Radiation-induced magnetic flux loss in permanent magnets

Abstract

This thesis reviews six publications studying the effects of radiation in permanent magnets. The radiation-induced magnetic flux loss has been measured in various types of experiment. The basic magnetic properties such as remanence (Bᵣ), coercive force (Hc) and maximum energy product ((BH)max) of the unalloyed Nd-Fe-B magnets has been measured at elevated temperatures. The magnetic flux loss has been measured as a function of the irradiation dose at 15 K and at 300 K with unalloyed and Nb- and Dy- alloyed Nd-Fe-B permanent magnets. The dependence of the radiation-induced magnetic flux loss on the irradiation temperature has been measured with proton and a-particle irradiations. The proton irradiations have been made in the temperature range of 20-300 K with Nd-Fe-B samples magnetized in two different ways and with Sm-Co permanent magnet samples at higher temperatures. In the a-particle irradiations we have used only one type of Nd-Fe-B samples. The dependence of the flux loss on the internal magnetic field has been studied using samples magnetized in different ways as well as doing the irradiations in an external magnetic field. The dependence of flux loss on the particle energy has been measured with 14-20 MeV proton beam. The experimental results show that the radiation-induced flux loss is highly dependent on the irradiation temperature and on the magnetic field inside the sample. We have developed a simple theoretical model to describe the mechanisms of the radiation-induced flux loss. The model is based on the assumption that the incoming particle collides with an atom of the crystal and causes a thermal spike in the crystal which allows a nucleation of a new domain in the opposite direction. The good agreement between experimental and theoretical results support our suggestion of the mechanism of radiation-induced magnetic flux loss.