Magnetostriction in Nickel Alloys

It is shown that the existence of an orbital degeneracy in the $3d$ band may lead to a change of sign of the magnetostriction, when this degeneracy goes through the Fermi level. Such a change of sign is observed in the Fe-Ni and Co-Ni alloy series (permalloy), at an electron concentration of 27.7 electrons per atom. As was shown in a previous publication, the same orbital degeneracy is probably responsible for the strong maximum of the magnetoresistance, and for the change of sign of the extraordinary Hall effect, observed in these alloys at the same value of the electron concentration.