the physics of rare-earth compounds has been of particular interest because of the variety of their magnetic properties, classified as valence fluctuations and heavy fermions. In Ce compounds, the valence changes between +3 (4f¹) and +2 (4f⁰) in some materials called valence fluctuation systems, although the valence is normally supposed to be +3 (4f¹). The very small valence excitation energy for Ce and Yb leads to an intermediate valence state by hybridization between the 4f states and neigh-boring atoms, and creates a resonance scattering band near the Fermi level E _F. The energy width of the band is about Δ = πN _F V ², where V is the hybridization amplitude and N _F is the density of states (DOS) of the conduction band near E _F. Therefore, the DOS at the Fermi surface is enhanced on the order of 1/Δ. Some of the Ce and Yb compounds show heavy-fermion behavior. A typical material is CeAl₃. As a result of the enhanced DOS at E _F, the effective mass is a hundred or a thousand times larger than that of the normal metals. At low temperatures, the mixing between the 4f orbital and the itinerant bands results in an excessively narrow quasi-particle band at the Fermi level. The Sommerfeld coefficient g is proportional to the DOS at E _F and γ = 1.6 JK⁻² mol⁻¹ in CeAl₃ [11]. While in the typical normal metal, copper, γ is about 0.7 mJK⁻² mol⁻¹. Therefore, very strong mass enhancement occurs in CeAl₃. We can know only the amplitude of the total interactions between electrons from the specific heat measurements.