Electron-phonon interaction and field-induced metamagnetism

We present a model describing the effect of electron-phonon interaction ill an itinerant electron system both in high and low magnetic fields. A part of the magnetic field splits the electronic energy levels and the remaining part is responsible for spin fluctuations. Starting from a variant of the Hubbard Hamiltonian, we obtain equations of motion for the number density and spin fluctuation amplitude. The equations are then solved in high-held ferromagnetic state including phenomenologically the damping of spin fluctuations and number density. A nonlinear equation connecting the applied field and the magnetic moment is obtained. In contrast to earlier works, we show that the transverse part of the field is responsible for the nonlinearity and the metamagnetism. The effect of electron-phonon interaction is found to suppress metamagnetism in an itinerant electron system. The effect of electron-phonon interaction is also considered on the low-field spin susceptibility by considering the electron self-energy as a function of frequency and magnetic field. It is found that modification brought about by the field dependence of self-energy is cancelled by the mass enhancement arising out of the frequency dependence of the self-energy. The results obtained are compared with some systematics of metamagnetic behaviour found in CeRu2Si2. (C) 1999 Elsevier Science B.V. All rights reserved.