Structural and magnetic properties of PrFe_12-xV_x and their nitrides

The structural and magnetic properties of novel permanent magnet materials, PrFe_12-xV_xN_y (x = 1.25 to approximately 2.0; y≈1.6), have been studied systematically. It is shown that the nitrides crystallize in the same ThMn₁₂-type structure as the parent alloys, with the unit cell volume expansion upon nitrogenation. Neutron diffraction performed on PrFe_10.5V_1.5N_0.4 at room temperature reveals that the vanadium atoms occupy the 8i sites, while the nitrogen atoms occupy the 2b sites and the iron atoms occupy the 8j, 8f and the rest 8i sites in the ThMn₁₂-type structure. Typical interstitial modification effects are observed in PrFe_12-xV_xN_1.6. The Curie temperature is raised to as high as 800 K and the saturation magnetization is increased to 1.54 T at room temperature. Moessbauer spectra studies also show that the hyperfine field and isomer shift are strongly enhanced upon nitrogenation. Nitrogenation changes the easy magnetization direction from the ab-plane to the c-axis with an anisotropy field up to 10.5 MA/m (132 kOe). Crystal field coefficients for Pr³⁺ in PrFe_12-xV_x and PrFe_12-xV_xN are calculated to explain this change in magnetocrystalline anisotropy behaviors. As a preliminary attempt, the magnetically hard powders based on Pr(Fe,V)₁₂N_1.6 are obtained with a maximum energy product of 135 kJ/m³ (16.9 MGOe) and a remanence of 1.18 T at room temperature.