Multimodal analysis of Zr substitution effects on magnetic and crystallographic properties in (Sm_1−xZr_x)(Fe_0.8Co_0.2)₁₂ compounds with ThMn₁₂ structure

SmFe₁₂-based compounds containing Zr atoms have been recognized as candidates for novel high-performance permanent magnet materials with a high saturation magnetization. Partial substitution of Sm with Zr in (Sm_1−xZr_x)(Fe_0.8Co_0.2)₁₂ has been known to enhance the magnetization of the compound. This study focused on (Sm_1−xZr_x)(Fe_0.8Co_0.2)₁₂ single-crystalline films to evaluate the effects of Zr substitution for Sm on their structural and magnetic properties via systematic multimodal analysis. X-ray absorption fine structure (XAFS) and scanning transmission electron microscopy indicated a selective occupation of the Sm site by Zr atoms without disturbing the Fe sublattices. The chemical shift in the Zr-K edge XAFS spectra suggested electron transfer from Zr to the Fe band. Soft X-ray magnetic circular dichroism analysis proved that the magnetic moment of Sm was two orders of magnitude smaller than the magnetic moments of Fe and Co, and its contribution to the total magnetization was negligible. The partial substitution of Sm with Zr resulted in a volumetric expansion and c-axis length elongation despite Zr having a smaller atomic radius, which is unique to (Sm_1−xZr_x)(Fe_0.8Co_0.2)₁₂ without structure-stabilizing elements such as Ti, V, and Si. This anisotropic expansion is explained by an increase in the magnetic moment and magnetic interactions in the Fe8f site, which is induced by the electron transfer from Zr to Fe8f atoms. Thus, we conclude that the electron transfer from Zr to the Fe8f atoms principally drives magnetization enhancement in (Sm_1−xZr_x)(Fe_0.8Co_0.2)₁₂ single-crystalline films.