Enhancement of negative capacitance effect in (CoFeZr)x(CaF2)(100−x) nanocomposite films deposited by ion beam sputtering in argon and oxygen atmosphere

The paper presents frequency f and temperature Tp dependences of phase shift angle Θ, admittance σ and capacitance Cp for the as-deposited and annealed (CoFeZr)x(CaF2)(100−x) nanocomposite films deposited by ion-beam sputtering of a compound target in a mixed argon-oxygen gas atmosphere in vacuum chamber. The studied films presented metallic FeCoZr “cores” covered with FeCo-based oxide “shells” embedded into oxygen-free dielectric matrix (fluorite). It was found for the metallic phase content within the range of 52.2 at.% ⩽ x ⩽ 84.3 at.% in low-f region that Θ values were negative, while in the high-f region we observed the Θ < 0o. It was obtained that the f-dependences of capacitance module displayed minimum at the corresponding frequency when the Θ(f) crossed its zero line Θ = 0o. It was also observed that the σ(Tp) dependence displayed the occurrence of two minima that were related to the values of Θ1 = 90° (the first minimum) and of Θ2 = −90° (the second one). Some possible reasons of such behavior of (CoFeZr)x(CaF2)(100−x) nanocomposite films are discussed.