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 (T_c) 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 T_c was calculated by the mean field theory. The calculated magnetic moment, MAE, and T_c are 3.63 μ_B/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 (H_ci) and the temperature dependence of magnetization (M_s(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 M_s.

The phase transformation in two modes, including both displacive and massive growth of τ-phase from ε-MnAl(C), was observed by in situ transmission electron microscopy. The exact temperature range for different phase transformation modes was determined by magnetic measurements. The displacive growth of ε→τ in Mn₅₄Al₄₆ (or Mn₅₄Al₄₆C_2.44) occurs at temperatures below 650 K (or 766 K), above which both modes coexist. One-third or less of the ε-phase can be transformed into τ-phase via displacive mode while the remaining two-thirds or more via massive mode. In bulk τ-phase, most τ-nanocrystals formed via displacive mode are distributed in the matrix of large τ-grains that formed via massive mode. The typical massive growth rate of the τ-phase is 8–60 nm/s, while the displacive growth rate is low. A more complete understanding of the ε→τ phase transformations in the MnAl-based magnets was provided in this work, based on which the annealing process for ε→τ was optimized and thus high purity τ-phase with high saturation magnetization was obtained.

High-pressure torsion (HPT) is a severe plastic deformation technique where a sample is subjected to torsional shear straining under a high hydrostatic pressure. The HPT method is usually employed to create ultrafine-grained nano-structures, making it widely used in processing many kinds of materials such as metals, glasses, biological materials, and organic compounds. Most of the published HPT results have been focused on the microstructural development of non-magnetic materials and their influence on the mechanical properties. The HPT processing of magnetic materials and its influence on the structural and magnetic properties have attracted increasing research interest recently. This review describes the application of HPT to magnetic materials and our recent experimental results on Mn₃O₄, Mn₄N, and MnAl-based alloys. After HPT, most magnetic materials exhibit significantly reduced grain size and substantially enhanced coercivity.

Little is known about the physical properties of Mn₄C for which is unstable and difficult to prepare. We herein report on the unusual thermomagnetic properties of high purity Mn₄C powders obtained by plasma melting and magnetic separation processes. The saturation magnetization of Mn₄C increases linearly with increasing temperature in the range of 50 K–590 K and remains stable at temperatures below 50 K. The anomalous magnetization increases of Mn₄C with increasing temperature can be considered in terms of the Néel's P-type ferrimagnetism. At temperatures above 590 K, the Mn₄C decomposes into Mn₂₃C₆ and Mn, which would be partially oxidized into manganosite when exposed to air. The remanent magnetization of Mn₄C varies little with temperature. The Curie temperature of Mn₄C is around ∼870 K. The positive temperature coefficient (∼0.0072 Am² kg⁻¹ K⁻¹) of magnetization in Mn₄C makes it potentially important in controlling the thermodynamics of magnetization in magnetic materials.