MICROMAGNETIC ANALYSIS OF NANOCRYSTALLINE GA-DOPED AND NB MO-DOPED NDFEB PERMANENT-MAGNETS/

Nanocrystalline as well as microcrystalline melt-spun ribbons of NdFeB alloys with small additions of Ga and Nb or Mo were prepared using th e single-roller technique. Average grain sizes of 25 nm up to 500 nm w ere obtained by varying the wheel speed and the parameters of the heat treatment after rapid quenching. Systematic investigations of the rel ationship between microstructural effects and magnetic properties were carried out according to micromagnetic concepts. The theory of nuclea tion in single-domain particles is able to explain the magnetization r eversal process, if several deteriorating effects of the microstructur e are taken into account. The additives Ga and Nb/Mo both improve the coercive field. Ga is known to form new intergranular phases leading t o a better decoupling of the grains. High-melting Nb/Mo-containing pre cipitates act as nucleation centers. Ga- and Nb/Mo-doped samples show therefore small magnetization reversal intervals and a high reversibil ity of the first part of the demagnetization curve, Furthermore the co ncept of microstructural parameters alpha(K) and N-eff is applied to a n extensive investigation of the temperature dependence of coercivity. For different microstructures it turns out that alpha(K) reaches satu ration as required by theory whereas a wide spectrum of N-eff is found , Nevertheless a strong correlation of both alpha(K) and N-eff restric ts the improvement of the coercive field. Best critical fields at room -temperature up to 2.5 T have been obtained for annealed Ga- and Nb-do ped samples with a fine-grained nanocrystalline and uniform microstruc ture.