Inductive type properties of FeCoZr–CaF₂ and FeCoZr–PZT nanocomposites

Nanocomposites (FeCoZr) _x (CaF₂)_(100−x) and (FeCoZr) _x (PZT)_(100−x), where the PZT acronym denotes a ferroelectric material of the composition (Pb₈₁Sr₄(Na₅₀Bi₅₀)₁₅(Zr_57.5Ti_42.5))O₃, have been produced in a vacuum chamber at the pressure of about 1 × 10⁻³ Pa using ion-beam sputtering of the target composed of a ferromagnetic alloy Co_0.45Fe_0.45Zr_0.10 plate and dielectric strips affixed to the plate. The nanocomposites have been produced according to the method described in [24, 25]. A beam of pure argon ions (oxygen-free materials) and a beam of mixed argon and oxygen ions (oxygen materials) have been used for the sputtering purposes. Oxygen-free nanocomposites (FeCoZr) _x (CaF₂)_(100−x) have been produced in the argon atmosphere at the pressure of p _Ar = 1.1 × 10⁻¹ Pa, while the oxygen nanocomposites have been produced in the mixed argon-oxygen atmosphere at partial pressures of p _Ar = 8.5 × 10⁻² Pa and p _O2 = 4.3 × 10⁻³ Pa, respectively. Due to low stability of the structure, at the production of the nanocomposites (FeCoZr) _x (PZT)_(100−x), the compounds cannot be obtained in the atmosphere of pure argon. The structures have been produced using combined beams of argon with a minimal oxygen content (p _Ar = 7.4 × 10⁻² Pa and p _O2 = 5×10⁻³ Pa) and with a high oxygen content (p _Ar = 6.6 × 10⁻² Pa and p _O2 = 3×10⁻³ Pa). The ion energy has been of about 5 keV.

Before the electric properties study, a structural characterization of the films was carried out basing on the results of transmission electron microscopy (TEM, Philips EM400T and Philips CM200), X-ray diffraction (XRD, PANalytical diffractometer, Cu K_α) and ⁵⁷Fe transmission electron Mössbauer spectroscopy (20 mCi ⁵⁷Co in Rh source). As was previously shown in [9, 26], (FeCoZr) _x (CaF₂)_(100−x) nanocomposites are of a granular structure with an average size of the ferromagnetic alloy grain of about 3–5 nm. Corresponding TEM images are presented in Fig. 1.

As can be concluded from electron diffraction (ED) (see inserts in Fig. 1), individual rings characterizing metallic nanoparticles and CaF₂ matrix are detected, i.e. clear phase separation is evident for films synthesized in both atmospheres. This result is also confirmed by XRD data presented in Fig. 2 and described in details in [26, 27]. Both XRD and electron diffraction reveal that nanoparticles are characterized by bcc structure of FeCo alloy which is labeled as α-FeCo(Zr) [9, 26, 28]. XRD detects nanoparticles disordered structure because only few broad peaks from α-FeCo(Zr) are on XRD patterns of films with x = 29–65 at.%.

It should be mentioned, that (FeCoZr) _x (CaF₂)_(100−x), 36 ≤ x ≤ 74, films obtained in argon-oxygen atmosphere demonstrate additional diffraction peak (ring) from oxide phase marked in Figs. 1 and 2 as a-(Fe,Co,Zr)_xO_y. Since it is possible to detect only one broad peak on XRD-pattern (single ring on ED image) from a-(Fe,Co,Zr)_xO_y oxide, it is impossible to identify its phase composition exactly. Probably, the oxide dislocates at nanoparticles surface and forms “metallic core—oxide shell” structures, as it was previously proved for (FeCoZr) _x (Al₂O₃)_(100−x) films synthesized by the same method and under the same conditions (p _O2) [28, 29].

Direct observation of FeCoZr-based nanoparticles inside PZT matrix is experimentally more complicated than in the cases of CaF₂ matrix (or Al₂O₃). The reason is that matrix and nanoparticles both contain heavy elements (Zr, Ti) and cannot be clearly separated by TEM. However, combination of techniques (Mössbauer spectroscopy, Raman spectroscopy, XRD, X-ray absorption) clearly reveals the formation of well-separated α-FeCo(Zr), FeCo-based oxides and PZT-like phases in films [11, 30‐32]. Mössbauer spectra of (FeCoZr) _x (PZT)_(100−x) films synthesized in atmospheres with two different oxygen pressures are shown in Fig. 3.

Mossbauer spectrum of (FeCoZr)₅₀(PZT)₅₀ film obtained in atmosphere with lower oxygen pressure p _O2 = 2.4 × 10⁻³ Pa (Fig. 3a) demonstrates full nanoparticles oxidation. It consists of two doublets characterizing different types of oxides, namely Fe(Co)₃O₄ or Fe(Co)₂O₃ (characterized with the superparamagnetic doublet, δ = 0.3–0.45 mm/s, ΔE = 0.9–1.2 mm/s for different x), and (Fe_xCo_1−x)_1−δO (characterized with the doublet, δ = 0.9-1.1 mm/s, ΔE = 1.7–1.8 mm/s) [11, 30, 31]. However, XRD of the film (see insert in upper Fig. 3a) detects also a diffraction line of low intensity from α-FeCo(Zr) structure in addition to the peaks characterizing PZT-matrix and FeCo-based oxide. This means that FeCoZr-based nanoparticles in a PZT-matrix can form metallic “core—oxide shell” structure [11, 31] typically observed for the FeCoZr-alumina nanocomposites [28, 29]. The main diffraction peaks from PZT-matrix and FeCo-based oxide cannot be clearly separated from each other because of their close position (2Ө ~ 31–33°). However, strong asymmetry of the detected peak at 2Ө ~ 32° increasing with x allows to conclude the phase separation.

Increase of FeCoZr concentration to x ≥ 67 at.% leads to appearance of additional sextet on Mössbauer spectra (Fig. 3a) characterizing ferromagnetic metallic α-FeCo(Zr) nanoparticles (or their agglomerations). This is accompanied with the significant increase of α-FeCo(Zr) peak intensity on XRD pattern of the studied film (insert in lower Fig. 3a). Some displacement of XRD peak characterizing both PZT-matrix and FeCo-based oxides to higher 2Ө values indicates the increase of FeCo-based oxides contribution with respect to PZT phase.

(FeCoZr) _x (PZT)_(100−x) films synthesized at higher oxygen pressure of p _O2 ≥ 3.7 × 10⁻³ Pa contain fully oxidized superparamagnetic nanoparticles of either Fe(Co)₃O₄ or Fe(Co)₂O₃ oxides over the whole range of studied x (x = 35–81 at.%) [30‐32]. This is confirmed by Mössbauer spectra shown in Fig. 3b for x = 43 and 67 at.%. Both spectra are characterized by the similar doublets with δ ~ 0.3 mm/s, ΔE = 1.0 mm/s. XRD analysis also proves the absence of non-oxidized α-FeCo(Zr) phase in (FeCoZr) _x (PZT)_(100−x) films obtained at higher oxygen pressure (see inserts in Fig. 3b). Thus, these films consist of a-(Fe,Co,Zr)_xO_y granules in PZT-matrix.

Testing of the electric properties has been performed at the AC, within the frequency range of 50 Hz –1 MHz. Measurements have been made for the resistance R _p and capacitance C _p in a parallel equivalent circuit as well as for the phase angle θ. Electric parameters have been measured using the measuring temperature range T _p from 77 K to 373 K at the temperature change step of 5 K. Component meters of the test stand are computer-connected and controlled by a program written in the Visual C++ software environment. Measurement results are saved in the computer memory.

Samples have been subjected to the 15-min annealing in a tubular furnace controlled with a thermoregulator, in the temperature T _a of 398 K and higher at the temperature interval step of 25 K. Temperature has been maintained with the accuracy of ±1 K.

Figure 4 presents frequency dependences of the phase angle θ for a nanocomposite (FeCoZr) _x (PZT)_(100−x), produced with the sputtering using argon ions with a minimal content of oxygen, of the metallic phase content x = 51.7 at.%, annealed in the temperature of 548 K. Figure 4 shows selected example curves obtained for four out of 63 measuring temperatures T _p .

As can be seen in Fig. 4a, in a low frequency area negative values of the phase angle θ occur, which means that capacitive conduction is a dominating type of conductivity there. Frequency increase makes the phase angle reach its minimum at a certain frequency value and then causes its further growth. It should be noted that over the whole examined frequency range the phase angle values remain negative. Conductivity (Fig. 4b) in the low (to about 500 Hz) and high (more than 50 kHz) frequency areas remains almost constant, while for the medium frequency range of 500 Hz < f < 50 kHz, conductivity rapidly increases along with the frequency increase. Such conductivity behaviour is characteristic for the hopping exchange of electric charges between metallic-phase nanoparticles and is consistent with the hopping carrier transport model on DC and AC [2, 9, 11]. In Fig. 4c it can be seen that in the low frequency area, capacitance (dielectric permittivity) remains constant. When frequency further increases and exceeds the value of f ₀, that approaches the inverse relaxation time τ, rapid decrease of capacitance occurs. Temperature dependence on permittivity can also be observed and specifically its increase along with the temperature growth. This is consistent with a model of thermally activated permittivity that is characteristic for the hopping exchange of electric charges [33, 34].

In the case when the ion beam applied to the nanocomposite production includes argon ions and a high content of oxygen (oxygen materials), frequency dependences of θ, σ and C _p for nanocomposites of a similar metallic phase content (52.0 at.%) are considerably differ. As can be seen in Fig. 5a, the phase angle θ changes from negative values in the low frequency area to the values higher than 90° in the high frequency area. Figure 5b presents dependencies of conductivity as a function of the frequency and shows that at the phase angle θ (Fig. 5a) transition through 0° in the C _p characteristic a strong minimum occurs (Fig. 4c), which is related to the occurrence of voltage resonance. Mutual compensation of capacitive and inductive voltage components takes place, which is expressed by the measured value of capacitance approaching zero. At the phase angle transition θ through 90° (Fig. 5c) current resonance occurs, which corresponds to the occurrence of a strong minimum in the conductivity versus frequency curve.

In this nanocomposite (Fig. 5) a phenomenon of the coilless inductance occurs. The phenomenon consists in that in a layer where there are no windings (no coil), the phase angle values are positive, which is characteristic for inductance.

Coilless inductance advantage is the ability to use a thin layer of the nanocomposite to shift the phase angle in the area of positive values. In conventional circuits for this purpose coils are used.

From Fig. 5 results that the temperature increase causes shift of minimums, which are located on dependence C _p(f) and correspond to frequency of voltage resonance (θ = 0°), in higher temperatures area. Similar situation is observed for minimums on dependence σ(f)—it is current resonance (θ = 90°). It is related to temperature changes of time τ.

Figure 6a presents frequency dependences of the phase angle for a nanocomposite (FeCoZr) _x (CaF₂)_(100−x) produced with the ion-beam sputtering using pure argon ions (oxygen-free material). As can be seen, over the whole range of the measured frequency, the phase angle values are negative (capacitive type of conduction). Conductivity curve shown in Fig. 6b exhibits two increase segments separated by a segment of an almost constant value. It means that conduction in that material is realized by hopping exchange of electric charges and that in the material there are two types of potential wells for the electrons to hop between them. It can also be seen in the C _p(f) curve of Fig. 6c, where two segments of the decreasing capacitance occur. It indicates that in the material two types of potential wells occur and they differ by the relaxation times. The maximum, that can be seen in the dependence θ(f) (Fig. 6a), is also related to the two types of potential wells.

Figure 7 presents frequency dependences of the phase angle, conductivity and capacitance for a nanocomposite (FeCoZr) _x (CaF₂)_(100−x), produced with the ion-beam sputtering using argon and oxygen ions (oxygen material) of the metallic phase content as in the previous case of the oxygen-free sample, that is x = 62.7 at.%. For the oxygen material, the obtained dependences θ(f), σ(f) and C _p(f) resemble the ones obtained for the nanocomposite (FeCoZr) _x (PZT)_(100−x) produced with the ion-beam sputtering using argon and oxygen ions. In this case, the coilless inductance phenomenon can also be observed. It consists in the occurrence of the phase angles of positive value. In this material current resonance occurs at the phase angle θ transition through the value of 90° and voltage resonance occurs when θ = 0°, which can be seen in the form of sharp minima in Fig. 7b, c.

In Fig. 7 it can be observed that on frequency dependences on conductivity and capacity minimums shift (related to voltage and current resonances) in area of higher frequencies occurs together with temperature increase.

There is no doubt, that the lack of positive phase angle values in the oxygen-free nanocomposites and in the nanocomposites with an insignificant oxygen content as well as their occurrence in the oxygen materials is related to the presence of oxygen in the sputtering beam. It follows from the results of our previous investigations on the coilless inductance phenomenon in the oxygen nanocomposites (FeCoZr) _x (Al₂O₃)_(100−x) that the occurrence of this phenomenon is related to the formation of a coating of metal oxides on the metallic-phase nanogranule surfaces [2, 3]. The occurrence of the coilless inductance phenomenon in nanocomposites (FeCoZr) _x (PZT)_(100−x) and (FeCoZr) _x (CaF₂)_(100−x), produced with the ion-beam sputtering using argon and oxygen ions, indicates that also in this materials on the nanogranule surfaces a film of metal oxides forms. That oxide layer makes a double barrier, and electrons hop through it (tunnelling) on their way from one metallic nanogranule core to another, the cores making the potential wells.

As was established in the works [10, 11, 31] in nanocomposites ferromagnetic alloy—dielectric electron hopping occurs between metallic phase nanoparticles, which are close to each other. Whereas electron hopping with variable jumps distance, which occurs at the temperature of liquid helium, is rather impossible because of the high measurement temperatures about 80 K and higher.