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

Abstract

SmFe 12 -based compounds have been considered as one of the most promising candidates for the next generation high performance magnetic materials. SmFe 12 -based compounds exhibit excellent intrinsic hard magnetic properties with lesser amount of rare earth elements compared to other hard magnetic materials, while synthesizing bulk SmFe 12 compounds faces a big difficulty due to the thermodynamic instability of these compounds. Additional elemental doping has been attempted to stabilize SmFe 12 compounds and Ti is currently one of the best elements to thermodynamically stabilize SmFe 12 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-Gru ¨neisen 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 SmFe 12 -based compounds with necessary thermodynamic stability and optimal magnetic properties.

Structures of Sm–Cu intermetallics with Fe as subphase candidates in SmFe12-based permanent magnets studied by first-principles thermodynamics

“…mFe 12 with the tetragonal ThMn 12 structure is expected to be a candidate for the main phase of strong rare-earth permanent magnets due to favorable magnetic properties of large magnetization and a large anisotropy field that are superior to those of Nd 2 Fe 14 B at high temperatures. [1][2][3][4][5] The bulk phase of SmFe 12 is attempted to be stabilized by partial substitutions of nonmagnetic elements for Fe.…”

“…[1][2][3][4][5] The bulk phase of SmFe 12 is attempted to be stabilized by partial substitutions of nonmagnetic elements for Fe. [6][7][8][9][10][11][12][13] On the other hand, to improve the performance of permanent magnets, it is necessary to design microstructures with nonmagnetic grain-boundary subphases. 5,[13][14][15][16][17][18] In fact, however, ferromagnetic bcc Fe often precipitates by substituting nonmagnetic elements for some of Fe atoms in SmFe 12 .…”

“…Recently, it has been reported experimentally that a Cu-41 at%Sm alloy equilibrates with SmFe 11 Ti. 20) Stoichiometric SmCu has been examined as a prototype subphase for SmFe 12 -based magnets by first-principles calculations, where the most stable B27 structure was used for SmCu intermetallics. 21) Binary SmCu intermetallics have two possible crystal structures: the B2 structure (CsCl type, Pm m 3 ) and the B27 structure (FeB-b type, Pnma).…”