Magnetovolume effects in invar alloys: Spontaneous and forced volume magnetostriction

Using a localized-electron model it is shown that the spontaneous and foreced volume magnetostriction, ωm and ∂ωm/∂H can be consistently interpreted assuming a volume dependence of the exchange interaction. This is expressed by J′1, the first derivative of the exchange intergal I1(r) with respect to the interatomic distance. Quantitative results are reported for Fe-Ni alloys. In the range 0.05 ≦ 1 – T/Tc ≦ 0.5 ωm and (I/I0)2, the relative magnetization squared, are fitted to a power law (1 – T/Tc)2β, where β increases from 0.31 for pure Ni to ≈ 0.5 for alloys in the Invar range.