On the reversible magnetization behavior in two phase NdFeB based melt spun ribbons

The present paper presents our studies on the reversible magnetization behavior of some two-phase alpha Fe/Nd2Fe14B thin melt spun ribbons (t=25 mum): Nd4Fe86CuNb3B6, Nd8Fe82CuNb3B6, Nd8Fe77Co5CuNb3B6 and Nd8Fe78Co5Hf3B6 with the volume ratio of Nd2Fe14B hard magnetic phase to alpha Fe soft magnetic phase of 28% and 70-73% respectively for the 4 at% Nd and 8 at% Nd in the investigated compositions. Exchange magnetic interactions are preponderant in samples with 8 at% Nd while in samples with 4 at% Nd, the dipolar type of interactions predominates at the investigated annealing conditions, With decreasing the volume ratio of the hard magnetic phase, the reversible behavior of the magnetization processes is accentuated and remanence becomes more sensitive to microstructure. All samples behave as exchange-spring magnets, the recoil permeability increasing with increasing the soft / hard volume ratio (e.g. from mu (rel) (rec)=0.8 for Nd8Fe77Co5CuNb3B6 to mu (rel) (rec)=1.7 for Nd4Fe86CuNb3B6, both treated at 750 degreesC for 1 min.). Nd8Fe73Co5Hf2B12 thick melt spun ribbons (t= 110 mum) obtained in nanocrystalline state due to the large content of B, exhibit a good exchange coupling between the grains of the soft and hard magnetic phases, which contributes to the value of the coercivity, H-i(c)=10.3kOe (not saturated at 1.6T), unusual high for the sub-stoichiometric Nd2Fe14B system.