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

TL;DR

The paper analyzes Mn2VAl and Mn2VGa in both L2_1 and B2 structures to understand finite-temperature magnetism and spin-dependent transport using a first-principles framework that incorporates transverse spin fluctuations via a disordered local moment method and CPA. It employs functional integral theory within DFT to obtain fluctuation probabilities and Green's functions, enabling DOS, magnetization, and spin-resolved conductivity calculations through the Kubo-Greenwood formalism. Key findings include systematic underestimation of Curie temperatures compared with experiment, higher Tc for B2 phases, and a temperature-driven loss of half-metallicity in the DOS, while conductivity-based spin polarization can improve at low temperature for Mn2VGa in L2_1 due to competing metallic transitions and spin-disorder scattering. A fixed-spin-moment analysis reveals strong longitudinal spin fluctuations, indicating itinerant d-electron behavior and signaling limitations of force-theorem approaches for these alloys, with implications for designing Mn-based Heusler spintronic devices at finite temperatures.

Abstract

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.

Figures (5)

• Figure 1: Temperature dependence of the total magnetization and site-resolved magnetic moments of the Mn and V sites of Mn$_2$VAl (MVA, closed circles) and Mn$_2$VGa (MVG, open squares) with (a) $L2_1$ and (b) $B2$ structures. Black, red, and blue lines correspond to the total magnetization and to Mn and V atoms, respectively. For both structures, the magnetic moments of the Mn atoms are multiplied by 2 because of the multiplicity of the position. Calculations were performed with LSDA.
• Figure 2: Temperature dependence of the DOS of (a) $L2_1$- and (b) $B2$- Mn$_2$VAl and (c) $L2_1$- and (d) $B2$- Mn$_2$VGa. Energy zero in the horizontal axis was chosen as the chemical potential at each temperature. Calculations were performed with LSDA.
• Figure 3: Calculated temperature dependence of spin-resolved conductivities of Mn$_2$VAl (red) and Mn$_2$VGa (blue) with the (a) $L2_1$ and (b) $B2$ structures, respectively. Squares correspond to majority spin channel UP and circles correspond to minority spin channel DN. Vertical lines correspond to Curie temperatures for each alloy. Calculations were performed with LSDA.
• Figure 4: Calculated temperature dependence of spin polarization calculated from (a), (b) spin resolved conductivities and (c), (d) DOS of Mn$_2$VAl (red) and Mn$_2$VGa (blue) with the $L2_1$ and $B2$ structures, respectively. Vertical lines correspond to Curie temperatures for each alloy. Calculations were performed with LSDA.
• Figure 5: Calculated total energy of paramagnetic states with different amplitude of magnetic moment of Mn$_2$VAl (red) and Mn$_2$VGa (blue) with the $L2_1$ structures for (a) Mn and (b) V sites, respectively. Calculations were performed with LSDA.