Theory of the ferromagnetic phase of heavy rare-earth metals

Abstract

The two-band model of an f metal is discussed. The one-dimensional d band is responsible for the existence of the flat region on the Fermi surface (FRFS) and the helical magnetic structure of the rare-earth metals. The low-temperature phase transition from ferromagnet to ferromagnetic helix under the influence of a magnetic field is considered. The three-dimensional sp band is responsible for the ferromagnetic tendency of the rare-earth metals and the absence of a perfect nesting of the d-electron band. The FRFS gives rise to anomalies in the spin wave, phonon and plasmon spectra. The strong crystal field is taken into account exactly and results in 2J branches of spin waves (J is the total angular momentum of the f ion). Only the lowest branch shows dispersion at low temperature. Charged nonmagnetic impurities act on the system through two mechanisms. The first give a finite lifetime to the conduction electrons, causes an erosion of the Fermi surface and increases the tendency of the system to ferromagnetism. The second mechanism is due to perturbation of the crystal field and is stronger than the first; it produces quasi-local magnetic excitation and can shift the instability point to any state. Landau damping of the spin waves due to d electrons is found to be large only in the nearest area of anomaly in the spin wave spectrum.