Effect of element doping and substitution on the electronic structure and macroscopic magnetic properties of SmFe12-based compounds

The mechanisms underlying the enhancement of magnetic anisotropies (MAs) of Sm ions, owing to valence electrons at the Sm site and the screened nuclear charges of ligands, are clarified using a detailed analysis of crystal fields (CF). In order to investigate the finite-temperature magnetic properties, we developed an effective spin model for SmFe12X (X=H, B, C, and N) and SmFe11M (M=Ti, V, and Co), where the magnetic moments, CF parameters, and exchange fields were determined by first-principle calculations. Using this model, the MA constants and magnetization curves at finite temperatures were investigated using a recently introduced analytical method [Yoshioka, Tsuchiura, and Novák, Phys. Rev. B 102, 184410 (2020)2469-995010.1103/PhysRevB.102.184410]. In SmFe12X, the doped light elements X are assumed to be at the 2b site, and in SmFe11M, the substitution site of Fe is systematically investigated for all inequivalent 8f, 8i, and 8j sites. We found that the first-order MA constant K1 is increased by a factor of about two when hydrogen is doped to the 2b site and when Fe is replaced by Ti or V at the 8j site, owing to the attraction of the prolate 4f electron cloud to the screened positive charges of the surrounding ligand ions. We found that when Fe is replaced by Co, the MA increases at all temperatures regardless of the substitution site. The substituted Co attracts electrons, which reduces the electron density in the region from the Sm site to the empty 2b site. This causes the 4f electron cloud at the Sm site to be fixed along the c-axis direction, which improves the MA. The calculated temperature dependence of K1(T) and K2(T) in SmFe11Co qualitatively reproduces the experimental results in the case of Sm(CoxFe1−x)12 for x=0.1 and 0.07. The first-order magnetization process is observed at low temperatures in SmFe12 itself and in many variations of SmFe12-based compounds prepared using element doping and substitution. This is mainly due to the competition between the conditions K1>0 and K2<0, and that of K1<−6K2 owing to the ThMn12 structure having a vacancy at the 2b site.