Submicron R₂Fe₁₄B particles

Mechanochemical synthesis of submicron R₂Fe₁₄B particles with R = Dy, Nd, Pr has been performed successfully via high energy ball milling of rare-earth oxides, iron oxide and boron oxide in the presence of a reducing agent (Ca) and a dispersant material (CaO), followed by annealing at 800 - 900 °C. In the R = Nd system, we were able to fabricate particles embedded in a CaO matrix with coercivity (H_c) of 10.3 kOe after annealing at 900 °C for 5 min. After washing off the dispersant, the H_c was decreased to below 1 kOe because of hydrogen absorption that leads to the formation of the hydrated R₂Fe₁₄BH_x phase that has a lower anisotropy. Upon removal of the hydrogen the coercivity was increased to 3.3 kOe. The average size of the Nd₂Fe₁₄B particles increases from 100 nm in a sample synthesized at 800 °C to 158 nm at 900 °C. The isotropic Dy₂Fe₁₄B particles showed a higher coercivity of 21 kOe in washed samples after annealing at 900 °C for 5 min. An average size of 71 nm is measured in samples synthesized at 800 °C and 107 nm at 900 °C. Fitting the high field M(H) measurements in Nd₂Fe₁₄B to the law of approach to saturation gave values for the magnetocrystalline anisotropy for the washed sample 2.23 × 10⁷ erg/cm³ and for the vacuum annealed sample 4.15 × 10⁷ erg/cm³, both of which are lower than the bulk values. This would explain the lower values of H_c observed in the particles.