JOURNAL OF APPLIED PHYSICS 113, 074103 (2013)
Ferroelectric and dielectric properties of ferrite-ferroelectric ceramic composites Cristina Elena Ciomaga,1,a) Alexandra Maria Neagu,1 Mihai Valentin Pop,1 Mirela Airimioaei,2 Sorin Tascu,3 Giorgio Schileo,4 Carmen Galassi,5 and Liliana Mitoseriu1,a) 1
Faculty of Physics, “Al. I. Cuza” University of Iasi 700506, Iasi, Romania Faculty of Chemistry, “Al. I. Cuza” University of Iasi 700506, Iasi, Romania and Dept. Chemistry & Process Engineering, University of Genoa, P-le Kennedy no. 1, I-16129, Genoa, Italy 3 RAMTECH Faculty of Physics, “Al. I. Cuza” University of Iasi 700506, Romania 4 Christian Doppler Laboratory for Advanced Ferroic Oxides, Sheffield Hallam University, Howard Street, Sheffield S1 1WB, United Kingdom 5 CNR—ISTEC, Via Granarolo no. 64, I-48018 Faenza, Italy 2
(Received 27 November 2012; accepted 1 February 2013; published online 20 February 2013) Particulate composites of ferrite and ferroelectric phases with xNiFe2O4 (NF) and (1 x)Pb0.988 (Zr0.52Ti0.48)0.976Nb0.024O3 (where x ¼ 2, 10, 20, 30, 50, 70, and 100 wt. %) were prepared in situ by sol-gel method. The presence of a diphase composition was confirmed by X-ray diffraction while the microstructure of the composites was studied by scanning electron microscopy revealing a good mixing of the two phases and a good densification of the bulk ceramics. The dielectric permittivity shows usual dielectric dispersion behavior with increasing frequency due to Maxwell-Wagner interfacial polarization. AC conductivity measurements made in frequency range 1 Hz-1 MHz suggest that the conduction process is due to mixed polaron hopping. The effect of NF phase concentration on the P-E and M-H hysteresis behavior and dielectric properties of the composites was investigated. At low NF concentration a sharp ferro-paraelectric transition peak can be observed at around 360 C while for higher NF concentrations a trend to a diffuse phase transition occurs. All the composite samples exhibit typical ferromagnetic hysteresis loops, indicating the presence of C 2013 American Institute of Physics. ordered magnetic structure. V [http://dx.doi.org/10.1063/1.4792494] I. INTRODUCTION
In the last years, the outstanding progress in miniaturization and multi-functionality of electronic devices led scientists towards composite materials.1–3 Composite materials are formed from combination of two or more single phase compounds. Physical properties of composites are determined by the properties of their constituent phases and the interaction between them.4 An important class of composites is magnetoelectric (ME) systems. The combination of a piezoelectric phase and a magnetostrictive one can produce a coupling between electric and magnetic properties, hence the magnetoelectric effect. This effect is defined as an induced dielectric polarization of a material in an applied magnetic field and/or an induced magnetization in an external electric field.5,6 The large number of possible application of ME composites, such as ME data storage and switching, microwave devices, phase shifters and attenuators, amplification and frequency conversion etc., makes them a very attractive research subject.1,6 Ferrite-piezoelectric composites exhibit unique and novel magnetoelectric phenomena which are facilitated by mechanical deformations. In this study, bulk composites with 0-3 connectivity pattern, which means Ni ferrite particles were embedded in a Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 (PZTNb) matrix, have been investigated. As a magnetic phase NiFe2O4 a)
Authors to whom correspondence should be addressed. Electronic addresses:
[email protected]. and
[email protected].
0021-8979/2013/113(7)/074103/7/$30.00
(NF) was chosen. This compound presents high piezomagnetic coefficient, large resistivity, initial permeability, and low anisotropy. It has an inverse spinel structure and does not react even at high sintering temperatures. The matrix PZTNb with a perovskite-like structure presents superior dielectric, piezoelectric, and pyroelectric properties. Niobium, Nb5þ, can be considered as a donor dopant for PZT materials, since it substitutes the Ti4þ/Zr4þ ions. It was proved that it enhances the electrical resisitivity, dielectric constant, Curie temperature and induces a better stability of properties with the temperature variation and a good stability in time, but the coupling piezoelectric coefficients are smaller. Besides the magnetoelectric phenomena, ferritepiezoelectric composites present unique electric, magnetic, and dielectric properties7,8 that depend on the structural compatibility of the components, grain size, and absence of porosity at the interfaces between the two phases. In the present work, the dielectric, ferroelectric, and magnetic properties of xNF-(1 x)PZTNb composite samples are investigated and discussed in correlation with the structural and microstructural features. II. EXPERIMENTAL DETAILS
The powder precursors and ceramic composites of xNiFe2O4-(1 x)Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 with various x ¼ 2%, 10%, 20%, 30%, 50%, 70%, and 100% concentrations were prepared by in situ processing based on sol-gel
113, 074103-1
C 2013 American Institute of Physics V
074103-2
Ciomaga et al.
method after sintering step of 1200 C/1 h. Preparation method was reported in Ref. 9. All the samples were structural characterized by a SHIMADZU XRD 6000 diffractometer ˚ ) with scan using Ni-filtered CuKa radiation (k ¼ 1.5418 A step increments of 0.02 and counting time of 1 s/step, for 2h ranged between 20 and 80 . The microstructures were investigated by using a scanning electron microscope (SEM) analysis performed with a SEM (Hitachi S-3400 N II microscope). The electrodes required for the electrical measurements were applied on both faces of the ceramic samples by applying conducting silver paint. Dielectric measurements as a function of temperature on different samples were performed using an Impedance/Gain Phase Analyzer (model: *1260, Solartron TM Instruments, Farnborough, UK*) linked to a *Carbolite (Hope Valley, UK*) tube furnace, at six different frequencies (1 MHz, 100 kHz, 10 kHz, 1 kHz, and 100 Hz) from 30 to 500 C. The P(E) loops at room temperature were recorded under a sinusoidal waveform of amplitudes E0 ¼ (0-8) kV/mm for 10 Hz frequency, by using a modified Sawyer-Tower circuit. The magnetic permeability was measured as a function of frequency using an E4991A RF Impedance/Material Analyzer, at room temperature in the range of frequency of 10 MHz-1 GHz. Magnetic hysteresis loops at room temperature were determined under magnetic fields in the range of (0-10 kOe) with a MicroMagTM magnetometer VSM (Vibrating Sample Magnetometer) model 3900 system from Princeton Measurements Co.
J. Appl. Phys. 113, 074103 (2013)
with the addition of ferrite phase in the composites. From figure 1 it is clear that we have obtained ceramic composites with no intermediate phases and also that the constituent phases do not suffer any structural changes. Figures 2(a), 2(b), and 2(c) show the SEM micrographs of some composite samples for a few representative NF concentrations, confirming the formation of two distinct phases: i.e., gray particle for PZTN and dark particle for Ni ferrite are randomly mixed together. With addition of NF phase the percolation limit is reached and there is no longer
III. RESULTS AND DISCUSSION A. Structural and microstructural studies of ferriteferroelectric composites
Typical XRD pattern is presented in Figure 1. The X-ray analysis of xNF-(1 x)PZTNb ceramic composites, with different ferrite concentrations, sintered at 1200 C reveal that all the peaks were identified for both ferrite and ferroelectric phases of the composite system (indexed using the powder diffraction files 86-2267 for NF and 33-0784 for PZTNb). It can be observed from the diffractogram that the intensity of the peaks corresponding to the NF phase gradually increases
FIG. 1. X-ray diffractograms of xNiFe2O4-(1 x)PZTNb with various x ¼ 2%, 10%, 20%, 30%, 50%, 70%, and 100% concentrations.
FIG. 2. SEM micrograph on surface and in fracture of xNiFe2O4-(1 x) PZTNb ceramic composites for (a) x ¼ 2%, (b) x ¼ 30%, and (c) x ¼ 70%.
074103-3
Ciomaga et al.
a distinct separation of the two phases (Fig. 2(c)). Thus, the microstructural analysis confirms the formation of the diphase ceramic composites with well compacted PZTNb and crystallized NF grains. The average grain size of NF dispersed in the PZTNb ferroelectric matrix is about 1-2 lm. B. Temperature-dependence of dielectric properties
The temperature dependence of dielectric constant of xNiFe2O4-(1 x)PZTNb composites (where x ¼ 2, 10, 20, 30, 50, and 70 wt. %) is illustrated in Figure 3. One can observe that the relative permittivity is found to increase with temperature, reach a maximum value at Curie temperature and after follow a decreasing trend indicating phase transition. The mobility of charge carriers increases with increasing temperature, which leads to an increase in the conductivity and polarization of the samples, hence increase in dielectric constant. The sharp and well defined peak corresponds to the transition from ferroelectric to paraelectric phase of the ferromagneticferroelectric composites. With increasing of Ni ferrite amount we have obtained a decreasing of dielectric constant and a displacement of TC for the composition with x ¼ 50% and 70% ferrite phase, a trend to a diffuse phase transition (DPT). In general, the DPT is caused by cation disorder in complex
J. Appl. Phys. 113, 074103 (2013)
perovskite and is related to the nanoscaled ordered microregions in the material. This can be explained due to the fact that the spontaneous polarization of the ordered microregions still survives above average transition temperature.10 In various ME composites from literature, the broadening of the transition has been attributed to disorder in arrangement of cation on one or more crystallographic structure, leading to a microscopic heterogeneity in composites and thus in the distribution of different localized Curie points. These can be related to the nanoscale ordered microregions existing in the material which are acting as locations of spontaneous polarization.10–13 Also, the decreasing of relative permittivity with addition of NF is attributed to the low resistivity of ferrite phase. The shift of TC from 363 C for x ¼ 2% to 329 C for x ¼ 70% ferrite content can be caused by the fact that the electric fieldinduced magnetic phase transition depends on the strength of interaction between electric and magnetic ordering, which in turn depends on the molar ratio of phases. The ferroelectric transition temperature is independent of the content of individual phases, suggesting that the ferroelectric character is maintained in the xNF-(1 x)PZTNb composite. Figure 3(b) shows the variation of dielectric loss as a function of temperature for the xNF-(1 x)PZTNb ceramic composites. The loss curves illustrate an increasing behavior with increase in temperature and with increasing of ferrite phase. Also, the increase in dielectric loss is a result of decreasing resistivity of the samples with temperature. C. Dielectric and ferroelectric properties at room temperature
FIG. 3. Temperature dependence of (a) relative permittivity (e) and (b) dielectric losses (tan d) for xNF-(1 x)PZTNb with different addition of Ni ferrite, at 1 MHz.
Figures 4(a), 4(b), and 4(c) show the variation of the complex dielectric constant (real and imaginary part of permittivity, e0 and e00 ) and loss tangent with frequency. The real part of permittivity decreases rapidly at lower frequencies (up to 103 Hz) and remains constant at higher frequencies, indicating dielectric dispersion (Fig. 4(a)). As we can observe from the frequency dependence of imaginary part of permittivity (e00 ), the dielectric properties of composites are dominated at low frequency by extrinsic effects:—Maxwell-Wagner phenomena at frequency of 1 Hz and—Debye relaxation in the range of 100 Hz-10 kHz (Fig. 4(b)). The Maxwell-Wagner relaxation mechanism is associated with uncompensated surface charges at the di-similar material (perovskite-spinel) interfaces inside the composite ceramics. When an electric field is applied to the investigated samples, the space charge provided by the ferrite phase accumulates at the interface of the two phases due to the presence of different permittivities and conductivities. The thermally activated relaxations are normally active at higher temperatures in single phase systems; it seems that by increasing the degree of inhomogeneity of the system, their activation energy is dramatically lowered and thus they manifest even at room temperature. With the addition of NF phase the real part of permittivity decreases while the dielectric losses increase. However, this is true only for ferrite concentrations lower than 30%
074103-4
Ciomaga et al.
J. Appl. Phys. 113, 074103 (2013)
x ¼ 2% Ni ferrite amount has the highest value of e0 1204 at f ¼ 10 kHz while for x ¼ 70% the system presents a significantly smaller value of permittivity of e0 109. The dielectric constant of the investigated composites decreases with the addition of x, as a consequence of the sum property and due to interfacial polarization this effect is visible for all the compositions, at frequencies higher than 1 kHz. Dielectric losses are proportional to the dielectric imaginary part of the permittivity (e00 ) and present values below 1 at 10 kHz for all the ceramic compositions and increase with NF amount (Fig. 4(c)). Similar dielectric properties were reported for PZT-Ni ferrite prepared by spark plasma sintering and in PZT-(Ni, Zn) ferrite ceramics prepared by powder-in-sol precursor hybrid processing route.11,12 The complex impedance spectra (Fig. 5) exhibit: (i) single semicircular arcs for small addition of NF (x ¼ 2%, 0%, 20%, and 30%) which indicates a good homogeneity of the dielectric and conductive properties and (ii) more than one component for x ¼ (50% and 70%) showing the local heterogeneity. The different components present in the complex impedance plot can be attributed to the regions with distinct values of conductivity and resistivity. To understand the conduction mechanism in composites and the types of polarons responsible for conduction the AC conductivity data was carried out at room temperature in the frequency range from 1 Hz to 1 MHz. The variation of AC conductivity (rAC) with frequency is shown in Figure 6(a). The plots are almost linear indicating that conductivity increases with frequency. Similar results were reported in literature.4,14 It is well known that the conduction in ferrite, ferroelectric, and their composites can be explained by polaron hopping process among the localized states. Hopping conduction is favored in ionic lattices in which the same kind of cation is found in two different oxidation states. Therefore, the hopping of 3 d electrons among Fe2þ and Fe3þ as well as between Ni2þ and Ni3þ could play an important role in the conduction process. If we look closer on the dependence of log (rAC-r (0)) with log(x2) (Fig. 6(b)) we no longer have an
FIG. 4. Frequency dependence of (a) real part of permittivity (e0 ), (b) imaginary part of permittivity (e00 ), and (c) tangent losses (tand) of xNF-(1 x) PZTNb composites with different concentration of Ni ferrite, at room temperature.
when there is no percolation effect and we have a distinct separation of the two phases. When the percolation limit is reached (in our case for x ¼ 50%) the dielectric constant increases significantly due to a major contribution from the interfacial polarization, at frequencies lower than 1 kHz. Therefore, only at high frequencies the intrinsic response of the material is expected to predominate. The composite with
FIG. 5. Complex impedance spectra of xNF-(1 x)PZTNb ceramic composites with x ¼ 2, 10, 20, 30, 50, and 70 wt. %, at room temperature.
074103-5
Ciomaga et al.
FIG. 6. (a) Frequency dependence of the effective conductivity and (b) plot of log(rAC-rDC) versus logx2 for xNF-(1 x)PZTNb ceramic composites with x ¼ 2, 10, 20, 30, 50, and 70 wt. %.
exact linear curve but just an increase of conductivity with frequency indicating mixed (small and large) polaron conduction. At lower frequencies the grain boundaries are more active and the role of polaron hopping process is minor. As the frequency of the applied field increases, the conductive phase becomes more active, thereby increasing the importance of mixed polaron hopping conduction.15 Also, the conductivity increases with addition of NF phase and for compositions higher than 30% the composites presents a semiconducting behavior rather than an insulating one. This is because when the ferrite particles make chains, the electrical resistivity of the composites is reduced significantly due to the low resistivity of the ferrite phase15 as well as the parallel connectivity between ferrite and ferroelectric grains in all composites.16 Figures 7(a) and 7(b) show the P-E loops for x ¼ 2, 10, 20, and 30 wt. % amounts of ferrite phase, at room temperature and 10 Hz frequency. The hysteresis measurements reveal that the value of remnant polarization (Pr) and the saturation polarization (Ps) of the samples decreases with increasing of Ni ferrite addition relates to the electromechanical coupling, low internal polarizability and strain.13 It can be observed that with increasing the amount of ferrite phase, the samples present a flattened loop which in general
J. Appl. Phys. 113, 074103 (2013)
FIG. 7. P-E hysteresis loop of (a) for pure PZTNb and (b) xNF-(1 x) PZTNb ceramic composites with x ¼ 2, 10, 20, and 30 wt. % at 10 Hz.
indicates compositional inhomogeneity and uniformity of the grain size of the component phases which is in good agreement with the microstructural analysis by SEM. Compared to the pure PZTNb, the investigated composites have lower coercivity since the ferrite addition was symmetrical distributed in ferroelectric matrix (Fig. 7(a)). The positive values of coercive field, remnant polarization, and their negative counter parts present an asymmetry about on the original point. This phenomenon may originate from the internal electric field that is caused by short-distance, off-center, and inherent movement traces of bound electrons in the composites. D. Magnetic properties
Figures 8(a) and 8(b) present the frequency dependence of the complex relative permeability (l0 , l00 ), for the xNF(1 x)PZTNb ceramic samples, with different concentration of NF. The permeability of nonmagnetic ferroelectric ceramics is always 1, while the soft magnetic ferrites have higher permeability. Due to the inverse proportion of permeability and cut-off frequency, the permeability turns lower in higher frequency range, which is associated with the magnetic structure of material. It could be seen that the real and
074103-6
Ciomaga et al.
FIG. 8. Frequency dependences of the real part (a) and imaginary part (b) of permeability at room temperature, in xNF-(1 x)PZTNb ceramic composites.
the imaginary parts of the relative permeability of the investigated composite samples are increased, when the percentage of ferrite is increased which indicates that the magnetization ability became stronger. The same values were reported in other different ferroelectric-ferrite ceramic systems.17–19 Figures 9(a) and 9(b) show the magnetization-magnetic field hysteresis loops of xNF-(1 x)PZTNb magnetoelectric composites, together with their saturation magnetizations and coercive fields. In Figure 9(a), all the composite samples exhibit typical ferromagnetic hysteresis loops, indicating the presence of ordered magnetic structure. The saturation magnetization (Ms) decreases from 45 emu/g for pure Ni ferrite to 32 emu/g for x ¼ 70% of Ni ferrite phase and 0.10 emu/g for x ¼ 2% (Fig. 9(b)). The remanent magnetization of the composites increases with increasing the amount of Ni ferrite phase from 0.1 emu/g for x ¼ 2% to 2.72 emu/g for x ¼ 70%. The increase in saturation magnetization with increasing NF content provides an indication that the spontaneous magnetization in xNF-(1 x)PZTNb ceramic composites originates from the unbalanced antiparallel spins (ferromagnetic character), which can give rise to the mentioned small coercivities. The coercive field (Hc) for all the composite samples is larger than that for the pure NiFe2O4
J. Appl. Phys. 113, 074103 (2013)
FIG. 9. (a) Magnetization-magnetic field hysteresis loops of xNF-(1 x) PZTNb magnetoelectric composites with different compositions and (b) their saturation magnetization and coercive fields.
phase as the results of mixing of non-magnetic phase with PZTNb phase. IV. CONCLUSIONS
Dense, homogenous, and fine-grained ferrite-ferroelectric composites consisting of NF ferromagnetic particles uniformly dispersed in a ferroelectric PZTNb matrix were prepared in situ by sol-gel method. The formation of the two phases with limited reactions at interfaces was tested by X-ray diffraction and SEM analyses. The variation of dielectric constant with temperature indicates a sharp ferroelectric to paraelectric phase transition peak at around 360 C for low NF concentrations and a diffuse phase transition at around 330 C for high amounts of ferrite in the composites. Thus, the addition of NF phase induces a microscopic heterogeneity in composites and a distribution of different localized Curie points. The dielectric losses also increase with temperature due to a semiconducting behavior of the composites at high temperatures. With respect to frequency the complex dielectric permittivity presents a dispersive behavior due to Maxwell-Wagner effect. The interfacial polarization has a major contribution for composites with high NF concentrations causing a significant increase of the real part of permittivity for frequencies below 1 kHz. So, we expect that intrinsic properties of the material to predominate at
074103-7
Ciomaga et al.
high frequencies. An interesting feature is the conduction mechanism in such composites. In this case, the variation of AC conductivity with frequency indicates that the conduction mechanisms are due to mixed polaron hopping. In what concerns the ferroelectric properties the ferrite phase acts as an inhibitor: with increase of NF concentration in the composites we obtained flattened P-E loops with smaller values of the remanent polarization. In contrast, the magnetic properties improve with addition of ferrite phase. However, the coercive field (Hc) for all the composite samples is larger than that of pure NiFe2O4 phase. An intriguing aspect is the influence of percolation between the two phases at high NF content on dielectric, ferroelectric, and magnetic properties. We demonstrated that along with interfacial polarization the percolation effect causes giant increase of permittivity at low frequencies, high dielectric losses, and diffuse phase transition of the composites. ACKNOWLEDGMENTS
This work was financial supported by the Romanian CNCSIS-UEFISCSU Project No. PN II-RU TE code 187/2010. One of the authors (Airimioaei M.) acknowledges the financial supported by POSDRU/89/1.5/S/63663 grant. The collaboration within COST Action MP0904 is highly acknowledged.
J. Appl. Phys. 113, 074103 (2013) 1
J. Ma, J. Hu, Z. Li, and C.-W. Nan, Adv. Mater. 23, 1062 (2011). L. Yan, Y. Yang, Z. Wang, Z. Xing, J. Li, and D. Viehland, J. Mater. Sci. 44, 5080 (2009). 3 R. A. Islam and S. Priya, Adv. Condens. Matter Phys. 2012, 29 (2011). 4 A. S. Fawzi, A. D. Sheikh, and V. L. Mathe, Solid State Sci. 11, 1979 (2009). 5 J. Zhai, N. Cai, Z. Shi, Y. Lin, and C.-W. Nan, J. Phys. D: Appl. Phys. 37, 823 (2004). 6 M. Fiebig, J. Phys. D: Appl. Phys. 38, R123 (2005). 7 M. Venkata Ramana, N. Ramamanohar Reddy, B. S. Murty, V. R. K. Murthy, and K. V. Siva Kumar, Adv. Condens. Matter Phys. 2010, 763406 (2010). 8 Q. Jiang, F. Liu, H. Yan, H. Ning, Z. Libor, Q. Zhang, M. Cain, and M. J. Reece, J. Am. Ceram. Soc. 94(8), 2311 (2011). 9 C. E. Ciomaga, M. Airimioaei, V. Nica, L. M. Hrib, O. F. Caltun, A. R. Iordan, C. Galassi, L. Mitoseriu, and M. N. Palamaru, J. Eur. Ceram. Soc. 32, 3325 (2012). 10 L. E. Cross, Ferroelectrics 76, 241 (1987). 11 H. Zhang, S. W. Or, and H. L. W. Chan, J. Appl. Phys. 104, 104109-1 (2008). 12 Q. H. Jiang, Z. J. Shen, J. P. Zhou, Z. Shi, and C.-W. Nan, J. Eur. Ceram. Soc. 27, 279 (2007). 13 A. S. Fawzi, A. D. Sheikh, and V. L. Mathe, Mater. Res. Bull. 45, 1000 (2010). 14 C. M. Kanamadi, G. S. R. Raju, H. K. Yang, B. C. Choi, and J. H. Jeong, J. Alloys Compd. 479, 807 (2009). 15 R. C. Kambale, P. A. Shaikh, C. H. Bhosale, K. Y. Rajpure, and Y. D. Kolekar, Smart Mater. Struct. 18, 085014 (2009). 16 J. Ryu, S. Priya, K. Uchino, and H.-E. Kim, J. Electroceram. 8, 107 (2002). 17 X. Qi, J. Zhou, Z. Yue, Z. Gui, L. Li, and S. Buddhudu, Adv. Funct. Mater. 14, 290 (2004). 18 J. Shen, Y. Bai, J. Zhou, and L. Li, J. Am. Ceram. Soc. 88, 3440 (2005). 19 Y. Bai, Ferroelectrics (InTech, 2010), Chap. 11, pp. 187–206. 2
Journal of Applied Physics is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/japo/japcr/jsp
