Nanostructured flexible magneto-dielectrics for radio frequency applications†
Flexible magneto-dielectric composites were prepared by dispersing high saturation magnetization (Ms), low coercivity (Hc), air-stable, Fe/citrate nanoparticles and heterostructures of Fe/Ag core–shell nanoparticles in an elastomer (polydimethylsiloxane) matrix. Fe/citrate nanoparticles with a polycrystalline Fe core, a thin oxide layer, and a citrate shell have a Ms of 144 emu g−1 and Hc of 420 Oe. Fe/Ag core–shell nanoparticles exhibit lower Ms of 98 emu g−1 and Hc of 210 Oe prior to annealing. The saturation magnetization of Fe/Ag core–shell nanoparticles increased to 120 emu g−1 and the Hc decreased to 58 Oe after annealing, due to formation of the heterostructure and changes in the chemical composition and size of the oxide layer. Magneto-dielectric composites fabricated with air-stable Fe/citrate nanoparticles possess high μ (3.0) and moderate ε (11.6) while maintaining low magnetic (0.29) and dielectric (0.051) loss. Moreover magneto-dielectric composites fabricated using Fe/Ag heterostructures combine high μ (2.25) and ε (22.3) with low magnetic (0.24) and dielectric (0.09) loss. Radio Frequency antennas made from Fe/citrate nanoparticle and Fe/Ag heterostructure composites have achieved size reductions of 44% and 38%, respectively.