Abstract
Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice constants (a, c) and bulk elastic modulus (B) below 100 K, spin reorientation and magnetic susceptibility maximum near 90 K, and, importantly for high-temperature magnetic applications, an increasing coercivity (unique to MnBi) above 180 K. We calculate the total energy and magneto-anisotropy energy (MAE) versus (a, c) using DFT+U methods. We reproduce and explain all the above anomalies. We predict that coercivity and MAE increase due to increasing a, suggesting means to improve MnBi permanent magnets.
Highlights
Spin reorientation arises from a change of sign in magnetic anisotropy energy (MAE), which depends on increasing a, This suggests simple means to control MAE: by thermal expansion, or by strain or alloying, e.g., coherent interfacing or doping
The calculated MAE changes sign with a small increase in a, which causes spin reorientation during thermal expansion. (iii) The magnetic susceptibility has a maximum at MAE = 0. (iv) Further increase of MAE with thermally expanding a increases coercivity at T > 180 K, where |MAE| > kT
Due to its sensitivity on a, the MAE can be altered by temperature, pressure, doping, or interfacial strain.[36,37,38,39]
Summary
(Received 9 December 2013; accepted 19 February 2014; published online 4 March 2014) We provide theoretical explanation for the long-standing experimental puzzles in the measured coercivity, spin orientation, lattice constants, and bulk modulus of MnBi. We suggest a means to further increase the MAE.
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