Magnetic and microstructural properties of nanocrystalline exchange coupled PrFeB permanent magnets

Nanocrystalline exchange coupled Pr(2)Fe(14)B single-phase and Pr(2)Fe(14)B + alpha-Fe two-phase magnets with grain sizes of about 20 nm were produced using the melt-spinning procedure. In the stoichiometric Pr(2)Fe(14)B composition a significantly enhanced remanence of J(R) = 0.95 T was achieved in comparison with conventional Pr-rich and therefore decoupled isotropic PrFeB magnets (J(R) less than or equal to 0.5J(S) = 0.78 T). In the composite magnets with overstoichiometric Fe a further enhancement of the remanence is possible. Values up to J, = 1.42 T and (BH)(max) = 180.7 kJ/m(3) were obtained. As there exists no spin reorientation in PrFeB magnets, our attention was not only directed to the magnetic behaviour at room temperature but also to the magnetic properties in the whole ferromagnetic temperature range. The microstructural parameters N(eff), alpha(K) and alpha(ex) describing the influence of the non-ideal microstructure and the effect of the exchange coupling on the coercive field were determined within the framework of the nucleation model from the temperature dependence of the coercive field. Furthermore, reversibility measurements of the demagnetization curves in the second quadrant give important information about the magnetization processes in exchange coupled magnets. Moreover, we have investigated the law of approach to ferromagnetic saturation of the single-phase magnet in comparison with the decoupled one. The magnetic results are correlated with TEM investigations of the real microstructure. (C) 1998 Elsevier Science B.V. All rights reserved.