Investigating High-frequency Electrical Transport Properties in Yttrium Iron Garnet (YIG)/Platinum (Pt) Using Picosecond Acoustic Pulses

Abstract

This thesis investigates high-frequency electrical transport properties in yttrium iron garnet (YIG)/platinum (Pt) heterostructures using picosecond acoustic pulses. Femtosecond laser excitation of aluminium (Al) transducers generates coherent strain pulses that propagate through gadolinium gallium garnet (GGG) substrates. These acoustic phonons couple magnetoelastically with the ferrimagnetic YIG layer, modulating local magnetic anisotropy and exciting spin waves. The resulting spin currents transfer to the adjacent Pt layer, where strong spin-orbit coupling converts them to measurable charge currents via the inverse spin Hall effect (ISHE), enabling electrical detection of acoustic-magnetic interactions. Samples comprised YIG films of 200 nm and 830 nm thickness deposited on both thick (500 𝜇𝑚) and thin (120-200 𝜇𝑚) GGG substrates, with 5 nm Pt detector strips and 25-60 nm Al transducers. Three temporally distinct signals emerge, which are optical excitation at t ≈ 3 ns from direct thermal spin Seebeck effect, primary acoustic response at t = 35.8-86.5 ns from magnetoelastic coupling, and acoustic echo signals confirming coherent phonon propagation. The antisymmetric voltage response upon magnetic field reversal (±1.66 mT to ±4.15 mT) demonstrates ISHE detection, representing the first electrical measurement of strain-induced spin currents in magnetic insulator/heavy metal bilayers. Key findings include temperature-dependent signal variations from 1.8 mV at 10 K to 1.25 mV at 250 K, field-tunable ferromagnetic resonance frequencies spanning 0.1-9.7 GHz, and acoustic standing wave resonances at 4 GHz and 8 GHz. Remarkably, 200 nm films at 50 K exhibit signal amplification over 4 𝜇𝑚 timescales, suggesting possible sound amplification by stimulated emission of radiation (SASER) action through magnetoelastic feedback mechanisms. These results establish picosecond acoustics as a powerful platform for investigating magnetoelastic coupling with sub-nanosecond temporal resolution, opening pathways for acoustic control of spin transport in next-generation spintronic devices