Effects of neutron irradiation on Nd-Fe-B magnetic properties

Nd-Fe-B permanent magnets from two different manufacturers were irradiated in the Omega West reactor at Los Alamos National Laboratory, with fast neutrons at temperatures of 426 K (153 degrees C) and 350 K (77 degrees C) to fluences of 5.0*10¹⁶ N/cm² and 6.1*10¹⁶ N/cm², respectively. At intervals during the irradiation the samples were removed from the reactor and the remanence measured at room temperature. The initial loss of remanence for irradiation at 426 K was 10% for a fluence of 10¹⁵ N/cm². At 350 K the initial loss rate was roughly half this value. The loss rates were nearly the same for samples from the two different manufacturers. These losses are due to the irradiation since the remanence does not decay with annealing at 426 K. Remagnetization after irradiation results in full recovery of the remanence and roughly a 20% increase in the coercivity. Evidence from this experiment suggests that the primary mechanism for loss of remanence is nucleation of reverse domains by the collision cascade and subsequent growth into the original domain.