Altermagnetic Flatband-Driven Fermi Surface Geometry for Giant Tunneling Magnetoresistance

TL;DR

This work demonstrates that flatband-driven, spin-polarized Fermi-surface geometry in altermagnets can dramatically suppress antiparallel-state transmission, enabling giant TMR in AMTJs. By comparing V$_2$Te$_2$O, RbV$_2$Te$_2$O, and KV$_2$Se$_2$O, the authors show that KV$_2$Se$_2$O’s quasi-2D, nodal-like spin overlaps yield an intrinsic $T_{ ext{MR}}$ of $4.3 imes10^{3} ext{%}$ with vacuum barriers, which is further amplified to $9.1 imes10^{4} ext{%}$ using a symmetry-matched Cr$_2$Se$_2$O barrier along [001]. Crystal orientation further tunes TMR, with a sizable but reduced $T_{ ext{MR}}$ of $3.3 imes10^{3} ext{%}$ along [100] using TiOF$_2$, highlighting the importance of Fermi-surface engineering and barrier selection. The study establishes a design paradigm for high-performance altermagnetic spintronics: select quasi-layered altermagnets with flatband-driven, symmetry-protected Fermi surfaces and tailor transport via crystallographic direction and barrier symmetry. These insights provide concrete guidelines for screening and optimizing altermagnetic materials for ultrahigh-TMR spintronic devices.

Abstract

Altermagnetism, characterized by zero net magnetization and symmetry-protected spin-split band structures, has recently emerged as a promising platform for spintronics. In altermagnetic tunnel junctions (AMTJs), the suppression of tunneling in the antiparallel configuration relies on the mismatch between spin-polarized conduction channels in momentum space. However, ideal nonoverlapping spin-polarized Fermi surfaces are rarely found in bulk altermagnets. Motivated by the critical influence of Fermi surface geometry on tunneling magnetoresistance (TMR), we investigate three experimentally synthesized altermagnets -- bulk $\mathrm{V_2Te_2O}$, $\mathrm{RbV_2Te_2O}$, and $\mathrm{KV_2Se_2O}$ -- to elucidate how flatband-driven Fermi surfaces minimize spin-channel overlap and boost AMTJ performance. Notably, $\mathrm{RbV_2Te_2O}$ and $\mathrm{KV_2Se_2O}$ host flat altermagnetic Fermi sheets, which confine spin degeneracy to minimal arc-like or nodal-like regions. Such Fermi surface geometry drastically reduces spin overlap, resulting in an unprecedented intrinsic TMR well over $10^3\%$ in the $\mathrm{KV_2Se_2O}$-based AMTJ. Incorporating an insulating barrier further enhances the TMR to $\sim10^5\%$, surpassing most conventional MTJs. These results not only establish $\mathrm{KV_2Se_2O}$ as a compelling candidate AMTJ material, but also highlight the critical role of flatband Fermi surface geometry in achieving high-performance altermagnetic-spintronic device technology.

Figures (6)

• Figure 1: Schematic illustration of tunneling magnetoresistance (TMR) mechanisms in MTJs with different electrode materials. a) Conventional ferromagnets. b) Typical altermagnets with overlapping spin conduction channels. c) AMs with ideally nonoverlapping spin conduction channels. (d-f) AMs with nonideal partially spin-degenerate Fermi pockets that allow residual AP tunneling. Two types of overlap are considered: spin-degenerate continuum and discrete spin-degenerate quasi-nodes. Red and blue denote spin-up and spin-down conduction channels, respectively.
• Figure 2: Crystal and magnetic structures of a) $\mathrm{V_2Te_2O}$, b) $\mathrm{RbV_2Te_2O}$ and $\mathrm{KV_2Se_2O}$. Red and blue indicate up and down spins of the V atoms, respectively. (c-e) Electronic band structures, three-dimensional spin-resolved Fermi surfaces and Fermi surface cross-sections at different $k_\mathrm{z}$ planes ($k_\mathrm{z}=0$ and 0.2) of different materials. Spin-degenerate bands are shown in black.
• Figure 3: Intrinsic transport properties of AMTJs with vacuum barriers based on $\mathrm{V_2Te_2O}$, $\mathrm{RbV_2Te_2O}$ and $\mathrm{KV_2Se_2O}$ electrodes. a) Schematic illustration of magnetization alignments in the parallel (P) and antiparallel (AP) configurations. (b-d) Calculated spin-resolved, $k_\parallel$-resolved transmission coefficients at the Fermi level for each AMTJ in both P and AP states. (e-g) Energy-dependent total transmission spectra for P and AP states, along with the resulting TMR ratio. Shaded regions highlight the energy ranges where AP tunneling $T_\mathrm{AP}$ is significantly suppressed due to changes in available conduction channels when energy is away from the Fermi level.
• Figure 4: Three-dimensional Fermi surfaces of bulk $\mathrm{V_2Te_2O}$ at a) $E=0.06$ eV, b) $E=0.10$ eV, c) $E=0.14$ eV and d) $E=0.18$ eV. Red and blue indicate spin-up and spin-down Fermi surfaces, respectively. The spin-degenerate continuum near the $\Gamma$ point progressively diminishes with increasing energy and entirely vanishes at $E = 0.18$ eV.
• Figure 5: Transport properties of $\mathrm{KV_2Se_2O}$-based AMTJs along the (a-f) [001] and (g-i) [100] directions. a) Crystal structure of the barrier material $\mathrm{Cr_2Se_2O}$. b) Electronic band structures of monolayer $\mathrm{Cr_2Se_2O}$, where red and blue denote spin-up and spin-down bands, respectively; spin-degenerate bands are shown in black. c) Atomic structure and layer-resolved density of states (LDOS) of the $\mathrm{KV_2Se_2O}|\mathrm{Cr_2Se_2O}|\mathrm{KV_2Se_2O}~(001)$ MTJ. Each panel corresponds to a single atomic layer of either $\mathrm{KV_2Se_2O}$ or $\mathrm{Cr_2Se_2O}$, with spin-up and spin-down components shown in the upper and lower subpanels. The Fermi level is marked by a dashed line. d) $k_\parallel$-resolved spin-polarized transmission coefficients at the Fermi level for the (001) AMTJ in both P and AP states. e) Total transmission spectra and f) TMR ratio as a function of energy. (g-l) Same as (a-f) but for the $\mathrm{KV_2Se_2O}|\mathrm{TiOF_2}|\mathrm{KV_2Se_2O}$ MTJ along the [100] direction.