Pure crystal field α-mode magnetostriction of the itinerant ferromagnet Y2Fe14B

We have experimentally isolated the pure single-ion crystal-field origin irreducible magnetostrictive strains epsilon(alpha,1) and epsilon(alpha,2), which respectively represent the volume and the shape cia ratio distortions, for the tetragonal itinerant ferromagnet Y2Fe14B. The unusual thermal variations of those strains, peaking at about 200 K and decreasing with temperature, have no sensible explanation within the Callen-Callen standard theory of magnetostriction for localized magnetic moments. Instead a developed simple rigid-band Stoner model of magnetostriction for itinerant 3d electrons explains quite well such dependencies and also allows us to extract the microscopic magnetoelastic coupling parameters M-12(alpha) and M-22(alpha), accounting for those strains respectively. These parameters are very large, about 10 meV/Fe atom, and of opposite signs.