Altermagnetic ferrovalley materials: Paradigm shift and frontier advances in valleytronics

Abstract

Valley degree of freedom, an intrinsic property of electrons, provides a novel dimension for information storage and processing. For most conventional valleytronic materials, however, stable and tunable valley polarization (VP) is usually realized based on spin−orbit coupling (SOC), which significantly limits material choices and operational flexibility. Altermagnetism — a novel magnetic order characterized by compensated magnetic order with vanishing net magnetization, broken inversion symmetry, and momentum-dependent spin splitting — offers a revolutionary solution to overcome this bottleneck. Originating from the coupling of altermagnetism and ferrovalley (FV) physics, the altermagnetic ferrovalley (AMFV) materials enable robust, tunable, and spontaneous spin-valley locking independently of strong SOC. This review systematically outlines the advances in AMFV materials. It begins by revisiting the types and limitations of traditional FV systems, then delves into the physical nature of altermagnetism and the symmetry principles underlying its integration with valley physics. Subsequently, it categorizes and reviews the latest theoretical and experimental progress on representative AMFV materials (e.g., V₂Se₂O, Fe₂MX₄), focusing on elucidating their multi-degree-of-freedom coupling characteristics (such as spin-valley-optical coupling) and their modulation mechanisms under external stimuli, including strain, electric field, sliding, twisting, and proximity effects. These AMFV systems hold enormous application potential in valley tunnel junctions and non-volatile memory, yet they still face challenges such as material scarcity, unclear mechanisms, and a lack of device-level exploration. Future efforts should leverage the synergy of high-throughput computation, advanced characterization techniques, and device development to drive breakthroughs and accelerate the development of next-generation information technologies.

Graphical abstract

Keywords

altermagnetism / ferrovalley materials / altermagnetic ferrovalley materials / valleytronics / spin-valley locking / multiferroic coupling

1 Introduction

The development of valleytronics stems from the in-depth exploration of the valley, an intrinsic degree of freedom of electrons [1−6]. Since the discovery of graphene in 2004, the exploration of its valley degree of freedom has gradually attracted widespread attention [7]. As band extrema of Bloch electrons in momentum space, valleys serve as stable binary information carriers, offering a new dimension for information encoding and storage [8−12]. In 2012, experimental verification of the valley Hall effect (VHE) and valley-dependent optical selection rules in monolayer MoS₂ marked the entry of valleytronics into experimental research [13−15]. Early studies mainly relied on external fields such as photoexcitation, magnetic field to induce VP, resulting in volatility in device information [16, 17]. In 2016, the concept of FV was formally proposed [18]. The coexistence of SOC and intrinsic magnetic exchange interaction can spontaneously break valley degeneracy, enabling stable valley polarization (VP) without external fields and laying the groundwork for nonvolatile valley devices [19−25]. Subsequently, ferroelectric ferrovalley (FV) materials (e.g., monolayer GeSe) were shown to generate tunable VP during ferroelectric switching, broadening the scope of FV research [26]. In recent years, FV physics has been discovered in various lattice systems, yielding rich multiferroic couplings with ferromagnetism and ferroelectricity (FE) [27−35]. Notably, recent studies have revealed that, contrary to traditional theoretical cognition, intrinsic FV behaviors can even exist in 2D bilayer systems with both time (T) and space (P) inversion symmetry, with VP switchable via ferroelasticity (FA) [36, 37]. Despite ongoing progress, VP modulation mostly relies on a single external field (e.g., electric field or strain) and generally depends on strong SOC. These limitations lead to insufficient modulation flexibility and a limited range of application material systems, which greatly hinder the development of high-performance and scalable valleytronic devices. The emergence of altermagnetism provides a promising pathway to overcome these bottlenecks.

As a magnetic order beyond the traditional ferromagnets (FM) and antiferromagnets (AFM) dichotomy, altermagnets (AM) exhibit zero net magnetic moment in real space but show significant non-relativistic spin splitting in momentum space [38, 39]. Crucially, their spin splitting originates from the broken PT symmetry rather than from strong SOC, opening up new opportunities for exploring spin and valley physics beyond heavy elements systems [40−45]. In recent years, altermagnetism has not only been experimentally confirmed in bulk materials such as CrSb, RuO₂, MnTe, and Rb_1−δV₂Te₂O via spin-polarized angle-resolved photoemission spectroscopy (ARPES) [46−52], but also shows rich tunability in the two-dimensional (2D) limits [53−56]. The convergence of altermagnetism and FV physics has spawned the emerging field of AMFV materials. In 2021, spin-valley locking protected by crystal symmetry was first discovered in systems such as V₂Se₂O, where strain alone can induce and regulate large VP, overcoming the traditional reliance on strong SOC in FV materials [57]. Since then, theoretical progress has advanced in tandem with experimental breakthrough in altermagnetism: synergistic piezoelectric, piezomagnetic, and piezovalley effects were demonstrated in V₂SeTeO [58]; an electric field-tunable VP state was proposed in layered A(BN)₂ materials [59]; spin-valley-optical triple locking was found in the Fe₂MX₄ system [60]; a new sliding engineering approach for modulation of VP in AMFV was developed.

Although valleytronics and altermagnetism have been reviewed separately, existing reviews focus either on conventional SOC-dependent FV materials or on magnetic and spintronic properties, neglecting valley physics and device applications. To date, no review has systematically summarized the rapidly advancing AMFV field, particularly the spin-valley locking independent of SOC, unified material design principles, and multi-degree-of-freedom coupling mechanisms driven by altermagnetic order. This review fills this critical gap. Overall, AMFV materials mark a pivotal shift in valleytronics. They enable VP control through magnetic order and symmetry engineering, thus eliminating the reliance on strong SOC and opening a new route to overcome the limitations of conventional materials in terms of available systems and tuning flexibility. This review aims to systematically summarize the research progress of AMFV materials, covering their physical mechanisms, modulation methods, and material systems, to provide a comprehensive perspective for researchers in related fields and inspire subsequent exploration and device innovation.

2 Traditional valley-polarized materials

Intrinsic VP is a core focus of FV material research. Its key advantage lies in achieving stable VP without modulation by external fields (magnetic/optical fields) modulation, relying solely on intrinsic spontaneous symmetry breaking of materials (e.g., magnetic order, electric polarization). This characteristic greatly reduces the dependence of traditional valleytronic materials on external stimuli. Based on the physical nature of polarization origins, such materials are currently categorized into three main types: ferromagnetic, ferroelectric, and ferroelastic materials, each of which exhibits unique advantages in mechanism innovation and device applications.

2.1 Ferromagnetic and ferroelectric FV materials

Ferromagnetic FV materials represent a breakthrough in valleytronic research. In conventional transition metal dichalcogenides (TMDs), the VP is largely an external-field-induced and volatile phenomenon [8, 13, 61−64], as the intrinsic SOC alone is insufficient to break the energy degeneracy between K⁺ and K⁻ valleys and induce spontaneous VP. In 2016, theoretical predictions revealed that the intrinsic ferromagnetism of 2H-VSe₂ endows the system with the intrinsic exchange interaction of V-3d electrons, which can spontaneously break time-reversal symmetry [Figs. 1(a, b)]. Combined with the inherent strong SOC of heavy transition metal TMDs that strongly couples the spin and valley degrees of freedom of electrons, this synergetic effect effectively lifts the energy degeneracy between K⁺ and K⁻ valleys and induces stable spontaneous VP at room temperature.

Originating from the SOC-induced spin-valley locking effect, the K⁺ and K⁻ valleys obey strict circularly-dependent optical selection rules, where the K⁺ and K⁻ valleys can only be excited by left-handed σ₊ and right-handed σ₋ circularly polarized light, respectively [Fig. 1(c)]. When an external magnetic field switches the magnetization direction of FV materials, both the VP and corresponding optical response reverse simultaneously. In addition, SOC modulates the valley-dependent distribution of Berry curvature in the k-space, and the spontaneous VP further leads to the asymmetric Berry curvature of K⁺ and K⁻ valleys, which is the core physical origin of the anomalous valley Hall effect (AVHE) observed in 2H-VSe₂ and similar FV materials. Based on the AVHE, prototype memory devices with electrical reading and magnetic writing functions can be constructed, realizing nonvolatile valleytronic information storage [Fig. 1(d)] [18].

A remarkable electronic state, known as the half-valley metal, has also been discovered in ferromagnetic FV systems [65−68]. Taking 2H-FeCl₂ as an example, by tuning the electron correlation strength, one valley can exhibit metallic behavior while the other remains semiconducting under the combined regulation of SOC and exchange interaction. In such a half-valley metal system, the spin-valley locking effect derived from SOC is still preserved, resulting in selective absorption of a specific helicity of circularly polarized light [65]. Theoretical calculations suggest that topological devices based on half-valley metals could achieve lower energy consumption and higher stability in quantum computing, making the experimental synthesis of related materials a current research focus [69].

Ferroelectric FV materials provide a low-power electrical route for manipulating VP, effectively overcoming the reliance on magnetic fields required in ferromagnetic systems [70−75]. In intrinsic orthorhombic ferroelectric GeSe [26], the inequivalent valleys are located at the X and Y points, enabling coupled FE and FV properties [Fig. 1(e)]. When GeSe transitions from the paraelectric to the ferroelectric phase, the system shifts from a paravalley (PV) to the FV state, inducing pronounced VP [Figs. 1(g, h)]. The valence-band maximum and conduction-band minimum of GeSe are mainly composed of p_x and p_y orbitals, which lead to optical selectivity dependent on x- and y-linearly polarized light [Fig. 1(f)]. In addition, sliding ferroelectric FV systems represent a recently developed class of materials in which interlayer atomic sliding enables switching between different stacking configurations, providing a versatile platform for coupling FE with FV properties [Fig. 2(a)] [20, 36, 37, 76−78]. For instance, when bilayer VS₂ slides from AA to AB stacking, the built-in electric field induced by sliding FE drives band splitting, induces pronounced VP and thereby achieving a three-way coupling among FM, FE, and FV order [78]. Similar behavior has been observed in bilayer VSiGeP₄ [Fig. 2(b)], YI₂ and other systems [20, 27, 79−81].

2.2 Other induction mechanisms of FV materials

Moreover, even in stacking structures that retain global inversion symmetry, specific sliding operations can break local symmetry and induce VP [36, 37]. This has led to the proposal of a new VP mechanism that does not rely on breaking either time-reversal or spatial-inversion symmetry, moving beyond the conventional requirement of ferromagnetic or ferroelectric order to lift valley degeneracy. For example, bilayer GaBr in a stable sliding-stacked state can achieve a valley splitting as large as 226.2 meV while preserving global inversion symmetry [Figs. 2(c, d)] [36]. It exhibits FV−FA coupling, enabling VP manipulation via shear strain [Fig. 2(e)]. Similarly, RhCl₃ and InI bilayers remove valley degeneracy by breaking the rotational symmetry within individual layers, yielding valley-splitting magnitudes comparable to those in conventional FV materials [37]. All these systems exhibit clear valley-dependent linear-polarization optical selectivity, offering a direct experimental signature for detection. In terms of control, VP in such sliding systems can be efficiently switched via shear strain or interlayer sliding, overcoming the limitations of electric-field manipulation in inversion-symmetric structures [Fig. 2(e)].

2.3 Properties and application prospects of VP materials

Valleytronics transcends the dependence of traditional electronics on charge and spin. By regulating valley states through various means such as light, electricity, magnetism, stress, and non-Hermitian physics, it demonstrates broad application potential in fields such as information storage, logic operations, quantum technologies, photoelectric conversion, and sensing [2, 6, 9, 82]. Currently, valleytronics has gradually progressed from fundamental research toward device implementation and functionalization, relying on properties such as symmetry breaking in two-dimensional materials, valley-spin coupling, and valley-selective optical selection rules. For instance, a non-Hermitian valley filter based on bilayer graphene moiré superlattices can achieve nearly 100% VP efficiency, and its gate-tunable characteristics offer a new approach for high-precision valley electron sources [Fig. 3(a)] [83]. As a core device, valley transistors have also developed various high-performance configurations [Fig. 3(b)]: the room-temperature type achieves valley-polarized injection and electrical detection via plasmonic antennas, with an on-off ratio of up to 10³ [84]; the piezoelectric type utilizes strain-induced piezoelectric fields to significantly enhance the operational frequency of valley quantum bits [85]. In optoelectronic devices and non-volatile memory, by tuning the twist angle of WSe₂/WS₂ heterostructures, electrically controlled VP switching and photon spin encoding can be realized, laying the foundation for the development of electrically controlled valley-spin memory [Fig. 3(c)] [86]. Additionally, valley information can couple with mechanical motion, enabling the transduction of valley states into mechanical displacement using a monolayer MoS₂ resonator [Fig. 3(d)] [2]. Moreover, the valley degree of freedom provides new insights for quantum computing encoding and topological state regulation. Through mechanisms such as valley-spin locking and the valley Zeeman effect, efficient manipulation and readout of quantum states are achieved [87−90]. Valleytronics is expected to overcome the power consumption and integration bottlenecks of traditional electronics, offering a novel solution for the next generation of highly efficient and low-power electronic technologies.

As listed in Table 1, the FV materials rely on strong SOC or symmetry breaking to achieve VP, which not only restricts material selection but also makes efficient electrical control challenging. Therefore, exploring new VP mechanisms that do not depend on strong SOC and are compatible with efficient electrical control is of great importance. In recent years, the emergence of altermagnetism, with its unique momentum-space spin splitting characteristics, offers a promising solution and has given rise to the new research direction of AMFV materials.

3 Altermagnetic materials

3.1 Overview of altermagnetism

Altermagnetism [see Fig. 4(a)] is an emergent class of magnetic materials that revolutionizes the traditional magnetic classification system. Its defining characteristic is non-relativistic spin splitting in reciprocal space, a unique property that opens new avenues for magnetic manipulation [38, 39, 42, 50, 91−101]. Compared to conventional magnetic materials, altermagnets possess a compensated magnetic order with zero net magnetic moment in real space, inheriting the stray field-free stability of antiferromagnets. Meanwhile, they show significant spin polarization (SP) in reciprocal space, retaining the readily tunable magnetoelectric response of ferromagnets. Based on differences in magnetic moment arrangement and coupling with electronic band characteristics, altermagnets can be classified into various types such as d-wave, g-wave, and i-wave [38, 39]. Each category shows distinct differences in spin splitting symmetry and charge transport efficiency, providing diverse options for targeted applications. Recently, the experimental confirmation of altermagnetism has been firmly established through the direct detection of momentum-space spin splitting via spin-resolved angle-resolved photoemission spectroscopy (ARPES) in bulk single crystals such as CrSb, RuO₂ and MnTe [48, 49, 102−108]. To date, the development of altermagnets has been particularly rapid, gradually giving rise to other types of altermagnetism, such as spin-cluster altermagnetism [109], spin-orbital altermagnetism [110], orbital altermagnetism [111], and charge-order-induced altermagnetism [112]. Application bottleneck of altermagnetism lies in achieving efficient and non-volatile external regulation of spin splitting. Current research mainly pursues breakthroughs through two pathways. The first is coupling AM with other ferroic orders such as FE and FA to construct multiferroic systems, and precisely regulating its spin properties via ferroic order switching (e.g., ferroelectric polarization switching and ferroelastic lattice rotation). The second is utilizing twist engineering to break the system symmetry of 2D bilayers, thereby inducing stable altermagnetism.

3.2 Altermagnetism and multiferroic coupling

From the perspective of the multiferroic coupling, different types of ferroic orders achieve flexible regulation of altermagnetism through unique pathways [113−121]. Among them, the coupling between FE and altermagnetism is the synergistic regulation of electron spin and magnons via polarization switching [Fig. 4(b)] [113]. For example, in monolayer CrPS₃, when the ferroelectric polarization is reversed from the P state to the −P state, the sign of spin splitting is simultaneously reversed with a switching barrier of only 0.17 eV/u.c. [116]. Systems such as wurtzite w-MnSe exhibit excellent room-temperature application potential, with a large ferroelectric polarization of 53 μC/cm² and a significant spin splitting of 245 meV [122]. In addition, altermagnetism can couple with antiferroelectricity (AFE) [Fig. 4(c)], giving rise to the antiferroelectric altermagnets (AFEAM) [114]. For example, in the AFE state of CuWP₂S₆, the spin splitting reaches 120 meV. When the system transforms to the FE state via a weak electric field, the spin splitting effect vanishing. Similarly, the G-type antiferromagnetic BiCrO₃ with AFE state is a typical AFMAM candidate, which transforms into an AFM in the FE phase [114]. Additionally, FA mainly dominates the regulation of altermagnetism through lattice distortion or lattice rotation [123]. For example, ferroelastic switching in pentagonal monolayer CoSe₂ not only drives a 90° rotation of the spin-split bands but also regulates the sign of the magneto-optical Kerr effect through the co-directional/counter-directional rotation of the lattice and Néel vector [Fig. 4(d)] [124]. Monolayers RuF₄ or CuF₂ can achieve two-state or three-state spin splitting switching via π/2 or 2π/3 lattice rotation with low switching barriers, showing promise for efficient encoding of multi-state spin information [Fig. 4(e)] [115]. A hydroxylated monolayer Mn₂B₂(OH)₂ further realizes the synergy between chemical and ferroelastic regulation, yielding a spin splitting as high as 1130 meV, while enabling simultaneous control of altermagnetism [123].

3.3 Twist-induced altermagnetism

Beyond the ferroic order coupling, twist technology provides a universal and efficient strategy for constructing altermagnets in 2D systems [125−128]. A 2023 study pioneered the core idea of inducing spin splitting via twisted interlayer AFM bilayers. Twisting can break spin degeneracy symmetry, inducing momentum-dependent non-relativistic spin splitting in the absence of SOC, with an effect comparable to SOC in heavy elements. This phenomenon has been verified in 2D materials like NiCl₂, CrI₃, CrN, and CrSBr [Fig. 5(b)], laying the experimental foundation [125]. A 2024 study further established a universal framework defining the three-step standardized process of stacking-flipping-twisting [Fig. 5(a)]. Stacking monolayers to form an interlayer AFM bilayer, then flipping the upper layer to introduce in-plane twofold rotational symmetry, and reversing the twist to break inversion symmetry, ultimately inducing stable AMs [126]. Compatible with all 2D Bravais lattices, this framework enables customization of various waveforms (e.g., d-wave, g-wave and i-wave), and can optimize performance by adjusting twist angle and Fermi level. These strategies enrich the regulatory dimensions of altermagnetism and promote its applications in low-power spintronic devices.

4 Altermagnetic ferrovalley materials

AMFV materials integrate altermagnetism with FV properties, enabling spontaneous VP within altermagnet systems [57, 60, 129−131]. Such non-relativistic valley polarization, independent of SOC, arises from broken PT symmetry in altermagnetic lattices, following the crystal symmetry-paired (C-paired) mechanism rather than the conventional SOC-dependent time-reversal symmetry-paired (T-paired) mechanism, and is supported by a minimal tight-binding model in many previous studies [57, 60, 132]. Their defining feature is the locked relationship between spin and valley degrees of freedom, which can be controlled through strain, stacking, sliding, twisting and proximity effects to create stable VP state in momentum space. Furthermore, the valley-dependent optical selection rules of polarized light in valleys provides a novel physical mechanism for information encoding and photoelectric detection. The coupling of spin, valley, optics, and layer degrees of freedom opens new directions for designing functional devices and shows broad application prospects in non-volatile memory and quantum information processing.

In AMFV systems, spin-group symmetries produce valley-dependent Berry curvature with an anisotropic distribution, which directly leads to the anomalous valley Hall effect. These systems also host rich topological band crossings, such as spin-resolved Weyl points and symmetry-protected second-order topological corner states. Fundamentally, altermagnetism-driven spin–valley locking unifies the crystal valley Hall effect, quantum anomalous Hall effect, and spin-resolved topological corner states into a complete topological-valley-spin framework [60, 101, 133−135]. The core modulation mechanisms of VP in AMFV materials are detailed in the following sections, along with a brief analysis of their relative strengths and limitations, thereby providing valuable references for the rational design of related device.

4.1 Strain manipulation of VP in AMFV

In 2021, the phenomenon of C-paired spin-valley locking (SVL) mediated by crystal symmetry was first revealed in V₂Se₂O [Figs. 6(a, b)] [57]. Unlike traditional transition metal dichalcogenides (TMDs), which rely on strong SOC and T-paired mechanisms, the spin splitting in V₂Se₂O-like materials originates from the strong exchange coupling between itinerant electrons and local magnetic moments of the antiferromagnetic order [58, 134, 136−139]. As shown in Figs. 6(c) and (d), the VP can be induced by breaking symmetry via uniaxial strain alone [140−142]. Under moderate electrostatic doping, it can also exhibit a giant piezomagnetic effect and generate large noncollinear spin currents [Figs. 6(e, f)]. Moreover, constructing a Janus-structured V₂SeTeO/V₂STeO monolayer not only retains the in-plane C-paired spin-valley locking, but also breaks the out-of-plane mirror symmetry, resulting in giant piezoelectric properties [58, 143]. This creates a multiply coupled system allowing independent tuning of piezoelectric, piezovalley, and piezomagnetic responses [58, 143, 144]. In addition, research on higher-order topological physics has further expanded the physical implications of AMFV materials, extending investigations from spin-valley manipulation to the field of corner electronics [133, 145, 146]. Related studies, based on effective models, have established a spin-corner locking mechanism and realized a spin-resolved second-order topological insulating state in two-dimensional AMFV systems such as CrO and Cr₂Se₂O.

4.2 Electric field tuning of VP in AMFV

The A(BN)₂-type materials represented by Ca(CoN)₂ are typical AMFV systems with layer-resolved spin-valley locking characteristics [59, 147]. Their core advantage lies in the efficient electric field modulation of VP via gate voltage, which simultaneously manipulates the spin polarization state [Figs. 7(a−c)]. By substituting the top-layer Co atoms in Ca(CoN)₂ with Fe to form the Janus CaCoFeN₂ structure [Fig. 7(d)], an orthorhombic ferrimagnetic ground state is obtained. This structure exhibits giant spontaneous valley splitting in the absence of SOC [Fig. 7(e)] [148]. This behavior originates from a triple mechanism: symmetry breaking, enhanced valley-layer locking driven by an internal electric field, and ferrimagnet-mediated spin-valley locking. The valley splitting strength can be flexibly tuned via uniaxial strain and external electric fields. Moreover, the material shows highly anisotropic spin-plasmon characteristics, with direction-dependent responses to both static electric fields and dynamic electromagnetic waves. Strain can reverse the VP direction [149, 150]. Studies on such layer-resolved AMFV materials have elucidated the core mechanism of layer-resolved spin-valley locking. Through multi-dimensional control strategies — such as electric fields and strain — along with device design, these materials provide a representative example for the practical application of AMFV systems in spintronics and valleytronics.

4.3 Interlayer sliding modulating of VP in AMFV

Sliding, as an effective means of inducing altermagnetism, also serves as one of the effective approaches for regulating AMFV state [132, 151−154]. The Fe₂MX₄ (M = W, Mo, etc.; X = S, Se, Te, etc.) family [Fig. 8(a)] has been identified as a new class of 2D tetragonal AMFV materials with room-temperature application potential [60]. In the absence of SOC, Fe₂MX₄ exhibit zero Berry curvature everywhere in the Brillouin zone. When SOC is included, M_xy mirror and S_4zT magnetic group symmetries ensure opposite Berry curvatures at the high-symmetry X and Y points, which forms the core origin of the crystal valley Hall effect. Under biaxial compressive stress, Fe₂MX₄ materials transform from the altermagnetic semiconductor phase into a bipolarized topological Weyl semimetal phase, exhibiting symmetry-protected Weyl points.

A key breakthrough of these d-wave altermagnets lies in the first identification of an observable physical quantity directly linked to the FV state: linearly optical dichroism [Fig. 8(b)]. In these systems, the spin-up X valley absorbs the y-polarized light (σ_y), while the spin-down Y valley absorbs only x-polarized light (σ_x) [Fig. 8(d)], forming a robust spin-valley-optical triple locking that is independent of SOC. This discovery provides a direct optical signal for non-contact and non-destructive detection of the FV state in AMFV materials, with distinct characteristics and practical implications compared to conventional optical probes: unlike circular dichroism in traditional SOC-based valleytronic materials (e.g., monolayer MoS₂, WSe₂ [10, 85]. The sensitivity of linear optical dichroism for AMFV detection is primarily constrained by valley splitting magnitude (reliably detectable above ~100 meV, e.g., 0.29 eV in Fe₂WTe₄ bilayers [132], while signals below 50 meV are easily masked by noise) and sample quality. Importantly, linear optical dichroism is well-suited for room-temperature detection due to that the altermagnetic order in Fe₂MX₄ (e.g., Fe₂WSe₄) remains stable up to 330 K.

In bilayer systems, sliding enables the transition from a PV state (AB stacking) to non-volatile FV states (AC₁/AC₂ stacking), inducing a giant VP of up to 0.29 eV and optical dichroism [Fig. 8(c)], which in turn triggers unequal absolute values of the Berry curvature leading to the AVHE [Fig. 8(e)] [132]. By adjusting the doping type, opposite transverse Hall voltages can be observed, offering a clear implementation pathway for designing non-volatile memory devices based on the mechanical sliding-based writing and electrical signal reading principle.

Furthermore, changing the magnetization direction constitutes another crucial dimension for inducing and switching the valley-polarized state in the presence of SOC. When the magnetic moment is oriented out-of-plane (along the z-axis) or at a 45° direction, the Fe₂MoS₂Se₂ system retains C_4zT magnetic group symmetry, leaving the X and Y valleys equivalent and non-polarized (PV state). However, when the magnetic moment rotates to an in-plane direction (x or y axis), it breaks the C_4z lattice rotational symmetry, inducing a VP of 1.6 meV (FV state), which can be further enhanced by heavy-element doping to strengthen SOC [155]. This mechanism provides another universal strategy for regulating FV states, further expanding the application prospects of such materials in multifunctional valleytronic devices.

4.4 Twisting modulating of VP in AMFV

In traditional 2D materials, twisting is often employed as an efficient way to manipulate degrees of freedom [86, 156, 157]. For AMFV materials, twisting can induce magnetic phase transition by modulating lattice symmetry and interlayer interactions, and when combined with an out-of-plane electric field, it enables flexible control of VP [Fig. 9(a)] [156]. Research in 2023 confirmed that twisting can break the fourfold spin degeneracy symmetry in two-dimensional antiferromagnetic materials [125]; the establishment of a standardized stacking-flipping-twisting framework for valley-related engineering in 2024 further demonstrated that this strategy is applicable to all two-dimensional Bravais lattices, laying the groundwork for developing valley-related functionalities [126]. In an AMFV bilayer formed via twisting, there exist degenerate valleys originating from different layers with opposite SP. Applying an out-of-plane electric field creates a layer-dependent electrostatic potential, causing the energy levels of these valleys to stagger and lift their degeneracy, thereby generating VP, and reversing the electric field direction allows for opposite control of VP. For instance, in bilayer VOBr with a twist angle of 48.16°, applying an out-of-plane electric field of 0.02 V/Å leads to pronounced valley splitting in the conduction band, successfully achieving SP and VP [Fig. 9(c)] [156]. Meanwhile, monolayer Ca(CoN)₂ [Fig. 9(b)], as a special altermagnet, exhibits valley splitting of 125 meV in the conduction band and 21 meV in the valence band under a 0.04 V/Å electric field, further confirming the universality of this approach [Fig. 9(d)] [156]. More importantly, beyond realizing VP, electric-field regulation can also induce key electronic states such as half-spin/valley metals. These states are ideal candidates for building high-performance integrated spintronic-valleytronic devices.

4.5 Proximity effect modulating of VP in AMFV

The altermagnetic proximity effect (AMPE) has emerged as a key method for regulating VP in AMFV materials [158]. Its core mechanism lies in utilizing the van der Waals interface coupling to efficiently transfer the spin texture from an altermagnet to an adjacent non-magnetic layer, thereby breaking valley degeneracy without external fields and enabling efficient VP control [Fig. 10(a)]. For instance, the altermagnetism of V₂Se₂O can transfer its non-relativistic spin texture across the interface to adjacent non-magnetic (NM) layers such as β-SnO, PbS, and PbO, converting them into proximity-induced altermagnets. This process breaks the original spin-valley degeneracy in the non-magnetic layer, with the valley splitting strength decaying monotonically with increasing interlayer distance [159]. The broken mirror symmetry of V atoms in the V₂Se₂O/β-SnO [Fig. 10(b)] induces a net magnetic moment, resulting in VP up to 100 meV. Furthermore, reducing the interlayer distance enhances the net magnetic moment of V atoms induced by AMPE, leading to a significant increase in VP. For example, in V₂Se₂O/β-SnO, reducing the interlayer distance by 0.5 Å can boost the VP to nearly 400 meV [Figs. 10(c, d)]. Additionally, uniaxial strain can further synergistically break the valley degeneracy, enabling fine-tuning of VP. In PbS/V₂Se₂O, applying 3% tensile strain along the x-axis produces a valence band valley splitting of 152 meV. Compared to chemical doping strategies such as substituting V atoms with Cr to form a ferrimagnetic monolayer, the AMPE approach avoids potential structural stability issues and offers greater tunability, providing crucial theoretical support for developing low-power consumption valleytronic devices.

Strain, electric field tuning, interlayer sliding, twisting engineering, and proximity effect represent the main strategies for regulating VP in AMFV materials. These tuning methods, together with their representative materials, achievable VP magnitudes, stability, and inherent pros and cons are systematically summarized and compared in Table 2. Strain engineering can generate large VP with good mechanical stability and compatibility with micro-nano fabrication, but suffers from low dynamic tunability and may cause lattice distortion or phase transition under excessive strain. Electric field modulation enables fast, reversible, low-power VP tuning compatible with semiconductor devices, but requires high-quality dielectrics and may be weakened by built-in electric fields. Interlayer sliding realizes non-volatile VP switching and multistable states for memory applications, but shows low speed, hysteresis, poor repeatability and difficulty in large-scale fabrication. Twisting engineering provides high flexibility and synergistic modulation with electric fields, but demands precise angle control, has low structural stability and produces small VP alone. The altermagnetic proximity effect achieves nondestructive and increases the complexity of device design. Notably, the synergistic combination of multiple tuning strategies can further break valley degeneracy and optimize VP performance, making it more suitable for practical device applications.

5 Potential applications and device concepts of AMFV materials

The unique synergy between FV physics and altermagnetism in AMFV materials provides a novel material foundation for valleytronic devices [59, 144, 160−162]. This fundamentally reduces the reliance on external fields and specific materials, driving innovation in device principles. In terms of device applications, such materials exhibit diverse and efficient design pathways with experimentally achievable high-performance metrics, far exceeding the performance of conventional valleytronic and spintronic devices. Monolayer Ca(CoN)₂, utilizing its valley-spin-layer coupling effect, enables the construction of a tunnel junction controllable only by gate voltage, inducing an ultrahigh effective magnetic field of ~10³ T and realizes complete spin/valley current switching (ON/OFF ratio for spin current approaching 100%) [Fig. 11(a)]. When electric fields are applied to both electrodes, the polarization states align, and the device exhibits a low-resistance state. Conversely, when opposite electric fields are applied, the device switches to a high-resistance state with a giant tunneling magnetoresistance (TMR) effect [Fig. 11(b)] [59]. Electron transport is feasible between valleys contributing the same spin, but not between valleys with different spins [Fig. 11(c)]. Notably, the compound KV₂Se₂O was successfully synthesized via the self-flux method (with KSe as flux agent, sintered at 1000 ℃ and slowly cooled at 2 K/h) , and it has been confirmed to be a room-temperature metallic altermagnet with d-wave spin-momentum locking [163]. If monolayer V₂Se₂O can be exfoliated from its parent compound KV₂Se₂O, it will provide direct experimental evidence for the application of V₂Se₂O monolayers in high-performance devices. Furthermore, through intercalation modification (Li or V intercalation), V₂Se₂O bilayer can exhibit ferrimagnetic-ferroelastic characteristics and a half-metallic electronic structure at above room temperature (Li: 358 K, V: 773 K), with magnetic order dynamics and no performance degradation during device operation at ambient temperature (300 K) device operation, and the ferroelastic switching barrier (0.202−0.210 eV) ensures long-term retention stability of the device state (thermal disturbance resistance at 300 K). Spintronic devices constructed from this intercalated V₂Se₂O bilayer achieve a giant magnetoresistance (GMR) of up to 877% (V-intercalated) and an ultrahigh thermal tunneling magnetoresistance of nearly 12000% under finite-temperature gradient conditions, which is two to three orders of magnitude higher than the thermal TMR of traditional 2D magnetic spintronic devices (typically < 100%) [Fig. 11(d)]. The Li-intercalated V₂Se₂O system can become a metallic phase with enhanced spin splitting, while the V-intercalated system changes to a half-metallic phase with nearly 100% spin-filtering efficiency (96%−98% at equilibrium), maintaining high spin polarization stability against finite-temperature fluctuations, and the latter enables nearly perfect spin-filtering efficiency [160]. Overall, the AMFV systems systematically address key challenges in generating, manipulating, detecting, and retaining valley information. This transitions valleytronics from a laboratory phenomenon to a developmental stage geared towards room-temperature operation, all-electrical control, and high-density integration, showing transformative potential for next-generation information devices.

6 Challenges and outlook

Although AMFV materials exhibit transformative potential in theory, their journey from concept to application faces a series of pressing key challenges. Currently, the most significant bottleneck lies in the extreme scarcity of material systems. Most candidates (e.g., V₂Se₂O, Ca(CoN)₂ and Fe₂MX₄) remain at the stage of theoretical calculations, with very few experimentally verifiable samples available, making it difficult to confirm their key physical properties. For instance, while bulk parent compound KV₂Se₂O has been experimentally synthesized, fabricating high-quality monolayer V₂Se₂O films remain challenging. This gap prevents experimental validation of key physical parameters, particularly the intrinsic spin-valley locking strength. Another fundamental challenge is the lack of a dedicated symmetry and group-theoretical framework for AMFV systems. Conventional magnetic space groups are insufficient to describe altermagnetic order, which requires spin-space groups for proper classification. However, no unified symmetry principles or group-theoretical classification schemes capable of simultaneously describing altermagnetic order and valley polarization have been established for AMFV materials. The roles of nonsymmorphic symmetries and anti-unitary operations in governing spin-valley locking and valley polarization also remain largely unexplored. Furthermore, the underlying physical mechanisms are still unclear. There is a lack of a unified theoretical framework for the complex multiferroic coupling between AMFV and other ferroic orders (such as FE and FA), as well as for the mutual regulation among multiple degrees of freedom (electron, spin, valley, optical and layer), which severely hinders the targeted design and performance modulation of such materials. Ultimately, device-oriented applications remains largely unexplored. How to translate their unique properties into feasible prototype devices (e.g., valley tunnel junctions, valley filters and high-density memories) and address key issues such as electrical writing, reading, and integration remains largely uncharted territory. Although electrically controlled tunnel junctions based on Ca(CoN)₂ have been proposed, there is still no experimental verification of low-power electrically controlled switching of VP.

To overcome these obstacles, future research should be strategically directed toward several key directions. First, revolutionizing material discovery and synthesis by utilizing high-throughput calculations and machine learning to expand the candidate library beyond current 2D systems (e.g., 2D wurtzite multiferroics, metal−organic frameworks), and combining this with advanced growth and characterization techniques to prepare high-quality samples, thereby constructing a reliable experimental material database. Second, establish symmetry-based design rules and group-theoretical classifications for AMFV materials, including the analysis of nonsymmorphic symmetries and anti-unitary operations, to enable rational material design. Third, deepen the understanding of the microscopic mechanisms by employing multi-scale theoretical modeling to elucidate the coupling mechanisms of multiple degrees of freedom. Specifically, focus on developing experimental strategies to verify SOC-independent valley splitting, thereby clarifying the physical origins. Finally, prioritize the development of device principles and integration by exploring compatible pathways campatible CMOS processes and novel spintronic architectures, starting with the development of unit devices such as electrically or mechanically controlled valley switches, to gradually promote their translation into practical applications.

7 Conclusion

In summary, the emergence of AMFV materials marks a critical paradigm shift in valleytronics This shift moves beyond a heavy reliance on intrinsic strong SOC toward the active utilization of synergistic design between altermagnetic order and crystal symmetry to generate and manipulate valley degrees of freedom. Such materials provide a new physical foundation and a versatile material platform to overcome long-standing bottlenecks in traditional valleytronic devices, including room-temperature operation, full electrical control, high-density integration, and low-power consumption. Theoretical studies have outlined multiple promising pathways — from V₂Se₂O to Fe₂MX₄ systems — and demonstrated efficient tuning of VP via strain, electric fields, sliding, twisting and proximity effects, yet translating this potential into practical technology remains constrained by multiple challenges in material preparation, mechanism understanding, and device implementation. Future progress will hinge on deeper integration among computational design, advanced characterization, material synthesis, and nanofabrication. Overcoming these challenges could not only enable novel information devices that transcend the limits of Moore’s Law but also establish a new research paradigm in which symmetry engineering harnesses intrinsic electronic degrees of freedom.

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