Abstract
We have performed first-principles calculations to obtain magnetic moment, magnetocrystalline anisotropy energy (MAE), i.e., the magnetic crystalline anisotropy constant (K), and the Curie temperature (Tc) of low temperature phase (LTP) MnBi and also estimated the maximum energy product (BH)max at elevated temperatures. The full-potential linearized augmented plane wave (FPLAPW) method, based on density functional theory (DFT) within the local spin density approximation (LSDA), was used to calculate the electronic structure of LPM MnBi. The Tc was calculated by the mean field theory. The calculated magnetic moment, MAE, and Tc are 3.63 μB/f.u. (formula unit) (79 emu/g or 714 emu/cm3), −0.163 meV/u.c. (or K = −0.275 × 106 J/m3) and 711 K, respectively. The (BH)max at the elevated temperatures was estimated by combining experimental coercivity (Hci) and the temperature dependence of magnetization (Ms(T)). The (BH)max is 17.7 MGOe at 300 K, which is in good agreement with the experimental result for directionally-solidified LTP MnBi (17 MGOe). In addition, a study of electron density maps and the lattice constant c/a ratio dependence of the magnetic moment suggested that doping of a third element into interstitial sites of LTP MnBi can increase the Ms.
Highlights
The maximum energy product (BH)max of a permanent magnet at elevated temperatures has become increasingly important because a motor for electric vehicles (EV) should be operational at473 K
The package is based on density functional theory (DFT) and uses the full-potential linearized augmented plane wave
The density of states (DOS) near the EF is a highly-degenerated energy state; slightly below the EF in the majority spin state and slightly above the EF in the minority spin state. These highly-degenerate energy states near EF are the origin of the magnetic moment of low temperature phase (LTP) MnBi by contributing to the difference between the number of electrons in the majority and minority spin states below EF
Summary
The maximum energy product (BH)max of a permanent magnet at elevated temperatures has become increasingly important because a motor for electric vehicles (EV) should be operational at473 K. The maximum energy product (BH)max of a permanent magnet at elevated temperatures has become increasingly important because a motor for electric vehicles (EV) should be operational at. The Nd-Fe-B magnets exhibit desirable magnetic flux density (B), intrinsic coercivity (Hci and (BH)max), a large negative temperature coefficient of Hci, a low Curie temperature (Tc) of 523 K [3]. The usage of Nd-Fe-B magnets is limited to a lower temperature than 473 K, due to their negative temperature coefficient of Hci and low. Low temperature phase (LTP) MnBi shows a positive magnetic anisotropy coefficient [4,5,6] resulting in Hci of about 1.5 T at 300 K and 2 T at 400 K [5]. The high Hci of LTP MnBi helps to make it usable at the operating temperature of the motor. Arc-melted and mechanically-milled LTP MnBi powder shows a low Br of 0.7 T and (BH)max of 11.00 MGOe [8] and Br of 0.7 T and (BH)max of 11.95 MGOe [9] at
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