Characterization of Magnetic Properties of Low-Temperature Phase (LTP) Synthesized by Surfactant-Assisted Cryo-Milling Process in MnBi Binary System

MnBi low-temperature phase (LTP) has excellent magnetic intrinsic properties and attractive characteristics as a permanent magnet material. It is necessary to synthesize fine particles of MnBi-LTP with a size close to that of its single magnetic domain (
$\approx 500$
nm) to improve the magnetic anisotropic characteristics. Although the room-temperature ball milling process has been reported as a common method to reduce MnBi particle size, it has an inevitable limitation in reducing the powder size due to the low milling energy, as well as the powder agglomeration by the cold welding phenomenon. In this study, we synthesized fine particles of MnBi-LTP by the cryo-milling process at a temperature of 77 K. Phase analysis and magnetic properties were examined as a function of the amount of stearic acid for the cryo-milled powders. As a result, it was found that stearic acid as a surfactant plays an important role in suppressing the decomposition of the magnetic phase that occurs during the cryo-milling process. Consequently, high-purity fine MnBi-LTP particles, with (
BH
)max of 9.25 MGOe, were obtained through this method. Thus, this indicates that the cryo-milling process has the potential to synthesize fine MnBi-LTP particles with superior magnetic properties.