[P2] Sm-based Magnets & Nitrides

[P2-35] Nanoscale Structural and Chemical Insights into the Magnetic Performance of SmCo Permanent Magnets

*Esmaeil Adabifiroozjaei¹, Stefan Giron⁷, Nikita Polin², Yangyiwei Yang⁸, Fernando Maccari⁷, Andras Kovács³, Trever Almeida⁴, Dominik Ohmer⁵, Kaan Üstünerg⁵, Aparna Saksena², Matthias Katter⁵, Iliya Angelov Radulov⁷, Christoph Freysoldt², Rafal E. Dunin-Borkowski³, Bai-Xiang Xu⁸, Oliver Gutfleisch⁷, Baptiste Gault^2,6, Konstantin P Skokov⁷, Leopoldo Molina-Luna¹ (1. Advanced Electron Microscopy Division, Institute of Material Science, Technical University of Darmstadt, 64287 Darmstadt, Germany (Germany), 2. Max Planck Institute for Sustainable Materials, Düsseldorf 40237, Germany (Germany), 3. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich, Jülich 52425, Germany. (Germany), 4. SUPA, School of Physics and Astronomy, University of Glasgow, Glascow G12 8QQ, United Kingdom (UK), 5. VACUUMSCHMELZE GmbH & Co. KG, 63450 Hanau, Germany (Germany), 6. Department of Materials, Royal School of Mines, Imperial College London, London, United Kingdom (UK), 7. Institute of Materials Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany (Germany), 8. Mechanics of Functional Materials Division, Institute of Materials Science, Technische Universität Darmstadt, Darmstadt 64287, Germany (Germany))

Keywords:

SmCo Magnets

Sm₂Co₁₇-type sintered magnets are well known for their exceptional magnetic properties, including high coercivity (>2.0 T), high Curie temperature (~800 °C), low temperature coefficients of coercivity (β ≈-0.2 to -0.3%/°C) and remanence (α ≈-0.03%/°C) [1]. These magnets are composed of the Sm₂(CoFe)₁₇ (2:17R) matrix phase, which is subdivided into nanoscale cells (~100 nm) by the Sm(CoCu)₅ (1:5H) cell boundary phase (~10 nm thick). Additionally, the Zr-rich forms lamellae (Zr-platelets) thinner than 10 nm, oriented perpendicular to the c-axis of Sm₂Co₁₇ [2]. Understanding the nanoscale structural and chemical features of these phases is crucial to elucidating the origins of their magnetic performance. In this study, we employed advanced transmission electron microcopy (TEM), atom-probe tomography (APT), Lorentz TEM, and electron holography to analyze the atomic-scale characteristics of bulk Sm₂(CoFeCuZr)₁₇ magnets with varying magnetic properties. Additionally, micromagnetic simulations were conducted to correlate the observed nanostructure and their magnetic behavior. Our TEM analysis reveals a correlation between pinning strength and cell structure. Regions with lower pinning strength exhibit a denser packing of smaller cells, whereas larger and more sparsely packed cells sizes are associated with stronger pinning. Moreover, in regions with lower coercivity, the content of 2:17′ phase is higher compared to that in the high-coercivity areas. APT data further confirm that regions with lower pinning strength have a denser arrangement of cells. High-angle annular dark field (HAADF) imaging shows that, regardless of local pinning strength, one or two layers of a 1:5H-likestacking appear on the Zr-platelets. APT analysis indicates that these layers are slightly enriched in Sm, Co and Cu, confirming their composition as a 1:5H phase. Micromagnetic simulation suggests that the presence of these thin layers of 1:5H on Zr-platelets enhances pinning strength. Furthermore, the HAADF imaging reveals that 1:5H cell boundaries are thinner in high-coercivity regions than in low-coercivity ones. Atomic-scale APT data show that the chemical profiles of all elements across the 1:5H phase and Zr-platelets are sharper in high-coercivity regions compared to low-coercivity regions. Lorentz TEM and electron holography confirm that the cell boundaries serve as the primary pinning phase in the Sm₂(CoFeCuZr)₁₇ magnets. These findings provide crucial insights into the nanostructural mechanisms governing the magnetic performance of SmCo permanent magnets, paving the way for further optimization of their coercivity and thermal stability.
1. H. Sepehri-Amin, et. al., Acta Materialia 126 (2017) 1-10.
2. M. Duerrschnabel, et. al., Nature Communications 8 (2017), 54.