Novel processing of high-performance MnBi magnets

Abstract

Rare-earth-free MnBi magnets have attracted much attention recently due to their positive temperature coefficient of coercivity. In this work, the preparation, microstructure and magnetic properties of bulk MnBi magnets have been investigated. A low-temperature (−120 °C), low-energy ball-milling (LTLEBM) process has been adopted in the initial MnBi powder preparation that reduces the particle size to 1–5 μm from the 35–75 μm size of raw material powders of the MnBi low-temperature phase (LTP) (∼97 wt%) made by melting and annealing. The LTLEBM process has significantly suspended the decomposition of the LTP MnBi that occurs excessively during ordinary room-temperature ball milling. After the LTLEBM, the coercivity iHc of the MnBi powder was increased from 1 kOe to 12 kOe while the LTP content in the powder was retained as high as 95 wt%. The as-milled powders were then aligned in an 18 kOe field and warm-compacted into a dense bulk magnet at 300 °C for 10 min to reach a mass density of ∼8.4 g cm−3. The bulk magnets have a maximum energy product of 7.8 MGOe and coercivity of 6.5 kOe at room temperature. When the temperature is increased to 475 K, the coercivity is increased to 23 kOe.