Coercivity mechanism and FORC analysis of MnBi-based permanent alloy

Abstract

The Mn55Bi45−xZrx (x = 0, 1, 3 and 5) permanent alloys are prepared by melt-spinning process with subsequently annealing. The microstructure evolution of Mn-Bi-Zr alloys was systematically studied by XRD, DSC and TEM, their coercivity mechanism was discussed by Kronmüller equation and magnetic behaviors were analyzed by FORC. The XRD results showed the proportion of the low temperature phase (LTP) MnBi in the as quenched MnBi ribbons increased with the addition of Zr, and improved by annealed treatment. According to the Kronmüller equation, the coercivity mechanism for the Zr-free and Zr-doped MnBi alloys is explained by the nucleation of reverse domains. The FORCs diagram showed that more than one major peak appeared for x = 0 sample, demonstrating the dispersive distribution of the interaction, resulting from isolated LTP MnBi grains by the Mn matrix from TEM HAADF image, and the dispersive distribution of the interaction led to collapsed hysteresis loop. Comparing to the Zr-free sample, one stronger peak is located at the lower Hc in the x = 1 sample, indicating a concentrated distribution of the interaction, which resulted in a single-phase magnetized loop and the lower coercivity.