Microstructural insights into the coercivity enhancement of grain-boundary-diffusion-processed Tb-treated Nd-Fe-B sintered magnets beyond the core-shell formation mechanism

• Published in: Journal of alloys and compounds 2021-05, Vol.864, p.158915, Article 158915
• Main Authors: Soderžnik, Kristina Žagar, Rožman, Kristina Žužek, Komelj, Matej, Kovács, András, Diehle, Patrick, Denneulin, Thibaud, Savenko, Aleksei, Soderžnik, Marko, Kobe, Spomenka, Dunin-Borkowski, Rafal E., Mayer, Joachim, Markoli, Boštjan, Šturm, Sašo
• Format: Article
• Language: English
• Subjects: Atom-probe tomography; Cascade chemical reactions; Coercivity; Core-shell structure; Electron energy-loss spectroscopy; Ferrous alloys; Grain boundaries; Grain-boundary diffusion; Magnetic domains; Magnetic induction; Magnetic materials; Magnetic properties; Magnetic saturation; Magnetism, Nd-Fe-B; Neodymium; Off-axis electron holography; Permanent magnets; Sintering; Transmission electron microscopy
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2021.158915

•Revealed dominant core-shell mechanism for grain boundary diffusion processed permanent magnet.•A structure-chemistry-magnetic-property analysis of Nd-Fe-B magnet used for electric vehicles and wind turbines.•Gaining high efficiency of Nd-Fe-B via grain boundary engineering.•Importance of high coercivity permanent magnets for production of electric components on a macro and nanoscale. We propose a dominant core-shell formation mechanism for grain-boundary-diffusion-processed (GBDP), Tb-treated, Nd2Fe14B sintered magnets. A depth-sensitive analysis of Tb-treated samples, relative to a non-GBDP Nd2Fe14B magnet, showed a 30% increase of the coercivity in the central part of the magnet. A structure-chemistry-magnetic-property analysis revealed the dominant GBDP mechanism. On the surface of the Tb-treated magnet, the Tb is released from the starting precursor following a cascade of chemical reactions between the Tb oxide and the Nd and/or the Nd-Fe-B. The released Tb diffuses along the grain boundaries, forming a core-shell structure. The calculated optimum concentration for a 30% increase in the coercivity was 50 ppm of Tb. Off-axis electron-holography measurements were used to quantitatively map the characteristic magnetic states of the samples, confirming a different magnetic domain structure in the shell than in the core. The magnetic induction in the core was found to be 26% higher than that of the shell, which has a lower magnetic saturation due to the presence of Tb. The results show that the measured increase in the coercivity is due to a structural effect, and not the magnetic contribution of the Tb. Our results pave the way towards grain-boundary-engineering studies that can be used to increase the coercivity of Nd-Fe-B magnets for e-mobility and eco-power applications.