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

Abstract

The high remanence and intrinsic coercivity of Nd-Fe-B magnets make them desirable for use in accelerators. Use in a radiation environment requires that the sensitivity of these magnets to radiation be determined and the mechanisms responsible be understood. We present results comparing the relative radiation sensitivity of a number of commercially available Nd-Fe-B magnets. Small samples of these magnets were irradiated to examine the decay of remanence. Our results show that the sensitivity to radiation varies widely among magnets from different vendors, indicating that microstructure and possibly composition play key roles in determining the loss of remanence. We examined optically the microstructure of several of the magnet materials and report on the correlation between microstructure and radiation-induced changes in magnetic properties. Samples were neutron irradiated at the Omega West Reactor in a neutron flux of 4×1012 n/cm2 s at a temperature of 350 K (77 °C). At various time intervals during the irradiation, samples were removed from the reactor and the open-circuit remanence measured. We also measured the quadrant II demagnetization curve of large samples before and after neutron irradiation to monitor changes in remanence and intrinsic coercivity. In general, these samples showed a decrease in the remanence and an increase in the intrinsic coercivity. We also present results comparing the relative decay of remanence for the large cylindrical samples having three different length-to-diameter ratios. This illustrates the effect of the demagnetizing field in determining the rate of remanence decay.