Epsilon-near-zero and magnetically driven properties for medium-entropy FeCoNi alloy particle-doped CNTs and their derivatives

Remarkably low permittivity at plasma frequency for epsilon-near-zero (ENZ) materials has garnered significant interest. This study reports a flexible magnetically driven radio frequency ENZ material. Cobalt nanoparticles, cobalt–nickel, and iron-cobalt–nickel alloy nanoparticles were in situ synthesized on the inner walls of carbon nanotubes (denoted as Co@CNTs, CoNi@CNTs, and FeCoNi@CNTs) and subsequently incorporated into thin films with waterborne polyurethane (PU). Notably, in the FeCoNi@CNTs-PU film, the real permittivity transfers from negative to positive, achieving ENZ performance at 45 MHz. Additionally, the findings indicate a gradual decrease in plasma frequency associated with the alloying process. The underlying mechanisms have been investigated through theoretical calculations, which reveal that as metals are synthesized and alloyed on the inner walls of CNTs, the band structure experiences a flattening effect and non-parabolic enhancement, leading to an increase in effective electron mass. Furthermore, the FeCoNi@CNTs-PU film exhibits exceptional magnetic driving capabilities and flexibility. This positions ENZ materials as promising candidates for applications in magnetically driven actuators and implantable electronic devices.