Self-diffusion and free volumes in Nd-Fe-B and Pr-Fe-B permanent magnets

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The present project aims at a combined study of self-diffusion and atomic free volumes in the rare-earth metal/transitionmetal permanent magnets Nd2Fe14B and Pr2Fe14B. Since intergranular melting occurs at hyperstoichiometric compositions these materials serve as application-oriented model system in order to study the diffusion behaviour of quasi-liquid intergranular films. Due to the high structure sensitivity of the material transport behaviour, the diffusion data are supposed to yield novel insight on the structure of confined liquid layers and the adjacent liquid-solid interfaces. In addition, these studies may elucidate the atomistic processes of liquidphase sintering and stress-induced magnetic texturing of these materials which are of substantial technical relevance. Specific information on the character (size, chemical environment) of interfacial free volumes and their variation upon the intergranular melting transition is gained from high-temperature measurements of the positron lifetime and the coincident Doppler broadening. The implantation of the radioisotopes 59Fe and 142Pr for the self-diffusion studies is performed at the off-line separator at the Institut für Strahlen- und Kernphysik of Universität Bonn in the framework of the common research project.