[Paper Review] Defects-driven appearance of half-metallic ferrimagnetism in Co-Mn--based Heusler alloys

K. Özdoğan, I. Galanakis+2 more|ArXiv.org|Feb 19, 2007

Heusler alloys: electronic and magnetic properties4 citations

TL;DR

This paper demonstrates that introducing Mn antisites—where Mn atoms occupy Co sites—in Co₂MnZ Heusler alloys induces robust half-metallic ferrimagnetism. Using first-principles calculations, it shows that these Mn impurities develop large antiparallel spin moments, preserving the half-metallic gap despite strong exchange splitting, leading to significantly reduced total spin moments ideal for low-energy spintronic devices.

ABSTRACT

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co$_2$MnZ where Z a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co$_{1-x}$Mn$_x$]$_2$MnSi is of particular interest since Mn$_3$Si is known to crystallize in the Heusler $L2_1$ lattice structure of Co$_2$MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.

Motivation & Objective

• To investigate the emergence of half-metallic ferrimagnetism in Co₂MnZ Heusler alloys due to Mn antisite defects.
• To understand how Mn atoms migrating to Co sites affect the electronic structure and magnetic ordering.
• To assess the stability and half-metallic character of the resulting ferrimagnetic compounds compared to their ferromagnetic parent phases.
• To evaluate the potential of these defect-engineered materials for low-energy spintronic applications due to reduced net magnetic moments.
• To explore the specific case of [Co₁₋ₓMnₓ]₂MnSi, given the known Heusler structure of Mn₃Si.

Proposed method

• Employed full-potential nonorthogonal local-orbital minimum-basis (FPLO) method for electronic structure calculations.
• Applied the coherent potential approximation (CPA) to model random substitutional disorder from Mn antisites.
• Used scalar-relativistic formulation to account for spin-orbit coupling effects in transition metal systems.
• Performed first-principles calculations on Co₂MnZ (Z = Al, Ga, Si, Ge, Sn) with increasing Mn concentration at Co sites.
• Analyzed density of states (DOS) and spin magnetic moments to confirm half-metallicity and ferrimagnetic ordering.
• Compared the evolution of spin moments and band gaps across different Z elements to assess robustness of half-metallicity.

Experimental results

Research questions

• RQ1 Can Mn antisites in Co₂MnZ Heusler alloys induce half-metallic ferrimagnetism while preserving the electronic gap?
• RQ2 How does the spin moment of Mn impurities at Co sites compare to those in perfect Co₂MnZ compounds?
• RQ3 What is the effect of increasing Mn defect concentration on the width of the minority-spin gap in these alloys?
• RQ4 Why is the half-metallic gap more robust in Mn-antisite Co₂MnZ compared to Cr-antisite Co₂CrX systems?
• RQ5 To what extent does the choice of Z element (Al, Ga, Si, Ge, Sn) influence the magnetic and electronic response to Mn defects?

Key findings

• Mn atoms occupying Co sites in Co₂MnZ alloys develop large negative spin moments (up to ~-2.8 μB in Al and Ga compounds), antiparallel to the other transition metal atoms.
• The half-metallic gap in the minority-spin channel remains robust despite strong exchange splitting of Mn impurities, with only marginal reduction in gap width.
• In Si-, Ge-, and Sn-based compounds, the Mn impurity spin moment increases in magnitude from ~-1.9 μB (Si) to ~-2.6 μB (Sn) at x = 0.025, due to a downward shift in the Fermi level.
• The total spin moment of the [Co₁₋ₓMnₓ]₂MnSi system is significantly reduced compared to the parent ferromagnetic compound, due to antiparallel coupling of Mn antisites.
• The half-metallic character is preserved across all Z elements studied, with the most stable behavior observed in Si, Ge, and Sn-based systems.
• Atomic swaps (Co on Mn sites) destroy half-metallicity, confirming that only Mn antisites at Co sites yield the desired ferrimagnetic half-metallic state.