Relation between anomalous magnetovolume behavior and magnetic frustration in Invar alloys

We present an exactly solvable microscopic model of the T=0 K magnetovolume properties (spontaneous volume magnetostriction, forced volume magnetostriction, compressibility, pressure-induced magnetization response, and paraprocess susceptibility) of a local moment binary alloy with two magnetic species, chemical disorder, and the possibility of magnetic frustration. The calculated properties are shown to have an explicit dependence on both the ground-state spin structure and the degree of magnetic frustration, evaluated as the fraction of magnetic exchange bonds that are energetically not satisfied. We apply this model to fcc Fe_1-yNi_y alloys (including classical Invar at y=0.35) in which we determine the spin structures by adjusting Monte Carlo simulation results to the measured magnetic properties. We find that Invar (Fe₆₅Ni₃₅) is a highly frustrated system and that this effectively changes the sign of the resulting magnetovolume expansion. Invar alloys based on high-moment γ-Fe are therefore understood as arising from an optimized blend of ferromagnetism, that causes most of the Fe-Fe bonds to be unsatisfied, and the antiferromagnetism of the magnetovolume active Fe-Fe bonds.