Radiation-induced changes in magnetic properties of Nd-Fe-B permanent magnets

Small magnets were irradiated in a fast neutron flux of 4×10 ¹² n/cm²-s at 350 K (77°C). At various time intervals during the irradiation, samples were removed from the reactor and the open-circuit remanence was measured. It is shown that the rate of remanence decay decreases monotonically with increasing length-to-diameter (L/D) ratio for samples of the same material. Samples of different materials having an L/D ratio of 4.4. were irradiated to study the decay of the remanence. It is shown that resistance to radiation-induced decay is improved by the substitution of Dy or Tb for Nd. The results also show that the sensitivity to radiation varies widely among magnets from different vendors. Possible reasons for this variation are discussed