Abstract
The high-temperature ferromagnet MnBi continues to receive attention as a
candidate to replace rare-earth-containing permanent magnets in applications
above room temperature. This is due to a high Curie temperature, large
magnetic moments, and a coercivity that increases with temperature. The
synthesis of MnBi also allows for crystals that are free of interstitial Mn,
enabling more direct access to the key interactions underlying the physical
properties of binary Mn-based ferromagnets. In this work, we use inelastic
neutron scattering to measure the spin waves of MnBi in order to characterize
the magnetic exchange at low temperature. Consistent with the spin
reorientation that occurs below 140~K, we do not observe a spin gap in this
system above our experimental resolution. A Heisenberg model was fit to the
spin wave data in order to characterize the long-range nature of the
exchange. It was found that interactions up to sixth nearest neighbor are
required to fully parameterize the spin waves. Surprisingly, the
nearest-neighbor term is antiferromagnetic, and the realization of a
ferromagnetic ground state relies on the more numerous ferromagnetic terms
beyond nearest neighbor, suggesting that the ferromagnetic ground state arises
as a consequence of the long-ranged interactions in the system.
