Grain boundary segregation, phase formation, and their influence on the coercivity of rapidly solidified SmFe11Ti hard magnetic alloys

$\mathrm{SmF}{\mathrm{e}}_{11}\mathrm{Ti}\text{\ensuremath{-}}\mathrm{based}$ 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 $\mathrm{S}{\mathrm{m}}_{1.25}\mathrm{F}{\mathrm{e}}_{11}\mathrm{Ti}$ 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 $\mathrm{Sm}{(\mathrm{Fe},\mathrm{Ti})}_{12}$ matrix phase are separated by grain boundaries exhibiting a different composition over 3--4 nm width. They contain $g75\phantom{\rule{0.16em}{0ex}}\mathrm{at}%$ 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 $\mathrm{SmF}{\mathrm{e}}_{11}\mathrm{Ti}$-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: $\mathrm{S}{\mathrm{m}}_{3}{(\mathrm{Fe},\mathrm{Ti})}_{29},\mathrm{SmF}{\mathrm{e}}_{2}$, and $\mathrm{F}{\mathrm{e}}_{2}\mathrm{Ti}$. 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 $\mathrm{Sm}{(\mathrm{Fe},\mathrm{Ti})}_{12}$ grains.