First-principles Study of Thermodynamic Stability in Light Elements (B, C and N) Doping SmFe12 Compounds

Abstract

SmFe12-based compounds have been considered 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 compare to other hard magnetic materials, while synthesizing SmFe12 compounds faces a big difficulty due to the thermodynamic stability of these compounds. Additional elements, typically transition elements, doping has been attempted to stabilize SmFe12 compounds. However, those elements tend to decrease the overall magnetization of the compound due to the replacement of Fe atom. Light elements such as B, C and N which tend to enter the interstitial site than substitution of the existing atom, thus, avoiding the reduction of Fe concentration. Recently some new experiments1 were reported to achieve the high coercivity in Sm(Fe0.8Co0.2)12 anisotropic magnetic thin film by B doping. Systematical investigation for finding suitable alloying elements is experimentally expensive and time consuming. By employing first-principles calculations, we explored zero K and finite temperature stability of doping light alloying elements (B, C and N) combing with several transition metal elements in SmFe12-based compounds for both specific site doping and random doping. The Special Quasirandom Structure (SQS)2 model is adopted to imitate the random atomic configurations. Debye-Grüneisen model3 is used to calculate vibrational contribution to the free energy. Thermal electronic excitation and configurational free energies are also included in to total free energy calculations. It is found that in case the light elements doping to SmFe12, at zero K, B has the possibility to stay in both interstitial site (2b) and substitutional site (8f) due to very small difference of formation energies although the interstitial site has slightly lower energy. C is unstable in unstable either in substitutional or interstitial sites. N is only at interstitial site. In co-doping of B and Co, similar to B doping, B is possible to be in both interstitial and substitutional states at zero K, which explains in a extend the experimental observation that the concentrators of Co and B are related each other. Further calculations revealed that at finite temperature, substitutional B becomes more stable than the interstitial one. The stability mechanism of those doping are discussed from electronic structures and lattice vibration effects. Reference 1 H. Sepehri-Amin, et al., Acta Mater. 194 (2020) 337-342 2 A. Zunger, et al., Phys. Rev. Lett. 65, 353 (1990). 3 V.L. Moruzzi, et al., Phys. Rev. B 37 (1988) 790.