Termination-Preserved Ultrahigh Tunneling Magnetoresistance in Altermagnetic KV₂Se₂O

Abstract

Altermagnets exhibit nonrelativistic spin splitting without net magnetization, offering a promising platform for next-generation spintronic devices. Although altermagnetic tunnel junctions (AMTJs) represent promising realizations, their practical applications are hindered by low tunneling magnetoresistance (TMR) ratios and strong sensitivity to interfacial configurations. Here, we systematically explore the transport properties and microscopic mechanisms of AMTJs based on the d-wave altermagnet KV₂Se₂O. Using first-principles calculations and orbital-resolved analysis, we demonstrate that the synergy between compressed nodal-point-like spin-degenerate channels and coplanar interfacial magnetic order yields an ultrahigh intrinsic TMR above 10⁵% for all interfacial terminations. More importantly, K-termination effectively preserves bulk spin polarization through its passivation characteristics, leading to an ultrahigh TMR up to 10¹²%. These results highlight the coupling between momentum-space topology and interfacial passivation as a reliable strategy for realizing giant magnetoresistive responses in altermagnetic spintronic devices.

Introduction

Magnetic tunnel junctions (MTJs) are the fundamental building blocks of magnetoresistive random access memory (MRAM), with each memory cell incorporating a discrete storage element. − Comprising two magnetic electrodes separated by a thin insulating barrier, an MTJ operates via the tunneling magnetoresistance (TMR) effect, in which the electrical resistance is governed by the relative magnetic configuration of the electrodes. , Given that a high TMR ratio is indispensable for robust data readout, MTJs have traditionally employed ferromagnetic (FM) electrodes to provide substantial spin polarization, thereby generating large TMR ratios. , However, the intrinsic stray magnetic fields associated with ferromagnetic order constrain device miniaturization and high-density integration. While antiferromagnetic (AFM) materials present an appealing alternative due to their zero net magnetization and immunity to external magnetic perturbations, their fully compensated spin structures lead to vanishing net spin polarization, which makes it difficult to realize sizable TMR signals suitable for practical memory applications. −

The recent identification of altermagnetic (AM) materials, which exhibit nonrelativistic spin splitting in their electronic band structure, provides a promising platform to break this deadlock. , Unlike conventional collinear antiferromagnets, altermagnets break PT or Uτ symmetries (where P, T, U, and τ denote space inversion, time reversal, spin flip, and lattice translation, respectively), inducing momentum-dependent spin polarization even in the absence of spin–orbit coupling. − This characteristic grants altermagnets ferromagnet-like spin-polarization transport capabilities, while their antiparallel magnetic sublattices ensure antiferromagnet-like zero net magnetization. Consequently, even when the net tunneling current is spin-unpolarized, altermagnetic tunnel junctions (AMTJs) can still exhibit a giant TMR effect. In AMTJs, the resistance state is determined by the relative orientation of the Néel vectors in the two altermagnetic electrodes, which can be configured as parallel (P) or antiparallel (AP). The TMR effect arises from the matching and mismatching of the spin-polarized Fermi surfaces of the two electrodes within the two-dimensional Brillouin zone (2DBZ), and the TMR magnitude is directly governed by the overlap between these Fermi surfaces. ,

Currently, AMTJs based on RuO₂, MnTe, NiF₂, and CrSb have emerged, including all-altermagnetic architectures and altermagnetic barriers. − However, TMR ratios in these systems fail to meet industrial standards. − Furthermore, the transport process is often dominated by interfacial magnetic moment configurations rather than the intrinsic Néel vector of the altermagnet, leading to undesirable antiferromagnet-like behavior. − Most importantly, the microscopic physical origin of altermagnetic TMR, particularly the contribution mechanisms at the orbital level, lacks in-depth theoretical elucidation. To address these research gaps, the d-wave altermagnet KV₂Se₂O is selected as the core subject of this study. This material possesses a Néel temperature above room temperature and quasi-two-dimensional flat Fermi surfaces that form only nodal-point-like overlaps between spin channels, providing a natural structural foundation for achieving ultrahigh TMR effects. Moreover, its coplanar altermagnetic order greatly enhances transport stability and ensures robustness against interference from interfacial magnetic moment arrangements.

Despite these intrinsic advantages, the full TMR potential of KV₂Se₂O remains largely underexplored. Existing studies mainly focus on bulk Fermi surface features. Although the flat-band Fermi surface has been demonstrated to play an important role in achieving large TMR, the mechanisms responsible for sustaining spin polarization during transport have not yet been clearly elucidated. More importantly, a significant discrepancy exists between idealized AMTJ models and real physical systems. As a K⁺-intercalated material, KV₂Se₂O tends to cleave along K or Se planes where interlayer bonding is weakest, whereas theoretical calculations adopt V–O atomic layers as the interface. Therefore, it is necessary to systematically investigate the influence of interfacial magnetic configurations and realistic cleavage surfaces on transport properties, while elucidating the underlying microscopic mechanisms from an orbital-resolved perspective. −

In this study, we demonstrate that KV₂Se₂O-based AMTJs can achieve ultrahigh TMR through the momentum-space selection mechanism and real-space interfacial magnetic moment matching. Using first-principles quantum transport calculations within a vacuum-barrier model, we elucidate the microscopic origin of this effect and quantify the role of interfacial terminations. Our findings demonstrate a universally ultrahigh TMR exceeding 10⁵% for all interface terminations. Notably, the K-terminated interface achieves a remarkably high TMR of 10¹²% by preserving bulk spin polarization through its passivation effect. Extending the investigation to realistic heterostructures with KTaO₃ barrier yields TMR values in the range of 10⁷–10¹²%, with the underlying transport mechanism remaining robust. These results highlight the critical roles of momentum-resolved spin channels and real-space interfacial configurations in enhancing TMR, and provide design principles for achieving extreme magnetoresistive responses in AMTJs.

Results and Discussion

Spin Channels and Design Principles

The TMR effect in MTJs originates from the symmetry and matching of spin-polarized states between the P and AP configurations. In AMTJs, where transport is governed by momentum-dependent spin splitting and alternating magnetic order, the TMR performance is primarily determined by the matching degree of momentum-space spin channels and the real-space arrangement of interfacial magnetic moments.

From the momentum-space perspective, the resistance originates from the matching and mismatching of the spin-polarized Fermi surfaces of the two electrodes projected onto the two-dimensional Brillouin zone (2DBZ) perpendicular to the transport direction. For a typical d-wave altermagnet, the two spin-resolved Fermi surfaces are related by a 90° rotation in momentum space (Figure a). In the P configuration (Figure b), the Néel vectors of the two electrodes are aligned, causing the spin-resolved Fermi surfaces to coincide, maximizing spin-matched transmission (Regions 1 and 2) and resulting in a low-resistance state. In contrast, in the AP configuration, reversal of one Néel vector interchanges the spin characters, leading to momentum-space mismatch of the spin-resolved transmission channels and suppressed coherent tunneling, thus producing a high-resistance state (Figure c). However, incomplete separation of opposite-spin Fermi surfaces inevitably leaves residual overlap regions (Region 0) in the projected 2DBZ. These states remain transmissive even in the AP configuration and contribute nearly equally to the conductance of both P and AP states, thereby limiting the maximum achievable TMR. Reducing the area of this overlapping region is therefore expected to enhance the TMR. In the ideal ballistic limit, with no overlap between opposite spin-resolved transmission channels, the AP state transmission vanishes and TMR formally diverges.

1.

Spin channels and design principles. (a) Schematic of the momentum-dependent spin distribution in a typical d-wave altermagnet. (b, c) Spin-resolved tunneling in altermagnet-based MTJs (AMTJs) under the ballistic transport framework. Regions 1 and 2 denote spin-up and spin-down channels; Region 0 represents the spin-degenerate overlap channel. (d, e) Tunnel magnetoresistance (TMR) dependent on interfacial magnetic moment configurations. (f, g) TMR independent of interfacial configurations in coplanar arrangements. The slashed regions represent the electronic states contributed by the interface. Pink and blue correspond to spin-up and spin-down electrons, respectively.

Beyond Fermi surface topology constraints, the real-space arrangement of interfacial magnetic moments modulates the intrinsic bulk transport characteristics. In AMTJs based on RuO₂, MnF₂, and CrSb, atoms with opposite moments are often arranged noncoplanarly along the transport direction. , Consequently, exposure of different magnetic atomic planes leads to a mismatch between interfacial moments and the electrode Néel vector. As illustrated in Figure d, when the bulk is in a parallel configuration, the interface exhibits antiparallel coupling, suppressing the total electronic state in the P state. Conversely, interfacial parallel alignment during a bulk AP state enhances leakage (Figure e). Such conflicts between the interface and bulk not only weaken the TMR effect but can also cause the AMTJs to degenerate into an interface-dominated antiferromagnetic-like junction, obscuring the intrinsic advantages of altermagnets. However, if the magnetic atoms with opposite spins are localized within the same plane along the transport direction (Figure f and g), the interfacial magnetic arrangement aligns with the electrode Néel vector. This decouples TMR from interfacial magnetic configuration interference, making it dependent solely on bulk altermagnetic features and specific atomic interfacial termination types.

Accordingly, achieving large TMR requires synergistic optimization by selecting altermagnets with minimized spin-degenerate regions on the Fermi surface and coplanar magnetic configurations. Meeting both criteria allows the AMTJ transport to bypass interfacial interference and recover its intrinsic bulk properties. This design principle establishes a clear strategy for optimizing the TMR effect and provides a clean physical model for investigating atomic-scale termination effects and orbital-related quantum transport mechanisms.

Electronic Structure of KV₂Se₂O

To validate the proposed design strategy, the d-wave altermagnet KV₂Se₂O is selected as the electrode material, aiming to achieve a breakthrough in TMR performance by leveraging its nodal-point-like spin-degenerate channels and coplanar alternating magnetic order.

The crystal structure of KV₂Se₂O features a tetragonal layered lattice belonging to the P4/mmm space group, as illustrated in Figure a. After geometric optimization, the calculated lattice parameters (a = b = 3.99 Å, c = 7.48 Å) are in excellent agreement with previous reports. From an atomic-structural perspective, the Se atoms are located directly above and below the V₂O planes, while adjacent V₂Se₂O layers are separated by K atomic layers, giving rise to a well-defined layered stacking configuration. Magnetism originates from the V atoms within the V₂O plane, where neighboring V atoms carry moments of equal magnitude but in opposite directions, with the Néel vector oriented along the [001] axis. The two opposite-spin sublattices can be interconverted via [C ₂||C _4z ] rotation or [C ₂||M _11̅0] operation, forming a planar d-wave altermagnetic order. This arrangement ensures that sublattices with opposite spins reside entirely within the same atomic plane, forming a quintessential coplanar magnetic configuration that provides a structural foundation for avoiding interfacial magnetic mismatch.

2.

Electronic structure of KV₂Se₂O. (a) Crystal structure of KV₂Se₂O. Purple, gray, yellow, and pink spheres denote K, V, Se, and O atoms, respectively. The arrows on V atoms indicate the coplanar alternating magnetic order. (b) Calculated spin-resolved band structure of KV₂Se₂O. (c) Fermi surfaces of KV₂Se₂O. The overlap of spin-up and spin-down branches is localized at four isolated degenerate nodal points (marked by orange circles). (d–g) Symmetry-resolved band structure and orbital projections along the [001] transport direction (Γ–Z). (d) Symmetry-resolved band structure. (e–g) Orbital-projected bands for the V–d _z ², V–d_xz , and V-d_yz orbitals, respectively.

The calculated band structure clearly demonstrates the momentum-dependent spin splitting in KV₂Se₂O (Figure b). Along the high-symmetry paths Γ–X–M and Γ–Y–M, the energy bands exhibit opposite spin splittings, with a maximum splitting magnitude of approximately 1.6 eV. Two bands (Band 1 and Band 2) cross the Fermi level and contribute to the Fermi surface (Figure c). Band 1 exhibits quasi-flat dispersions along the [001] direction, whereas Band 2 forms spin-polarized Fermi pockets near the X and Y valleys. The flatness of Band 1 plays a crucial role in yielding a large TMR effect, because it compresses the extended spin-degenerate regions into four isolated degenerate nodal points, thereby suppressing the leakage current in the AP state. As a result, when charge transport occurs parallel to these nodal lines (i.e., along [001]), the projected overlap between opposite spin-resolved transmission channels in the 2DBZ becomes effectively nodal-point-like (Figure S4a and b). Thus, zero spin polarization occurs only at four isolated nodal points, while all remaining transmission channels are fully spin-polarized (Figure S4c).

To further elucidate the microscopic origin of spin-dependent tunneling, the symmetry-resolved band structure of KV₂Se₂O along the transport direction ([001]) is analyzed (Figure d–g). Calculations reveal that transport channels near the Fermi level are mainly governed by the Δ₁ and Δ₅ irreducible representations, which correspond to symmetry-resolved states classified by the C _{4 v} point-group symmetry along the transport direction and associated with distinct orbital compositions (Figure d). The Δ₁ channel, primarily originating from the V–d_z ² orbital (Figure e), exhibits spin degeneracy along this high-symmetry path. Nevertheless, its contribution to the background current is negligible due to the aforementioned nodal-point-like characteristics. Meanwhile, the Δ₅ states exhibit a strong orbital symmetry constraint, which is essential for the altermagnetic effect. The spin-up channel is dominated by the d _xz orbital of V atoms (Figure f), while the spin-down channel is governed by the d _yz orbital (Figure g). Since the d _xz and d _yz orbitals are spatially orthogonal and have distinct transverse momentum distributions, such an orbital-dominated spin transport mechanism offers a strict symmetry-matching basis for employing KV₂Se₂O as a high-performance AMTJ electrode.

Termination-Dependent TMR

To explore the intrinsic transport properties of KV₂Se₂O, we construct AMTJs with a 6 Å vacuum barrier (Figure a). Three typical interfacial terminations, namely K-termination, Se-K-termination, and Se-termination, are considered due to their lower formation energy and experimental feasibility (Figure S3 and Table S2). Owing to the characteristic crystal structure of KV₂Se₂O, the interfacial magnetic moment orientation is independent of the exposed magnetic layer, which eliminates interfacial magnetic interference, enabling the transport properties to directly reflect the synergistic effects of bulk altermagnetic features and interfacial termination atomic types.

3.

Termination-dependent TMR. (a) Interfacial configurations of three typical terminations: K-termination, Se-K-termination, and Se-termination. (b) Transverse momentum ( k _||)-resolved transmission for the K-termination configuration at the Fermi level. (c, d) Energy-dependent transmission spectra for the P state (c) and AP state (d) across the three termination types. (e) Calculated TMR ratios of the three terminations as a function of energy.

Figure b illustrates the transverse momentum ( k _||)-resolved transmission for the K-termination AMTJ at the Fermi level. In both P and AP configurations, the transmission spectra for spin-up and spin-down electrons exhibit identical shapes with only a π/2 spatial rotation difference, directly visualizing the d-wave symmetry inherent in KV₂Se₂O bulk bands. In the P state, the transmission spectra present high-value regions with contours that highly coincide with the conduction channels. Conversely, in the AP state, overall transmission is suppressed due to the spin mismatch in momentum space, with the spectra shrinking into four isolated nodal-point-like spots. This strong contrast in the tunneling landscapes leads to transmission in the P state markedly outweighing that in the AP configuration (Figure c and d). Thus, the K-termination achieves an exceptionally large TMR ratio of 10¹²% (Figure e).

Other interfacial terminations render transmission probability extremely sensitive to the atomic layer type. As shown in Figure c–d, transitioning from K-terminated interface to Se-terminated one significantly reduces P state transmission and increases AP state leakage current. Consequently, the TMR of the Se-K-termination drops to 4.51 × 10⁶%, while full Se-termination further plummets to the order of 10⁵% (Figure e). This performance degradation is further corroborated by the local density of states (LDOS) distributions (Figure S5), which confirm that the replacement of K-terminations triggers a drastic attenuation of the effective tunneling DOS within the barrier region. For all three terminations, TMR increases within the energy window, as the nodal-point-like features shift toward the M point and eventually vanish (Figure S4d–g). Nevertheless, despite large transmission fluctuations across terminations, k _|| -resolved transmission spectra still retain the bulk Fermi surface feature (Figure S6). Furthermore, the total current remains inherently spin-neutral regardless of the termination type (Figure S7), which is caused by the d-wave symmetry of the altermagnetic system. It is worth noting that the inclusion of spin–orbit coupling (SOC) does not alter the overall transport mechanism or the intrinsic origin of the ultrahigh TMR, as shown in Figure S9 and Table S4. Accordingly, results without SOC are mainly presented in the subsequent analysis to more clearly highlight the intrinsic transport mechanism of the system and avoid SOC-induced perturbations to the main physical behavior.

Ultrahigh TMR realization depends not only on bulk Fermi surface structure but also crucially on interfacial atomic species. For comparison, transport properties along the [100] direction are also investigated (see Figures S10 and S11). Calculations show that while [100]-oriented AMTJs exhibit distinct spin splitting and certain robustness against interfacial variations, their TMR remains significantly lower than that along the [001] direction. This contrast highlights the application potential of the [001] orientation when coupled with specific interfacial terminations. Notably, it should be emphasized that even under the most unfavorable interfacial configurations, KV₂Se₂O-based AMTJs still outperform currently known AMTJ systems, fully demonstrating the great potential of this material in spintronic applications.

Termination-Preserved TMR

Before analyzing the microscopic mechanisms underlying termination-dependent TMR, the thermodynamic stability of the considered surface terminations is first evaluated. Two surface slab models of KV₂Se₂O corresponding to the K-termination and Se-termination are presented in Figure a. The surface grand potential (Ω) is employed to systematically assess the relative stability of these terminations as a function of the potassium chemical potential (Δμ _K ), with computational details provided in the Supporting Information (Section 3). The resulting chemical potential phase diagram concerning Δμ _K and Δμ _Se is displayed in Figure b. The calculations reveal that under K-rich conditions (Δμ_K > −0.889 eV), the K-termination exhibits the lowest surface grand potential (Ω _K = 0.0236), serving as the thermodynamic ground state of the system. In contrast, under Se-rich or K-poor conditions (Δμ_K < −0.889 eV), the Se-termination becomes the more stable configuration. However, the experimental synthesis of KV₂Se₂O single crystals mainly relies on the KSe self-flux method, which provides a K-rich environment favorable for the formation of thermodynamically stable K-terminated interfaces. Moreover, analysis of competing secondary phases including K₂Se, V₂O₅, and VSe₂ shows that these phases all lie outside the stability region of KV₂Se₂O, further verifying the experimental feasibility of realizing high-quality K-terminated interfaces.

4.

Microscopic physical mechanism of termination-dependent transport in KV₂Se₂O AMTJs. (a) Surface slab models of K-termination and Se-termination. (b) The chemical potential (Δμ _K , Δμ _Se ) phase map. The shaded area is the thermodynamically stable range for the equilibrium growth of KV₂Se₂O, with yellow representing the stability region of the Se-terminated surface and purple representing that of the K-terminated surface. (c, d) Electronic band structures of the K-termination (c) and Se-termination (d) slab models. (e–g) Atom-resolved projected density of states (PDOS) for the spin-up and spin-down electronic states of V (e), Se (f), and K (g) atoms.

Subsequently, electronic structures of different surface slab models are compared (Figure c–d). Since the focus is on tunneling along the Z direction, the analysis primarily considers the band structure near the Γ point to assess transmission properties. The results show that the K-terminated interface band features are highly consistent with bulk KV₂Se₂O, indicating that K-termination maximally preserves the bulk’s intrinsic physical properties. In contrast, the Se-termination induces significant impurity levels within the band gap near the Fermi level, introducing deep-level defect characteristics to the system. This result is further validated by the comparison of the DOS in Figure S12a, where K-termination does not alter DOS energy distributions, only slightly increasing intensity due to more atoms, while Se-termination generates substantial charge accumulation in the valence band region, directly corresponding to the defect levels in the band structure. Additionally, real-space electronic states confirm that the Se-terminated surface has numerous extra electronic states from dangling bonds, which significantly weaken the intrinsic altermagnetic properties (Figure S12b and c).

Deep analysis of the orbital contributions from each atom in bulk KV₂Se₂O further elucidates the microscopic mechanism, as depicted in Figure e–g. In bulk KV₂Se₂O, altermagnetism primarily originates from the two orthogonal d-orbitals (d _xz and d _yz ) of the V atoms (Figure e), followed by the p-orbital contribution from Se atoms (Figure f), while K atoms’ s-orbital contribution is extremely weak (Figure g). This orbital weighting disparity determines the sensitivity of the electronic structure at the interface. Since Se atoms primarily contribute via p-orbitals, Se-termination exposure induces strong p-d orbital hybridization with the V atoms. This hybridization disrupts the local tetragonal symmetry of the V atoms and introduces incoherent scattering channels, acting as a bottleneck for TMR enhancement. This is further corroborated by the projected local density of states (PLDOS) along the transport direction (Figure S8). With the substitution of Se atoms, the effective density of states penetrating the vacuum barrier increases significantly in the AP state, leading to a non-negligible leakage current. Conversely, in the P state, previously continuous conduction channels are disrupted by hybridization-induced scattering, substantially reducing tunneling state density. In contrast, K-termination exhibits high electronic transparency. Since the DOS of K atoms mainly consists of s and p _z orbitals (Figure g), both extending along the Z-direction (the transport direction) and contributing minimally near the Fermi level, electrons of different spin orientations experience negligible transverse scattering when traversing the interface. In this near-ideal passivation environment, P-state conduction channels maintain continuous resonant tunneling features, while strict orbital orthogonality is preserved in the AP state, ultimately achieving an intrinsic TMR effect of up to 10¹²%.

Enhanced TMR via Barrier Engineering

To investigate KV₂Se₂O performance within realistic device architectures, AMTJs are constructed using KTaO₃ as the tunneling barrier (Figure a). KTaO₃ is selected for its excellent lattice matching with KV₂Se₂O (a mismatch of less than 1%) and wide bandgap characteristics. Its band structure (Figure b) exhibits no crossing bands at the Fermi level, confirming its superior insulating nature. Complex band structure calculations reveal that KTaO₃ also possesses Δ₁ symmetry, which can enhance the selectivity of the tunneling process (Figure S12e). Moreover, k _||-resolved transmission calculations indicate that the rotational symmetry of KTaO₃ is highly compatible with that of KV₂Se₂O, and a high-transmittance window appears near the center of the Brillouin zone (Figure S12f). These features ensure effective lateral momentum conservation and the momentum-space polarization of the KV₂Se₂O electrodes during transport, thereby enhancing the TMR effect.

5.

Enhanced TMR in KV₂Se₂O/KTaO₃/KV₂Se₂O AMTJs. (a) Device configurations of AMTJs with KTaO₃ barriers for K-termination and Se-termination. The outermost K and Se atoms at the interfaces are indicated in angstroms (Å). (b) Band structure of KTaO₃. (c, d) Projected local density of states (PLDOS) for the K-terminated device in the P state (c) and AP state (d). (e, f) Comparison of transmission spectra between vacuum-barrier and KTaO₃-barrier AMTJs for K-termination (e) and Se-termination (f). (g) Comparison of TMR ratios between this work and previously reported AMTJ systems.

The tunneling transport mechanism is intuitively reflected in the PLDOS of the device. As illustrated in Figure c–d, taking the K-termination as an example, a clear energy gap persists at the Fermi level within the barrier region for P and AP states. This confirms that the KTaO₃ layer effectively decouples the Bloch states of the two electrodes, ensuring that transport is dominated by coherent tunneling. The calculated layer-resolved DOS of the supercell also supports this mechanism (Figure S13). In the P state, the barrier layer optimizes the interfacial matching (Figure c), further enhancing the Fermi-level transmission coefficient compared to the vacuum-barrier case (Figure e). Conversely, in the AP state, the absence of effective conduction channels between the left and right electrodes severely hinders electron propagation (Figure d), dramatically reducing transmission (Figure f). These dual factors substantially elevate the TMR value to 3.3 × 10¹²%. Furthermore, the barrier layer also increases the TMR value of the Se-terminated AMTJ to 10⁷%. Although redundant surface states at the Se-interface increase tunneling probabilities for both P and AP states (Figure S14a–b), the overall TMR remains high because the KTaO₃ barrier effectively preserves electron resonant tunneling characteristics in the P state (Figure f). Additionally, k _||-resolved transmission coefficients show that the bulk KV₂Se₂O Fermi surface features are precisely preserved after the introduction of the KTaO₃ barrier (Figure S14c–d), with the total transmission remaining spin-neutral in both the P and AP states (Figure S15). This further confirms that the giant TMR effect originates from the perfect synergy between the intrinsic physical properties of the electrodes and the high-quality interface.

To objectively assess the potential of KV₂Se₂O-based AMTJs, we compare them with recent representative theoretical studies on AMTJs (Figure g). ,,− ,,, Existing AMTJs, whether optimized through barrier engineering, half-metallic electrodes, or specific crystallographic orientations, typically exhibit TMR values below 10⁵%. However, the devices designed in this study achieve TMR values above 10⁷%, with the potential to reach 10¹²% through rational interfacial termination design. Compared with recently reported AMTJs, particularly KV₂Se₂O-based AMTJs, this study elucidates the role of specific interface terminations in preserving spin polarization in altermagnetic systems. Specifically, the K-termination, owing to its passivation characteristics, maximally preserves the spatial orthogonality and intrinsic spin polarization of the V-3d orbitals in bulk KV₂Se₂O, thereby enabling exceptionally high TMR. Meanwhile, the intrinsically compressed nodal spin-degenerate channels, together with the coplanar interfacial magnetic order, provide the underlying physical basis for this termination-preserved behavior, allowing the ultrahigh TMR to be fully realized. Overall, these results not only establish the theoretical performance upper limit of KV₂Se₂O-based AMTJs, but also provide clear design principles and guidelines for regulating altermagnetic tunneling transport via interface termination engineering. Additional discussion on experimental feasibility and device realization is provided in the Supporting Information (Section 11).

Conclusions

In summary, this work reveals the ultrahigh TMR in KV₂Se₂O-based AMTJs and its underlying microscopic mechanism. The quasi-flat dispersion along [001] compresses spin-degenerate regions into isolated nodal lines, minimizing opposite-spin channel overlap and suppressing transmission in the AP configuration. Meanwhile, the robust coplanar interfacial magnetic order ensures the alignment of interfacial magnetic moments with the bulk Néel vector, eliminating the interfacial magnetic mismatch. Through atomic-level interfacial engineering, K-termination yields an ultrahigh TMR of 10¹²% by maximally preserving the spatial orthogonality and spin polarization of bulk V-3d orbital states. Even for Se-termination, where dangling bonds induce surface states, the system still exhibits a high TMR of 10⁷%, which is comparable to the highest values reported for altermagnetic systems. The performance enhancement upon introducing a KTaO₃ barrier further validates the robust transport stability of the junction under realistic device conditions. This study provides theoretical guidance and effective regulation strategies for the design of high-performance altermagnetic spintronic devices.

Methods

The geometric optimization, electronic structure calculations, and quantum transport simulations of KV₂Se₂O are performed using the QuantumATK package. , The exchange–correlation interaction is described within the spin-polarized generalized gradient approximation (SGGA) using the Perdew–Burke–Ernzerhof (PBE) functional. All atoms are described using a High basis set together with PseudoDojo pseudopotentials.

For geometry optimization and electronic structure calculations, the Brillouin zone is sampled using a dense 25 × 25 × 15 Monkhorst–Pack k-point mesh. The real-space grid cutoff for the charge density is set to 80 Hartree, and the electronic temperature is fixed at 300 K. Structural relaxation is continued until the residual force on each atom is smaller than 0.01 eV/Å. With these parameters, both the optimized geometries and the resulting band structures show good agreement with available experimental data, ensuring the reliability of the calculations. The effects of Hubbard-U and SOC are further tested, and both are found to have negligible influence on the band structure (Figure S1).

Quantum transport properties are investigated within the density functional theory (DFT) combined with the nonequilibrium Green’s function (NEGF) formalism. − In the Green’s function approach, an infinitesimal imaginary broadening parameter of 0.001 Hartree is introduced to ensure numerical stability of the energy integration and to avoid unphysical oscillations. The device is modeled as a two-probe system, consisting of a central scattering region sandwiched between two semi-infinite symmetric electrodes. For transport calculations, the real-space grid cutoff is increased to 150 Hartree. In the self-consistent transport calculations, a 15 × 15 × 150 k-point mesh is used. For the calculation of k _||-resolved transmission, the two-dimensional Brillouin zone is sampled using a dense meshes of 500 × 500 k points, which has been verified by convergence tests (Figure S2 and Table S1).

In crystalline MTJs operating in the ballistic regime and in the absence of SOC, both spin and the transverse wave vector k _|| is conserved. Consequently, the longitudinal tunneling conductance per unit area for spin σ is given by $g^{\sigma }=\frac{e^2}{h}\sum _{{\mathit{k}}_{\parallel }}{N}_{\parallel }^{\sigma }({\mathit{k}}_{\parallel })$ . Here $N_{\parallel }^{\sigma }({\mathit{k}}_{\parallel })$ represents the spin-resolved number of conduction channels at transverse wave vector k _||. We then define the k _||-resolved spin polarization p _||( k _||) as

$$p_{\parallel }({\mathit{k}}_{\parallel })=\frac{N_{\parallel }^{\uparrow }({\mathit{k}}_{\parallel })-N_{\parallel }^{\downarrow }({\mathit{k}}_{\parallel })}{N_{\parallel }^{\uparrow }({\mathit{k}}_{\parallel })+N_{\parallel }^{\downarrow }({\mathit{k}}_{\parallel })}$$ 1

The TMR effect in MTJs fundamentally arises from the k _||-resolved spin polarization p _||( k _||) matching conditions of two metal electrodes in the P and AP configurations.

TMR is defined as

$$TMR=\frac{T_P-T_{AP}}{T_{AP}}\times 100\%$$ 2

where T_P and T_AP denote the total transmission in the P state and AP state, respectively.

A version of this manuscript has been deposited as a preprint: Guo, J.; Mavani, H.; Fang, W.; Tang, J.; Li, W.; Wu, W.; Li, H.; Tsymbal, E.Y.; Zhang, L. Termination-Preserved Ultra-high Tunneling Magnetoresistance in Altermagnetic KV₂Se₂O, 2026, arXiv:2605.18493, arXiv. https://arxiv.org/abs/2605.18493 (accessed June 13, 2026).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.6c05425.

• Hubbard-U and spin-orbit coupling tests for KV₂Se₂O; convergence test of the k-point mesh density; interfacial terminations and stability of KV₂Se₂O; transport mechanisms along the [001] and [100] directions; influence of spin-orbit coupling on transport properties; electronic structures of KV₂Se₂O and KTaO₃; transport mechanism of KV₂Se₂O/KTaO₃/KV₂Se₂O AMTJs; robustness of TMR under realistic perturbations; angle-dependent TMR in KV₂Se₂O/KTaO₃/KV₂Se₂O AMTJs; experimental feasibility and device realization of KV₂Se₂O-based AMTJs (PDF)

J.G. performed the first-principles and transport calculations, analyzed the results, prepared the figures, and wrote the initial draft. H.M. performed additional calculations and contributed to data analysis. W.F., J.T., and W.L. contributed to discussion and analysis. W.W., H.L., E.Y.T., and L.Z. acquired funding for the project. E.Y.T. contributed to manuscript review and editing. L.Z. conceived the idea, supervised the project, and contributed to manuscript revision. All authors approved the final version of the manuscript.

The authors declare no competing financial interest.