Ab Initio Theory of Thermal Spin-Lattice Disorder in Iron and Invar

Abstract

Despite its deceptive simplicity and because of its scientific and technological importance, bcc Fe is still the subject of research and debate. We develop an ab initio theoretical framework and apply it to calculate temperature-dependent phonon modes and magnetic interaction parameters in bcc Fe. This framework incorporates realistic thermal disorder in a coupled spin-lattice system. Thermal spin-lattice coupling is found to significantly renormalize the phonon modes and magnetic interaction strength, resulting in significant temperature-dependencies. A method for treating magnetic systems of unknown entropy is developed and applied to calculate phonon modes and investigate the anomalous thermal expansion of the classical invar alloy, Fe0.65Ni0.35. Results over the temperature range 50K to room temperature are consistent with the observed low thermal expansion of this material. Excellent agreement with measured data is achieved for calculated phonon modes in both bcc Fe and the invar alloy.