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.