Free energy of bcc iron:: Integrated ab initio derivation of vibrational, electronic, and magnetic contributions

We present ab initio derived thermodynamic properties of ferromagnetic bcc iron up to the bcc-fcc phase-transition temperature (1200 K), including vibrational, electronic, and magnetic contributions. The quasiharmonic approximation and finite-temperature density-functional theory are employed to account for vibrational and electronic excitations. The magnetic contribution is derived from the solution of the quantum Heisenberg model within many-body theory using the mean-field and random-phase approximation. The calculated thermodynamic properties show an excellent agreement with available experimental data and reveal the necessity to consider all three types of excitations.