Effect of lattice vibrations on magnetic phase transition in bcc iron

The most widely taught example of a magnetic transition is that of Fe at 1043 K. Despite the high temperature most discussions of this transition focus on the magnetic states of a fixed spin lattice with lattice vibrations analyzed separately and simply added. We propose a model of α iron that fully couples spin and displacement degrees of freedom. Results demonstrate a significant departure from models that treat these coordinates independently. The success of the model rests on a first principles calculation of changes in energy with respect to spin configurations on a bcc-iron lattice with displacements. Complete details of environment-dependent exchange interactions that augment the Finnis-Sinclair potential are given and comparisons to measurements are made. We find that coupling has no effect on critical exponents, a small effect on the transition temperature, T_c, and a large effect on the entropy of transformation.