Cavity Magnon Hybrids in Terahertz Spintronics

• Cavity magnon hybrids are hybrid light–matter excitations formed by strong coupling between cavity photons and antiferromagnetic magnons in NiO.
• They are realized using a self-formed Fabry–Perot cavity in a thinned NiO slab, with THz time-domain spectroscopy and external magnetic fields to tune multiple resonance modes.
• These structures enable advanced terahertz spintronic and quantum devices by demonstrating robust anticrossing signatures and strong coupling parameters.

Cavity magnon hybrids are hybridized light–matter quasiparticles formed from the strong coupling of electromagnetic cavity photons and magnon excitations in magnetic materials. In the terahertz regime, these hybrid states have recently been realized by exploiting a self‐formed Fabry–Perot cavity in a thinned single‐crystal nickel oxide (NiO) slab, an antiferromagnetic insulator. The intrinsic cavity modes of the NiO sample interact with high‐frequency antiferromagnetic magnons, paving the way toward advanced terahertz spintronics and hybrid quantum devices.

1. Experimental Setup and Cavity Realization

The system employs two single‐crystal NiO wafers (10 mm diameter, 491 μm thick) cut along specific crystallographic planes [(110) and (111)]. Terahertz time‐domain spectroscopy (THz‐TDS) is used in combination with magnetic fields up to 25 T provided by a RAMBO pulsed magnet system. The NiO slab itself forms a Fabry–Perot cavity due to its finite thickness and parallel surfaces; no external mirrors are needed. With mode spacing of 89 GHz and a full width at half maximum of 28 GHz, the cavity supports discrete resonances in the 0.1–1.5 THz spectral range, critical for achieving multiple resonance conditions with the magnon modes.

2. Coupling Mechanism and Frequency Tuning

In this system the magnon excitations—specifically the infrared‐active modes—are enabled by the antiferromagnetic order of NiO. Two infrared-active modes are observed: one near 0.13 THz and another near 1 THz. The higher-frequency magnon at approximately 1 THz is tunable; its frequency increases (blue-shifts) with magnetic field according to the two-sublattice model for an easy-plane antiferromagnet, obeying

$\omega = \gamma_r \sqrt{2 H_E H_A + H_0^2},$

where $H_E$ is the exchange field, $H_A$ is the anisotropy field, $H_0$ is the external magnetic field applied perpendicular to the easy plane, and $\gamma_r$ is the gyromagnetic ratio. As the magnon is tuned in frequency by the external field it sequentially becomes resonant with several Fabry–Perot cavity modes, thereby enabling investigation of coupling at multiple terahertz frequencies.

3. Evidence for Strong Coupling

When the resonant frequency of the magnon coincides with that of a Fabry–Perot cavity mode, the measured transmission spectra show a clear anticrossing signature. This anticrossing is a hallmark of strong coupling and polariton formation because the vacuum Rabi splitting exceeds the linewidths of the uncoupled modes. Quantitatively, the magnon–photon coupling strength is measured to be

$g = 0.014\,\text{THz} \quad (14\,\text{GHz}),$

the magnon decay rate is

$\gamma = 0.015\,\text{THz} \quad (15\,\text{GHz}),$

and the cavity decay rate is

$\kappa = 0.028\,\text{THz} \quad (28\,\text{GHz}).$

These parameters yield a cooperativity of

$C = \frac{4g^2}{\gamma\,\kappa} = 1.87>1,$

and the normalized coupling strength

$\frac{g}{\omega_0} = 0.014,$

where $\omega_0$ is the resonant frequency. In the reported spectral window up to 1.5 THz, three anticrossings are observed as the magnon is tuned through consecutive cavity modes, confirming the robustness of the strong coupling regime.

4. Implications for Spintronics and Hybrid Quantum Systems

The demonstrated strong photon–magnon coupling at terahertz frequencies in an antiferromagnet marks the first such observation in a prototypical AFM material at room temperature. This is significant given that previous strong-coupling demonstrations were limited to ferromagnetic or ferrimagnetic systems operating in the gigahertz range. The high natural frequency of AFM modes in NiO and the inherent tunability afforded by both magnetic field variation and sample thickness adjustment provide a simple and robust platform for designing terahertz cavity magnonic devices. Such devices have promising applications in:

• Quantum information processing and hybrid quantum networks, where coherent exchange of information between spin, photon, and potentially phonon modes can be exploited.
• Spintronic devices operating at terahertz frequencies, enabling fast, energy-efficient signal processing.
• Quantum transduction and advanced sensing, as strong cooperativity and robust coupling are crucial for interfacing disparate quantum systems.

5. Theoretical Framework and Modeling

The experimental results can be modeled using a standard coupled-oscillator Hamiltonian:

$$\hat{H} = \hbar\omega_{\text{cav}} \hat{a}^\dagger \hat{a} + \hbar\omega_m \hat{m}^\dagger \hat{m} + \hbar g (\hat{a}^\dagger \hat{m} + \hat{a} \hat{m}^\dagger),$$

where $\hat{a}$ and $\hat{m}$ are the photon and magnon annihilation operators, respectively. The hybridized eigenfrequencies given by

$$\omega_{\pm} = \frac{1}{2}(\omega_m+\omega_{\text{cav}}) - \frac{i (\gamma+\kappa)}{4} \pm \sqrt{g^2 + \left(\frac{i(\kappa-\gamma)}{4} + \frac{\omega_m-\omega_{\text{cav}}}{2}\right)^2}$$

capture the vacuum Rabi splitting observed in transmission spectra. This theoretical framework forms the basis for understanding the interplay of cavity engineering, magnetic anisotropy, and external field tuning in achieving and manipulating strong coupling.

6. Future Directions in Terahertz Cavity Magnonics

The successful realization of terahertz cavity magnon polaritons in NiO opens several avenues for future research. Advances may include refinement of cavity design for improved quality factors, integration with other quantum elements such as superconducting qubits or optical systems for transduction, and exploration of nonlinear phenomena in strongly coupled regimes. Moreover, the scalability and simplicity of the sample-cavity approach suggest that antiferromagnetic materials could underpin future spintronic and quantum devices operating at unprecedented frequencies, with potential for robust, tunable, and room-temperature quantum platforms.

Cavity magnon hybrids thus represent a promising frontier in terahertz spintronics and hybrid quantum technology, providing both a testbed for studying fundamental light–matter interactions in antiferromagnets and a versatile building block for next-generation quantum devices.