Influence of a Reduction Process on the Phase Component and Magnetic Properties of NdFeB Magnetic Nanoparticles

NdFeB magnetic nanoparticles with a main phase of Nd₂Fe₁₄B have successfully been synthesized. The preparation includes the following processes. First, the NdFeB intermediate was synthesized by a wet-chemical method. The NdFeB intermediate was annealed at 800 °C, which resulted in the formation of an NdFeB oxide. Then, the NdFeB oxide was reduced into NdFeB nanoparticles by a second-step reduction annealing with a CaH₂ reductant. The second-step reduction annealing temperature was a key factor in preparing the NdFeB nanoparticles. A lower reduction temperature of 900 °C could not completely reduce the NdFeB oxide into Nd₂Fe₁₄B. There were some residual unreduced nonmagnetic phases in the prepared materials, which resulted in obvious decreases of coercivity. A sufficiently high second-stage reduction temperature resulted in an increased reduction, and more of the Nd₂Fe₁₄B phase could be obtained. In this work, nanoparticles with a uniform morphology and an increased Nd₂Fe₁₄B phase could be obtained at an optimum reduction temperature of 940 °C, achieving a high coercivity of 5.4 kOe.