Magnon–Orbitronics Coupling

• Magnon–orbitronics coupling is the interaction between spin-wave excitations and orbital degrees of freedom in magnetic systems, enabling conversion between magnonic, spin, and orbital currents.
• It utilizes mechanisms like magnetoelastic interactions, orbital hybridization, and scalar spin chirality to amplify spin–orbit torques and drive dissipationless transport phenomena.
• This coupling underpins advanced device applications, from enhanced SOT–MRAM and pure orbital torque devices to low-power, high-speed circuits based on orbital Hall effects.

Magnon–orbitronics coupling refers to the interplay between spin-wave (magnon) excitations and the orbital degrees of freedom—either electronic or magnonic orbital angular momentum—in low-dimensional magnets, magnetic heterostructures, and topological materials. This coupling enables conversion between magnonic, spin, and orbital currents and is central to the emerging field of magnon orbitronics, which seeks to harness the orbital sector for information transmission and manipulation, exploiting dissipationless channels and unique selectivity offered by the orbital quantum number. The phenomenon encompasses both fundamental theoretical mechanisms (e.g., scalar spin chirality, magnon–phonon and magnon–orbital hybridization) and experimentally verified amplification of spin–orbit torques (SOTs), magnon-mediated orbital Hall effects, and nonreciprocal magnon transport.

1. Microscopic Mechanisms of Magnon–Orbitronics Coupling

Magnon–orbitronics coupling arises via a variety of microscopic processes, including magnetoelastic interactions, orbital Rashba–Edelstein effects, and scalar spin chirality:

• Magnetoelastic (Phonon–Magnon) Coupling: The fundamental interaction is described by $$H_{mp} = \sum_{q} g_{mp}(q)\left(b_{q}a^{\dagger}_{q} + b^{\dagger}_{q}a_{q}\right)$$, where $a_q$ ($b_q$) are the magnon (phonon) annihilation operators and $g_{mp}(q)$ encodes the mode-dependent coupling. High-frequency (MHz) elastic drives can increase the magnon population by resonant transfer of angular momentum from phonons, thereby enhancing the magnon contribution to spin–orbit torque (Zhang et al., 2023).
• Orbital Hybridization at Interfaces: At inversion-symmetry-broken interfaces, orbital hybridization between metallic $d$-states and oxides' $p$-states generates a Rashba-type orbital splitting. The resulting orbital Rashba–Edelstein effect (OREE) produces an interfacial orbital accumulation that is subsequently converted into spin accumulation via local atomic spin–orbit coupling, leading to efficient magnon injection into adjacent antiferromagnetic insulators (Pu et al., 9 Jan 2026).
• Scalar Spin Chirality and Orbital Nernst Effect: In noncoplanar or thermally fluctuating spin textures, magnons dynamically realize nontrivial scalar spin chirality, $$\chi_{ijk} = \mathbf{S}_i \cdot (\mathbf{S}_j \times \mathbf{S}_k)$$, coupling to itinerant electrons through ring-exchange terms. This mechanism generates topological orbital magnetization (TOM) in the electronic subsystem and supports transverse orbital transport (orbital Nernst effect) of magnonic angular momentum (Zhang et al., 2019, Zhang et al., 2020).

2. Magnon–Phonon–Orbitronics: Magnetoelastic Amplification of SOT

Recent experiments have demonstrated a threefold enhancement of spin–orbit torques in conventional SOT bilayers (e.g., Pt/Py, W/CoFeB) when the write current is modulated at MHz frequencies. This effect is quantitatively explained by the creation of additional magnons through phonon–magnon coupling. The theory is corroborated by:

• Measured increase in the Gilbert damping parameter $\alpha$ and a decrease in effective magnetization $M_{\mathrm{eff}}$ by ∼8% when ramping modulation frequency from $<$0.5 MHz to 1.6 MHz.
• Extraction of the effective spin Hall angle $\theta_{\mathrm{SH}}$ from ST-FMR spectra, showing an increase from $\approx0.067$ to $\approx0.19$ in Pt/Py, and from $-0.28$ to $-0.97$ in W/CoFeB.
• The observed enhancement persists across material systems and bilayer thicknesses, indicating a general amplification of magnon channel conductance via phonon pumping (Zhang et al., 2023).

These findings imply that the critical current for SOT-driven switching is reduced by ∼$2/3$, paving the way for lower-power, high-speed SOT–MRAM and minimal-invasiveness device architectures solely by operating in the appropriate frequency regime.

3. Interfacial Magnon Torque via Orbital Hybridization

At interfaces between light metals (e.g., Cr) and antiferromagnetic insulators (NiO), strong $p$–$d$ orbital hybridization and inversion symmetry breaking create substantial Rashba orbital fields ($\alpha_R$) that enable large orbital Hall conductivities and efficient orbital-to-spin conversion:

• Application of an in-plane electric field $E_x$ generates an orbital accumulation $m_L^y = \alpha_R D(0) E_x$, where $D(0)$ is the Fermi-level density of states.
• This orbital moment is converted to spin accumulation $m_S^y = (\lambda/\Delta) m_L^y$ by atomic spin–orbit coupling $\lambda$.
• The interfacial spin accumulation then excites magnons in the NiO via exchange coupling, with injection rate scaling as $$\Gamma_m \propto (\lambda/\Delta)^2 (\alpha_R E_x)^2$$.
• Effective spin Hall conductivity values of $2.45 \times 10^{5}\ \hbar/(2e\ \Omega\cdot m)$—twice the best conventional magnon torque systems—are achieved without relying on heavy-metal spin–orbit coupling (Pu et al., 9 Jan 2026).

This mechanism supports reliable, low-power, room-temperature switching of perpendicular CoFeB layers and demonstrates that orbital engineering at interfaces can replace or augment conventional SOT approaches.

4. Intrinsic Magnon Orbital Hall and Orbital Nernst Effects

Theoretical analyses show that magnon–orbitronics coupling enables dissipationless transport of orbital angular momentum, orders of magnitude stronger than spin-based responses:

• In honeycomb antiferromagnets without explicit spin–orbit coupling, the magnon orbital Berry curvature,

$$\Omega^L_n(\mathbf{k}) = \sum_{m \neq n} \frac{2\hbar^2\,\mathrm{Im}[\langle n| j^L_{z,y} | m\rangle \langle m | v_x | n\rangle]}{(\bar{\epsilon}_{n,\mathbf{k}} - \bar{\epsilon}_{m,\mathbf{k}})^2},$$

gives rise to orbital Hall and Nernst conductivities (Go et al., 2023).

• For realistic exchange and anisotropy parameters, the magnon orbital Nernst conductivity $\alpha^L_z$ reaches $\sim10^{-2}k_B$, roughly $10^3$ times the spin Nernst contribution.
• Magnetoelectric coupling translates edge magnon orbital accumulation into experimentally accessible voltages, suggesting a direct electrical probe of pure magnon orbital currents.

In kagome ferromagnets, the scalar-chirality-driven orbital Nernst effect similarly leads to electronic orbital currents and associated torques, with predicted orbital magnetizations up to $0.1\ \mu_B$ per unit cell and $\kappa_{ONE}^{xy}$ on par with the best known spin-Hall systems (Zhang et al., 2019, Zhang et al., 2020).

5. Spin–Orbit Coupling Mechanisms for Magnons

Various forms of magnon spin–orbit coupling underpin the nontrivial band topology and transport phenomena in magnon–orbitronics:

• Dipole-Dipole Interaction: In bilayer van der Waals antiferromagnets, long-range dipolar forces induce a $k$-dependent spin–orbit field, resulting in linear band crossings (2D Weyl points) and facilitating nonreciprocal magnon propagation.
• Rashba/Dresselhaus Analogs: The DDI-induced field $\boldsymbol{\Delta}^-_{\mathbf{k}}$ acts as a magnonic analog to electronic Rashba spin–orbit coupling, enabling momentum-space manipulation of magnon pseudospins and thermally or field-driven valley-selective currents (Liu et al., 2020).
• Dzyaloshinskii–Moriya Interaction (DMI): Surface ferromagnetism and symmetry-induced DMI in topological semimetals (e.g., CoSi) support nonreciprocal spin-wave dispersion, which, under applied current, yield Edelstein-driven spin–orbit torques and coherent magnon emission (Esin et al., 2021).

These mechanisms allow for topologically protected magnon edge states, robust against disorder, and enable integration with spintronic and orbitronic circuits.

6. Device Implications and Applications

Magnon–orbitronics coupling facilitates a new class of devices, including:

• Enhanced SOT–MRAM: MHz-frequency phonon pumping reduces write currents and enables faster, lower-power devices without the need for heavy metal layers or interface engineering (Zhang et al., 2023).
• Pure Orbital Torque Devices: Cr/NiO/CoFeB heterostructures realize efficient room-temperature switching with minimized Joule heating, exploiting magnon-driven torques mediated by orbital hybridization (Pu et al., 9 Jan 2026).
• All-Electrical Magnon Emitters/Detectors: Chiral topological semimetals with surface ferromagnetism function as magnonic sources/detectors via current-induced SOC mechanisms, enabling nonreciprocal and reconfigurable spin-wave channels (Esin et al., 2021).
• Dissipationless Orbital Hall Circuits: The dominance of orbital angular momentum transport over spin transport in certain magnets suggests low-loss signal paths for information processing and storage, with electrical readout via magnetoelectric effects or conversion to spin in hybrid heterostructures (Go et al., 2023, Zhang et al., 2019, Zhang et al., 2020).

A summary of representative mechanisms and device archetypes is given below.

Mechanism/Structure	Key Coupling	Device Implication
Pt/Py, W/CoFeB (MHz mod.)	Phonon–magnon (magnetoelastic)	Amplified SOT, reduced MRAM write current
Cr/NiO/CoFeB	Orbital hybridization/OREE	Pure orbital-torque switching, efficient magnon injection
CoSi surface	Surface ferromagnetism + SOC	All-electrical magnon generation, nonreciprocal waveguides
Honeycomb/kagome magnonics	Orbital Berry curvature, scalar chirality	Dissipationless orbital Nernst current, topological control

7. Outlook and Perspectives

Magnon–orbitronics coupling, realized via both interfacial and bulk mechanisms, enables the utilization of orbital degrees of freedom for highly efficient, low-dissipation transport and torque generation in magnetic and spintronic devices. The strong enhancement of SOT by MHz phonon pumping, tunable magnonic orbital Hall effects, and distinct symmetry-based pathways for magnon injection and detection suggest avenues for further scaling, energy reduction, and the integration of magnonics with orbitronics. Exploration of nontrivial magnon band topology, electrically readable orbital accumulations, and new interfacial materials (e.g., light-metals/oxides with engineered hybridization) is likely to extend the domain of magnon–orbitronics into topological quantum technologies and ultrafast, low-power logic implementations (Zhang et al., 2023, Pu et al., 9 Jan 2026, Go et al., 2023).