Microstructural insights into the coercivity enhancement of grain-boundary-diffusion-processed Tb-treated Nd-Fe-B sintered magnets beyond the core-shell formation mechanism
Kristina Žagar Soderžnik, Kristina Žužek Rožman, Matej Komelj, András Kovács, Patrick Diehle, Thibaud Denneulin, Aleksei Savenko, Marko Soderžnik, Spomenka Kobe, Rafal E. Dunin–Borkowski, Joachim Mayer, Boštjan Markoli, Sašo Šturm
Abstract
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.