Manganese-bismuth (MnBi) compounds are promising candidates for permanent magnets. MnBi has been known to exhibit permanent magnetism since the 1950s. Today powerful samarium-cobalt (SmCo) and neodymium (NdFeB) magnets are typically used for high-performance magnetic applications. However, their rare-earth elements can be expensive and ecologically harmful to extract from ores. In contrast, manganese-bismuth materials are abundant and cheaper, and constitute excellent platforms for investigating the next generation of higher-flux permanent magnets. In these binary compounds, the overall magnetic field arises from both the electron spins of the lighter element (manganese) and the heavier element's (bismuth) spin orbital momenta. 

This study focused on studying a new manganese-bismuth phase, MnBi₂. One challenge surrounding MnBi₂ is that it requires very high pressures to synthesize and cannot be recovered to ambient conditions (returned to standard atmospheric pressure). In order to detect the magnetic moments in high-pressure MnBi₂, the researchers turned to X-ray magnetic circular dichroism (XMCD) performed at beamline 4-ID-D of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, using a sample of MnBi₂ within a diamond anvil cell (DAC). The XMCD technique revealed a substantial magnetic anisotropy due to spin-orbit coupling that gave rise to permanent magnet behavior. These results corroborate leveraging high Z elements to develop next-generation magnets for a variety of uses.

Permanent magnet behavior derives from two distinct atomic-level spin effects. First, permanent magnets require alignment of spin from individual electrons that sum up to create larger magnetic moments. If these collective spins stay aligned after a magnetizing force is removed, then the material can form a permanent magnet.

A second key spin dynamic is an electron's motion (orbital motion) around its nucleus. The combination of electronic orbital and spin momenta, called spin-orbit coupling (SOC), can strongly resist spin flipping. This inherent property, known as magnetic anisotropy, constrains the direction of the permanent magnet's field and imparts high coercivity. Magnets with higher coercivity require a larger magnetic field to demagnetize them.

Scientists strive to create magnets with large coercivity and magnetic flux. Elements with a high number of protons, like bismuth with Z = 83, are good components for making such magnetic materials because they have strong SOC behavior. MnBi is well known for its high coercivity at elevated temperatures. And the same researchers who uncovered permanent magnetism in MnBi also predicted that MnBi₂ forms a new Mn–Bi magnetic phase. Comparing the MnBi and MnBi₂ magnetic phases may prove most helpful in designing new magnetic materials, since scientists can relate their magnetic characteristics to their distinct crystalline and electronic structures.

However, it has proven difficult to characterize the MnBi₂ phase since it only persists at very high pressures, and many conventional methods for measuring magnetic properties cannot be used to probe samples within a diamond anvil cell. So the researchers turned to synchrotron XMCD.

XMCD employs both right and left circularly polarized X-rays. The technique is highly element-specific because the energies of the x-rays are tuned to match the absorption edge of selected core electrons of a specific element. In this experiment XMCD probed the spin polarization of bismuth's 6d states. Because Bi atoms have no inherent unpaired spins, any signal detected reflects an interaction with the unpaired spins of manganese.

The XMCD data revealed significant orbital momenta associated with the 6d electronic states within MnBi₂, which exceeded the corresponding orbital moments in MnBi. The researchers attribute the relatively greater orbital momenta of the MnBi₂ 6d states to increased spin-orbit coupling, arising from the greater proportion of bismuth in MnBi2 versus MnBi.

Hysteresis data (Fig. 1) was gathered from MnBi₂ at 10 kelvin and 9.4 gigapascals. Under these conditions MnBi₂ is a permanent magnet. This magnetism weakened with increasing temperature, but the compound still retained remnant magnetism at room temperature. In contrast, magnetic behavior in MnBi is quite unusual as magnetic flux increases with increasing temperatures. This divergent temperature-dependent magnetic behavior of two chemically similar compounds highlights the dramatic effects due to different crystalline structures (Fig. 2).

This research constitutes an important step towards developing advanced permanent magnets. The results confirm the strategy of capitalizing on high-Z elements that possess significant spin-orbit coupling, with the goal of merging orbital and electronic spin moments from two distinct atomic species to achieve strong magnetocrystalline anisotropy. Future investigations will involve more complex ternary systems, featuring three distinct atomic species. – Philip Koth

See: C.K. Badding¹, E.A. Riesel¹, R.A. Murphy¹, D. Puggioni², D. Popov³, G. Fabbris³, D. Haskel³, J.M. Rondinelli², A.B. Altman⁴, D.E. Freedman¹, “MnBi₂ is a permanent magnet,” J. Am. Chem. Soc. 2025, 147, 29, 25129-25135 (2025) https://doi.org/10.1021/jacs.5c06874

Author affiliations: ¹Massachusetts Institute of Technology; ²Northwestern University; ³Argonne National Laboratory; ⁴Texas A&M University.

Authors D.E.F., A.B.A, C.K.B., E.A.R., and R.A.M. acknowledge the Department of Energy (DE-SC0023292) for support on all high-pressure synthesis and work creating and studying new permanent magnets. Portions of this work were performed at HPCAT (Sector 16) and Sector 4, Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA’s Office of Experimental Sciences, with partial instrumentation funding by NSF. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory, under Contract No. DE-AC02-06CH11357. E.A.R. and C.K.B. acknowledge support from the National Science Foundation Graduate Research Fellowship, under Grant No. DGE-2141064). This work was carried out in part using MIT.nano’s facilities. D.P. and J.M.R. were supported by the National Science Foundation (NSF), under Award No. DMR-2413680. They thank Max Hauschildt and Dawson Smith for early computational work on MnBi₂ supported by NSF through the Center for the Mechanical Control of Chemistry (No. CHE-2303044), part of the National Science Foundation (NSF) Centers for Chemical Innovation (CCI) program, Research Experience for Undergraduates (REU) program. We gratefully acknowledge Dr. Michael K. Wojnar for the insightful discussions and helpful comments on the manuscript. We thank Curtis Kenny-Benson for technical support setting up the resistive heating for our synthesis. C.K.B. and E.A.R. acknowledge support from the NSF-GRFP..

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