Effect of Ho Substitution on Magnetic Properties and Microstructure of Nanocrystalline Nd-Pr-Fe-B Alloys

The inevitable thermal demagnetization of magnets at high-temperatures is a key issue for Nd-Fe-B based permanent magnetic materials, especially for electric motors. Here, we report the effect of partially substituting the element Holmium (Ho) on the magnetic properties and microstructure of nanocrystalline melt-spun [(NdPr)(1-x)Ho-x](14.3)Fe76.9B5.9M2.9 (x = 0-0.6; M = Co, Cu, Al and Ga) alloys. It shows that Ho can enter into the main phase and significantly enhance the coercivity (H-cj). A large coercivity of 23.9 kOe is achieved in the x = 0.3 alloy, and the remanent magnetization (M-r) remains in balance. The abnormal elevated temperature behavior of M-r is observed in the alloys with a high amount of Ho substitution, in which the M-r of the x = 0.6 alloy increases with rising temperature from 300 K to 375 K owing to the antiparallel coupling between Ho and Fe moments. As a result, the positive value (0.050%/K) of temperature coefficient alpha of M-r is achieved in the x = 0.6 alloy within the temperature range of 300-400 K, in excess of that of existing Nd-Fe-B magnets. The temperature coefficient beta of H-cj is also improved by Ho substitution, indicating the introduction of Ho in Nd-Fe-B magnets is beneficial for thermal stability. The microstructure observation of x = 0, 0.3 and 0.6 alloys confirmed the grain refinement by Ho substitution, and Ho prefers to remain in the 2:14:1 phase than Nd and Pr. The present finding provides an important reference for the efficient improvement of the thermal stability of Nd-Fe-B-type materials.

Automated summary

The introduction of Holmium into Nd-Fe-B permanent magnetic materials can significantly enhance coercivity and improve the thermal stability of the magnets. High levels of Ho substitution can lead to an increase in remanent magnetization at elevated temperatures, attributed to the antiparallel coupling between Ho and Fe moments. Additionally, the coercivity temperature coefficient beta is also improved with Ho substitution, indicating the beneficial effects of Ho on thermal stability in Nd-Fe-B magnets.