Natural oxidation process of magnetic LTP in MnBi alloys. Challenges and issues for preserving the magnetic phase

The low-temperature phase (LTP) of MnBi alloy is considered a promising low-cost magnetic material, yet its practical application is hindered by challenges in the manufacturing process and susceptibility to oxidation. The alloy manufacturing process presents challenges, primarily due to the peritectic reaction to the alloying process of Mn and Bi. A controversial issue of the LTP phase currently is attributed to the presence of Bi, Mn and MnO as a consequence of alloy processing and its phase transition at temperatures over 355 °C. In this work, we investigate the natural oxidation behaviour of LTP-MnBi by analysing various types of samples over a 4-week period. Additionally, in order to enhance the oxidation resistance of the LTP, 3 types of protective coatings were tested against natural corrosion in MnBi samples. The oxidation state and the impact on the structural and magnetic properties of the alloy were closely monitored, and the results indicated a drop up to 55% in magnetization values. Our findings provide new insights into the oxidation mechanism of LTP-MnBi, challenge the existing idea that the increase in Bi content is solely due to the phase transition, and highlight the need for improved protective strategies to extend the lifespan and effectiveness of MnBi-based magnets. • Time dependent oxidative process of magnetic LTP of MnBi alloys • Evidence of Bi raising in MnBi alloys • Oxidative proposed mechanism of MnBi • Protective strategies of corrosion prevention in MnBi.