Negative capacitance of nanocomposites with CoFeZr nanoparticles embedded into silica matrix

Abstract

The study presents frequency dependences of real part of admittance Z′ (f) and phase shift angle θ(T, f) in nanogranular films containing CoFeZr nanoparticles with “core-shell” structure embedded into SiO2 matrix. The 3 μm thicknesses (Co41Fe49Zr10)x(SiO2)100−x films with 20 ≤ x ≤ 80 at.% were deposited in vacuum chamber evacuated either with pure Ar (Set 1 samples) or Ar + O2 gas mixture (Set 2) using ion-beam sputtering technique. After characterization by X-Ray diffraction, Mössbauer spectroscopy, scanning electron microscopy and magnetization studies, the films both of Set 1 and Set 2 samples were subjected by admittance measurements at 300 K in the frequency range of 0.1–1000 kHz. Mössbauer spectroscopy have shown that oxidized CoFeZr nanoparticles in Set 2 samples contain semiconducting iron-based oxides with Fe3+ charge states of iron ions. The observed Z′ (f) and θ(f) dependencies for the Set 2 films below the xc have shown dielectric regime of carrier transport. They also exhibited that at weak AC electric fields inductive-like contribution to reactive part of admittance (with positive θ values) prevails over the capacitive one f > 10 kHz. This effect of the so-called “negative capacitance” was explained by the delay of current, formed by electrons hopping between nanoparticles, relative to applied bias voltage. This delay is forced by formation of dipoles of charged FeCoZr nanoparticles with native Fe-based oxide “shells” around them that results in the increase of mean life time of hopping electrons on nanoparticles with “core-shell” structure.