Fabrication of Freestanding Polycrystalline Yttrium Iron Garnet Thin Films Using a Sacrificial Layer

The emergence of epitaxial lift-off techniques as a novel method for layer transfer and hetero-integration has enabled the synthesis of freestanding oxide membranes with distinctive functional properties and promising practical applications. Freestanding yttrium iron garnet (Y₃Fe₅O₁₂, YIG) films, in particular, have garnered increased attention within the spintronics community due to their diverse range of magnetic and magnonic phenomena—such as low damping, spin pumping, spin orbit torque and spin wave propagation—that not only hold potential for enhancement compared to their bulk counterparts but also for integration into advanced devices and new technologies.<br/>Despite their significant promise, there remains a gap in exploring reliable techniques for producing YIG membranes. While single-crystalline freestanding garnets have been previously fabricated using graphene-based remote epitaxy [1], the use of 2D materials for membrane fabrication in general is not well developed for garnet-based structures. The challenges arise due to the difficulty in achieving high-quality graphene and other 2D material transfer onto garnet substrates, the sensitivity of the 2D materials to the high temperatures and oxygen required for complex oxide growth, and the variety of other competing phases such as orthoferrites or iron oxides that can form from the constituents of the garnet.<br/>In this work, we demonstrate an alternative method for the fabrication and transfer of freestanding garnet membranes using high-quality water-soluble Sr₃Al₂O₆ (SAO), which is commonly used for making freestanding epitaxial perovskite films. An SrTiO₃ (STO)/SAO (30 nm)/STO (8 nm) heterostructure is first grown using pulsed laser deposition in which the upper STO protects the SAO during subsequent steps, followed by the deposition of 5 nm silica using sputtering. The silica forms an amorphous layer upon which a 150 nm layer is grown from a YIG target by PLD without heating. The as-grown YIG-composition layer is amorphous, but it is then rapid thermal annealed at 900°C for 5 min in oxygen to obtain polycrystalline garnet without secondary phases according to x-ray diffraction, and with grain diameters of several µm. Use of the silica layer is necessary to prevent a Fe-rich YFeO₃ orthoferrite forming epitaxially on the STO layer [2]. The stack is then immersed into deionized water for one day to dissolve the SAO sacrificial layer, releasing the STO/silica/YIG, and hydrochloric acid is used to dissolve the STO. The as-grown and transferred YIG films exhibit magnetic hysteresis loops characteristic of polycrystalline garnet with an in-plane easy axis resulting from the dominant shape anisotropy. This method aims at inspiring alternative methods to remote epitaxy in garnet systems as means of exploring exotic physics, and developing innovative spintronic and magnetooptical applications.<br/>[1] Kum at al., Nature 578 p75 (2020)<br/>[2] Gross et al., Appl. Phys. Lett. 121, 252401 (2022)