Morphology dependent negative dielectric permittivity in spinel ferrite nanostructures

Metamaterials with negative dielectric permittivity has become a research hotspot due to their potential efficacy in various electromagnetic utilizations, such as, filter and antenna design, novel capacitance and inductor design, electromagnetic absorber etc. This study unfurls immense possibilities of inducing metamaterial properties such as negative dielectric permittivity in Magnesium Ferrite, a naturally occurring compound, just by restructuring the morphology. Herein, we report the correlation between structural, dielectric, and magnetic properties of the system in the same chemical composition but different morphologies, namely, nano solid spheres (NSSs) and nano hollow spheres (NHSs). Strikingly, only NHSs show negative dielectric permittivity owing to the Lorentz dielectric resonance in the vicinity of a metamagnetic transition. Further investigations show that the hollow cores and greater number of interfaces in NHSs, in comparison to NSSs, cause the uncompensated charges to accumulate within them and release at some frequencies and temperatures, leading to the backscattering of dipoles, which in turn induces the resonance. NSSs with stronger anti-ferromagnetic interaction and impermeable morphology do not show any sign of negative dielectric permittivity.