First-principles study of phase stability and magnetic properties of B2 AlCr, AlMn, AlFe, AlCo and AlNi aluminides

TL;DR

This work uses first-principles DFT within the EMTO-CPA framework to map the phase stability and magnetic ordering of equiatomic AlX aluminides (X = Cr, Mn, Fe, Co, Ni) across sc, bcc, and B2 structures. It demonstrates that the ordered B2 phase is energetically preferred over disordered bcc for all five systems, with Co acting as the strongest B2 stabilizer, while magnetic tendencies vary from AFM in AlCr to NM in AlCo and AlNi. Stoner analysis and DOS considerations rationalize the observed magnetic trends, and electrostatic (Madelung) contributions help explain the stability differences among late- versus early-transition-metal aluminides. Notably, the B2 AlCr–Co system can transition from AFM to FM with Cr moments dominating the magnetization, suggesting a potential magnetocaloric material design route analogous to FeRh. The results provide mechanistic insight into how crystal structure, chemistry, and magnetism interplay to govern phase stability in Al–3$d$ aluminides, and point to Co-doped AlCr as a promising ferromagnetic B2 intermetallic.

Abstract

Using ab initio Density Functional Theory (DFT) calculations, we investigate the electronic structure, phase stability, and magnetic properties of equiatomic binary alloys between Al and 3d magnetic transition elements (Cr, Mn, Fe, Co, and Ni). Thermodynamically, all five binary aluminides are more stable in the ordered B2 phase than in the disordered body centered cubic phase, and Co is found to be the strongest B2 forming element with Al. The AlCo and AlNi compounds with B2 structure are verified to be non-magnetic, whereas AlFe turns out to be weakly magnetic, which is consistent with other DFT calculations employing similar exchange-correlation approximations. Magnetic simulations based on the Heisenberg Hamiltonian predict an antiferromagnetic ground state for the hypothetical B2 AlCr, which is also confirmed by direct DFT calculations. Doping AlCr with Co leads to an antiferromagnetic to ferromagnetic transition, where ferromagnetism is to a large extent attributed to Cr atoms. The phase stability and magnetic trends are explained using electronic structure arguments. The present findings contribute to a deeper understanding of the phase stability and magnetic properties of AlX binary alloys, providing insights into the formation mechanisms of the B2 structure with 3d magnetic transition metals.

Figures (6)

• Figure 1: Theoretical total energies as a function of Wigner-Seitz radius for simple-cubic Cr (a), Mn (b), Fe (c), Co (d), and Ni (e). Results are shown for FM (blue), NM (green), and PM (red) states. In panel (a), the energy curves overlap for the three magnetic states.
• Figure 2: Theoretical total energies as a function of Wigner-Seitz radius for AlCr (a), AlMn (b), AlFe (c), AlCo (d) and AlNi (e) for B2 (closed symbols) and bcc (open symbols) structures. Results are shown for FM (blue), NM (green), and PM (red) states. All curves are plotted relative to the B2 FM energy minimum. In panels (a) & (e), the bcc energy curves overlap for the three magnetic states, while in panel (b), they overlap at Wigner-Seitz radii below 2.74 Bohr. In panels (d) & (e), the B2 energy curves overlap with each other for all three magnetic states.
• Figure 3: The calculated DOS of non-magnetic sc Cr, Mn, Fe, Co, and Ni for sc (a), B2 AlCr, AlMn, AlFe, AlCo, and AlNi (b), and bcc AlCr, AlMn, AlFe, AlCo, and AlNi (c). All calculations were carried out at the corresponding theoretical equilibrium volumes. Panel (d) collects the DOS values at the Fermi level for the three structures.
• Figure 4: The calculated DOS of ferromagnetic B2 AlCr, AlMn, AlFe, AlCo, and AlNi (a), and bcc AlCr, AlMn, AlFe, AlCo, and AlNi (b). All calculations were carried out at the corresponding theoretical equilibrium volumes. Panel (c) collects the DOS values at the Fermi level for the two structures.
• Figure 5: (a) The calculated Cr-Cr exchange interaction $J_{\text{0j}}$ for B2 AlCr. $\mathrm{R}_{\mathrm{0}j}$ represents the distance between atom $j$ and atom at the origin in units of B2 lattice parameter $a$. (b) The theoretical total energies of AlCr for the B2 structure in FM, AFM, and PM states. The FM and PM curves are identical to those in Fig. \ref{['fig:2']} and all energies are plotted relative to the AFM minimum. (c) The calculated magnetic energy difference ($\Delta E$ = $E^{\mathrm{FM}}$-$E^{\mathrm{AFM}}$) and total magnetic moment (per formula unit) in the FM state for B2 $\text{Al}_{50}\text{Cr}_{1-x}\text{Co}_x$ as a function of Co content.