Replacement of Dy and substitution of Nd in NdFeB-based permanent magnets by Ce, the most abundant and lowest cost rare earth element, is important because Dy and Nd are costly and critical rare earth elements. The Ce, Co co-doped alloys have excellent high-temperature magnetic properties with an intrinsic coercivity being the highest known for T ≥ 453 K.

Rare-earth (R) permanent magnets of R2Fe14B have technological importance due to their high energy products, and they have two R-sites (Wyckoff 4f and 4g, with four-fold multiplicity) that affect chemistry and valence. Designing magnetic behavior and stability via alloying is technologically relevant to reduce critical (expensive) R-content while retaining key properties; cerium, an abundant (cheap) R-element, offers this potential. We calculate magnetic properties and Ce site preference in (R1−xCex) 2Fe14B [R = La,Nd] using density functional theory (DFT) methods—including a DFT + U scheme to treat localized 4f-electrons. Fe moments compare well with neutron data—almost unaffected by Hubbard U, and weakly affected by spin-orbit coupling. In La2Fe14B, Ce alloys for 0≤x≤1 and prefers smaller R(4f) sites, as observed, a trend we find unaffected by valence. Whereas, in Nd2Fe14B, Ce is predicted to have limited alloying (x≤0.3) with a preference for larger R(4g) sites, resulting in weak partial ordering and segregation. The Curie temperatures versus x for (Nd,Ce) were predicted for a typical sample processing and verified experimentally.

Magnetic properties of Ce and Co co-doped (Nd 1-x Ce x) 2 Fe 14-y Co y B compounds have been investigated both in bulk polycrystalline and rapidly solidified nanostructured ribbon forms. For certain Ce concentrations the materials exhibit spin reorientation transitions below 140 K. The Curie temperatures, saturation magnetizations, and other magnetic properties relevant for applications as permanent magnets are controlled by Ce and Co substitutions for Nd and Fe, respectively. Most importantly, the results show that Ce, Co co-doped compounds are excellent replacements for several Dy-based high performance permanent magnets (dysprosium is one of the critical elements and is, therefore, in short supply). The high temperature (375 K) magnetic properties for Nd-Ce-Fe-CoB based alloys show promise not only as a replacement for Dy-doped Nd 2 Fe 14 B permanent magnets, but the new alloys also require significantly lower amounts of Nd, which too is the critical element that can be replaced by a more abundant Ce.

Scanning electron microscopy, and magnetization measurements reveal that as cast (Nd 1-x Ce x ) 2 Fe 14 B alloys contain significant amounts of -Fe that can be dramatically reduced by annealing the alloys at 1000 o C for 3 days. The room temperature intrinsic coercivity, Hci, of (Nd 0.8 Ce 0.2 ) 2.2 Fe 14 B melt spun ribbons was found to be 11 kOe, which is 32 to 10% higher in comparison to that of Nd 2 Fe 14 B (Hci = 8.3 kOe), and (Nd 0.8 Ce 0.2 ) 2.0 Fe 14 B (Hci = 10 kOe), respectively. The substitution of Co for Fe in (Nd 0.8 Ce 0.2 ) 2 Fe 14-z Co z B significantly increases both T C and the maximum energy product, (BH)max. Our study shows that both Co-containing and Co-free Ce-substituted Nd 2 Fe 14 B alloys have excellent magnetic properties at room temperature and above. The experimental results also demonstrate the potential of Nd-Ce-Fe-TM-B based alloys as alternative to expensive Dy-containing high performance rare earth magnets. a corresponding author: pathak138@ameslab.gov

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