MnBi2 Is a Permanent Magnet

Abstract

Creating and understanding new permanent magnets requires an understanding of the impact of orbital angular momentum on coercivity. A simple approach to interrogating this relationship is by incorporating high Z (where Z is the atomic number) elements into binary compounds to maximize spin-orbit coupling. The Mn-Bi system is an appealing platform for these studies since it contains MnBi, a permanent magnet with a large coercive field. We previously identified a new compound in the Mn-Bi system, MnBi₂, but could not elucidate its magnetic properties ex situ due to its decomposition upon decompression. Here, we harnessed synchrotron X-ray magnetic circular dichroism to probe the magnetism of MnBi₂ at high pressure within a diamond anvil cell. Our results indicate that MnBi₂ exhibits ferromagnetic hysteresis at both 10 K and room temperature. Through calculations and experiments, we show that orbital angular momentum and spin-orbit coupling from Bi impart magnetic anisotropy in MnBi₂. Comparing the Mn-Bi family of compounds, we consider the Bi p and d orbitals to explain the differences in magnetic behavior within the system. Collectively, these results validate leveraging high-Z elements in the synthesis of new hard permanent magnets.