  The characteristic microstructure of Sm(Co,Fe,Cu,Zr)$_z$ 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 the 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 mechanism 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 using 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 and thus form different magnetic textures.
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
transition temperature 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.
