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Strain-induced Dynamic Spin-Phonon Coupling in Epitaxial RuO2 Films

This study demonstrates that anisotropic strain in epitaxial RuO2 films induces dynamic spin-phonon coupling, enabling the photo-induced frequency modulation of both sub-THz acoustic and optical phonons to facilitate new ultrafast quantum opto-spintronic applications in altermagnets.

Original authors: In Hyeok Choi, Seung Gyo Jeong, Jae Hyuck Lee, San Kang, Sreejith Nair, Changyoung Kim, Dirk Wulferding, Bharat Jalan, Jong Seok Lee

Published 2026-10-02

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CC BY 4.0

Original authors: In Hyeok Choi, Seung Gyo Jeong, Jae Hyuck Lee, San Kang, Sreejith Nair, Changyoung Kim, Dirk Wulferding, Bharat Jalan, Jong Seok Lee

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Inside the solid world of crystals, atoms are not frozen in place; they vibrate constantly, jiggling in rhythmic patterns that travel through the material like sound waves. In many magnetic materials, these vibrations are tightly linked to the tiny magnetic spins of the atoms, creating a partnership where the movement of the lattice and the orientation of the spins influence one another. Scientists have long known that if you squeeze or stretch a crystal, you can change its magnetic properties, but the reverse is also true: the magnetic state can change how the crystal vibrates. This connection, known as spin-phonon coupling, is a fundamental interaction that could allow researchers to control magnetism using light or sound, potentially leading to faster, more efficient technologies for data storage and computing. However, observing this interaction in real-time, especially in materials that are magnetic but have no overall magnetic field, has remained a significant challenge.

A team of researchers has now captured a glimpse of this dynamic relationship in a specific material called ruthenium dioxide. While bulk samples of this material are generally considered non-magnetic, the researchers found that when they grow it as an ultra-thin film on a specific type of crystal substrate, the material behaves differently. The film is forced to stretch and compress in uneven ways due to the mismatch with the substrate, creating a state of internal strain. By carefully controlling the thickness of these films, the scientists induced a specific type of strain relaxation that created tiny defects, or dislocations, within the crystal structure. It is within these strained regions that the material exhibits a hidden magnetic order, allowing the researchers to probe the connection between its magnetism and its vibrations.

To investigate this, the team used an ultrafast laser technique that acts like a high-speed camera for atomic motion. They fired a short pulse of laser light at the thin films to excite the atoms, causing them to vibrate in a coordinated way. They then used a second, weaker laser pulse to measure how the film's surface reflected light over time, allowing them to track the vibrations as they evolved. The results revealed two distinct types of vibrations. The first was a low-frequency wave, similar to a deep rumble, which appeared only in films thicker than a certain point where the strain had partially relaxed. This vibration was identified as a transverse acoustic mode, a type of sound wave where atoms move side-to-side. Crucially, the speed of this vibration changed depending on the temperature. As the film was cooled below approximately 500 Kelvin, the vibration became stiffer and faster, a phenomenon known as hardening. This change in stiffness occurred in the same temperature range where other studies have suggested magnetic transitions happen in strained ruthenium dioxide, strongly implying that the magnetic spins are stiffening the atomic lattice.

The researchers also looked for a second type of vibration, a higher-frequency optical mode where atoms move against each other. This vibration was not visible in the standard reflection measurements but appeared clearly when they measured a subtle rotation in the polarization of the reflected light, a signal known as the magneto-optic Kerr effect. This specific vibration, which occurs at a frequency of about 4.2 terahertz, showed the opposite behavior: as the temperature approached the critical point near 500 Kelvin, the vibration slowed down significantly, or softened. This softening is a classic signature of a material approaching a magnetic transition, where the lattice becomes more susceptible to the influence of the magnetic order. The fact that this vibration was only observed in the strained films and not in fully relaxed, bulk-like films further confirmed that the local strain fields created by the dislocations were essential for stabilizing the magnetic state that drives these changes.

The study carefully ruled out other potential causes for these dramatic shifts. The researchers demonstrated that simple heating of the material by the laser pulse could not account for the magnitude of the frequency changes observed. They also showed that the effects were not present in films that were fully relaxed and free of the specific strain patterns, nor in the bulk single crystals of the material. Instead, the evidence points to a direct coupling between the magnetic order and the lattice vibrations. The researchers suggest that the magnetic spins in the strained film interact with the lattice through a mechanism called magnetoelastic coupling, where the magnetic state directly alters the stiffness of the material. This interaction happens on an incredibly fast timescale, with the magnetic system responding to the laser pulse within picoseconds, or trillionths of a second.

By mapping out how these vibrations change with temperature and film thickness, the team has provided strong experimental evidence that strain can be used to engineer the magnetic properties of ruthenium dioxide. The ability to induce and control a magnetic state through the precise manipulation of crystal strain opens new possibilities for manipulating materials at the atomic level. The findings suggest that by designing specific patterns of strain in thin films, scientists could potentially create materials where magnetic properties can be switched or tuned using ultrafast light pulses. This work does not just confirm a theoretical concept but offers a practical pathway to explore the interplay between spin and lattice in a new class of magnetic materials, paving the way for future devices that operate at the speed of light.

Technical Summary: Strain-induced Dynamic Spin-Phonon Coupling in Epitaxial RuO2 Films

Problem and Context
Ruthenium dioxide (RuO2) has been the subject of conflicting reports regarding its magnetic ground state, with studies suggesting it is either a non-magnetic metal or an altermagnet exhibiting non-relativistic spin splitting without net magnetization. These discrepancies suggest that extrinsic factors, particularly strain and defects, play a critical role in stabilizing magnetic order. While static strain is known to modify the electronic band structure and induce magnetic states in epitaxial RuO2 films, the dynamic interplay between lattice vibrations (phonons) and magnetic order (spins) under non-equilibrium conditions remains poorly understood. Specifically, the ability to manipulate phonon dynamics through spin-phonon coupling in these materials is an open area of investigation.

Methodology
The authors employed ultrafast coherent phonon spectroscopy and time-resolved magneto-optic Kerr effect (tr-MOKE) measurements to probe the dynamics of epitaxial RuO2 thin films grown on TiO2 (110) substrates via hybrid molecular beam epitaxy (MBE). The study utilized a systematic variation of film thickness (ranging from 7 nm to 17 nm) to control the strain state, inducing anisotropic strain relaxation.

Excitation and Detection: The films were excited with 785 nm ultrafast laser pulses (approx. 50 fs). Transient reflectivity ( $\Delta R$ ) was measured to detect acoustic phonons, while transient Kerr rotation ( $\Delta \theta_K$ ) was used to probe optical phonons and magnetic order parameters.
Strain Engineering: X-ray diffraction (XRD) reciprocal space mapping confirmed that films exhibited anisotropic strain: fully strained along the [11 $\bar{1}$ 0] direction and partially relaxed along the [001] direction. This relaxation introduces edge dislocations and local shear strain fields.
Analysis: The researchers analyzed the frequency, amplitude, and time-evolution of coherent oscillations across a temperature range from 10 K to 560 K. They utilized Fast Fourier Transform (FFT) for frequency analysis and quadratic fitting of peak positions to determine time-dependent frequency shifts (softening or hardening).

Key Contributions and Results

Observation of Strain-Induced Transverse Acoustic (TA) Phonons:
The study identified a coherent sub-THz oscillation in films thicker than 4 nm, which is absent in fully strained or fully relaxed films. This mode exhibits a two-fold anisotropy in amplitude relative to the probe polarization, identifying it as a transverse acoustic (TA) standing wave confined within the film. The authors attribute the generation of this mode to non-zero shear strain ( $\eta_{xz}$ ) arising from edge dislocations associated with anisotropic strain relaxation.
Temperature-Dependent Dynamic Frequency Modulation:
A significant finding is the pronounced temperature dependence of the TA phonon frequency. Below a critical temperature ( $T_c \approx 500$ K), the TA mode exhibits transient softening immediately following photo-excitation, whereas above $T_c$ , it shows transient hardening (or a lack of softening). This crossover behavior is distinct from typical electron-phonon decoupling or carrier diffusion mechanisms. The transition temperature aligns with previous observations of magnetic anomalies in strained RuO2 films.
Evidence of Spin-Phonon Coupling:
The authors propose that the observed frequency anomalies arise from magnetoelastic coupling. The TA mode frequency decreases by approximately 10% as temperature increases from 10 K to $T_c$ , a magnitude consistent with magnetoelastic phonon renormalization in other magnetic oxides. The time-varying frequency shift is interpreted as a transient response where the photo-excited spin temperature ( $T_s$ ) follows the electron temperature ( $T_e$ ), leading to a temporary softening of the mode near $T_c$ before recovery.
Detection of B1g Optical Phonons via MOKE:
In the transient Kerr rotation signal, the authors observed a coherent optical phonon oscillation centered at 4.2 THz, identified as the $B_{1g}$ mode. This mode is not observable in transient reflectivity ( $\Delta R$ ) due to symmetry selection rules (the $B_{1g}$ Raman tensor contains only off-diagonal elements for the (110) orientation). The $B_{1g}$ mode also exhibits significant frequency softening near $T_c$ (450 K). Crucially, this anomaly is absent in single-crystal RuO2 and fully relaxed 40 nm films, reinforcing the role of local strain and dislocations in stabilizing the magnetic correlations necessary for this coupling.

Significance and Claims
The paper claims to demonstrate the ultrafast engineering of THz phonons in anisotropically-strained epitaxial RuO2 films. By controlling film thickness to induce specific strain relaxation states, the authors successfully generated and detected coherent acoustic and optical phonon modes that are dynamically modulated by magnetic correlations.

The significance of these findings lies in:

Verification of Spin-Phonon Coupling: The temperature-dependent anomalies in both acoustic and optical phonon frequencies provide indirect but strong evidence for dynamic spin-phonon coupling in RuO2, specifically linking the magnetic state to lattice dynamics in the presence of strain.
Role of Strain and Defects: The results highlight that local strain fields from dislocations are not merely structural imperfections but are essential for inducing the magnetic state and enabling the observed spin-phonon interactions in thin films.
Ultrafast Control: The study suggests that phonon dynamics in the THz range can be manipulated on ultrafast timescales through epitaxial design, offering a pathway for controlling lattice and spin dynamics in crystal-symmetry-coupled magnets (altermagnets).

The authors conclude that these dynamic frequency modulations offer a new avenue for "ultrafast quantum opto-spintronics," where phonons can be manipulated via spin-phonon coupling controlled by epitaxial strain engineering.

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