Influence of preparation regimes and composition on structure, phase state and electric properties of nanogranular (FeCoZr)x(PZT)100-x (30 ≤ x ≤ 85 at.%) films

Granular films containing Fe₅₀Co₅₀Zr₁₀ alloy nanoparticles inside the ferroelectric matrix Pb_0.81Sr_0.04(Na_0.5Bi_0.5)_0.15(Zr_0.575Ti_0.425)O₃ (PZT) are characterized by a complex of functional magnetic and electrical characteristics that can be effectively controlled by an external electric or magnetic field. The formation of the necessary granular structure in the case of a PZT matrix is possible only during synthesis in an oxygen-containing atmosphere, leading to significant oxidation of metal nanoparticles. In this regard, an urgent task is to study the degree of oxidation of metal nanoparticles depending on the synthesis conditions, as well as the influence of the forming phases on the electrical properties of films.
The relationship of the phase composition and electrical characteristics of granular films (FeCoZr)_x(PZT)_100-x (30 ≤ x ≤ 85, at.%) obtained in an oxygen-containing atmosphere at a pressure of PO in the range (2.4—5.0) ⋅ 10^-3 Pa was studied by X-ray diffraction analysis, EXAFS spectroscopy (Extended X-ray Absorption Fine Structure) and four-probe electrical resistivity measurements.
A comparative complex analysis of the structural-phase composition and local atomic order in films (FeCoZr)_x(PZT)_100-x for the first time showed the fundamental influence of oxygen pressure during synthesis on the oxidation of nanoparticles and their phase composition. It is shown that in the case of oxygen pressure up to the values of P_O = 3.2 ⋅ 10^-3 Pa, a transition from nanoparticles of complex Fe(Co,Zr) oxides occurs with increasing x to the superposition of complex oxides and ferromagnetic nanoparticles α-FeCo(Zr,O) (or their agglomerations). At a higher oxygen pressure P_O = 5.0 ⋅ 10^-3 Pa, complete oxidation of nanoparticles is observed with the formation of a complex oxide (Fe_xCo_1-x)_1-δO with a wustite structure.
The observed structural-phase composition allows us to explain the measured temperature dependences of the electrical resistance of granular films, characterized by a negative temperature coefficient of electrical resistance (TKR) in the entire range of film compositions at high oxygen pressure (P_O = 5.0 ⋅ 10^-3 Pa), and the transition to positive TKR at lower oxygen pressure (P_O = 3.2 ⋅ 10^-3 Pa) in the synthesis atmosphere and the value x ≤ 69 at.% in films. The transition from negative to positive TKR, indicating the presence of a metallic contribution to conductivity, is fully correlated with the detection by XRD and EXAFS methods of non-oxidized ferromagnetic nanoparticles α-FeCo(Zr,O) or their agglomerations.

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