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The characteristic microstructure of Sm (Co , Fe , Cu , Zr) z (z = 6.7 – 9.1) alloys with SmCo 5 cell walls in Sm 2 Co 17 cells, all intersected by Zr-rich platelets, makes them some of the best performing high-temperature permanent magnets. Plentiful research has been performed to tailor their microstructure at the nanoscale; but due to its complexity, many questions remain unanswered about the effect of the individual phases on the magnetic performance at different temperatures. Here, we explore this effect for three different Sm (Co , Fe , Cu , Zr) z alloys by deploying high-resolution magnetic imaging via in situ transmission electron microscopy and three-dimensional chemical analysis via atom probe tomography. We show that their microstructures differ in terms of SmCo 5 cell-wall and Z-phase size and density as well as the Cu concentration in the cell walls and demonstrate how these features influence the magnetic domain size and density, thus forming different micromagnetic spin structures. Moreover, we illustrate that the dominant coercivity mechanism at room temperature is domain-wall pinning and show that magnets with a denser cell-wall network, a steeper Cu gradient across the cell-wall boundary, and thinner Z-phase platelets have a higher coercivity. We also show that the coercivity mechanism at high temperatures is domain-wall nucleation at the cell walls. Increasing the Cu concentration inside the cell walls decreases the temperature of transition between pinning and nucleation, significantly decreasing the coercivity with increasing temperature. We, therefore, provide a detailed explanation of how the microstructure on the atomic to nanoscale directly affects the magnetic performance and provide detailed guidelines for an improved design of Sm (Co , Fe , Cu , Zr) z magnets. [ABSTRACT FROM AUTHOR]

Some of the best-performing high-temperature magnets are Sm–Co-based alloys with a microstructure that comprises an Sm 2 Co 17 matrix and magnetically hard SmCo 5 cell walls. This generates a dense domain-wall-pinning network that endows the material with remarkable magnetic hardness. A precise understanding of the coupling between magnetism and microstructure is essential for enhancing the performance of Sm–Co magnets, but experiments and theory have not yet converged to a unified model. Here, transmission electron microscopy, atom probe tomography, and nanometer-resolution off-axis electron holography have been combined with micromagnetic simulations to reveal that the magnetization state in Sm–Co magnets results from curling instabilities and domain-wall pinning effects at the intersections of phases with different magnetic hardness. Additionally, this study has found that topologically non-trivial magnetic domains separated by a complex network of domain walls play a key role in the magnetic state by acting as nucleation sites for magnetization reversal. These findings reveal previously hidden aspects of magnetism in Sm–Co magnets and, by identifying weak points in the microstructure, provide guidelines for improving these high-performance magnetic materials. [ABSTRACT FROM AUTHOR]