An analysis of neutron radiation effects on NdFeB permanent magnets

Radiation-induced demagnetization of permanent magnets can result in the failure of magnet-based devices operating in high-radiation environments. To understand the mechanism underlying radiation-induced demagnetization, NdFeB magnets were irradiated with fast and fast plus thermal neutrons. After irradiation, magnetic flux losses were measured and were shown to increase with the fluence. Compared with samples irradiated only with fast neutrons, the samples exposed to the fast plus thermal neutrons had higher magnetic flux losses, which was attributed to the thermal neutron capture reaction of boron. Hysteresis loops of the NdFeB magnets were analyzed using the Jiles Atherton model and revealed an increase in both the coercivity, and the interaction between the domains in the magnet post irradiation and also a decrease in the local magnetic moments. Full remagnetization of the samples after irradiation was possible, which indicates that structural damage is unlikely to be an important factor in the demagnetization process at these levels of neutron energy and fluence. This also indicated that thermal spikes and domain reversal in the affected areas play an important role in the process.;A Molecular Dynamics (MD) simulation was performed on a cube of iron to obtain a better understanding of the thermal spike mechanism and to get a better idea of the time and length scales involved in the process. The results led to time scales on the order of a hundred femtoseconds and length scales on the order of a few nanometers. Finally, an electronic structure calculation based on density functional theory was performed on NdFeB to characterize the role of defects in magnetism loss. The results of the simulation clearly showed that the change of the magnetization depends on the type of structural defect being introduced. However, it was shown that defects introduced to the perfect structure to mimic the effects of fast and thermal neutrons can lead to an observable change in the magnetization. This agreed with previous experimental observations. The effect of defects on the magnetization is expected to be more pronounced with higher energy and higher flux neutron radiation. Finally, it is recommended that a Monte Carlo simulation be developed to predict magnetism loss as a result of radiation. This model should account for multiple thermal spikes and domain reversals throughout the magnet and should also account for structural damage to the magnet after a certain cutoff in radiation fluence and energy.