Model calculation of two-ion magnetostriction in the itinerant uniaxial ferromagnet Y2Fe17.

Abstract

Two-ion contribution to the magnetoelastic (MEL) coupling in the itinerant hexagonal iron-rich ferromagnet ${\mathrm{Y}}_{2}$${\mathrm{Fe}}_{17}$ is investigated within a Stoner-like model with an orbitally degenerate rigid $3d$ band. The Spontaneous irreducible magnetostrictive strains ${\ensuremath{\epsilon}}^{\ensuremath{\alpha},1}$ (volume distortion) and ${\ensuremath{\epsilon}}^{\ensuremath{\alpha},2}$ ($c$-axis pure shape distortion) are calculated as well as their temperature dependences. The agreement with the experiment is reasonably good. The model includes contributions to the strains from the electron hopping within the dumbbell of $4f$ site Fe atoms and from isotropic exchange between the $4f$ Fe and $12j$ site nearest-neighbor Fe atoms, treated within the mean-field approximation. The strain derivative of the exchange integral between $4f$ and $12j$ iron atoms is obtained by the fitting of the experimental data. The one-electron hopping has been dealt with using a two-site Hubbard Hamiltonian with orbital degeneration, and within the Hartree-Fock approximation for the repulsive $U$ term. We have also obtained the temperature dependence of the four macroscopic two-ion MEL parameters ${\overline{D}}_{\mathrm{ij}}^{\ensuremath{\alpha}}$ (in Callen and Callen notation), appropriate for uniaxial symmetry.