Measurement of radiation-induced demagnetization of Nd–Fe–B permanent magnets

Nd–Fe–B permanent magnets are highly desirable for use in the insertion devices of synchrotron radiation sources due to their high remanence, or residual magnetic induction, and intrinsic coercivity. However, the radiation environment within high-energy storage rings makes essential the determination of the degree of radiation sensitivity as well as the mechanisms of radiation-induced demagnetization. Sample Nd–Fe–B permanent magnets were irradiated at the Advanced Photon Source with bending magnet X-rays up to an absorbed dose of approximately 280 Mrad (1 Mrad=10kGy). Sample magnets were also irradiated with 60Co γ-rays up to an absorbed dose of 700 Mrad at the National Institute of Standards and Technologyʹs standard gamma irradiation facility. Changes in the residual induction were found to be within the experimental uncertainties for both the X-ray and γ-ray irradiations. Sample Nd–Fe–B permanent magnets were then irradiated at Oak Ridge National Laboratoryʹs Californium User Facility for Neutron Science with fast neutrons up to a total fast fluence of 1.61×1014 n/cm2, and with thermal neutrons up to a total thermal fluence of 2.94×1012 n/cm2. The fast-neutron irradiation revealed changes between residual induction measurements of the sample magnets before and after irradiation of 0.6% and greater for fast-neutron fluence levels of 2×1013 n/cm2 and above. Thermal-neutron irradiation revealed changes in the residual induction measurements of the sample magnets before and after irradiation that were within the experimental uncertainties for thermal-neutron fluence levels up to 3×1012 n/cm2.