First-principles study of magnetic and spin-dependent transport properties of Mn2VZ (Z = Al, Ga) with negative spin polarization using a disordered local moment approach at finite temperatures

First-principles studies were performed on two Mn-based ferrimagnetic Heusler compounds with L21 and B2 structures, that is, Mn2VZ (Z = Al or Ga). The aim was to investigate their magnetic properties, electronic structures, and spin-resolved longitudinal conductivity at finite temperatures. Density functional theory (DFT) and functional integral theory were used. This approach incorporates transverse spin fluctuations through a disordered local moment method and the coherent potential approximation. In all cases, the calculated theoretical Curie temperatures were lower than the experimental values. Alloys with a B2 structures exhibit higher Curie temperatures compared to compounds with an L21 structures. Calculations of the temperature dependence of the density of states (DOS) indicate that the half-metallic electronic structure collapses owing to the renormalization of transverse spin fluctuations at a finite temperatures. However, the spin-resolved longitudinal conductivities demonstrated an improved spin polarization, particularly for Mn2VGa with an L21 structure. This result contradicts predictions based on the temperature-dependent DOS. The competition between the metallic transitions, which are caused by a modification of the DOS, and scattering coming from spin-disorder explains this phenomenon. Both of these effects are induced by transverse spin fluctuations. Additionally, the results show that half-metallicity, as defined by the DOS or conductivity, is inconsistent at finite temperatures. Finally, the total energy landscape of the paramagnetic state was calculated using the fixed spin moment method to investigate the strength of the longitudinal spin fluctuations. These results suggest that the alloys may exhibit strong longitudinal spin fluctuations.

💡 Research Summary

This work presents a comprehensive first‑principles investigation of the finite‑temperature magnetic, electronic, and spin‑resolved transport properties of two Mn‑based ferrimagnetic Heusler alloys, Mn₂VAl and Mn₂VGa, in both the ordered L2₁ and the partially disordered B2 crystal structures. The authors combine density‑functional theory (DFT) with a functional‑integral formalism that explicitly treats transverse spin fluctuations via a disordered local moment (DLM) picture and the coherent‑potential approximation (CPA). The zero‑temperature electronic structure is obtained with the Korringa‑Kohn‑Rostoker (KKR) method within the atomic‑sphere approximation (ASA) and tight‑binding linear muffin‑tin orbitals (TB‑LMTO). From the DLM‑CPA framework the temperature‑dependent probability distribution of local moment orientations ω(T, eᵢ) is derived, allowing the calculation of the temperature‑dependent density of states D(T,E), site‑resolved magnetizations Mᵢ(T), and spin‑resolved longitudinal conductivities σ↑(T) and σ↓(T) via the Kubo‑Greenwood formula. Vertex corrections are evaluated to assess the role of spin‑disorder scattering.

Key magnetic findings: the calculated Curie temperatures (T_C) are 440 K (L2₁‑Mn₂VAl), 520 K (L2₁‑Mn₂VGa), 500 K (B2‑Mn₂VAl) and 580 K (B2‑Mn₂VGa) within LSDA; GGA raises these values by roughly 150 K but still underestimates the experimental T_C≈ 770 K. The B2 phase consistently yields a higher T_C than the L2₁ phase, reflecting the influence of chemical disorder on exchange interactions. Magnetization curves follow a Langevin‑type mean‑field behavior due to the single‑site static approximation.

Electronic structure evolution: at 0 K both compounds exhibit half‑metallic character with a gap in the majority‑spin channel, leading to a negative spin polarization. As temperature increases, transverse spin fluctuations broaden the majority‑spin states, fill the gap, and destroy half‑metallicity. This trend mirrors previous DLM‑CPA and DMFT studies on Heusler alloys.

Transport results reveal a striking discrepancy between DOS‑based and conductivity‑based spin polarizations. For L2₁‑Mn₂VGa, the majority‑spin conductivity first drops with temperature, then recovers at low temperatures, causing the spin polarization Pσ(T) = (σ↑−σ↓)/(σ↑+σ↓) to increase despite the DOS indicating a loss of half‑metallicity. The authors attribute this to a competition between a metal‑insulator‑like transition driven by DOS changes and enhanced spin‑disorder scattering that preferentially suppresses the low‑mobility d‑derived majority‑spin carriers. Consequently, the minority‑spin channel dominates transport at intermediate temperatures, while the majority channel regains prominence at lower temperatures, yielding an apparent improvement in spin polarization. In the B2 phase, conductivity follows the DOS trend more closely, and vertex corrections significantly affect the results, underscoring the sensitivity of transport to chemical disorder.

The study also examines longitudinal spin fluctuations by performing fixed‑spin‑moment calculations of the paramagnetic energy landscape. The shallow curvature around zero magnetization indicates strong longitudinal fluctuations, suggesting that both transverse and longitudinal spin dynamics contribute to the observed transport anomalies.

Overall, the paper demonstrates that transverse spin fluctuations can simultaneously collapse the half‑metallic DOS and, through spin‑disorder scattering, modify the spin‑resolved conductivity in a non‑trivial way. This leads to an inconsistency between half‑metallicity defined by the DOS and that defined by transport, especially in the L2₁‑Mn₂VGa alloy. The findings highlight the necessity of incorporating realistic spin‑fluctuation and disorder effects when predicting spin‑polarized transport for spintronic applications, and they point to the importance of considering both electronic structure and scattering mechanisms rather than relying solely on zero‑temperature DOS predictions. Future work should address spin‑phonon coupling, dynamic longitudinal fluctuations, and more accurate exchange‑correlation treatments beyond ASA to further close the gap between theory and experiment.