[Paper Review] Nearly perfect spin polarization of noncollinear antiferromagnets

Gautam Gurung, Elekhtiar, Mohamed+2 more|arXiv (Cornell University)|Jun 5, 2023

Magnetic and transport properties of perovskites and related materialsMaterials Science3 citations

TL;DR

This paper proposes that noncollinear antiferromagnets, despite having zero net magnetization, can exhibit nearly perfect spin polarization across broad regions of their Fermi surface due to momentum-dependent spin textures. Using first-principles calculations, the authors demonstrate that Mn₃GaN supports nearly 100% spin-polarized states that enable an extraordinary tunneling magnetoresistance (ETMR) of up to 10⁴% in antiferromagnetic tunnel junctions with SrTiO₃, revealing a hidden spintronic functionality in noncollinear antiferromagnets.

ABSTRACT

Ferromagnets with high spin polarization are known to be valuable for spintronics--a research field that exploits the spin degree of freedom in information technologies. Recently, antiferromagnets have emerged as promising alternative materials for spintronics due to their stability against magnetic perturbations, absence of stray fields, and ultrafast dynamics. For antiferromagnets,however, the concept of spin polarization and its relevance to the measured electrical response are elusive due to nominally zero net magnetization.Here, we define an effective momentum-dependent spin polarization and reveal an unexpected property of many noncollinear antiferromagnets to exhibit nearly 100% spin polarization in a broad area of the Fermi surface. This property leads to the emergence of an extraordinary tunneling magnetoresistance (ETMR) effect in antiferromagnetic tunnel junctions (AFMTJs). As a representative example, we predict that a noncollinear antiferromagnet Mn$_{3}$GaN exhibits nearly 100% spin-polarized states that can efficiently tunnel through low-decay-rate evanescent states of perovskite oxide SrTiO$_{3}$ resulting in ETMR as large as $10^{4}$%. Our results uncover hidden functionality of material systems with noncollinear spin textures and open new perspectives for spintronics.

Motivation & Objective

• To identify and characterize hidden spin polarization in noncollinear antiferromagnets, which lack net magnetization but may still support spintronic functionality.
• To resolve the ambiguity in defining spin polarization for antiferromagnets by introducing an effective momentum-dependent spin polarization measure.
• To explore the potential of antiferromagnetic tunnel junctions (AFMTJs) for achieving extreme magnetoresistance effects.
• To establish a link between noncollinear spin textures and high spin polarization, enabling novel spintronic applications.

Proposed method

• The study employs first-principles density functional theory (DFT) calculations to compute electronic band structures and spin textures in noncollinear antiferromagnets.
• An effective momentum-dependent spin polarization is defined as a function of wavevector (k) to quantify spin alignment on the Fermi surface.
• The spin-polarized transport properties are analyzed using the Landauer-Büttiker formalism to model tunneling current across AFMTJs.
• The model system Mn₃GaN is studied in conjunction with SrTiO₃ as a tunnel barrier to simulate tunnel junction behavior.
• The tunneling magnetoresistance (TMR) is calculated by comparing conductance for parallel and antiparallel spin configurations.
• The analysis focuses on evanescent states in SrTiO₃ and their coupling to spin-polarized states in Mn₃GaN to assess transmission efficiency.

Experimental results

Research questions

• RQ1 Can noncollinear antiferromagnets exhibit high spin polarization despite having zero net magnetization?
• RQ2 How can spin polarization be meaningfully defined and quantified in antiferromagnetic systems?
• RQ3 What is the role of momentum-dependent spin textures in enabling high spin-polarized tunneling in antiferromagnetic junctions?
• RQ4 Can antiferromagnetic tunnel junctions achieve magnetoresistance values significantly exceeding conventional ferromagnetic devices?
• RQ5 What materials and interface configurations maximize spin-polarized tunneling and ETMR in noncollinear antiferromagnets?

Key findings

• Noncollinear antiferromagnets such as Mn₃GaN exhibit nearly 100% spin polarization across broad regions of their Fermi surface due to momentum-dependent spin textures.
• The effective momentum-dependent spin polarization reveals that spin-polarized states are robust and spatially extended in momentum space.
• In Mn₃GaN/SrTiO₃ heterostructures, nearly perfect spin-polarized states tunnel efficiently through low-decay-rate evanescent states in the oxide barrier.
• The calculated extraordinary tunneling magnetoresistance (ETMR) reaches up to 10⁴%, significantly exceeding typical TMR values in ferromagnetic junctions.
• The results demonstrate that noncollinear spin textures in antiferromagnets can enable high-performance spintronic devices without net magnetic moments.