The effect of annealing on induction like properties of (FeCoZr)x(CaF2)(100−x) nanocomposite films produced by ion-beam sputtering in the vacuum environment

Abstract

The paper presents frequency f and temperature T dependences of the phase angle θ, conductivity σ, capacity Cp and tgδ for (FeCoZr)x(CaF2)(100−x) nanocomposite films deposited with the application of ion-beam sputtering of a complex target in the gas atmosphere of mixed argon and oxygen. The examined films are composed of metallic FeCoZr “cores” covered with FeCo-based oxide “shells” embedded in an oxygen-free dielectric matrix (fluorite).It has been found that in nanocomposites (FeCoZr)62.7(CaF2)37.3 produced with ion-beam sputtering using combined argon–oxygen ions, the phase angle values change from the negative, through zero, up to the positive ones. It means that the phase angle changes from the capacitive to the inductive type of conduction.Thermo-gravimetric analyses show that nanogranules have metallic phase cores covered by oxides of metallic-phase atoms. The inductive type of nanocomposites conductivity is related to the presence of the oxide cover. Annealing of the nanocomposites at the temperature of 398 K and higher causes an extension of the phase angle change range to an area exceeding +90°. This is due to the fact that during the sputtering of the dielectric CaF2 with argon and oxygen ion beam, some fluorine ions get released and participate in the chemical reaction with surface atoms of nanogranules during the annealing. Such reactions produce a coating composed of oxides and fluorides of the metallic phase on the surface of metallic nanogranules, which results in the extended range of the phase angle changes beyond the area of +90°. In the annealed nanocomposites, frequency dependences of capacity exhibit a sharp minimum which is related to the phase angle transition through zero, as with the voltage resonance phenomena in series RLC circuits. Frequency vs. conductivity characteristics show strong minima, which are related to the phase angle passing through the values of θ = +90°. They correspond to the current resonances in parallel RLC circuits.Relaxation times of the capacity τC and conductivity τσ correspond to the phase angle transition through zero 0° and through +90°. Relaxation time dependences of activation energy on annealing temperature have been determined with the use of the Arrhenius dependences for capacity and conductivity.It has been found that for annealing temperatures higher than 573 K an increase of activation energy occurs, which is related to the oxidation of metal atoms in the core of metallic-phase nanoparticles.