Electron-phonon interaction mechanism of magnetovolume and magnetoelasticity effects in itinerant electron ferromagnets

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Abstract
While the volume and elastic constants of a system are determined from its free energy, we note that the free energy of a metal consists of the electron part and the thermal phonon part, and that the phonon part also depends on magnetization. Starting from such an observation, we study the role of electron-phonon interaction in the volume and elastic behaviors of a ferromagnetic metal in the same spirit as we recently studied the role of electron-phonon interaction in determining the magnetic properties [D. J. Kim, Phys. Rev. Lett. 47, 1213 (1981); Phys. Rev. B 25, 6919 (1982); J. Appl. Phys. 55, 2347 (1984)]. First, for the magnetovolume effect we find that in the ferromagnetic state of metals the phonon Grüneisen constant behaves in quite diverse ways sensitively depending upon their electronic structure near the Fermi surface, and can become negative to cause zero or negative thermal expansion. We propose that such behavior of the Grüneisen constant can be an important mechanism in the anomalous volume behavior of ferromagnetic metals including the Invars. Next, as for the magnetoelastic effect, in the ferromagnetic state the role of thermal phonons is found to be of secondary importance compared with that of electrons; the dominant cause of the magnetization dependence of elastic constants comes from that of electron energy as we previously noted. In the paramagnetic state, however, the temperature dependence of the elastic constant is found to be determined principally by the effect of thermal phonons.