Mechanical Strain-Tunable Microwave Magnetism in Flexible CuFe₂O₄ Epitaxial Thin Film for Wearable Sensors

Purely mechanical strain-tunable microwave magnetism device with lightweight, flexible, and wearable is crucial for passive sensing systems and spintronic devices (noncontact), such as flexible microwave detectors, flexible microwave signal processing devices, and wearable mechanics-magnetic sensors. Here, a flexible microwave magnetic CuFe₂O₄ (CuFO) epitaxial thin film with tunable ferromagnetic resonance (FMR) spectra is demonstrated by purely mechanical strains, including tensile and compressive strains, on flexible fluorophlogopite (Mica) substrates. Tensile and compressive strains show remarkable tuning effects of up-regulation and down-regulation on in-plane FMR resonance field (H_r), which can be used for flexible tunable resonators and filters. The out-of-plane FMR spectra can also be tuned by mechanical bending, including H_r and absorption peak. The change of out-of-plane FMR spectra has great potential for flexible mechanics-magnetic deformation sensors. Furthermore, a superior microwave magnetic stability and mechanical antifatigue character are obtained in the CuFO/Mica thin films. These flexible epitaxial CuFO thin films with tunable microwave magnetism and excellent mechanical durability are promising for the applications in flexible spintronics, microwave detectors, and oscillators.