Nanosized Anisotropic Sm–Fe–N Particles with Metastable TbCu7-Type Structures Prepared by an Induction Thermal Plasma Process

TbCu₇-type Sm-based compounds can be produced in bulk and potentially surpass Nd₂Fe₁₄B as permanent magnets. However, as the processes to prepare anisotropic magnetic particles are limited, the full potential of TbCu₇-type Sm-based compounds cannot be exploited. In this study, metastable TbCu₇-type phases of anisotropic Sm-Fe-N ultrafine particles were prepared using the low-oxygen induction thermal plasma (LO-ITP) process. X-ray diffraction analysis revealed that the obtained TbCu₇-type Sm-Fe alloy nanoparticles exhibited a c/a value of 0.8419, with an Fe/Sm atomic ratio of ~8.5. After nitrogenation, the obtained Sm-Fe-N nanoparticles were aligned under an external magnetic field, indicating that each alloy particle exhibited anisotropic magnetic properties. A substantially high degree of alignment of 91 ± 2% was achieved, quantitatively estimated via pole figure measurements. Numerical analysis following Sm-Fe nanoparticle formation showed that, compared with Fe condensation, Sm condensation persisted even at low temperatures, because of a significant difference in vapor pressure between Sm and Fe. Though this led to a relatively large compositional distribution of Sm within particles with a Sm concentration of 9-12 at%, the preparation of single-phase TbCu₇-type Sm-Fe-N particles could be facilitated by optimizing several parameters during the LO-ITP process.