A novel computational framework to calculate Gibbs energy and phase transitions under external magnetic fields applied to the Bi-Mn system

Applying an external magnetic field can obtain unique microstructure and thus enhanced properties, which are unattainable without such a field. The thermodynamic description is essential for focused de-velopment of such materials under magnetic field. Based on the molecular field theory of Weiss in the mean field approximation using the Heisenberg model, a predictive computational framework for ma-terials under external magnetic field is developed for the first time. Only three magnetic material pa-rameters, atomic magnetic moment (gJ mu B), total angular momentum ( J ), and Curie temperature (TC) are used to generate explicit functions of normalized dimensionless quantities, establishing basic equations for quantitative thermodynamic calculations of the Gibbs energy, phase transitions and phase diagrams in multicomponent and multiphase systems under external magnetic field with established and potent software packages. The approach is demonstrated for the Bi-Mn system, where strong interactions of the compounds with magnetic field are known. The four-phase equilibrium existing in this system is proven to be a true peritectic-type formation reaction, Liquid + Mn + BiMn = BiMn1.08, at the predicted state point T 4 = 719.6 K, B 4 = 48.56 T, and mu Mn, 4 =-29.288 kJ/mol. Manipulation of phase transition and microstructure under external magnetic field guided by thermodynamic considerations is also discussed. The present work provides a fundament for quantitative calculations of phase equilibria and thermody-namics in multicomponent and multiphase systems under external magnetic field.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

Applying an external magnetic field can lead to unique microstructure and improved properties in materials. The thermodynamic description is crucial for the development of such materials. A predictive computational framework has been developed for the first time, based on the molecular field theory and the mean field approximation using the Heisenberg model. This framework allows for quantitative thermodynamic calculations and the study of phase transitions and phase diagrams in multicomponent and multiphase systems under an external magnetic field.