Research Projects

The proposal aimed to investigate magnetoelectric (ME) coupling in van der Waals (vdW) two-dimensional (2D) multiferroic heterostructures, specifically FePS₃/CuInP₂S₆, using confocal laser Doppler vibrometry (LDV). ME coupling, characterized by the bilinear energy term ( E_me} = \alpha_{ij} M_i P_j ), links electric polarization ( P_j ) and magnetization (M_i ), enabling field-driven control of ferroic order parameters. This interaction is pivotal for energy-efficient spintronic applications, ferroelectric memory, and quantum sensors, yet quantifying the ME coefficient ((\alpha_{ij} \sim 10^{-11} , \text{s/m})) in ultrathin systems remains challenging due to interface dominance and limitations in conventional methods like Kerr rotation (MOKE), piezo force microscopy (PFM), second harmonic generation (SHG), and electrical transport. MOKE averages over large areas (~10–100 μm), PFM risks sample damage, and SHG lacks mechanical insight, whereas LDV’s sub-picometer displacement sensitivity offers a contactless solution to probe spin-phonon and dipole-phonon interactions. Building on LDV’s success in graphene nanomechanics (Nano Lett. 2018), this project applies it to detect nanoscale strain responses under electric and magnetic stimuli, addressing a critical gap in 2D material characterization. The hypothesis is based on the FePS₃/CuInP₂S₆ heterostructure, with FePS₃’s antiferromagnetism (( T_N \approx 120 K)) and CuInP₂S₆’s ferroelectricity (~200 K), exhibits strain-mediated ME coupling at their sub-2 nm interface. LDV, enhanced by cryogenic conditions (4–150 K) in a cryostation, is expected to resolve picometer vibrational shifts (from frequency domain measurement) linked to ME-driven strain, surpassing conventional probes. This is supported by the proposing group’s expertise in 2D magnet excitations (FePS₃) and ferroelectric ordering (CuInP₂S₆), evidenced by publications in Physical Review B (2021, 2023, 2024, 2025), Advanced Functional Materials (2024), and ACS Applied Nanomaterials (2024), alongside FET/CMOS device physics (Applied Physics Letters 2023, Physical Review Applied 2025, ACS Nano 2021) and ultrasound propagation in MoS₂ devices (Advanced Materials 2025). The project’s objectives are: (1) Develop an LDV setup to resolve SAW-driven displacements with picometer sensitivity, extracting (\alpha_{ij}) via power spectrum analysis; (2) Map interface inhomogeneities (defects, strain gradients) modulating (\alpha_{ij}) with ~1 μm resolution; (3) Investigate temperature (down to 4 K), magnetic field (0–0.8 T), and electric field dependencies in microelectronic/FET geometries; (4) Integrate NV center magnetometry with LDV to correlate nT magnetic field detection with pm SAW movements using a low-temperature confocal setup. Key experiments include: fabricating FePS₃/CuInP₂S₆ heterostructures via exfoliation and annealing, exciting SAWs with interdigital transducers (IDTs) on LiNbO₃ substrates, and measuring displacements with a LDV set up with interferometers under controlled fields. Spatial mapping will use piezo-scanning, while temperature and field studies will employ a cryostat and electromagnet/source-meter. NV-LDV integration will involve ODMR and cross-correlation analysis. Models like the damped harmonic oscillator and Landau-Ginzburg framework will guide data interpretation. Success would significantly advance fundamental understanding by providing a high-resolution, non-invasive ME coupling metric, revealing strain-phonon dynamics in 2D systems. Application-wise, it could be important for the biomedical implant maintenance by enabling ultra-sensitive biosensors to monitor tissue-electrode interfaces, improving implant longevity and patient outcomes, thus bridging materials science and healthcare innovation.