Altermagnetic Multilayers: Novel Spin Phenomena

• Altermagnetic multilayers are layered heterostructures characterized by compensated collinear spin order and momentum-dependent spin splitting without net magnetization.
• Their engineered stacking and symmetry breaking enable unconventional magnetotransport, quantum spin Hall effects, and multiferroic phenomena observable via advanced spectroscopic and transport techniques.
• Precise tuning of crystal symmetry, interfaces, and interlayer coupling in these systems offers actionable insights for developing low-power spintronic and multifunctional device applications.

Altermagnetic multilayers are layered heterostructures composed of materials exhibiting altermagnetism—a magnetic ordering distinguished by compensated collinear spin alignment accompanied by momentum-dependent nonrelativistic spin splitting in the band structure. These systems uniquely combine antiferromagnet-like absence of stray magnetic fields with ferromagnet-like spin polarization of electronic states, resulting in unconventional magnetotransport, topological, and magnetoelectric phenomena. The emergent functionality of such multilayers is governed by the interplay of crystal symmetry, stacking sequence, interface structure, and the coupling between spin, lattice, and orbital degrees of freedom.

1. Fundamental Concepts and Symmetry Principles

Altermagnetism is defined by collinear, spatially compensated magnetic moments on symmetry-inequivalent sublattices that are not related by inversion, distinguishing it from both ferromagnetism and conventional (𝒫𝒯-symmetric) antiferromagnetism. The absence of global magnetization coexists with momentum-dependent spin splitting, expressed mathematically as

$$E_{\uparrow,\downarrow}(\mathbf{k}) = E_0(\mathbf{k}) \pm \Delta(\mathbf{k}),$$

where $\Delta(\mathbf{k})$ is dictated by the underlying magnetic and crystal symmetry and does not require relativistic spin–orbit coupling (Sattigeri et al., 2023).

Crucially, the breaking of certain symmetries—particularly inversion and horizontal mirrors—in multilayers leads to distinct classes of spin splitting and enables the emergence of type-II multiferroicity, quantum spin Hall effects with multiple edge channels, and nonvolatile ferroelastic control of spin states (Guo et al., 4 May 2025, Chen et al., 5 Aug 2025, Peng et al., 27 May 2025).

2. Material Platforms and Structural Engineering

A diversity of material systems support altermagnetic multilayer phenomena:

• Transition Metal Dichalcogenides and Manganese Compounds: For example, CoNb₄Se₈ shows easy-axis A-type antiferromagnetism with a 2D superlattice of Co atoms, resulting in g-wave–like alternating spin splitting, confirmed by both DFT and neutron diffraction (Regmi et al., 16 Aug 2024). MnTe, in both bulk and multilayered thin-film forms, displays robust compensated order and pronounced altermagnetic spin textures, observable through X-ray magnetic dichroism and angle-resolved photoemission (Yamamoto et al., 25 Feb 2025, Chilcote et al., 6 Jun 2024).
• Synthetic and Stacked Systems: “Synthetic altermagnets” constructed by stacking two anisotropic ferromagnetic layers at a relative π/2 rotation generate d-wave–like spin splitting, even without net magnetization. The Hamiltonian encodes layer anisotropy and interlayer hopping as critical design parameters (Asgharpour et al., 3 Dec 2024). Twisted van der Waals bilayers or those engineered via direct stacking according to the General Stacking Theory (GST) allow prediction and control of altermagnetic states by an analysis of magnetic space groups, requiring specific point symmetry elements such as C₂, D₂, D₃, D₄, D₆, D₂d, or S₄ (Pan et al., 11 Sep 2024).
• Quasi-1D Chain Arrays: Monolayers assembled from single-atomic chains (e.g., AA-stacked AFM B-XY₃ prototypes) form a family of Q1D altermagnets, where inter-chain ferromagnetic coupling is crucial for achieving altermagnetic behavior. The electronic structure, including nodal-line semiconducting and altermagnetic phases, is highly tunable by spacing and external fields (Guo et al., 9 Feb 2025).

3. Transport, Magnetoelectric, and Topological Phenomena

Altermagnetic multilayers display rich transport and coupling behaviors:

• Magnetoelectric Coupling and Multiferroicity: Altermagnetic systems can host spontaneous electric polarization that is locked to the Néel order parameter, a paradigm shift for type-II multiferroics. The macroscopic polarization $\mathbf{P}$ acquires a quadratic dependence on the Néel vector ($\mathbf{L}$) following

$$p_\text{tot}^\alpha = [K_A + K_B]_{\beta\gamma}^{\alpha} L^\beta L^\gamma,$$

where the difference in sublattice tensors ($K_A \neq -K_B$) is only possible in noncentrosymmetric altermagnets (Guo et al., 4 May 2025). First-principles studies of MgFe₂N₂ monolayers confirm large out-of-plane polarization (∼15 μC/m²), switchable by in-plane Néel vector rotation at sub-30 μeV barriers.

• Ferroelastic Control: The coupling between ferroelasticity and altermagnetism enables nonvolatile, multistate switching of spin splitting. In RuF₄ and CuF₂ monolayers, ferroelastic lattice reorientation (via 90° or 120° rotation) switches the direction or magnitude of spin splitting. The resulting “altermagnetoelastic effect” leads to state-dependent spin conductivity, supporting mechanical encoding of spin information (Peng et al., 27 May 2025). Switching barriers are on the order of a few to tens of meV/atom, supporting efficient low-energy switching.
• Quantum Spin Hall Extensions: Altermagnetic multilayers circumvent the conventional ℤ₂ quantum spin Hall (QSH) constraint, allowing for multiple pairs of gapless helical edge states. The interplay between spin–orbit coupling and d-wave altermagnetic order yields a mirror-spin Chern number ($C_m$), causing the number of edge state pairs to scale with layer count. Fe₂Se₂O multilayers display two or three pairs of edge states for bilayer and trilayer structures, respectively, with quantized spin-Hall conductance ($\sigma_{xy}$) plateaus at $4e/4\pi$ and $6e/4\pi$, directly observable by transport and ARPES (Chen et al., 5 Aug 2025).

4. Experimental Characterization, Control, and Device Implications

• Magneto-optical and Spectroscopic Identification: Domain and Néel vector orientation can be identified via optical conductivity anisotropy and Faraday rotation microscopy, thanks to robust, symmetry-specific signatures in the optical response. The angular dependence of magneto-optical effects serves as a probe for the Néeel order $\phi$ in systems such as MgFe₂N₂ (Guo et al., 4 May 2025).
• Multifunctional Spintronic Devices: Altermagnetic multilayers exhibit high tunability for spintronic architectures. For example, CrSb/In₂Se₃/Fe₃GaTe₂ multiferroic tunnel junctions achieve TMR up to 2308%, TER up to 707%, and nearly 100% spin filtering even at above-room-temperature conditions (Zhang et al., 18 Mar 2025). The synergy of altermagnetic electrode, ferroelectric barrier, and half-metallic FM contact allows independent or dual electrical and magnetic control of tunnel resistance, with significant potential for logic, memory, and quantum computing elements.
• Ferroelastic/Straintronic Nanodevices: The combination of multistate, nonvolatile strain-driven spin splitting and robust antiferromagnetic order enables nanomechanical switches immune to external magnetic fields. The angularly dependent spin conductivity in ferroelastic altermagnets such as RuF₄ and CuF₂ can be exploited for multibit mechanical memory and straintronic logic (Peng et al., 27 May 2025).

5. Dimensionality Effects and Magnetic State Competition

• Dimensionality-Induced Phenomena: Reducing altermagnetic materials to the 2D limit does not necessarily preserve bulk altermagnetic order. Monolayer MnTe exhibits frustrated spin-glass–like states, while bilayer MnTe recovers a layered AFM structure with no net magnetization but strong internal spin splitting. This dimorphism points to unique opportunities and challenges for designing low-dimensional altermagnetic multilayers with precise control over long-range order and frustration (Cuxart et al., 15 Apr 2025).
• Coexistence and Competition of Magnetic States: In GdAlGe, the admixture of altermagnetic and weak ferromagnetic order in films down to a single monolayer leads to spontaneous exchange bias and tunable anomalous Hall effects, scaling with reduced thickness. The interplay of competing collinear orders can be precisely manipulated by controlling growth, stoichiometry, and substrate interaction, opening routes to scalable memory and sensor technologies (Parfenov et al., 19 May 2025).

6. Theoretical Frameworks and Predictive Design

• Symmetry-Based Classification: General stacking theory (GST) provides group-theoretical criteria for identifying and designing altermagnetic bilayers and multilayers, specifying seven possible point group symmetries where altermagnetism naturally emerges (Pan et al., 11 Sep 2024). Combined with first-principles methods, this enables high-throughput prediction of viable systems (e.g., hundreds in 2D materials repositories).
• Microscopic Mechanisms: The quadratic dependence of electric polarization on Néel order, exchange-striction-driven mechanisms (without spin–orbit coupling), and the sensitivity of band splitting to stacking, interlayer distance, and point group operations all contribute to the design knobs available for functionality tuning. The distinction from conventional antiferromagnetic multiferroics (which are limited by strict symmetry constraints) is especially pronounced for type-II coupling, as found in monolayer MgFe₂N₂ and LiMnO₂/RuF₄-derived systems (Cao et al., 29 Dec 2024).

7. Outlook and Future Directions

Altermagnetic multilayers offer an extensive platform for exploring emergent phenomena that merge spintronic, magnetoelectric, and topological functionalities. Ongoing progress involves:

• Experimental verification and characterization in exfoliable materials and synthetic heterostructures.
• Integration of multiferroic, ferroelastic, and quantum spin Hall uses within a single device stack.
• Elucidation of precise symmetry, interface, and dimensionality requirements for robust altermagnetic behavior.
• Theoretical generalization to complex stacking sequences, heterovalent alloying, and control by gating, strain, or optical means.

A plausible implication is that further advances in material synthesis, symmetry engineering, and stacked device architectures will accelerate the implementation of altermagnet-based memory, logic, and quantum information systems distinguished by ultrafast switching, low-power operation, and topological robustness.