Curie temperature of Nd2Fe14B phase is only around 313 degrees Celsius which much lower than first and second generation of rare earth permanet magnet, SmCo5 and Sm2Co17. The saturation magnetization of Nd2Fe14B drops quickly with the increasing temperature which means temperature coefficient has relatively higher absolute value. For sintered Neodymium magnets, its temperature coefficient of remanence αBr is in the range of -0.08%/degree Celsius and -0.12%/degree Celsius from -50 degree Celsius to 150 degree Celsius. In reality, low temperature coefficient Neodymium magnets have certain market, especially precision instruments and aerospace.
Both Curie temperature and temperature coefficient of Nd2Fe14B can be significantly enhanced via substituting Cobalt for Iron, but it will result to the decreasing of intrinsic coercivity. In the meanwhile, the room-temperature magnetocrystalline anisotropy field decreased little when substitution is small. Such phenomenon could not be due to the magnetocrystalline anisotropy field changes of Nd2(Fe1-xCox)14B. It is obvious that changes in the phase structure and microstructure are a critical factor to reduce temperature coefficient.
Both SmCo5 magnets and Sm2(Co, Cu, Fe, Zr)17 magnets are well-known for their heavy rare-earth temperature compensation effect, and similar effect is also existing in the sintered Neodymium magnet which also plays an important role in the enhancement of the temperature stability, therefor, adding moderate Dysprosium and Holmium to substitute Neodymium will compensate negative temperature coefficient of the Neodymium magnets.
Besides substituting Cobalt for Iron or substituting Dysprosium or Terbium for Neodymium, the temperature coefficient of sintered Neodymium magnets also can be improved by adding Aluminum, Gallium, Copper, and Niobium on the foundation of above two substitutions. Such low melting point metal or refractory metal will adjust the intrinsic magnetic properties after entered into the Nd2Fe14B main phase.
