Novel microwave assisted chemical synthesis of Nd₂Fe ₁₄B hard magnetic nanoparticles

The high coercivity and excellent energy product of Nd₂Fe ₁₄B hard magnets have led to a large number of high value added industrial applications. Chemical synthesis of Nd₂Fe₁₄B nanoparticles is challenging due to the large reduction potential of Nd ³⁺ and the high tendency for Nd₂Fe₁₄B oxidation. We report the novel synthesis of Nd₂Fe₁₄B nanoparticles by a microwave assisted combustion process. The process consisted of Nd-Fe-B mixed oxide preparation by microwave assisted combustion, followed by the reduction of the mixed oxide by CaH₂. This combustion process is fast, energy efficient and offers facile elemental substitution. The coercivity of the resulting powders was ∼8.0 kOe and the saturation magnetization was ∼40 emu g^-1. After removal of CaO by washing, saturation magnetization increased and an energy product of 3.57 MGOe was obtained. A range of magnetic properties was obtained by varying the microwave power, reduction temperature and Nd to Fe ratio. A transition from soft to exchange coupled to hard magnetic properties was obtained by varying the composition of Nd _xFe_1-xB₈ (x varies from 7% to 40%). This synthesis procedure offers an inexpensive and facile platform to produce exchange coupled hard magnets.