High-performance nanocrystalline PrFeB-based magnets produced by intensive milling

The intensive milling technique has been used to produce nanocrystalline isotropic PrFeB-based powders. Highly coercive powders were obtained using the base composition R16T76B8 (R: rare earth, T: transition metal) with Dy and Zr additions achieving a high value of μ0Hc=2.66 T for Pr15Dy1Fe75.9B8Zr0.1. The influence of Dy and Zr, and the substitution of Pr by Nd, on the microstructural and magnetic properties have been studied. Reduction of the R content, increase of the T content, and the presence of Co in the starting composition gave rise to an improved performance with values of Jr=0.92 T, μ0Hc=1.25 T, and (BH)max=140 kJ/m3 for Pr9Nd3Dy1Fe72Co8B6.9Zr0.1, originating from a very fine microstructure with a mean grain size of 20 nm. Furthermore, intensive milling has been shown to be a very versatile technique to produce high-performance nanocomposite magnets by blending this latter alloy with different fractions of soft magnetic α-Fe (x=5–35 wt % Fe). A 25 wt % Fe addition resulted in an optimum combination of magnetic properties with a very high (BH)max value of 178 kJ/m3 due to an effective exchange coupling between the hard- and soft-magnetic phases. A Curie temperature of about 370 °C was observed for this magnet. Demagnetization recoil loops of both single-phase and nanocomposite magnets showed clear differences with relatively open minor loops in the case of the latter due to the exchange-spring mechanism present. δJ plots for the nanocomposite magnets showed a negative deviation of the demagnetizing remanence from the Wohlfarth model, indicative of exchange-coupling interactions being dominant.