Unconventional magnetization textures and domain-wall pinning in Sm--Co magnets.

The strongest magnets for high-temperature applications are Sm-Co based alloys characterized by a cellular microstructure with Sm$_2$Co$_{17}$ cells and SmCo$_5$ precipitates, intersected by elongated Zr-rich platelets. This elaborate geometry endows the material with remarkable magnetic hardness because it constitutes a dense domain-wall-pinning network. A precise understanding of the pinning mechanisms could enable the formation of stronger Sm-Co magnets, but experiment and theory have not yet converged to a unified model. Here, we show by comparing high-resolution Lorentz microscopy and micromagnetic simulations that the magnetization processes in Sm-Co magnets are an interplay between curling instabilities and pinning effects. Specifically, domain walls are nucleated at the intersections between Zr-rich platelets and Sm$_2$Co$_{17}$ cells, and become pinned at the SmCo$_5$ precipitates, through which they can tunnel only via the Zr-rich platelets. Further, we have observed exotic domain-wall structures, such as topologically non-trivial 2{\pi} and 3{\pi} domain walls involved in domain-wall annihilation. Based on our findings, we propose how the microstructure of Sm-Co magnets could be optimized to increase the coercivity and remanence of the Sm-Co magnets.