Long-lived spin waves in a metallic antiferromagnet

The study addresses whether metallic antiferromagnets can host long-lived, high-frequency (THz) magnons suitable for ultralow-power magnonic devices. Magnons in insulators often have low damping but are limited by low carrier densities and reduced exchange interactions, restricting candidate materials. In metallic magnets, indirect exchange can yield high ordering temperatures and THz magnon frequencies, but strong electron–magnon interactions with the Stoner continuum typically cause heavy damping and short lifetimes. Antiferromagnets are attractive for spintronics due to ultrafast dynamics, robustness to stray fields, and multiple modes, yet direct observation of short-wavelength, high-frequency modes is challenging. The authors propose that if low-energy Stoner excitations are suppressed by a spin-dependent gap (Stoner gap) in the magnetically active states near the Fermi level, metallic systems could exhibit long magnon lifetimes at THz energies. They focus on the lanthanide intermetallic CeCo2P2 (TN = 440 K), hypothesizing it combines a reduced Co-3d DOS at EF (effective Stoner gap) with strong exchange, enabling long-lived THz magnons, and compare it to isostructural LaCo2P2, expected to show typical metallic damping.

The paper reviews how magnon lifetimes are limited by electron–magnon, phonon–magnon, magnon–magnon interactions, spin–orbit damping, and impurities, with electron–magnon coupling to the Stoner continuum dominant in metals. Long lifetimes can occur when low-energy Stoner excitations are forbidden by a gap between occupied and unoccupied magnetically active states near EF (Stoner gap), as in insulators, half-metals, or select metals where sp bands provide metallicity while d states are gapped. Synthetic antiferromagnets offer detectable spin-wave modes but typically in the GHz range due to weak interlayer coupling. Prior work on Heusler alloys and other systems suggests reduced damping when half-metallicity or gaps suppress Stoner transitions, though half-metallicity is fragile to disorder. Antiferromagnets exhibit intrinsically higher characteristic frequencies and multiple modes, making them promising for THz magnonics, but direct measurements at finite q are challenging. Within the Co and Fe pnictide families, spin-wave energies and damping vary widely and can be tuned by pressure, chemical substitution, or strain, indicating opportunities to engineer reduced damping by modifying the electronic structure near EF.

• Electronic structure and exchange: First-principles density functional theory (DFT) using a Green-function multiple-scattering method within GGA, with Co 3d states treated using GGA+U to include correlations. The Hubbard U was chosen to reproduce key magnetic properties (e.g., Néel temperature). Exchange coupling constants were determined via the magnetic force theorem (Heisenberg model parameters J), identifying dominant interlayer antiferromagnetic coupling in CeCo2P2 and near-zero interlayer coupling in LaCo2P2. Spin-wave dispersions and magnon lifetimes were calculated; theoretical dispersions were scaled by 0.9 to best match experiment, attributing residual differences to correlation effects beyond standard DFT.
• RIXS measurements: Resonant inelastic X-ray scattering at the Co L3 edge on ESRF ID32. Experimental geometry tuned momentum transfer q by varying incidence and scattering angles. Measurements performed at T = 20 K with incident energy hv ≈ 780 eV. Energy resolution ΔE = 28 meV (FWHM). High-quality single crystals were freshly cleaved; comparable spectra were obtained on uncleaved samples, confirming bulk origin. Spectra were acquired over accessible q along high-symmetry directions, including (H01) and paths enabling in-plane momentum transfers up to and beyond 200 meV magnon energies.
• Spectral analysis: Low-energy RIXS spectra were modeled as the sum of an elastic peak at zero loss and a magnon peak described by a damped harmonic oscillator (DHO) response characterized by peak energy ħω0 and damping factor γ, plus a background. Both peaks were convolved with a Gaussian of FWHM 28 meV to account for instrumental resolution. Slight residual phonon contributions around 30–40 meV were small and neglected. Dispersion relations were extracted from peak positions; damping factors were obtained from DHO fits. Temperature-dependent measurements (up to room temperature) were performed at representative q to assess robustness.
• Comparison system: Parallel first-principles calculations and RIXS measurements were performed on isostructural LaCo2P2 to benchmark typical metallic magnon damping and differences in dispersion and lifetimes.

• Electronic structure: DFT reveals CeCo2P2 has a reduced total DOS at EF with Co 3d occupied and unoccupied states separated by a pseudogap (effective Stoner gap), while LaCo2P2 shows large 3d DOS at EF in both spin channels. This implies significantly weaker electron–magnon coupling in CeCo2P2 than in LaCo2P2.
• Exchange and order: In CeCo2P2, dominant interlayer antiferromagnetic coupling (negative J⊥) and ferromagnetic intralayer coupling yield AFM order consistent with experiment; LaCo2P2 exhibits near-zero interlayer J⊥ and ferromagnetic order.
• Magnon dispersion (CeCo2P2): RIXS detects a linear, antiferromagnetic-like dispersion with spin-wave energies exceeding 400 meV along in-plane directions (THz regime), while dispersion along Γ–Z is smaller. Theoretical dispersions agree well after a 0.9 scaling factor.
• Damping and lifetimes (CeCo2P2): Extracted damping factors γ are minimal across a wide energy range; for ħωq up to about 200 meV, 2γ < ΔE (28 meV), making peaks symmetric and resolution-limited (undamped regime). Calculations predict notable damping (γ ≥ 10 meV) only above ~200 meV. Experimental measurements off the (H01) line confirm increased damping only at magnon energies ≳200 meV.
• Robustness vs temperature: At q = (−0.24, 0, 1) the magnon properties in CeCo2P2 remain essentially unchanged up to room temperature, consistent with its high TN = 440 K.
• Comparison with LaCo2P2: At similar magnon energies (~34 meV), CeCo2P2 shows a sharp, symmetric, nearly resolution-limited peak, whereas LaCo2P2 exhibits a broad, asymmetric, strongly damped feature merging with the quasielastic line. At the same q = (−0.24, 0, 1), CeCo2P2 has ħωq ≈ 120 meV vs ~34 meV in LaCo2P2, with CeCo2P2 still showing low damping (γ below resolution). The calculated inverse lifetimes in LaCo2P2 are of the order of the excitation energy, indicating strong damping.
• Damping ratio: CeCo2P2 exhibits large inverse damping ratios ħωq/(2γ) > 1 up to high energies, indicating long-lived magnons, standing out compared to other metallic magnets reported in literature.
• Mechanism: Long lifetimes in CeCo2P2 arise from a weak electron–magnon interaction due to a suppressed Co 3d DOS at EF (opening of an effective Stoner gap), while sp states at EF maintain metallicity.

The results directly address the challenge of achieving long-lived THz magnons in metals by demonstrating that a metallic antiferromagnet (CeCo2P2) can host high-energy, weakly damped spin waves when low-energy Stoner excitations are suppressed by a gap in the magnetically active d-states near EF. RIXS measurements validate the theoretical prediction of reduced electron–magnon coupling, showing linear AFM-like dispersions to >400 meV with minimal damping below ~200 meV and only moderate damping above. The stark contrast with LaCo2P2, which lacks a Stoner gap and shows strong damping and weaker dispersion, corroborates the role of the electronic structure near EF in controlling magnon lifetimes. The persistence of low damping up to room temperature underscores practical relevance. These findings suggest a materials-design pathway: engineer metallic systems where sp states provide conduction while d-states near EF are gapped for both spins participating in Stoner excitations. Tuning via chemical pressure, substitution (e.g., La→Ce), external pressure, or strain can modify the DOS and exchange, enabling control over magnon damping and frequency.

The study demonstrates long-lived, high-energy (THz) magnons in the metallic antiferromagnet CeCo2P2 (TN = 440 K), verified by Co L3-edge RIXS and supported by first-principles theory. The key enabling factor is a reduced Co 3d DOS at the Fermi level, opening an effective Stoner gap that weakens electron–magnon coupling, in contrast to LaCo2P2 which exhibits strong damping due to large d-DOS at EF. The work establishes that bulk metallic systems can support undamped THz magnons, providing a route for energy-efficient, high-speed magnonic devices and offering a computational and experimental framework to identify similar materials. Future research directions include: systematic computational screening of Co and Fe pnictides and related intermetallics; tuning electronic structure via chemical substitution, pressure, or epitaxial strain to induce Stoner gaps; engineering thin-film heterostructures combining 3d and 4f sublattices to realize long-lived magnons in the THz range.

• Theoretical dispersions required a uniform 0.9 scaling to match experiment, indicating limitations of standard DFT (even with +U) in capturing correlation effects quantitatively.
• RIXS energy resolution (ΔE = 28 meV) limits direct determination of very small damping; many peaks are resolution-limited, so only upper bounds on γ are obtained below ~200 meV.
• Phonon contributions around 30–40 meV were neglected due to small intensity; residual effects could slightly influence low-energy fits.
• q-space access is constrained by experimental geometry (e.g., regions near magnetic Bragg conditions omitted), potentially missing parts of the dispersion.
• Experimental conditions were primarily at low temperature (20 K), with limited temperature-dependent measurements reported (details in Supplementary), though room-temperature robustness at a representative q was shown.
• The study focuses on a single material pair (CeCo2P2 vs LaCo2P2); generality to broader classes, while plausible, remains to be established by further screening and experiments.