Composition and property optimization of rare-earth-free Mn-Al-C magnet by phase stability and magnetic behavior analysis

Mn-Al-C system offers a possibility of rare-earth-free permanent magnets with the reduced temperature-dependent deterioration of magnetic properties. The Mn-Al-C alloy composition and magnetic properties have been optimized via calculations of electronic structures and phase stability of L1₀-ordered ferromagnetic τ-phase (Mn₀.₅Al₀.₅)_100−xC_x. The WIEN2k program package, Vienna Ab-initio simulation package (VASP), and Alloy Theoretic Automated Toolkit (ATAT) were used to calculate and identify the optimal carbon content for the most stable τ-phase of the L1₀-structured Mn-Al-C. We used the Brillouin function and Callen-Callen semiempirical relation to obtain the saturation magnetization and magnetocrystalline anisotropy constant at elevated temperatures. It was found that a carbon content of 2.33 at% gives the most stable τ-phase L1₀ (Mn₀.₅Al₀.₅)_100−xC_x that has the lowest formation energy and highest saturation magnetization among the studied carbon contents (x = 0–3.03 at%). The magnetocrystalline anisotropy constant at 0 K increases with increasing the carbon content. Therefore, the carbon-doped Mn-Al becomes magnetically harder than the pure Mn₅₀Al₅₀. However, the anisotropy constant decreases at 300 K as the carbon content increases. The Curie temperature decreases to 590 K at x = 2.33 from 685 K at x = 0.0. The estimated saturation magnetization was approximately 130 emu/g at 300 K, leading to 18 MGOe under B_s = B_r and H_ci> B_r/2. Therefo_re, it is highly probable that Mn-Al-C potentially fills the gap between 10 and 30 MGOe magnets. The results in this study quantify and explain the reason for widely studying the approximately 2 at% carbon-doped Mn-Al systems.