Negative capacitance switching in size-modulated Fe3O4 nanoparticles with spontaneous non-stoichiometry: confronting its generalized origin in non-ferroelectric materials
A persistent low-frequency negative capacitance (NC) dispersion has been detected in half-metallic polycrystalline magnetite (Fe3O4) nanoparticles having a size-variation: 13 – 236 nm, under the application of moderate dc bias. Using Harvriliak–Negami model, 3D Cole-Cole plot is employed to recapitulate the relaxation-time (τ) of the associated oscillating dipoles, related shape-parameters (α,β) and resistivity for different sizes. Universal Debye relaxation (UDR) theory requires a modification to address the shifted quasi-static NC-dispersion plane in materials showing both +ve and –ve capacitance about a transition/switching frequency (f0). A consistent blue-shift of ‘f0’ is observed with increasing external dc-field and decreasing particle-size. Based on this experimental data, a generalized dispersion scheme is proposed to fit the entire positive and negative capacitance regime including the diverging transition point. In addition, a comprehensive model is discussed using phasor-diagrams to differentiate the underlying mechanisms of continuous transition from –ve to +ve capacitance concerning localized charge recombination or time-dependent injection/displacement currents; adequately explored in scientific literature and the newly proposed ‘capacitive switching’ phenomenon. An inherent non-stoichiometry due to iron-vacancies [Fe3(1-δ)O4], duly validated from first principles calculations; builds up p-type nature, which consequently promotes more covalent and heavier dipoles slowing down dipolar relaxations; incommensurate to Maxwell-Wagner interfacial polarization (MWIP) dynamics. This combinatorial effect is apprehended for the sluggish response of the associated dipoles and stabilization of NC.