Thermodynamic fluctuations between magnetic states from first-principles phonon calculations: The case of bcc Fe

The longstanding issue of magnetic thermodynamics containing anomalies can be resolved using a reliable model at finite temperatures: thermodynamic fluctuations among the competing collinear magnetic configurations (especially the low energy ones) in accordance with canonical partition function. Based on first-principles phonon calculations, we shed light on the magnetic materials (as exemplified in bcc Fe): the Schottky anomaly of heat capacity and the pressure-dependent Curie temperature stem from the magnetic configurational entropy due to the competition of various magnetic states.