Effect of Cu redistribution in grain boundary on magnetic properties of Sm(Co0.665Fe0.25Cu0.06Zr0.025)7 permanent magnets

Highlights: • Sm(Co_0.665Fe_0.25Cu_0.06Zr_0.025)₇ magnets were prepared with different conditions. • Correlation between preparation conditions and microstructure/magnetic properties was studied. • Sintering temperature increment promotes grain amalgamation and Cu redistribution. • Cu dilution leads to improved demagnetization curve squareness and enhanced energy product. • Cu dilution leads to coercivity reduction indicated by magnetic domain pattern modification. The correlation between preparation conditions and microstructure as well as magnetic properties were studied for Sm(Co_0.665Fe_0.25Cu_0.06Zr_0.025)₇ magnets. Increase of sintering temperature from 1468 to 1493 K promotes the amalgamation of small grains into big ones, resulting in an enlargement of average grain size from 10 to 25 μm and diffusion of Cu from inner grains to Cu-lean grain boundaries. As a result, the demagnetization curve squareness of the magnets improves substantially, leading to a remarkable enhancement of the maximum energy product from 21.3 to 27.1 MGOe. The coercivity of the magnets drops from 25.3 to 16.0 kOe at the same time, which was verified by the increase of the average domain width from 1.1 μm in small grains to 2.5 μm in big grains. It is therefore concluded that tuning of the preparation conditions causes a more homogeneous distribution of Cu, which is crucial for the improvement of magnetic properties.