FeNdB-based magnets are the most powerful permanent magnets today. Due to aspects such as resource efficiency and sustainability, the critical rare earth (RE) metal Nd is increasingly being partially substituted by the better available and significantly cheaper RE metal Ce. However, the Fe-Nd-B and Fe-Ce-B systems differ in that the Laves phase Fe₂Ce additionally occurs in the latter system and the hard magnetic Fe₁₄Ce₂B phase has poorer intrinsic magnetic properties such as saturation polarization J_s and anisotropy constant K₁ (Fe₁₄Ce₂B: J_s = 1.17 T, K₁ = 1.44 MJ/m³; Fe₁₄Nd₂B: J_s = 1.61 T, K₁ = 4.3 MJ/m³). In typical sintered magnet alloys, up to 30% of the Nd can be replaced by Ce without significant amounts of the Laves phase forming. At higher Ce contents, this is formed to a larger extent and the free Fe₂Ce usually occurs in the form of flat aggregates, which only wet the hard magnetic 14:2:1 grains to a limited extent. This also reduces the amount of the hard magnetic phase and the RE-rich grain boundary phase. However, it is possible to modify the stability window of the Fe₂Ce phase by adding the additives La, Y and Si [1, 2].
In this work, single-alloy sintered magnets with a degree of substitution >50 % of the composition (Ce, La, Nd)-Fe-B-M (M e.g. Co, Al) with different La contents are produced and systematically analyzed. The focus is on the influence of La on the type and quantity of the occurring phases and the magnetic properties of the sintered magnets. The quantification of the sintered microstructures and phase proportions is carried out using quantitative microstructure analysis (QMA). It is shown that the La-richest composition contains more 14:2:1 phase and significantly less Fe₂Ce phase than the La-free variant. La also has a positive effect on the remanence, which could be increased by approximately 25%. In contrast, the coercivity shows the opposite behavior and is highest in the La-free sintered magnet. In summary, the microstructure and magnetic properties of sintered magnets with a high Ce content can be tailored depending on the La content.

[1] J. S. Zhang et al., J. Magn. Magn. Mater. 552, 169217, 2022
[2] Z. Li et al., J. Magn. Magn. Mater. 505, 166747, 2020