Many-body electron origin for the forced magnetostriction in iron-rich Y2Fe17 and Y2Fe14B itinerant ferromagnets

We report on the observed high-field (up to 30 T) volume, partial derivativ e epsilon (alpha,1)/partial derivative H, and shape, partial derivative eps ilon (alpha,2)/partial derivative H, forced magnetostrictions in the iron-r ich Y2Fe17 and Y2Fe14B itinerant uniaxial intermetallics. The magnetostrict ions, which increase rapidly with temperature, are rather strong if compare d with iron metal, amounting (in 10(-6)T(-1)) for Y2Fe14B up to congruent t o 37 and congruent to-2.2 and for Y2Fe17 up to 82 and 15, respectively. A s imple Hubbard-like model of forced magnetostriction, within the Hartree-Foc k approximation, has been developed in order to interpret the experiments. The most significant result is that the dominant mechanism for forced magne tostriction in those ferromagnets is the strong strain dependence of the 3d intraband effective many-body electron Coulomb repulsion potential, U-eff, which amounts to (in eV Fe atom) partial derivative U-eff/partial derivati ve epsilon (alpha,1) congruent to -(0.12+/-0.01) and partial derivative U-e ff/partial derivative epsilon (alpha,2) congruent to 0.04+/-0.01 for Y2Fe14 B, and - (0.04+/-0.002) and - (0.006+/-3 X 10(-4)) for Y2Fe17, respectively . The origin for this effective potential strain dependence is mainly relat ed to the strain dependence of the 3d electron bandwidth. Dependencies of m agnetization and high-field susceptibility with strictions also play a subs tantial role.