Improving Thermodynamic Stability of SmFe12-Type Permanent Magnets from High Entropy Effect

SmFe12-based compounds have been considered as one of the most promising candidates for the next generation high performance magnetic materials. SmFe12-based compounds exhibit excellent intrinsic hard magnetic properties with lesser amount of rare earth elements compared to other hard magnetic materials, while synthesizing bulk SmFe12 compounds faces a big difficulty due to the thermodynamic instability of these compounds. Additional elemental doping has been attempted to stabilize SmFe12 compounds and Ti is currently one of the best elements to thermodynamically stabilize SmFe12 compound, but it degrades the magnetic properties. Multi-element random alloying of selected elements gives a possible pathway to improve the thermodynamic stability making use of the high entropy effect while minimizing the degradation of the magnetic properties. Various special quasirandom structures (SQS) are created to imitate the random atomic configurations in multi-element doping systems. Free energies as the function of temperature are calculated from the electronic structure and total energy at 0K and the finite temperature effects using Debye-Gruneisen model. By carefully balancing among the intrinsic formation energy at 0K, vibrational contribution, configurational entropy and thermal electronic excitation contribution of various alloying elements sets, it should be possible to achieve the multi-element alloying SmFe12-based compounds with necessary thermodynamic stability and optimal magnetic properties.