Switching of Coercivity Process in MnBi Alloys

Abstract

MnBi-based alloys represent an interesting choice for developing rare earth-free permanent magnets due to the high magnetocrystalline anisotropy of their characteristic low-temperature intermetallic phase (LTIP) with hexagonal structure. In this work, we discuss the switching of coercivity mechanism in MnBi alloys by modulation of their phase distribution and microstructure. As-cast MnBi alloys obtained by suction-casting technique exhibited LTIP interspersed within Bi- and Mn-rich areas. A noticeable coercivity field of 282 kA/m was observed. The coercivity mechanism for this alloy was explained in terms of the nucleation of reverse domains after saturation, by means of the Kronmuller equation, which incorporates the detrimental effect of microstructure defects through fitting parameters associated to reduced intrinsic magnetic properties at grain size boundaries, interfaces, and local demagnetizing fields. Subsequent annealing at 583 K for 24 h produced a marked reduction of coercivity (down to 16 kA/m), reflecting a switching of coercivity process from nucleation to pinning of domain walls. The key microstructural feature determining this variation is the formation/suppression of Bi-rich areas, which promotes the nucleation and growth of the initial MnBi intermetallic phase.