Magnetic X-Ray Spectroscopy and FMR Spin-Pumping Studies of PLD Amorphous YIG

In this thesis, the magnon diffusion length in amorphous YIG (a-YIG) has been studied in a series of YIG(45nm)/a-YIG/Pt(5nm) trilayer structures, fabricated on Gadolinium Gallium Garnet (GGG) substrates by pulsed laser deposition (PLD). VNA-FMR spectroscopy shows a large change in Gilbert damping with the addition of either Pt, or a-YIG plus Pt; indicating spin-pumping through the a-YIG spacer into the Pt. No significant enhancement in damping is observed when only a-YIG is grown on YIG for thicknesses between 0 and 30nm, without the Pt layer. As the thickness of the a-YIG layer is increased, the additional damping due to spin pumping into the Pt is reduced. The relationship between damping and a-YIG thickness resembles one described by diffusive magnon transport. However, a significantly longer magnon diffusion length of (16±2)nm for a-YIG is observed; approximately four times larger than that observed in previous spin-pumping studies.
Additionally, spectroscopic magnetometry offered by XMCD has been performed on a series of a-YIG thin films to study magnetization at the two Fe3+ sites. The Fe environment near the surface of the amorphous material is shown to be different to that in the crystalline state. Atomic multiplet calculations using Cowan’s ab initio Hartree-Fock method suggest a-YIG magnetism is 80% dominated by Oh-coordinated Fe species, compared to the crystalline YIG which approaches the expected Fe3+ Td:Oh ratio of 3:2, with no Fe2+. Ratios of Fe2+:Fe3+ in a-YIG could not be determined with certainty from fits of the XMCD by atomic multiplet calculations. Oh-dominated magnetism from a-YIG XMCD is shown to be weak and stable with thermal annealing, only changing significantly with recrystallisation of the YIG at 650°C, where Fe3+ Td dominates over Oh Fe species. Bulk-sensitive Fe K-edge XAS and XANES provides experimental evidence for a Fe3+/Fe ratio approaching 1 in both a-YIG and YIG films.