Theory of High-Temperature Magnetostriction

In contrast to the rapid decrease predicted by conventional theory, the magnetostriction λ₁₀₀ of iron has a large maximum just below the Curie temperature. We propose a mechanism based on the fact that an ellipticity in the quasiparticle spectrum permits a lowering of the free energy by distortion; an equivalent mechanism arises from the anisotropic magnon-phonon interaction near the zone boundary. This latter interaction is large at temperatures such that magnon renormalization (due to magnon-magnon interaction) lowers the magnon spectrum to degeneracy with phonons at the zone edge. The degeneracy temperature agrees well with the temperature of the λ₁₀₀ maximum in iron. Adding silicon raises impurity states from the phonon spectrum and thence lowers the degeneracy temperature, but increases the range of temperature over which near-degeneracy occurs; this agrees with the observed shift and broadening of the λ₁₀₀ peak. The mechanism also predicts a monotonically decreasing λ₁₁₁ of the opposite sign to λ₁₀₀, as observed in iron.