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• Ferromagnetic MnBi with high coercivity could be synthesized by liquid phase sintering (LPS). • The complex nature of LPS processes for producing MnBi was demonstrated. • The formation of MnBi is mainly governed by diffusion processes. • The diffusion coefficient was found to follows a well-known Arrhenius equation. Low-temperature phase manganese bismuth (LTP-MnBi) was synthesized by low-temperature liquid phase sintering in ultrahigh vacuum at 275, 325 and 375 °C. The magnetic and structural properties of the sintered MnBi powders with two sets of different particle size ranges (<20 μm and 20–53 μm) were studied. It was found that the smaller particles exhibit twice the coercivity (H c), with approximately 15% lower saturation magnetization, compared to the larger ones. Powder LTP-MnBi with a maximum H c of 4.98 kOe could be produced at sintering temperature of 325 °C. The elemental depth profile in the MnBi layers at the inside-surface of the cracks in Mn powder particles were measured by using scanning electron microscopy combined with energy dispersive X-ray spectroscopy. MnBi is seeded at cracks through which liquid Bi flows during sintering and grows via diffusion. Variation of the liquid Bi flow rate in cracks results in a distribution of effective diffusion lengths. The diffusion coefficient was estimated from the maximum diffusion length to be 3.47 × 10-14, 7.68 × 10-14 and 15.30 × 10-14 cm2/s at 275, 325 and 375 °C, respectively. The coefficient follows a well-known Arrhenius equation with the pre-exponential factor of 5.33 × 10-10 cm2/s and activation energy of 0.45 eV. [ABSTRACT FROM AUTHOR]

This study investigated the evolution of magnetic properties in low-temperature vacuum-sintered MnBi samples over an 18-month period at room temperature. Two sample conditions were examined: one exposed to ambient air in powder form and the other softly compressed within a sealed tube. Over time, both samples demonstrated an enhanced remanence ratio, resulting in a significant increase of over 60% in the energy product (BH) max. The compressed sample in the tube exhibited greater magnetic alignment and slightly higher values of (BH) max. Distinct evolution patterns of the M-H curves were observed in the two samples. Oxides were detected in the as-prepared samples, showing a slight increase during the exposure period. Furthermore, the particle size experienced a substantial growth from approximately 10 µm to 30 µm. These changes in magnetic properties were attributed to the continuous formation of MnBi, governed by the diffusion mechanism, and the creation of new complex oxides. The magnetic and phase stabilities of MnBi could benefit from its Mn/MnBi/Bi core/double-shell structure. The diffusion mechanism was proposed as the primary factor contributing to the long-term enhancement of (BH) max in the preserved MnBi samples. To describe the increase in (BH) max , a relation between (BH) max and diffusion length was established. • The evolution of magnetic properties in sintered MnBi samples over an 18-month period was observed. • A significant increase of over 60% in (BH) max was observed in the 18-month old sample compared to the fresh one. • The magnetic property changes were attributed to continuous MnBi formation via diffusion and the creation of new oxides. • The magnetic and phase stabilities of MnBi could benefit from its Mn@MnBi@Bi core@double-shell structure. • A relation between (BH) max and diffusion length was established to describe the increase of its magnetic performance. [ABSTRACT FROM AUTHOR]

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 °C. The incorporation of up to 10 at% Al led to a significant enhancement in coercivity (H c), 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 H c 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. [Display omitted] • Large-scale synthesis of MnBi can be achieved through sintering in a vacuum. • An increase of up to 16 % in (BH) max was observed in MnBi with 5 at% Al composites. • 3–5% Al interacts with the MnBi phase, with excess forming an Al-rich phase. • The formation of Mn-Bi-Al ternary alloys was confirmed by XPS analysis. • The AlMn/MnBi slab-DFT model shows enhanced MnBi magnetization with Al inclusion. [ABSTRACT FROM AUTHOR]