Magnetic excitation and thermodynamics of BaFe₂As₂

Magnetic excitation of the iron-based superconductor parent compound BaFe₂As₂ at finite temperature are addressed with a first-principles formulation of the Helmholtz energy, which can account for the finite temperature mixture of many magnetic configurations. We find that it is the spin exchange coupling in the interplane c direction that dictates the spin density wave ordering. We have also quantitatively predicted the pressure dependence of the spin density wave ordering temperature, the Schottky anomaly, and the temperature dependence of thermal populations of several low-energy spin configurations, all in agreement with available experimental data.