Diffusion-Driven Mechanical Exfoliation of Rare-Earth Iron Garnet Nanosheets

Rare-earth iron garnet thin films have advanced the development of on-chip non-reciprocal photonic and prototype spintronic devices due to their large Faraday rotations and spin diffusion lengths, respectively. However, non-epitaxial garnets deposited using pulsed laser deposition (PLD) or sputtering require an annealing temperature of 800°C or greater, and epitaxial garnet films are limited to lattice-matched substrates that are not foundry compatible. Here, a modified rapid thermal annealing process enabled mechanical exfoliation of high-gyrotropy cerium-doped terbium iron garnet (Ce 0.25 Tb2.75 Fe 4.75 O12) from a silicon substrate without the need for an intermediary graphene layer, such as used in remote-epitaxy. Exfoliation is enabled by vacancy diffusion that followed a Nabarro-Herring model where strain was varied using different thicknesses for the garnet films. An increase in the tension between the film and the substrate resulted in a diffusion of vacancies away from the substrate. The diffusivities calculated by fitting the model to the strain rate-stress data identify iron and terbium as the rate-determining lattice diffusants. Furthermore, cross-section energy dispersive X-ray spectroscopy shows an accumulation of cations to form an amorphous Fe-Tb-Si-O interlayer at the substrate, leaving behind vacancies that create an exfoliation plane ~30nm into the film. This distance is comparable to the cation diffusion length, verifying the model. Magnetic and optical measurements from the exfoliated nanosheets reveal a saturation magnetization of 18 emu cc -1 and a Faraday rotation of -2900°cm-1 at 1550 nm, both of which are comparable to bulk values. Exfoliation through controlled creep-diffusion will open non-traditional pathways for incorporating garnets with other crystalline wafer platforms and protect devices from the high-temperature processes often used in crystallizing garnet films.