Ferrimagnetism and Ferroelectricity of the Composite Matrix: SrBi2Nb2O9(SBN)X-BaFe12019(BFO)100–X ()
1. Introduction
The ceramics composite matrix are uniform, multiphases materials that have been strongly studied in recent works because of the possibility to obtain materials with desirable properties, specially for application in electronics devices. This properties wouldn’t be easily observed for a single phase material, requesting extreme conditions, for example, low temperatures, to its observation thus limiting its applications.
In this work, our main goal is to develop a dielectric material that is able to respond to both electric and magnetic stimulus, i.e. that is ferroelectric and ferromagnetic. To do so, we use the Aurivillius ceramic SrBi2Nb2O9 and the Hexaferrite BaFe12O19. Such a material could be applied in the same way that common dielectrics (as dielectric resonator antennas, for example) but opening a wide range of possibilities to make the application of ceramics to electronic devices, memories and telecommunications more useful and powerful.
The Aurivillius family of compounds that can be represented by the general formula [(Bi2O2)2+(Am–1BmO3m+1)2–]in which A can be a monovalent, divalent or trivalent cation or a combination of those in suitable proportions, B can be a tetravalent or pentavalent cation with m having values of 2, 3, 4… etc. Their structures comprise a stacking of n peroviskite units of normal composition Am–1BmO3m+1 interleaved with Bi2O3 layers along the pseuedotetragonal c-axis known as bismuth layered ceramics (BLFC).
They are potential materials for microelectronic applications when desirable properties are reached such as high dielectric permittivity, high Curie temperature, high fatigue resistance [1-3]. However, dielectric loss is still too high for practical applications for electronic devices. Fortunately, the structure of peroviskite allows easy replacement of cations and combinations among phases as well.
Recently, some researchers reported additions of impurities in bismuth-layered composites [1-3]. This work focused on a new alternative for a composite matrix by using barium hexaferrite.
M-type ferrite has a hexagonal structure and has been largely used for ferrofluids and magnetic devices. It has some interesting properties such as high saturation magnetization and high Curie temperature and has attracted interest for many years. By combining ferroelectric SBN ceramics and M-type ferrite, it was possible to produce the magneto-dielectric composite SrBi2Nb2O9(SBN)X- BaFe12O19(BFO)100–X. By bringing these two materials together, it is expected to develop a bi-phase material, which would be ferroelectric and ferromagnetic. To do so, it would require SBN not to interact with BFO changing their characteristic properties, and the original phases can remain in the composite. The microstructure investigation is able to reveal whether the materials interact with each other forming new phases or not. If there is no undesirable chemical interactions between SBN and BFO, the new bi-phase ceramic will be formed and it will probably be ferroelectric and ferromagnetic.
The behavior of this new class of ceramics is not clearly known so far. Considering such context a precise investigation should be carried out for each composite combination. These matrix composites have interactions with to magnetic and electric fields which are not completely understood because of cumbersome procedures for getting both ferroelectric and ferromagnetic properties under simple physical conditions. By using a composite matrix it is possible to mitigate those drawbacks and two non-interacting ferromagnetic and ferroelectric ceramics would provide magneto-dielectric properties. In this work, we have studied the microstructure, dielectric and magnetic properties of SrBi2Nb2O9(SBN)X-BaFe12O19 (BFO)100–X matrix.
2. Experimental Procedure
SBN (SrBi2Nb2O9) were synthesized from stoichiometric quantities of Bi2O3 (99.99%), Nb2O5 (99.99%), SrCO3 (99.99%) and BFO (BaFe12O19) from BaO (99.99%) and Fe2O3 (99.99%) derived from solid-state reaction method. Both reactions of the uncalcined powders are shown below:
Bi2O3 + Nb2O5 + SrCO3 → SrBi2Nb2O9 + CO2 BaO + 6Fe2O3 → BaFe12O19
Bi2O3, Nb2O5, SrCO3 for SBN have been mixed and ball-milled with zirconium balls in a polyethylene container by using a planetary mill for 8 hours, in order to lower it’s grain size and enlarge it’s surface area aiming to achieve more reactive powders. The same calcination conditions were carried out for ferrite for 2 hours. SBN and BFO samples were calcinated at 800˚C for 5 hours and at 1000˚C for 24 hours, respectively.
SBN and BFO powders were mixed as the following rule SBNxBFO100–x (x = 0, 25, 50, 75 and 100 wt%). Three series of samples were produced for different binders, namely polyvinyl alcohol (PVA), glycerin and tetraethyl orthosilicate (TEOS). The two powders in the defined proportions are mixed with 3 wt% of one of the binders. The powders were used to make cylindrical pellets under an uniaxial pressure of about 0.1 MPa for 5 minutes using a hydraulic press.
The bulks were sinterized at 1050˚C for 5 hours. Samples were named in proportion of BFO and SBN (BFO100 -100% BFO, BFO75-75% BFO + 25% SBN, BFO50- 50% BFO + 50% SBN, BFO25-25% BFO + 75% SBN and SBN100-100% SBN) and also depending on the binder’s name used during their formation (P, T, G, polyvinyl alcohol, tetraethyl orthosilicate and glycerin respectively). The sintered pellets were polished with fine emery paper in order to make both the surfaces flat and parallel and were electroded with high-purity silver paste for dielectric and electrical measurements to ensure a good electrical contact by using an impedance analyzer to cover the frequency range of interest.
The samples powders were analysed using Mössbauer spectroscopy and X-ray diffraction, both at room temperature. The Rietveld analysis was performed by the refinement program DBWS. The bulk samples were analized with Raman and infrared spectroscopy and Scanning electron microscope, for investigation of the structure and radiofrequency measurements, as well as, magnetic and electric hyteresis for investigation of the electromagnetic properties.
2.1. X-Ray Diffraction
The X-ray diffractograms were obtained at room temperature, by using non-sinterized powder samples and a Siemens D501 graphite-monochromator diffractometer using a Cu-K radiation detector (k = 0.1542 nm) with a Bragg-Brentano-geometry scintillation counter operating at 40 kV and 25mA. The ferrite powders were scanned through the 2θ angle ranging from 20˚ to 85˚ and the SBN powder ranging from 20˚ to 70˚ during five seconds for each step of counting time. The Rietveld analysis was performed by the refinement program DBWS. A Modified Thompson-Cox-Hasting pseudo-Voigt profile function was used to fit the calculated curve into the experimental diffraction data. The obtained density with such refinement was used for calculating the relative density (%) of the samples by the Archimedes method.
2.2. Raman and Infrared Spectroscopy
Infrared spectroscopy was obtained with a Mattson 7000 (FTIR) spectrometer, while Raman spectrum was obtained with a FTIR spectrometer VERTEX 70 equipped with RAM II Bruker FT-Raman module adapted for the rough surfaces (diffusion reflection spectroscope EasyDiff), using bulk sintered samples in both cases.
2.3. SEM
Scanning electron microscope (SEM) images for microstructure observation of the sintered pellets were obtained with a scanning electron microscope Vega XMUT/Tescan, Bruker.
2.4. Radiofrequency Measurements
Both surfaces of pellets were painted with silver paste for impedance at radiofrequency measurements. The permittivity and loss tangent were measured by using Agilent E4991A RF impedance/material analyzer connected to a personal computer, for frequency ranging from 1 MHz to 3 GHz, at room temperature.
2.5. Magnetic Hysteresis Measurements
For the measurement of the magnetic moment (M) versus the applied magnetic field (H) a VSM (vibrating sample magnetometer) was used. All measurements were done at room temperature. Magnetization was normalized by the mass of each sample in order to provide M in emu/g. The samples were measured over a maximum applied field of 8000 Oersted.
2.6. Electric Hysteresis Measurements
The electric hysteresis were obtained from a system composed by a Sawyer-Tower circuit connected to a Agilent 54622A digital oscilloscope, a Agilent 33220A function generator and a Trek 610E high-voltage font. All hysteresis loops were done at room temperature with disc shaped samples.
2.7. Mössbauer Spectroscopy
Mössbauer measurements were performed at room temperature by a FAST (ConTec) Mössbauer Systems spectrometer through transmission geometry and radioactive source of 57Co in Rh matrix and isometric shifts are related to metallic iron α-Fe. The NORMOS program was used for adjusting iron sites in Barium hexaferrite structure and for determining hyperfine parameters by using powdered samples.
3. Results and Discussion
3.1. X-Ray Diffraction
The X-ray diffraction patterns for BFO100, BFO50 and SBN100 are shown in Figure 1. The diffraction peaks presented by BFO100 sample were identified by JPCDS (file 78-0133), and powder diffraction patterns of BaFe2O4 located at 33, 16˚ and 46, 61˚ were identified by JPCDS
Figure 1. X-ray diffractograms for SBN100, BFO50, BFO100 samples.
(file 20-0132). Some low-intensity peaks observed at 2θ < 30˚ were attributed to magnetite identified by JPCDS (file 76-0958). Mali and Ataie [4] suggest that the formation of barium hexaferrite comes from the reaction of intermediate phases of iron oxide and a spinel monoferite BaFe2O4, and the percentage of crystalline BaFe12O19 grows monotonically as increasing the temperature. This would explain the residual phases and a pure hexaferrite would be obtained for higher calcination temperatures. The presence of residual phases for barium hexaferrite was also reported by other authors [5]. The crystal structure found has hexagonal symmetry belonging to the P63/mmc space group. The average crystallite size was calculated by using the Debye-Scherrer’s equation around 37, 16 nm.
For the SBN100 sample all the diffraction peaks were indexed according to JCPDFS (file 49-0607), without residual phase evidences. The crystal structure found is orthorhombic belonging to the A21am space group with crystalline size estimated about 45 nm.
The diffraction peaks for the present composite shows only a combination of the diffraction of the two base compounds. The representative peaks for SBN100 and BFO100 and spinel monoferrite were indexed by using S, B and F letters respectively.
3.2. Rietveld Refinement
Rietveld Refinement was performed only for the SBN100 and BFO100 samples.
The standard R-factors Rp, Rwp, Rexp, and goodness-offit parameter S, as well as the lattice parameters (a, b, c), the unit cell volume (V) and the density are given in Table 1. A low percentage of Rwp and S value close to unit (<1.3 normally) indicates that the refinement was successful. Better results were obtained to SBN100 refinement. The density and volume of unitary cell were close to values reported [3] and also from PDF database.
To the BFO, the presence of a small amount (~0.04%) of secondary phases was not considered in the refinement which increases the refinement parameters values. The density, unit cell and volume presented here are in good agreement with the expected results (JPCDS file peaks number 78-0133) already reported [6,7].
3.3. Archimedes Method
The relative densities of the present samples were measured by Archimedes method and are shown in Table 2. The expected density was achieved and better densification is related to TEOS binder use. The only difference between using TEOS, Glycerin and PVA as binders is the densification factor of the samples, since the organic
Table 1. Rietveld refinement parameters.
Table 2. Relative density of the samples obtained from the Archimedes method.
material should vanish around 500˚C and it is supposed not to interfere with the material atomic arrangement. Therefore, no structural differences should be observed, even though electrical and magnetic properties are influenced if any consistent change is observed as causing undesirable effects.
3.4. Raman Spectroscopy
Figure 2 shows the Raman spectra for the composites, SBN100T, and BFO25T, BFO50T, BFO75T and BFO100T, respectively. The characteristic bands for SBN were all in agreement with data from the literature [8]. It is usual to all Aurivillius ceramics, once they are peroviskite like materials, a vibration mode over the 800 cm–1 - 900 cm–1 frequency range (associated to an octahedral sub-structure). The 835.95 cm–1 mode was identified to be associated to NbO6 octahedral stretching mode A1g. The 585 cm–1 mode is associated to Eg modes related to the stretching of oxygen bonds in the octahedron. The lower frequency modes are probably related to lattice vibrations or backscattered photons.
A detailed analysis of hexaferrite vibration modes was carried out by Kreisel [9]. According to his work, 683.7 cm–1 mode was clearly observed, which is related to the symmetric stretching of bi-pyramid shaped sub-structure of FeO5. This mode is commonly between 670 - 710 cm–1 for M-type ferrite. This is the strongest band in fer-
Figure 2. Raman spectra for the SBN100T, BFO75T, BFO50T, BFO25T, BFO100T samples.
rites containing such structure. No other Raman active modes are expected for M-type ferrite above 800 cm–1 frequency, thus 845.3 cm–1 band should be related to BaFe2O4 monoferrite. Some expected bands for BaFe12O19 are not observed in spectrum [9], this fact is a conesquence of strong noise in the spectrum, hiding lower intensity bands. Opaque and dark samples usually show this sort of spectrum in FT-Raman measurements.
The composite spectra show no new band, indicating that the vibration modes remain invariant when the two materials are mixed. An apparent displacement of the 585 cm–1 SBN band in composite samples may be a result of insufficient resolution to identify this band on SBN from 683.7 cm–1 band on BFO. The bands are located at the same frequency, just varying their intensities proportionally to the sample composition, showing that both are non-interacting materials and that they form a bi-phase ceramics.
3.5. Infrared Spectroscopy
Figure 3 shows the IR spectra. The NbO6 structure could be related to the 600 - 640 cm–1 bands which are expected for octahedral stretching. The bismuth layers bands are hidden once they would be located about the same frequency range where NbO6 octahedral stretching bands is found, and are wide enough to hide bismuthlayer bands.
Similar results were already reported for the ferrite spectra [10,11-13]. The small displacement of the bands could be related to different methods of sample preparation [11-13]. The identified bands for barium hexaferrite in this present work are 609.4 cm–1, 430 cm–1 and 543.89 cm–1, which are all in good agreement with the characteristic bands for crystalline BaFe12O19 (552, 434 and 583 cm–1) [11-13]. These bands are related to Fe-O vibration bonds. Again, the composites did not apparently show new bands.
3.6. Mössbauer Spectroscopy
The Mössbauer Spectroscopy was carried out only for BFO100, since there is no iron atoms in SBN and the presence of strontium atoms caused great difficulties to obtain the spectra when this material is added into ferrite.
Figure 4 shows five observed sextets related to a char-