Exchange coupling effect and magnetic properties in sintered RE-Fe-B magnets

Abstract

Preparing the magnetic materials bearing both a high coercive field and a high energy product is a great challenge. In this paper the content of Nd was reduced in Nd-Fe-B for enhancing the interaction effect as well as for increasing the energy product, and Tb-Fe-B powders were mixed to enhance the coercivity. In the sintered RE-Fe-B magnets some of Tb atoms diffuse into the outer regions of Nd-Fe-B grains, and Tb-rich RE2Fe14B grains also exist with the crystal size at micrometer scale. The recoil loops bear the spring behaviors, and the reversal susceptibility implies the availability of exchange coupling effect among the micro grains. The activation volume also indicates that the exchange coupling is effective in magnetization reversal. So the magnetization reversal in Nd-Fe-B grains could be restrained by the high anisotropy phase of Tb-rich core. Therefore, besides the effects of Tb-diffusion into grain outer region and the intergranular phase, the exchange coupling effect could not be neglected in improving the coercivity in the Nd-content-reduced magnets, and both a high coercive field of 20.0 kOe and a high energy product of 50.1 MGOe are achieved in the micro-crystal composite magnets.