Mechanics of Nanoscale ϵ-Fe₂O₃/Organic Superlattices toward Flexible Thin-Film Magnets

State-of-the-art atomic layer deposition (ALD) thin-film technology which is well-integrated with microelectronics and beyond is currently strongly extending toward hybrid materials where organic fragments are inserted within the inorganic matrix through so-called molecular layer deposition (MLD) cycles. This allows the fabrication of nanoscale inorganic-organic superlattices and multilayers directly from the gas phase. These materials are particularly promising for enhancing the mechanical flexibility of rigid inorganics. Here we demonstrate for ALD/MLD-grown ϵ-Fe2O3/Fe-terephthalate superlattice structures that nanoscale (1-10 nm) Fe-terephthalate interlayers significantly improve the flexibility of ϵ-Fe2O3 thin films without sacrificing their unique high-coercive-field magnetic characteristics. Nanoindentation evaluation indicates that the elastic modulus can be reduced by a factor of 2 down to 70 ± 20 GPa, while in situ tensile fragmentation testing demonstrates that the crack onset strain and critical bending radius can be tuned by a factor of 3. Modeling of the tensile fragmentation patterns through Weibull statistics shows that cohesive and interfacial shear strain, following the trend for crack onset strain, increase with increasing organic content, while gains for the respective strengths are limited by the simultaneous reduction in elastic modulus. The Fe-terephthalate film exhibits high interfacial shear strain/strength and low tendency for buckling, which highlights its potential to act as an adhesion layer for the ferrimagnetic ϵ-Fe2O3 layers. The results are particularly interesting for magnetic recording applications in sustainable next-generation flexible electronics.