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DARIUSZ BOCHENEK, PRZEMYSŁAW NIEMIEC, RADOSŁAW ZACHARIASZ, PIOTR GUZDEK, ARTUR CHROBAK, GRZEGORZ ZIÓŁKOWSKI

Ferroelectric and ferromagnetic properties of the (1–x)NiZnFeO4 –(x)Pb(Fe1/2Nb1/2)O3

composite

INTRODUCTION

According to the thermodynamic terminology of the multiferroics, they are some ferroics of II and III order which have simultaneously at least two spontaneously ordered subsystems among the states: ferromagnetic FM, ferroelectric FE, ferroelastic FES and ferrotoroidal FT [1÷3]. Possible application of the multiferroics depend, first of all, on a degree of mutual coupling of particular subsystems (magnetic, electric and elastic) [4].

Materials showing simultaneous electric (FE – ferroelectric, AFE – antiferroelectric or FIE – ferrielectric) and magnetic ordering (FM – ferromagnetic, AFM – antiferromagnetic or FIM ferrimagnetic) are called ferroelectromagnetics (FEM) [5]. In the ferroelectromagnetics both magnetic and electric properties can be controlled by external factors, such as: a magnetic field, an electric field, stress or temperature [6]. If a mutliferroic material is the ferromagnetic and the ferroelectric at the same time, then it will be characterized by magnetic response to a variable electric field, or inversely, a polarization change in the external magnetic field. This effect can be strengthened by making composite based on the ferrites and the ferroelectric materials (multiferroics, biferroics, ferroics). According to the classification, made by D. Khomsky, the mutliferroic materials can be divided into multiferroics of type-I and type-II [5]. PbFe1/2Nb1/2O3 (PFN) is one of the well-known ferroelectromagnetic materials. It has a structure of the perovskite type of the ABO3 general formula, where A positions of the unit cell are filled with big ions of lead Pb, whereas B positions are filled alternately with (in a random way) ions of iron Fe and niobium Nb [7, 8]. According to the classification of D. Khomsky [5] PFN belongs to mutiferroics of type-I, in which ferroelectricity is connected with a displacement of ferro-electrically active ions d0 (Nb) from the centre of regular octahedrons – O6, whereas magnetism is connected with presence of ions dn (Fe). Such a situation in the „mixed” perowskites is a polarization source which co-exists with the magnetic ordering (antiferromagnetic). However, coupling of those subsystems in PFN is weak. The multiferroics with the strong coupling of the magnetic and electric subsystems are included into the multiferroics of type-II, where magnetism causes ferroelectricity formation, but the spontaneous polarization is relatively low (about 10–2 C/cm2). Ferroelectricity in those multiferroics occurrs only if they show the magnetic ordering. TbMnO3 and TbMn2O3 are the examples of the materials from this group [5].

Technology and results of comprehensive examinations of the ferroelectromagnetic composite based the NiZnFeO4 nickel-zinc ferrite and the PbFe1/2Nb1/2O3 multliferroic with the general formula of (1–x)NiZnFeO4-(x)PFN, for x = 0.6 (a ratio of the component content 40:60) are presented in this work.

Ferrite NiZnFeO4 (Fe2Ni0.63Zn0.37O5) belongs to the soft ferrite of the work frequency range (50÷1000 MHz) and it is used, among others, for signal treatment (in telecommunication filters, distance sensors, pulse transformers, in delay lines), for damping EMI (while damping broad-band electromagnetic interferences in EMI filters, in current-compensated chokes), for broad-band trans-formers, elements, which do not require usage of higher power levels, higher frequencies. Recently, soft nickel-zinc ferrites with high permeability and high Curie temperature have been widely studied both for scientific reasons and for LTCC (low temperature co-fired ceramics) applications [9, 10].

An aim of this work was to obtain the (1-x)NiZnFeO4-(x)PFN ferroelectromagnetic composite and to improve the coupling of the magnetic and electric sub-systems.

EXPERIMENT

The test material was the (1–x)NiZnFeO4-(x)Pb(Fe1/2Nb1/2)O3 ferroelectromagnetic composite, for x = 0.6, consisting of the zinc-nickel ferrite of high magnetic permeability and high resistance (NiZnFeO4) (Fig. 1) and the synthesized PbFe1/2Nb1/2O3 (PFN) ferroelectromagnetic powder. In the further part of the work the composite was marked as NZF-PFN.

The columbite method (a two stage synthesis of the PbFe1/2Nb1/2O3 components) by powder calcination was used to obtain the synthesized PFN [11]. In the first stage, the FeNbO4 iron niobate was obtained according to the Fe2O3+Nb2O5 2FeNbO4 reaction (synthesis conditions: TsynthI = 1000°C and tsynthI = 4 h), whereas in the second stage the final product was obtained in a form of the synthesized PbFe1/2Nb1/2O3 powder according to the reaction: FeNbO4+2PbO2PbFe1/2Nb1/2O3 (synthesis conditions: TsynthII = 800°C and tsynthII = 3 h). The nickel-zinc ferrite was

Fig. 1. SEM image of the ferrit powder Rys. 1. Zdjęcie SEM proszku ferrytu NiZnFeO4

Dr hab. Dariusz Bochenek, mgr Przemysław Niemiec, dr Radosław Zachariasz (radoslaw.zachariasz@us.edu.pl) – Departament of Materials Science, University of Silesia, dr Piotr Guzdek – Institute of Electron Technology Cracow Division, dr inż Artur Chrobak mgr Grzegorz Ziółkowski – Institute of Physics, University of Silesia

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obtained by the powder calcination method in the conditions: Tsynth = 1000°C and tsynth = 4 h.

The component powders were mixed in the planetary mill of the FRITSH Pulwerisette 6 type at all stages of the technological process, for 15 h, on wet, in the ethanol (400 rev/min).

On the basis of the derivatographic examinations (Fig. 2) on the Q-1500D derivatograph (with the system of F. Paulik, J. Paulik, L. Erdey), the following synthesis conditions of the component powder mixture of the composite were selected: Tsynth = 900°C and tsynth = 2 h. The composite powder was compacted by the free sintering method (FS) in the conditions: Ts = 1050°C/ts = 2 h. Electrodes were applied on the composite specimen surfaces by silver paste burning method for electrical tests.

The X-ray measurements of the crystal structure were made on a diffractometer of the Phillips firm. The microstructure examinations of the specimen fractures were performed on the SEM scanning electron microscope with the field emission, HITACHI S-4700 and EDS Noran Vantage system, and the temperature measurements of the dielectric parameters were made on the capacity bridge of the QuadTech 1920 Precision LCR Meter type. The measurements of the magnetic properties were carried out by the SQUID magnetometer (MPMS XL-7 Quantum Design) in the temperature range from –271°C to 27°C, magnetic field to 7 T, and the Faraday magnetic susceptibility balance in the temperature range from 27°C to 750°C. These examinations of relationships of the Q–1 internal friction and the Y Young’s modulus were made on the automatic acoustic frequency relaxator of the RAK-3 type.

The ceramic specimens were subjected to poling in the silicon oil by the high voltage method (the low temperature method) using the Matsusada Precision Inc. HEOPS-5B6 high voltage supply in the following conditions: poling field Epol = 25 kV/cm, poling time tpol = 3 h at poling temperature Tpol = 50°C. Examinations of the piezoelectric parameters were carried out by the resonance-antiresonance method.

The magnetoelectric effect (ME) was evaluated at room temperature with the dynamic lock-in method described in Ref. [12]. The ME effect was measured by applying a sinusoidal magnetic field HAC produced by Helmholtz-type coils excited with an HD amplifier directed by lock-in amplifier SR 830 (Stanford Research System, USA), and superimposed on a bias magnetic field HDC produced by an electromagnet (AGH, Poland) working in tandem with a power supply (Dacpol Service, Poland). The magnetic fields were applied perpendicularly to the polished surfaces of the pellets onto which conductive electrodes had been applied. Measurements of the magnetoelectric coefficient ME were performed by recording voltage output (V) as a function of bias magnetic field HDC and the frequency of sinusoidal magnetic

Fig. 2. DTA, TG and DTG curves of the composite NZF-PFN Rys. 2. Krzywe DTA, TG I DTG dla kompozytu NZF-PFN

field HAC. The ME coefficient was calculated from the voltage output by the following relation (1):

dH

V

ACME (1)

where V is the voltage generated due to the magnetoelectric effect, HAC is the amplitude of the sinusoidal magnetic field and d is the thickness of the sample as presented here [12, 13] for both the single phase and composite magnetoelectric media. The induced voltage between sample surfaces was measured using the SR 830 lock-in amplifier with an input resistance of 100 M and capacitance of 25 pF. The lock-in amplifier worked in the differential mode. The Hall probe system SM 102 (Asonik, Poland) was employed to measure the DC and AC magnetic fields.

RESULTS

At room temperature the X-ray diffraction patterns show a typical single phase cubic spinel for the ferrite powder NiZnFeO4 (Fig. 3). The diffraction peak of PFN component was identified as a tetragonal perovskite structure with a P4mm space group without pyrochlore phase. The X-ray analysis of the NZF-PFN composite showed that besides the peaks coming from the component phases (PFN and ferrite NiZnFeO4) there was also a low quantity of a foreign pyrochlore phase, which was formed during the technological process (Fig. 3).

The EDS examinations of the NZF-PFN composite and its particular components (Fig. 4), showed presence of characteristic maxima coming from the elements constituting the compounds in question. The examinations also confirmed a similar percentage composition of the components in the examined compounds according to the stoichiometric calculations.

In spite of the long mixing and milling in the planetary ball mill, the NZF-PFN composite microstructure shows high heterogeneity. It confirms the non-simultaneous and non-uniform growth of the grains of the PFN powder and NiZnFeO4 ferrite. Bigger grains grow quicker at the expense of smaller grains what is visible in the SEM microstructural images of the composite specimen fracture (Fig. 5). The NiZnFeO4 ferrite powder grains are characterized by rectangular boundaries in a shape of polygons, whereas the PFN ferroelectric powder grains have irregular edges on the grain boundaries (Fig. 5b).

Fig. 3. XRD patterns of the NZF-PFN composite, PFN and NiZnFe powders Rys. 3. Widma rentgenowskie kompozytu NZF-PFN oraz proszków składowych PFN i NiZnFeO4

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Fig. 4. EDS microanalysis of element for PFN and NiZnFeO4 powders and NZF-PFN composite Rys. 4. Wyniki mikroanalizy EDS pierwiastków dla PFN, NiZnFeO4 oraz kompozytu PFN-NiZnFeO4

Fig. 5. Fracture of composite NZF-PFN samples: a) magnification ×5000, b) magnification ×25000 Rys. 5. Przełam próbek kompozytu NZF-PFN: a) pow. ×5000, b) pow. ×25000

The temperature examinations of the M(T) relationship of the

NZF-PFN composite have shown high values of magnetic susceptibility (Fig. 6), decreasing with an increase in temperature. The M(T) curve is typical for ferro/ferrimagnetic behaviour with

Fig. 6. Magnetization temperature dependences for NZF-PFN composites Rys. 6. Temperaturowe zależności magnetyzacji dla kompozytu NZF-PFN

the Curie temperature near 400°C. In the temperature range from 80C to 120C an anomaly, connected with a slight inflexion of the M(T) course, is observed. That behaviour can show an influence of the electrical sub-system on the magnetic properties of the NZF-PFN composite. Above 400C the NZF-PFN composite is the paramagnetic.

Magnetic hysteresis loops M(H) for the NZF-PFN composite, made at –271C, –173C and 27C are presented in Figure 7. The M(H) courses are characteristic for magnetically soft materials. At –271C the NZF-PFN composite shows a narrow magnetic loop with high saturation. Spontaneous magnetization at this temperature is MS = 36.42 emu/g, magnetization remanence MR = 18.24 emu/g, whereas the coercion field value is about HC = 0.11 T With an increase in temperature the spontaneous magnetization value decreases and at 27C it is MS = 27.84 emu/g, whereas the magnetization remanence equals MR = 7.98 emu/g. With an increase in temperature the magnetic hysteresis loop image narrows, and the coercion field value decreases to the value of HC = 0.03 T. Similar results of the magnetic examinations were obtained by the authors in article [14], but the values of magnetization were lower.

Examinations of the dielectric properties of the NZF-PFN confirmed presence of a few maxima of electric permittivity in the (T) relationship – Figure 8. The first maximum present at a lower temperature is connected with the ferroelectric-paraelectric phase transition (coming from PFN). The second maximum (strongly diffused), present at a higher temperature, is connected to an increase in activation mechanism of electric conductivity. In the (T) diagram anomalies can be also seen in the area of the magnetic – paramagnetic phase change (about 390°C), visible for lower frequencies of the measurement field. Like in the PFN ceramics, the phase transition from paraelectric to ferroelectric phase shows a diffused character (Fig. 8b) in the NZF-PFN composite. The maximum of the m electric permittivity value in the phase transition is about 12 times lower comparing to the PFN ceramics [e.g. 15]. The characteristic maxima present in the area of the magnetic phase transition in the NZF-PFN composite show an influence of the magnetic subsystem on its dielectric properties.

Dielectric losses decrease with an increase in the frequency of the measurement field. It is visible in the tan(T) temperature relationships of the NZF-PFN composite (Fig. 9). However, values of the dielectric losses are rather high. The temperature increase (above 100°C) results in their rapid increase, which is particularly visible for low frequencies of the measurement fields.

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Fig. 7. Magnetic hysteresis loops for NZF-PFN composites at temperatures: a) –271°C, b) –173°C and c) 27°C Rys. 7. Magnetyczne pętle histerezy dla kompozytu NZF–PFN w temperaturach: a) –271°C, b) –173°C i c) 27°C

The phase transition taking place in the NZF-PFN composite

are also confirmed on the temperature relationships of mechanical losses Q–1 and Young’s modulus Y (Fig. 10), determined by the internal friction method [16]. In the diagrams distinct changes of the Y values in a form of the AFC-E minimum, correlating with the maximum of the PFC-E internal friction (the Q–1 maximum diffused at about 100C) can be observed.

Fig. 8. Temperature dependences on dielectric permittivity for NZF-PFN composite Rys. 8. Temperaturowe zależności przenikalności elektrycznej dla kompozytu NZF–PFN

Fig. 9. Temperature dependences on the tan for NZF-PFN composite Rys. 9. Zależności tangensa kąta strat dielektrycznych tan od temperatury T dla kompozytu NZF–PF

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Such behaviour of the PFC-E peak connected with the rapid increase in the Y Young’s modulus value above the AFC-E minimum shows that the phase transition takes place (ferroelectric paraelectric). Similar regularities are observed at higher temperature, where there is the subsequent Y minimum (AFC-M), overlapping with the internal friction maximum (PFC-M). Temperature of the Q–1 maxima correspond rather well to temperature of the electrical permittivity maxima (Fig. 8a) and the magnetic phase transition in the M(T) diagram (Fig. 6). Those anomalies can confirm an influence of the magnetic subsystem on the electrical properties and inversely.

The results of the magnetoelectric coefficient measurements for NZF-PFN composite are plotted versus the HDC bias magnetic field (Fig. 11a) and the frequency f of the HAC magnetic field (Fig. 11b).

As can be seen in Figure 11, the magnitude of magnetoelectric effect decreases with the increase in bias magnetic field HDC (Fig. 11a), while the increase in the ME value with the frequency of HAC magnetic field (Fig. 11b) is observed. The magnetoelectric coefficient exhibits hysteresis in the range of magnetic field from 500 Oe to 2000 Oe. The magnetoelectric hysteresis was observed in other magnetoelectric materials [17]. The αME coefficient strongly decreases with increase in DC field in the range 300÷2000 Oe and above HDC = 2000, Oe is practically constant. The dependence αME vs. HDC is possibly related to the piezomagnetic coefficient dλ/dH, where λ is the magnetostriction.

For the NZF-PFN composite, the αME values grow with frequencies changing from 10 Hz to 6 kHz and reach an almost constant maximum level in the range of 6÷10 kHz. In general, magnetoelectric coupling between a magnetostrictive phase and a ferroelectric phase increases with the frequency of AC magnetic field rising up to the value corresponding to the resonance magnetoelectric effect, and then decreases [18]. However, the frequency dependencies of the magnetoelectric coefficient obtained using measurement or theoretical calculations for magnetoelectric composites by others authors are similar to our dependencies [19, 20]. The authors suggested, that the magnetoelectric coefficient in the low frequency region depended on the composite composition and conductivity of the ferrite phase.

SUMMARY

The X-ray examinations of the (1–x)NiZnFeO4–(x)Pb(Fe1/2Nb1/2)O3 composite showed a slight quantity of the pyrochlore phase besides the main composition phases (coming from PFN and NZF), and the examinations of the magnetic properties showed their significant increase (comparing to PFN).

Fig. 10. Diagrams of the Q–1(T) and Y(T) relationships for NZF-PFN composites Rys. 10. Zależności strat mechanicznych Q–1(T) oraz modułu Younga Y(T) dla kompozytu NZF–PFN

Fig. 11. The magnetoelectric coefficient ME determined for NZF-PFN composite as function of the bias magnetic field HDC (a) and as function of the f frequency of the HAC magnetic field (b). The values of HAC amplitude, frequency f of the HAC and HDC are introduced Rys. 11. Zależność współczynnika magnetoelektrycznego ME w funkcji polaryzacji pola magnetycznego HDC (a) i funkcji częstotliwości f pola magnetycznego HAC (b) dla kompozytu NZF-PFN. Odpowiednie wartości HAC, HDC oraz f podano na wykresach The conducted comprehensive examinations of the dielectric, magnetic, magnetoelectric and elastic properties showed charac-teristic anomalies, present in the phase transition area of the elec-tric sub-system (ferroelectric paraelectric) and in the area of the magnetic sub-system phase transition (ferromagnetic para-magnetic). These characteristic changes visible in the temperature relationships M(T), (T), Q-1(T) and Y(T) can confirm a mutual influence of the magnetic and electric sub-systems. In the future possibility of controlling of electric and magnetic properties by both an external electric and magnetic factor can be used for building magnetoelectric transducers.

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