Spin-lattice coupling in the classical invar alloy from first principles

We present a method for treating magnetic systems and use it to investigate the anomalously low thermal expansion of the classical Invar alloy, Fe_0.65Ni_0.35. Results at and below room temperature are consistent with the experimentally observed low thermal expansion of this Invar alloy. Phonon dispersions are also calculated, and renormalization of the phonon spectra by magnetic thermal disorder is found to be significant. The calculated phonon dispersions are in excellent agreement with measurement.