Twins – A weak link in the magnetic hardening of ThMn12-type permanent magnets

Nd₂Fe₁₄B-type materials exhibit the highest energy product around room temperature and hence dominate the high-performance permanent magnet market. Intensive research efforts aim at alternative material systems containing less critical elements with similar or better magnetic properties. Nd- and Sm-based compounds with a ThMn₁₂-type structure exhibit intrinsic properties comparable or even superior to Nd₂Fe₁₄B. However, it has not been possible to achieve technically relevant coercivity and remanent magnetization in ThMn₁₂-based bulk sintered magnets. Using SmFe₁₁Ti as a prototypical representative, we demonstrate that one important reason for the poor performance is the formation of twins inside micro-crystalline grains. The nature of the twins in SmFe₁₁Ti was investigated in twinned "single crystals" and both bulk and thin film poly-crystalline samples, using advanced electron microscopy and atom probe tomography as well as simulations and compared with benchmark Nd₂Fe₁₄B. Both micro-twins and nano-twins show a twin orientation of 57±2° and an enrichment in Sm, which could affect domain wall motion in this material. Micromagnetic simulations indicate that twins act as nucleation centers, representing the magnetically weakest link in the microstructure. The relation between twin formation energies and geometrical features are briefly discussed using molecular dynamic simulations.