Extended Magnetic Exchange Interactions in the High-Temperature Ferromagnet MnBi

TL;DR

This study uses inelastic neutron scattering to analyze magnetic exchange interactions in MnBi, revealing long-range ferromagnetic interactions up to sixth nearest neighbors that underpin its high-temperature ferromagnetism.

Contribution

It provides the first detailed characterization of long-range magnetic exchange interactions in MnBi, highlighting the importance of extended interactions for its ferromagnetic ground state.

Findings

Interactions up to sixth nearest neighbor are significant.

Nearest-neighbor interaction is antiferromagnetic.

Long-range ferromagnetic order arises from multiple ferromagnetic interactions.

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.