[Paper Review] Ab-initio search for half-metallic Co-based full Heusler alloys: Linear-response-based DFT+U study

Kenji Nawa, Yoshio Miura|arXiv (Cornell University)|Mar 1, 2019

Heusler alloys: electronic and magnetic propertiesMaterials Science3 citations

TL;DR

This study employs a linear-response-based DFT+U method to identify half-metallic (HM) ferromagnetism in Co-based full Heusler alloys, revealing that Co-Mn $t_{2g}$ orbital hybridization governs the minority-spin band gap. The approach predicts Co₂(Fe₀.₂₅,Mn₀.₇₅)Si as a highly promising HM material with a large gap centered at the Fermi level, ensuring high thermal stability for spintronic applications.

ABSTRACT

The first-principles calculations for exploring half-metallic (HM) ferromagnets with wide band gap around Fermi energy in minority spin channel remains challenging in treatment of correlation effect. The DFT+U method, in which the Hubbard-type effective on-site Coulomb parameters ($U_{eff}$) is theoretically determined from linear response (LR) approach, is a suitable method on a practical level because of an advantage of efficient computational cost. We apply this method for Co-based ternary Heusler alloys Co$_2$YSi where Y is $3d$ transition metal. Based on detailed analyses of calculated band structures for Co$_2$MnSi, we propose an energy diagram that reveals an important atomic-orbital hybridization of Co-Mn $t_{2g}$ state, which is newly discovered and governs minority HM gap around Fermi energy. This unified diagram provides a new insight in that the HM property is possible to be tuned by $t_{2g}$ coupling through a choose of Y atom. We also investigated the role of $U_{eff}$ parameters on the Mn and Co sites independently and found that the correlation effect of Mn site is more significant than that of Co to reproduce the ground-state electronic and magnetic structures in consistent of experimental results. Our LR-based DFT+U method were, further, extended to other compounds including quaternary system Co$_2$(Y,Mn)Si, where a part of Mn is substituted with Y, to explore a possibility of the HM ferromagnetism in Co-based full Heusler alloys. The results indicate that the quaternary compound have a potential to be a HM ferromagnet when a composition of substituting element is appropriately selected, i.e., Y$= m{Ti}$, V, Fe. Particularly, Co$_2$(Fe$_{0.25}$,Mn$_{0.75}$)Si shows great promise in the spintronics applications due to a sizable minority HM gap and Fermi energy position being at center of the gap, which lead to high thermal stability in terms of the HM band structure.

Motivation & Objective

• To identify half-metallic ferromagnets with wide band gaps in the minority spin channel in Co-based full Heusler alloys.
• To address the challenge of accurately treating electron correlation effects in transition metal systems using a computationally efficient method.
• To explore the tunability of half-metallic behavior through substitution of Mn with 3d transition metals in Co₂YSi compounds.
• To determine the relative importance of U_eff on Mn versus Co sites for reproducing experimental electronic and magnetic ground states.
• To extend the method to quaternary systems like Co₂(Y,Mn)Si to assess the potential for stable half-metallicity with optimized composition.

Proposed method

• Application of the linear-response (LR) approach to self-consistently determine effective Hubbard U parameters (U_eff) for Co and Mn atoms.
• Use of DFT+U with site-specific U_eff values to improve treatment of electron correlation effects in Co₂YSi and Co₂(Y,Mn)Si systems.
• Calculation of electronic band structures and spin polarization to identify half-metallic gaps in the minority spin channel.
• Analysis of orbital hybridization, particularly the Co-Mn $t_{2g}$ state coupling, to understand the origin of the half-metallic gap.
• Systematic substitution of Mn with Y elements (Ti, V, Fe) in Co₂(Y,Mn)Si to explore tunability of half-metallic behavior.
• Evaluation of Fermi level position relative to the band gap to assess thermal stability of the half-metallic state.

Experimental results

Research questions

• RQ1 What is the role of Co-Mn $t_{2g}$ orbital hybridization in generating and stabilizing the minority-spin band gap in Co-based Heusler alloys?
• RQ2 How do site-specific U_eff parameters for Mn and Co affect the accuracy of predicted electronic and magnetic ground states?
• RQ3 Can quaternary Co₂(Y,Mn)Si compounds exhibit half-metallic ferromagnetism when Y is selected from Ti, V, or Fe?
• RQ4 What composition of Y substitution leads to a half-metallic gap centered at the Fermi level for optimal thermal stability?
• RQ5 To what extent can the half-metallic gap be tuned via chemical substitution while preserving the ferromagnetic ground state?

Key findings

• The Co-Mn $t_{2g}$ orbital hybridization is identified as the key mechanism responsible for the formation of the minority-spin band gap in Co₂MnSi.
• The correlation effect at the Mn site is more significant than at the Co site for correctly reproducing the experimental electronic and magnetic structures.
• Co₂(Ti,Mn)Si, Co₂(V,Mn)Si, and Co₂(Fe,Mn)Si quaternary systems show potential for half-metallic ferromagnetism with appropriate Y-element substitution.
• Co₂(Fe₀.₂₅,Mn₀.₇₅)Si exhibits a sizable minority-spin band gap with the Fermi level positioned at the center of the gap, indicating high thermal stability.
• The LR-based DFT+U method successfully predicts half-metallic behavior with good agreement to experimental trends, validating its use for screening new materials.
• The unified energy diagram based on $t_{2g}$ coupling provides a predictive framework for tuning half-metallicity via elemental substitution.