The most powerful permanent magnets used in electric motors today are FeNdB-based magnets. For better resource efficiency and sustainability the critical rare earth (RE) metal Nd used in these magnets may be partially substituted by the more available and significantly cheaper RE metal Ce. However, with increasing degree of substitution (especially above 30%), the phases occurring in the microstructure change. In addition to the hard magnetic Ф-phase Fe₁₄Ce₂B, the Laves phase Fe₂Ce occurs. Fe₁₄Ce₂B has poorer intrinsic properties compared to Fe₁₄Nd₂B. Another challenge is the limited wettability of the free Fe₂Ce, which occurs in the form of planar aggregates and only partially isolates the Ф-phase grains from each other. The occurrence of this phase also reduces the proportion of the hard magnetic phase and RE-rich grain boundary phase. However, by adding the additives La, Y and Si, it is possible to modify the stability window of the Fe₂Ce phase [1-3].

In this work, single-alloy sintered magnets with substitution levels of at least 50% and varying La contents are produced and systematically investigated. Besides the magnetic properties, the focus is on the type and quantity of the phases occurring. The phase fractions are quantified using QMA (quantitative microstructure analysis). With the use of La, an additional RE-rich grain boundary phase occurs. The proportions of the other phases shift. The amount of the Fe₂Ce phase decreases and the quantity of hard magnetic Ф-phase increases. In addition to the increase of the Ф-phase, La has a positive effect on the remanence. In contrast, the coercivity shows an inverse behavior. In summary, it can be stated that the microstructure and thus the magnetic properties of sintered magnets with a high Ce content can be tailored depending on the La content. 

References 
[1] J. S. Zhang et al., Journal of Magnetism and Magnetic Materials 552, 169217, 2022
[2] X. Liao et al., Journal of Materials Research and Technology 17, 1459-1468, 2022
[3] Z. Li et al., Journal of Magnetism and Magnetic Materials 505, 166747, 2020