Formation and characterization of Mn-Bi-Al ternary alloys of enhanced magnetic performance in MnBi/Al composites

Low-temperature phase manganese bismuth (LTP-MnBi) and a series of aluminum (Al) composites were synthesized using a potentially facile and scalable low-temperature liquid-phase sintering method in a vacuum at 300 degrees C. The incorporation of up to 10 at% Al led to a significant enhancement in coercivity (Hc), increasing from 2.32 +/- 0.04 to 4.15 +/- 0.07 kOe, while saturation magnetization showed a slight decrease of less than 3 %. However, beyond this concentration, a dramatic reduction in Hc was observed. The density of the freshly compacted powders, which included up to 10 at% Al, remained relatively constant at 7.47-7.58 g/cm3 but decreased with excess Al. A maximum energy product (BH)max of 1 MGOe was achieved in the fresh sample, with a 16 % enhancement in (BH)max in the MnBi composite containing 5 at% Al. Scanning electron microscopy revealed distinct MnBi and Bi-rich regions, while Al-rich areas became prominent at Al concentrations above 10 at%. Energy dispersive spectroscopy confirmed that only 3-5 at% Al could be effectively incorporated into Mn-Bi regions, with excess Al unevenly distributed on the surface. X-ray photoelectron spectroscopy indicated the formation of Al, Al2O3, and potential Mn-Bi-Al ternary alloys. Additionally, slab-DFT models, such as AlMn/ MnBi, indicate that Al inclusion enhances the magnetization in MnBi composites, providing insights into its effects on the magnetic properties of Mn-Bi systems. These findings offer promising strategies to address the challenges posed by excess Bi in the MnBi structure, potentially optimizing the magnetic performance of similar non-magnetic or soft-magnetic composite systems.