In this work, the influence of Cu powders doping on the magnetic and mechanical properties of Sm ₂ Co ₁₇ permanent magnets is systematically investigated. For high-energy-product Sm ₂ Co ₁₇ (low initial Cu content), the intrinsic coercivity is significantly improved from 11.7 kOe to 25.7 kOe with 1 wt.% Cu powders doping. The flux loss (soaking at 300 °C for 2 h) is reduced from 7.4% to 3.8%, and the average flexure strength is improved from 60.2 MPa to 74.1 MPa. As for high-temperature-resistant Sm ₂ Co ₁₇ (high initial Cu content), the intrinsic coercivity is slightly reduced from 39.2 kOe to 38.3 kOe with 1 wt.% Cu powders doping. The flux loss (soaking at 300 °C for 2 h) is increased from 1.4% to 3.4%, and the average flexure strength is improved from 86.7 MPa to 93.2 MPa. The differences in magnetic properties change of these two Sm ₂ Co ₁₇ permanent magnets are closely related with their initial Cu content. The doped Cu is prone to diffuse into intragranular cell boundaries in magnet with low initial Cu content. The increased Cu content in intragranular cell boundaries benefits the improvement of intrinsic coercivity, thus enhancing the thermal stability and reducing the flux loss. Doping Cu powders may increase the Cu-rich phase around ground boundaries, which will hinder the grain growth during heat treatment. The decrease of grain size will enhance the flexure strength, while it will increase the flux loss.

The effects of Nd doping on the microstructures and magnetic properties of Sm_1–x Nd _x (Co_0.695Fe_0.2Cu_0.08Zr_0.025)_7.2 (x = 0, 0.3, 0.5, 0.7, 1.0) permanent magnets are studied. The scanning electron microscope (SEM) analysis of the solid solution states of the magnets shows that with the increase of Nd content, the distribution of elements becomes inhomogeneous and miscellaneous phase will be generated. Positive temperature coefficient of coercivity (β) appears in each of the samples with x = 0.3, 0.5, and 0.7. The corresponding positive β temperatures are in ranges of about 70 K–170 K, 60 K–260 K, 182 K–490 K for the samples with x = 0.3, 0.5, and 0.7, respectively. Thermomagnetic analysis shows that spin-reorientation-transition (SRT) of the cell boundary phase is responsible for this phenomenon. On the basis of this discovery, the Sm_0.7Nd_0.3 (Co_0.695Fe_0.2Cu_0.08Zr_0.025)_7.2 magnet possessing thermal stability with β ≈ –0.002 %/K at the temperature in a range of 150 K–200 K is obtained.

Magnetocrystalline anisotropy has many advantages over shape anisotropy regarding coercivity in permanent magnets, making it a promising approach to enhance the coercivity of AlNiCo magnets. In this work, AlNiCo magnets with NdFeB-nanocrystalline phase were prepared by spark plasma sintering (SPS), and the effect of the NdFeB phase on coercivity was uncovered. AlNiCo powder with a spinodal structure and NdFeB powder with a nanocrystalline structure, which exhibit shape anisotropy and magnetocrystalline anisotropy, respectively, were sintered by SPS. With the advantages of low-temperature densification achieved by the SPS process, the spinodal and nanocrystalline structures were mostly retained. The microstructure analysis indicated that the SPS-ed magnet primarily consisted of AlNiCo regions with a spinodal structure, NdFeB regions with a nanocrystalline structure, and a transition region approximately 1~7 µm wide between them. A significant effect of the magnetic anisotropy of the NdFeB phase on magnetization behavior was found. The hysteresis loop of the SPS-ed magnets became single-phase magnetization, in contrast with the double-phase magnetization observed in the simple mixed powder. As the magnetocrystalline anisotropy of the NdFeB phase possesses higher coercivity, the coercivity of the SPS-ed magnet increased from 1250 Oe (of the AlNiCo raw powder) to 2490 Oe. This work provides valuable information for the coercivity enhancement of AlNiCo magnets.