In our previous article, Effects of Heavy Rare Earth Elements (Dy, Tb, Gd, Ho) on Sintered NdFeB Magnet Properties, we discussed how heavy rare earth elements are used to enhance coercivity and high-temperature performance—often at the expense of cost and magnetic output.
Building on that discussion, this article focuses on a different but equally important approach: substituting neodymium (Nd) and iron (Fe) with light rare earth elements and selected metallic additives. Elements such as La, Ce, and Pr, as well as Co, Al, Cu, and other alloying elements, are increasingly used to optimize cost, temperature stability, and overall magnetic performance.
By examining how these substitutions influence key magnetic properties, this article aims to provide practical insights into the performance–cost trade-offs involved in modern sintered NdFeB magnet design.
Substitution of Light Rare Earth Elements for Nd
Light rare earth elements such as lanthanum (La), cerium (Ce), and praseodymium (Pr) have relatively abundant reserves and lower costs compared with neodymium. As a result, substituting part of Nd with light rare earth elements has attracted increasing interest, particularly for cost-sensitive NdFeB magnet applications.
La Substitution for Nd
Lanthanum can form the compound La₂Fe₁₄B, but its formation is more difficult and requires relatively high processing temperatures. Once formed, this phase remains stable below approximately 860 °C.
In sintered NdFeB magnets, Nd typically accounts for 65–75% of the total rare earth content. Since the cost of La is often less than half that of Nd, partial substitution with La can significantly reduce raw material costs while promoting more balanced utilization of rare earth resources.
However, increasing La content leads to a noticeable decline in key magnetic properties. Magnetic polarization (Js), remanence (Br), coercivity (Hcj), and maximum energy product ((BH)max) all decrease as La concentration rises. Because La is a non-magnetic rare earth element, its magnetic dilution effect causes (BH)max to drop more rapidly than Br, limiting its use in high-performance magnets.
Ce Substitution for Nd
Cerium is even more abundant than La, but its application in NdFeB magnets is more challenging. The compound Ce₂Fe₁₄B has poor thermodynamic stability, making it difficult to form and maintain a stable crystal structure.
As Ce content increases, nearly all magnetic properties decline, including remanence, coercivity, and energy product. In addition, Ce substitution significantly lowers both the Curie temperature (Tc) and the temperature stability of the magnet. For these reasons, Ce is generally unsuitable for applications requiring stable performance over a wide temperature range.
Pr Substitution for Nd
Praseodymium offers a more favorable alternative among light rare earth elements. Pr₂Fe₁₄B meets the basic structural requirements for permanent magnet materials and can achieve good magnetic properties after sintering at approximately 1060 °C.
Magnets produced using (Pr,Nd)–Fe–B alloys can exhibit magnetic performance comparable to conventional NdFeB magnets. However, Pr is more prone to oxidation than Nd, which means that careful control of Pr content is necessary—especially for applications requiring high environmental or thermal stability.
Substitution of Other Elements for Fe
Although sintered NdFeB magnets offer high magnetic performance, they typically suffer from low coercivity, low Curie temperature, and limited thermal stability. Their practical operating temperature is often restricted to around 80 °C, and corrosion resistance can also be a concern. To address these limitations, extensive research has focused on partially substituting iron (Fe) with other metallic elements to improve overall performance.
Co Substitution for Fe
Cobalt (Co) is commonly used to replace part of Fe in NdFeB magnets. As Co content increases, the Curie temperature (Tc) rises significantly, while the temperature coefficients of remanence and coercivity decrease, improving thermal stability.
- When Co content is below ~5 at.%, Br and (BH)max remain largely unchanged.
- When Co content exceeds 30 at.%, most magnetic properties decline sharply.
- In practice, adding less than 10 at.% Co is considered optimal, as it raises Tc while maintaining acceptable magnetic performance.
Al Substitution for Fe
Studies have shown that adding a small amount of aluminum (Al) can significantly improve coercivity in NdFeB magnets. In Nd–Fe–Co–B systems, Al can compensate for the coercivity loss caused by Co addition.
As a result, Nd–Fe–Co–Al–B alloys often exhibit a better balance of coercivity, thermal stability, and overall magnetic performance.
Cu Substitution for Fe
Copper (Cu) plays an important role in enhancing coercivity through microstructural modification. Research indicates that adding small amounts of Cu to (Nd,Dy)–Fe–B or (Nd,Dy)–(Fe,Co)–B systems can significantly increase Hcj, while causing little to no reduction in Br.
This makes Cu particularly valuable for producing magnets with high coercivity and high (BH)max, especially in combination with other alloying elements.
Other Element Substitutions (Nb, Ga, etc.)
Additional elements such as niobium (Nb) and gallium (Ga) are sometimes used in small quantities to further refine magnetic properties:
- Nb or Zr substitution can improve coercivity and squareness of the demagnetization curve. However, excessive Nb content can destabilize the Nd₂Fe₁₄B phase; experimental results suggest an upper limit of approximately 3 at.% Nb.
- Ga addition has been shown to significantly enhance coercivity and reduce irreversible losses. In Nd–Fe–Co–B systems, Ga can counteract the coercivity reduction normally associated with higher Co content.
- Combined additions of Ga and Nb can further improve temperature stability.
Practical Considerations
Substituting Fe with selected alloying elements is an effective way to improve thermal stability, coercivity, and reliability of sintered NdFeB magnets. However, excessive substitution often leads to reduced remanence and energy product, making careful composition optimization essential.
In practice, these substitutions are widely used in motor, automotive, and high-temperature applications, where performance stability is more critical than maximizing magnetic output.
Element Substitution Effects Comparison Table for Sintered NdFeB Magnets
To better summarize how different element substitutions influence the performance and cost of sintered NdFeB magnets, the table below provides a side-by-side comparison of commonly used rare earth and metallic additives. It highlights their primary functions, benefits, limitations, and typical application scenarios, offering a quick reference for both technical and purchasing decisions.
Substituted Element | Replaced Element | Main Purpose | Positive Effects | Negative Effects | Typical Use Case |
La | Nd | Cost reduction | Lower raw material cost; abundant supply | Significant reduction in Br, Hcj, and (BH)max | Low-cost, low-performance magnets |
Ce | Nd | Cost reduction | Very abundant; low price | Poor phase stability; reduced Tc, Br, and temperature stability | Limited practical use |
Pr | Nd | Performance tuning | Comparable magnetic performance to NdFeB | Higher oxidation sensitivity | Medium to high-performance magnets |
Dy | Nd | Coercivity enhancement | Strong increase in Hcj; improved high-temperature stability | Reduced Br and (BH)max; high cost | High-temperature, anti-demagnetization applications |
Tb | Nd | Maximum coercivity | More effective than Dy in raising Hcj | Extremely high cost; scarce | Critical, extreme-condition applications |
Gd | Nd | Cost reduction | High Tc; relatively abundant | Low Js and HA; limited performance gain | Generally not recommended |
Ho | Nd | Coercivity tuning | Similar effects to Gd | High cost; low efficiency | Rarely used |
Co | Fe | Thermal stability | Raises Curie temperature; improves temperature coefficients | Excess Co reduces Br and (BH)max | Motors, high-temperature environments |
Al | Fe | Coercivity balance | Improves Hcj; offsets Co-related losses | Excess Al lowers Br | Nd–Fe–Co–B systems |
Cu | Fe | Microstructure control | Increases Hcj with minimal Br loss | Limited effect alone | High-coercivity magnets |
Nb/Zr | Fe | Grain refinement | Improves coercivity and squareness | Excess destabilizes Nd₂Fe₁₄B phase | Precision performance tuning |
Ga | Fe | Coercivity & stability | Enhances Hcj; improves thermal stability | Cost consideration | High-reliability applications |