Disproportionate influence of site disorder on the evolution of magnetic phases in anti-Heusler alloy Al$_2$MnFe

Anti-Heusler alloys, being a new addition to the Heusler alloys family, exhibit atomic disorders, and almost all of them are reported as a re-entrant spin-glass system. Although such spin-glass feature is generally attributed to the inherent atomic disorder, a comprehensive and extensive investigation on the individual roles of different types of disorders in magnetic interactions remains lacking for any of the reported anti-Heusler systems. As an illustrative case, we have carried out an in-depth experimental as well as theoretical investigation of structural, magnetic, and transport properties of a polycrystalline anti-Heusler alloy, Al$2$MnFe. While the major atomic disorder is found to be among Fe and Mn atoms, which are randomly distributed among the two octahedral sites, 4$a$ and 4$b$ (B2-type disorder), a relatively small fraction ($\sim$12%) of Mn atoms also replace Al atoms at the tetrahedral 8$c$ site. Magnetically, the system undergoes two transitions: a paramagnetic to a ferromagnetic transition at $T{\rm C}\sim$113K, followed by a spin-glass phase transition below $T_{\rm f}\sim$20K. Here, the magnetic moment is primarily confined to Mn atoms. Very interestingly, our theoretical analysis reveals that the ferromagnetic spin arrangement remains rather robust in spite of the 50% disorder of moment-carrying Mn atoms between the two octahedral sites, but a much smaller ($\sim$12%) cross-distribution of Mn atoms between octahedral and tetrahedral sites are sufficient to impose a reentrant spin-glass state at low temperature. Our analysis brings forth the importance of understanding the role of individual types of swap-disorder on magnetic properties in the anti-Heusler family of materials.

💡 Research Summary

This work presents a comprehensive experimental and theoretical investigation of the anti‑Heusler alloy Al₂MnFe, focusing on how distinct types of atomic disorder influence its magnetic phases. Polycrystalline samples were synthesized by arc‑melting with a slight excess of Mn to compensate for evaporation. Room‑temperature X‑ray diffraction confirms a single‑phase cubic structure (space group Fm‑3 m, a = 5.893 Å). Rietveld refinement initially suggests an ordered L2₁‑type anti‑Heusler lattice, but detailed analysis of the (111), (200) and (220) Bragg intensities reveals two co‑existing disorder mechanisms: (i) a B2‑type disorder in which Fe and Mn are randomly distributed over the two octahedral sites (4a and 4b), amounting to roughly 50 % site mixing; and (ii) a smaller (~12 %) Mn‑Al antisite disorder in which Mn atoms occupy the tetrahedral 8c site normally reserved for Al. ⁵⁷Fe Mössbauer spectroscopy corroborates the mixed Fe/Mn environment at the octahedral positions and the presence of Mn on the Al site.

Magnetization measurements show a clear paramagnetic‑to‑ferromagnetic transition at TC ≈ 113 K, followed by a low‑temperature spin‑glass freezing at Tf ≈ 20 K. Zero‑field‑cooled and field‑cooled curves split below Tf, and AC susceptibility exhibits a frequency‑dependent peak, confirming a re‑entrant spin‑glass state. Heat‑capacity data display an anomaly at Tf, indicating a thermodynamic signature of the glassy transition. Electrical resistivity remains metallic over the whole temperature range but shows a subtle upturn below 20 K, consistent with enhanced spin‑disorder scattering in the glassy regime. Hall measurements reveal dominant electron carriers with a weak temperature dependence.

First‑principles density‑functional theory (DFT) calculations were performed using the projector‑augmented wave method (VASP) with the PBE‑GGA functional. To model disorder, a 128‑atom special quasi‑random structure (SQS) was constructed for three cases: (a) perfectly ordered L2₁, (b) pure B2 disorder (Fe/Mn 50 % mixing on 4a/4b), and (c) B2 disorder plus 12 % Mn‑Al antisite swaps. All structures retain a metallic density of states, but the Mn‑Al antisite introduces localized Mn‑Mn interactions on the tetrahedral sublattice. Exchange parameters were extracted via the Liechtenstein formalism using the SPR‑KKR Green‑function method and mapped onto a classical Heisenberg Hamiltonian. In the pure B2 model, the dominant Mn–Fe nearest‑neighbor exchange (J₁) remains ferromagnetic, preserving the high TC. When Mn occupies the 8c site, the Mn–Mn nearest‑neighbor exchange becomes antiferromagnetic (negative J₁), creating competing ferromagnetic and antiferromagnetic bonds. Monte‑Carlo simulations based on these parameters reproduce a robust ferromagnetic order at high temperature and a spin‑glass‑like frozen state at low temperature, mirroring the experimental observations.

The key insight is that the large‑scale Fe/Mn mixing between the two octahedral sites does not significantly disrupt the ferromagnetic network; the system tolerates up to 50 % B2 disorder while maintaining a TC near 110 K. In contrast, a relatively minor fraction of Mn atoms swapping onto the Al tetrahedral sites is sufficient to generate competing antiferromagnetic interactions that destabilize the ferromagnetic ground state at low temperature, leading to a re‑entrant spin‑glass phase. This demonstrates that not all disorder is equally detrimental: the crystallographic nature of the swapped sites determines whether the magnetic order is preserved or frustrated.

The study thus provides a clear mechanistic picture of how specific atomic swaps control magnetic interactions in anti‑Heusler alloys. It establishes a design principle for engineering complex magnetic states—such as coexisting ferromagnetism and spin‑glass behavior—by selectively tuning site disorder. The findings are relevant for spintronic applications where controlled magnetic frustration and tunable transition temperatures are desirable, and they open avenues for exploring other anti‑Heusler compounds with tailored disorder profiles.