Grain boundary segregation, phase formation, and their influence on the coercivity of rapidly solidified SmF e11Ti hard magnetic alloys

Palanisamy, D. and Ener, S. and Maccari, F. and Schäfer, L. and Skokov, K.P. and Gutfleisch, O. and Raabe, D. and Gault, B.

Volume: 4 Pages:
DOI: 10.1103/PhysRevMaterials.4.054404
Published: 2020

SmFe11Ti-based alloys have potential as permanent magnet materials; however, until now, crystallographically textured bulk permanent magnets have not yet been produced from this alloy system. This is partly due to the lack of information on the morphology and composition of grain boundary phases present in the Fe-rich Sm-Fe-Ti alloys. Here we investigated the microstructure of a Sm1.25Fe11Ti alloy by using correlative transmission electron microscopy and atom-probe tomography, combined with magneto-optical Kerr effect (MOKE) probing to relate the material's micro- and nanostructure to its properties. The grains of the Sm(Fe,Ti)12 matrix phase are separated by grain boundaries exhibiting a different composition over 3-4 nm width. They contain >75at% of the ferromagnetic element Fe, with an enrichment of Sm of up to 16.6 at% and a depletion in Ti, down to approx. 3.4 at%. We believe that the grain boundary is ferromagnetic at room temperature, which makes the magnetic decoupling of the grains practically impossible, which, in turn, leads to a low coercivity of SmFe11Ti-based alloys. MOKE measurements reveal the strong ferromagnetic coupling across the grain boundary, causing the nucleation of reversal magnetic domains when exposed to low magnetic fields. In a triple-junction area we identified three other ferromagnetic phases: Sm3(Fe,Ti)29,SmFe2, and Fe2Ti. These details bring out the scope of further adjustment of the coercivity in the Sm-Fe-Ti alloy system by grain boundary segregation engineering through the reduction of the presence of ferromagnetic phases to ensure a magnetic decoupling of the micrometer-sized Sm(Fe,Ti)12 grains. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the ""Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

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