[Paper Review] Ultrasensitive strain modulation of terahertz magnons at a magnetic phase transition

Lichen Wang, Sajna Hameed+9 more|arXiv (Cornell University)|Feb 14, 2026

Magnetic properties of thin films0 citations

TL;DR

The paper demonstrates uniaxial-strain control of the antiferromagnetic ground state and THz magnons in Ca2RuO4, revealing a strain-induced A-to-B magnetic transition with a magnon energy jump of ~0.3 THz (over 10%). The authors explain this via strain-tuned spin-orbit admixture and interlayer exchange sign reversal, suggesting transition-edge magnonic devices.

ABSTRACT

Antiferromagnets typically host spin-wave (magnon) excitations in the terahertz (THz) regime, offering a promising platform for high-speed magnonic information technologies. Harnessing these excitations requires sensitive control of their spectral properties. Here we use resonant x-ray diffraction and Raman scattering to demonstrate uniaxial-strain control of the antiferromagnetic (AFM) ground state and THz magnon excitations in the layered Mott insulator Ca$_2$RuO$_4$. Although the states separated by the strain-induced phase transition differ only by the sign of the weak and partially frustrated interlayer interaction, their magnon energies differ by more than 10% (~ 0.3 THz). Our theoretical analysis explains this surprising observation by tracing the origin of both the sign reversal of the interlayer coupling and the magnon energy to the spin-orbital composition of the Ru valence electrons. The extreme strain sensitivity of the THz magnon energy near a magnetic phase transition opens up pathways towards a new generation of transition-edge magnonic devices.

Motivation & Objective

• Explore how uniaxial strain controls the AFM ground state in Ca2RuO4.
• Quantify strain-induced changes in THz magnon energies across a magnetic phase transition.
• Develop a theoretical framework linking strain, spin-orbit coupling, and interlayer exchange to magnon spectra.
• Demonstrate reversibility and identify potential for strain-tunable magnonic devices.

Proposed method

• Use resonant x-ray diffraction (RXD) at the Ru L3 edge to identify A-centered vs B-centered AFM stacking under strain.
• Perform Raman scattering to track zone-center magnon and phonon modes under uniaxial strain along [100] and [1-10].
• Define an effective strain variable epsilon from lattice orthorhombicity to model magnon energy dependence.
• Apply a minimal pseudospin-1 Hamiltonian including exchange anisotropy, single-ion anisotropy, and orthorhombic field (Eq. 2).
• Derive the magnon gap (Eq. 3) showing strong sensitivity to in-plane anisotropy kappa which itself depends on crystal fields Delta and Delta_ort.
• Fit parameters from existing literature to reproduce observed strain responses, including intrinsic magnon gap omega0 and orthorhombicity epsilon0.

Experimental results

Research questions

• RQ1 How does uniaxial strain along different crystallographic directions influence the AFM ground state in Ca2RuO4?
• RQ2 What is the magnitude and nature of the magnon energy shift across the strain-induced magnetic phase transition?
• RQ3 What mechanisms couple strain to spin-orbit entangled Ru pseudospins to produce a sign change in interlayer exchange Jc?
• RQ4 Can the observed phenomena be described quantitatively by a spin-orbit–lattice coupling model linking crystal fields to magnon spectra?

Key findings

• A strain-induced magnetic phase transition from A-centered to B-centered stacking occurs near epsilon ~ 0.15%.
• Raman under [1 1 0] compression shows an abrupt magnon-energy jump, with the dominant mode up by ~0.3 THz (>10% of the pre-transition energy) at the transition.
• RXD confirms the A→B transition and reveals residual A-centered phase coexisting with B-centered phase up to ~0.22% strain.
• The magnon energy under strain is captured by a pseudospin-1 model, with the gap omega ≈ sqrt[kappa(4J(2−tau)+0.5 tau E + kappa)], highlighting strong sensitivity to the orthorhombic field and crystal-field Delta, Delta_ort.
• The sign reversal of interlayer coupling Jc is explained by strain-induced enhancement of xy-orbital character in the spin-orbit entangled wave functions, switching the interlayer exchange from AFM to ferromagnetic.