Effect of composition and heat treatment on MnBi magnetic materials

Abstract

The metallic compound MnBi is a promising rare-earth-free permanent magnet material, unique among all candidates for its high intrinsic coercivity (Hci) and its large positive temperature coefficient. The Hci of MnBi in thin-film or powder form can exceed 12 and 26 kOe at 300 and 523 K, respectively. Such a steep rise in Hci with increasing temperature is unique to MnBi. Consequently, MnBi is a highly sought-after hard phase for exchange coupling nanocomposite magnets. However, the reaction between Mn and Bi is peritectic, and hence Mn tends to precipitate out of the MnBi liquid during the solidification process. As result, when the alloy is prepared using conventional induction or arc-melting casting methods, additional Mn is required to compensate the precipitation of Mn. In addition to composition, post-casting annealing plays an important role in obtaining a high content of MnBi low-temperature phase (LTP) because the annealing encourages the Mn precipitates and the unreacted Bi to react, forming the desired LTP phase. Here we report a systematic study of the effect of composition and heat treatments on the phase content and magnetic properties of Mn-Bi alloys. In this study, 14 compositions were prepared using conventional metallurgical methods, and the compositions, crystal structures, phase content and magnetic properties of the resulting alloys were analyzed. The results show that the composition with 55 at.% Mn exhibits both the highest LTP content (93 wt.%) and magnetization (74 emu g À1 with 9 T applied field at 300 K).

Single-crystal structure determination of two new ternary bismuthides: Rh6Mn5Bi18 and RhMnBi3

“…One promising candidate is the intermetallic phase -BiMn; unfortunately, it has not been possible to synthesize this phase as a single-phase bulk material in spite of intensive research (e.g. Liu et al, 2004;Rama Rao et al, 2013;Cui et al, 2014;Chen et al, 2015;Marker et al, 2018). A possible approach to circumvent these problems was considered to be the addition of a third component, e.g.…”

Structural and Ferromagnetic Properties of an Orthorhombic Phase of MnBi Stabilized with Rh Additions

“…As result of the peritectic decomposition at high temperatures, when LT MnBi is prepared by using conventional induction or arc-melting casting methods, additional Mn as well as postcasting annealing is required to form the desired LT phase. The HT Mn 1.08 Bi is often obtained during the preparation of LT MnBi by various metallurgical methods [9]. It is therefore essential to elucidate its structural and magnetic properties as well as to investigate the circumstances under which this phase can become more or less stable.…”

Extended magnetic exchange interactions in the high-temperature ferromagnet MnBi

“…It also displays favorable magneto-optical properties such as a large Kerr effect, which may be useful as a magnetic storage medium in thin films 9,10 . For this reason, there is an ongoing, concerted effort to understand the growth conditions to optimize its properties for these applications 11 . Despite the recent interest in MnBi, the microscopic origin of its magnetic properties have not been completely determined.…”