Preparation and characterization of BaTiO3+MgCuZnFe2O4 nanocomposites

Preparation and characterization of BaTiO3+MgCuZnFe2O4 nanocomposites

Journal of Magnetism and Magnetic Materials 341 (2013) 112–117 Contents lists available at SciVerse ScienceDirect Journal of Magnetism and Magnetic ...

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Journal of Magnetism and Magnetic Materials 341 (2013) 112–117

Contents lists available at SciVerse ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Preparation and characterization of BaTiO3+MgCuZnFe2O4 nanocomposites M. Kanakadurga, P. Raju, S.R. Murthy (n) Department of Physics, Osmania University, Hyderabad 500007, India

art ic l e i nf o

a b s t r a c t

Article history: Received 25 September 2012 Received in revised form 6 April 2013 Available online 17 April 2013

MgCuZn ferrite and BaTiO3 powders with the crystallite sizes 88 nm and 82 nm were prepared using a high energy mechanical milling and sintering method. The prepared powders were characterized using X-ray diffractometer (XRD), Fourier transform infrared spectrometer and scanning electron microscope (SEM). The nanopowders were mixed to obtain the composites with composition xBaTiO3+(1−x) Mg0.48Cu0.12Zn0.4Fe2O4 (where x ¼ 0–1) using a mechanical milling. The presence of ferroelectric (BaTiO3) phase and ferrimagnetic (MgCuZn ferrite) phase has been confirmed using XRD and SEM. Ferroelectric hysteresis loops and magnetic hysteresis loops have been recorded at room temperature. In polarization– electric-field curves (P–E), the remanent polarization and coercive fields display little asymmetry. When the amount of ferrite phase is increased, the ferroelectric coercive field also increases. The saturation magnetization decreases with an increase of phase fraction of BaTiO3, because the interaction between magnetic grains is weakened by the existence of nonmagnetic (ferroelectric) phase that is distributed in the magnetic phase. The electrical properties were measured on the composites at 1 MHz. The static magnetoelectric (ME) voltage coefficient (dE/dH)H was measured by change in ME output voltage with respect to dc bias magnetic field at a constant applied magnetic field. & 2013 Elsevier B.V. All rights reserved.

Keywords: Composites Ferrite Ferroelectrics Mechanical milling Magnetic properties Magnetoelectric coefficient

1. Introduction Ferroelectric–ferromagnetic composite materials have attracted wide attention from many researchers because of their interesting electromagnetic properties and magnetoelectric effects [1–3]. Ferrite–ferroelectric composite materials can provide both inductance and capacitance. Hence, these materials can be used to design and produce the passive electromagnetic frequency interference (EMI) filters, integrating inductive and capacitive elements in one package [4,5]. These components have intensive industrial applications for suppressing electromagnetic frequency interference in electronic circuitry. The filters performance can be optimized by adjusting the inductive and capacitive properties of ferrite–ferroelectric composite materials through compositional variation [6]. Compositional variation means, customizing filters properties. The important factor, in the preparation of capacitive– inductive composites for multilayer chip inductors and EMI filters, is the selection of proper composition of ferrite and ferroelectric materials [7]. It is also important that the chemical reactions between two constituent materials should not take place during the preparation of a composite material [8,9]. This will help to stop the degradation of dielectric and magnetic properties of the composite. High energy mechanical milling, a physical method,

(n )

Corresponding author. E-mail address: [email protected] (S.R. Murthy).

0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.04.037

is chosen for preparing the materials constituting the composite, since other methods such as chemical methods involve chemical manipulations and rections during the preparation of the constituents of the composite, which may influence the degradation of magnetic and electrical properties [10,11]. Moreover, for physical methods, the raw materials used for the preparation of the constituents, are more economical when compared with those of other chemical methods. With the rapid development of mobile communication and information technology, passive electronic components, such as surface mount devices (SMD), with small size, high efficiency and low cost, are required [12]. Chip inductors, one of the passive SMD, are important components for the electronic products such as notebook computers, hard disk drive, video cameras, mobile phones, etc., which require small dimensions, light weight and better functions [12–15]. As a result of fast development of wireless communication industry, mobile handsets continue to shrink in size and simultaneously offer increasingly complex functions. This raises the requirements both for miniaturization and performance on all components used. In particular, multilayer chip inductor (MLCI) needs to be made even smaller in size while providing high performance for high frequency circuit applications. The traditional wire wound inductors, with no magnetic shielding, can be miniaturized to a certain limit [9]. This leads to the development of new materials for MLCI. From the earlier work done on ferrites, NiCuZn ferrites and MgCuZn ferrites were found to be suitable materials for the use in

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MLCI applications due to better electromagnetic properties [16– 19]. However, MgCuZn ferrite was found to be a better material than NiCuZn ferrite for MLCI, owing to its high resistivity, Curie temperature and low cost. Moreover, the magnetostriction constant for MgCuZn ferrite is lower than that of NiCuZn ferrites and hence, the ferrite possesses better magnetic properties [20–22]. As a result, further miniaturization of multilayer chip inductors can be achieved with MgCuZn ferrites [19]. Multiferroic composite materials that display a coexistence of ferroelectric and ferromagnetic responses attracted the current interest because of their potential for several novel device applications such as high frequency MLCI applications, EMI filters and sensors etc [7]. In a multiferroic composite, electromagnetic coupling is facilitated by elastic interaction between ferroelectric and ferrimagnetic components via piezoelectric effect and magnetostriction [7]. In the current work, the constituent materials selected were: BaTiO3, a ferroelectric material with large piezoelectricity and Mg0.48Cu0.12Zn0.4Fe2O4, (MCZ ferrite) a ferrimagnetic material with low magnetostriction, so that composites of BaTiO3– Mg0.48Cu0.12Zn0.4Fe2O4 combine the ferroelectricity and ferrimagnetism. When both ferrimagnetic phase and a ferroelectric phase coexist in a single material, novel properties such as magnetoelectric, magneto-optic and other coupling mechanisms are expected due to interaction between the magnetization and electric polarization [4,5]. The possibility of these interesting coupling effects motivates us to study ferrite–ferroelectric composites. In the present investigation, the composites were prepared using a high energy mechanical milling method. To our knowledge, there are no reports on the preparation of MgCuZn ferrite and BaTiO3 nanocomposites using the high energy mechanical milling method and their electrical and magnetic studies. The prepared samples were characterized using XRD, FTIR and SEM. Ferroelectric hysteresis loops and magnetic hysteresis loops have been recorded at room temperature and the obtained results are presented in this paper.

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conditions mentioned above. The powders were milled for 40 h to reach a steady state where the particles have become homogenized in size and shape. The milling parameters (speed, milling time and purity of chemicals) have been controlled carefully to reduce the defects and then to obtain nanocomposites with desired morphology. The 40 h milled powders were uniaxially pressed into toroids and pellets and then sintered at 850 1C/2 h in air atmosphere. X-ray diffractometer (XRD) with CuKα radiation was used to identify the structure of the composites. The bulk density of the sintered composites was measured using the Archimedes principle. Magnetic measurements were carried out using vibrating sample magnetometer (VSM; Lakeshore, Model 7404). Ferroelectric hysteresis loops were recorded under an alternative electric field using a ferroelectric test system. The microstructure of sintered composite materials was studied using scanning electron microscope (SEM; Model JEOL, Tokyo, Japan). For electrical measurements, the disk samples were coated with silver paste on both sides. The dielectric constant (ε), dissipation factor (D), initial Permeability (mi) and quality factor (Q) of the sintered composite specimens were measured using LCR meter (Kokuyo Electric Co., Japan model no. KC-605) at 1 MHz. The composites are poled electrically and magnetically before measuring the magnetoelectric coefficient (ME effect). The electric poling was carried out in a 1.5 kV/cm dc field, during constant cooling of samples from 140 1C to room temperature. Magnetic poling was done at a constant dc magnetic field (5 kOe) for 20 min by mounting the sample centrally in between the pole pieces of a dc electromagnet using a sample holder. The stray charges developed during poling were removed by grounding the plates of the sample holder. The ME signals were measured by means of electric potential that is developed across the sample, as a function of applied increasing dc magnetic field. The output ME voltage generated in the sample was measured using Keighley's electrometer (Model 614). The static ME voltage coefficient (dE/dH)H is measured by change in ME output voltage with respect to dc bias magnetic field at a constant applied magnetic field.

2. Experimental method For the preparation of Mg0.48Cu0.12Zn0.4Fe2O4 (MCZ), a mixture of Fe2O3 (99.8% purity), CuO (99.7%), MgO (99.0%) and ZnO (99.9%), all from Aldrich, were used as received without further purification. The required amounts were weighed and mixed accordingly in a vial to achieve the stoichiometry. Mechanical milling was carried out for 20 h in the hardened WC vial together with 10 12 mm WC balls, using a Retsch Co. high energy planetary ball mill. Ball to powder mass charge ratio of 14:1 was chosen. The speed of the mill was set at 400 rpm with interval at 40 min. The powders were analyzed by Fourier Transform Infrared spectrometer [(FTIR, Brucker tensor 27] at intervals of 5 h grinding time to confirm the ferrite phase formation. Finally, the powder grounded for 20 h, has been examined from the X-ray diffraction (XRD) spectrum using X-Pert PAN Analytical diffractometer. Pure barium oxide (BaO) and titanium dioxide (TiO2) powders were taken in stoichiometric ratio and mixed in the mechanical mill for the preparation of BaTiO3 (BT). The total grinding time used was 16 h. At the end of 5 h, 10 h and 16 h grinding time, the powders were analyzed using XRD and FTIR. In this case also, ball to powder mass charge ratio of 14:1 with mill speed of 400 rpm were used. The prepared powders of MCZ and BT were mixed at different mol%, to obtain composites, xBaTiO3+(1−x)Mg0.48Cu0.12Zn0.4Fe2O4 {where x ¼[0 (MCZ), 0.2 (MCZBT1), 0.4 (MCZBT2), 0.5 (MCZBT3), 0.6 (MCZBT4), 0.8 (MCZBT5), 1.0 (BT)]}. The mixed powders were milled in a Retsch Co. high energy planetary ball mill for 10 h, 15 h, 20 h and 40 h under air atmosphere with the same milling

3. Results and discussion The formation of spinel structure in the ferrite sample, can be confirmed using FTIR spectral analysis [23]. Fig. 1 shows the FTIR spectra of MgCuZn ferrite powder at different grinding times (10 h, 15 h and 20 h). For 10 h and 15 h milled sample, the spinel cubic structure bands are not formed, suggesting that it is a intermediary phase consisting of Mg, Cu, Zn, and Fe, which nucleates at interfaces and grows by interdiffusion under interfacial metastable equilibrium. For 20 h milled powders, the formation of cubic spinel structure is confirmed. The vibrational frequencies of IR bands of MCZ ferrite are observed in the range ν1 ¼ 582 cm−1 and ν2 ¼443 cm−1, which are attributed to tetrahedral and octahedral sites of the spinel structure [23,24], are in well agreement with the reported values [25]. The figure also reveals the confirmation of metal–oxide bonding of MgCuZn ferrite [26,27], in which the absorption bands vary with the milling time of the samples. A change in the intensity of absorption bands as seen from the FTIR spectra is attributed to the difference in the ferrite phase formation during milling. The additional bands observed in the figure are due to partial phase formation. It is observed from the figure that there exist prominent bands near 3400 cm−1 and 1450 cm−1, suggesting the presence of considerable amounts of water (H2O) and OH−1 in the sample [28] which are attributed to the stretching modes of H–O–H bending vibrations of free or absorbed water [29,30].

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Fig. 1. FTIR of Mg0.48Cu0.12Zn0.4Fe2O4 powder milled at different hours.

Fig. 2. FTIR of BaTiO3 powder milled at different hours.

Fig. 2 gives FTIR spectra of as prepared BaTiO3 powder after 10 h and 16 h grinding. From the figure, it is clear that there are two absorption bands, due to stretching vibrations of Ti–OI along polar c axis, in the wave number range 600–470 cm−1 and OI–Ti–OII and a bending vibration of TiO6 octahedra in the range 470– 375 cm−1[31]. The two bands with center positions near 582 cm−1 and 442 cm−1 were found to be corresponding to Ti–OI vertical stretching vibration and OI–Ti–OII bending vibration of TiO6 octahedra of BaTiO3 [32]. The intensities of peaks at 3488 and 1443 cm−1, which are due to O–H–O stretching and bending vibrations of water respectively [29,30], were significantly reduced after 16 h grinding. Fig. 3(a) gives XRD pattern of MgCuZn ferrite. The spectrum of the sample was recorded at room temperature using CuKα radiation (λ¼ 1.5418 Å), in the 2θ range 10–801 with step size 0.031 operating at 40 kV and 30 mA. The X-ray diffraction pattern of the sample shows good crystallization with seven characteristic peaks (2 2 0), (3 1 1), (2 2 2) (4 0 0), (4 2 2), (5 1 1) and (4 4 0), which reveals the formation of single phase cubic spinel structure and the peaks are tallied with the reported values of JCPDS card no. 080234. Crystallite size has been calculated using Scherer's equation (Dm ¼Kλ/β cos θ, K is a constant, λ wavelength of x-rays, β the full width half maximum and θ the diffraction angle), and the value is found to be 88 nm. The XRD analysis of Fig. 3(b) shows that the BaTiO3 sample is pure perovskite structure and no pyrochlore-type phase is formed. The d-spacing values are indexed to the obtained peak values and it is also observed that the obtained powder has cubic perovskite structure, because no peak splitting occurs at 2θ—45o and 50.8o as a result of tetragonal phase. All the XRD peaks of BaTiO3 match well with the reported values of JCPDS card no. 50626. Using Scherrer's equation, the crystallite size was calculated to be 82 nm. The XRD patterns of 20 h milled MCZ, 16 h milled BT powders sintered at 850 1C/2 h and xBaTiO3+ (1−x)Mg0.48Cu0.12Zn0.4Fe2O4 sintered composites are shown in Fig. 4. The composites are composed of MCZ ferrite phase with spinel structure and BT phase with perovskite structure. The intensity of the major peaks, such as (311) for MCZ and (110) for BT, depends on the amount of their individual phase fraction in the composite sample. With an increase of x, the intensity of BT peaks increases while the intensity of ferrite peaks decreases. Fig.5 gives the SEM photographs along with EDAX, for the samples. It can be seen from the figure that there are BT grains (white regions) and MgCuZn ferrite grains (black regions). Energydispersive X-ray spectroscopy (EDAX) has been used to confirm the grains of these two phases. It can be seen from the figures that, in the white regions the BT element concentration is higher and in the black regions the Fe element concentration is higher. Since the cell structures of two materials are different, the two phases coexist forming a composite material. From the images the average grain size of the composites is found to be in the range 98–156 nm (Table 1). Table 1 gives the values of bulk densities for the composites. The bulk density linearly decreases with increasing x. From the appearance of the sintered samples, no mismatch has occurred in the composites, which indicates that the two phases have better cofiring properties. The lattice constant of ferrite phase increases from 8.239 Å to 8.443 Å with decreasing BT phase in the composite. The average porosity of the composites has been estimated to be 8% from the theoretical and bulk density values. The dc electrical resistivity (ρdc) of all the samples has been measured using the four probe method and results are presented in Table 2. It is clear from the table that on increase of BT phase, the resistivity increases up to x ¼0.5 (MCZBT3) and for x>0.5, it decreases. Table 2 gives the values of dielectric constant (ε), dissipation (D), initial permeability (mi ) and quality factor (Q), at

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Fig. 3. XRD patterns of (a) MCZ ferrite and (b) BaTiO3 as prepared powder.

Fig. 4. XRD of BaTiO3 and Mg0.48Cu0.12Zn0.4Fe2O4 composite samples.

1 MHz and at room temperature, for the sintered composite samples. It can be seen from the table that pure BT sample with no ferrite component, had relatively high dielectric constant and loss. With increasing ferrite content for the composite samples, the dielectric constant and loss decreases. The presence of ferrite phase has diluted the ferroelectric property of the composites, resulting in the reduction of the dielectric constant. From Table 2, it is clear that, on increasing the ferroelectric phase in the composites, the initial permeability (μi) decreases and quality factor (Q) increases at room temperature. Fig. 6 shows magnetic hysteresis loops of MCZ, MCZBT1, MCZBT2, MCZBT3, and MCZBT4 samples at room temperature. The samples exhibit typical magnetic hysteresis, indicating that the composites are magnetically ordered. The values of saturation

magnetization (MS) and coercivity (HC) are obtained from the hysteresis loops and are presented in Table 1. It can be seen from the table that the coercivity increases with the increase of BT component up to x ¼0.4 (MCZBT2), which indicates that the magnetization ability becomes weak because of the existence of nonmagnetic BT phase, in which domain wall pinning occurs, which leads to the increase of coercivity. For x>0.4, the coercivity decreases indicating that no domain wall pinning occurs. The saturation magnetization of the composites linearly decreases with the increase of BT phase. Our results are consistent with the Bruggeman theory [1,10,21]. According to this theory, given the properties of the individual components ε1, m1, and ε2, m2, predictions for effective dielectric and magnetic properties of the composite material that consists of the two component (MCZBT in the present case) phases, are best described by the equation f {(ψ1−ψeff)/(ψ1+2ψeff)}+(1−f){(ψ2−ψeff)/ (ψ2+2ψeff)}¼ 0, where, f is the volume fraction, ψ1 and ψ2, the frequency-dependent complex permittivity or permeability of the individual composite constituents, and ψeff the effective complex permittivity or permeability of the composites [1]. In the present situation, the above equation can be written as {3(1−f)/[2+ψ2/ψeff]}+3f/2 ¼ 1, where, ψ1 is the permeability of one phase (BT in the present case), ψ2 the permeability of second phase (MCZ), and ψeff the effective complex permeability of composites. The ψ1 value of BT is unity because of its inherent nonmagnetic nature. The above equation is deduced based on the assumptions that the ferrite phase is dominant and ψeff≫1. However, because BT predominates, the ψ1:ψeff ratio cannot be neglected. From the above theory, it appears that with an increase of nonmagnetic volume fraction (f), i.e., the increase of BT content, the effective permeability (ψeff) of the composites decreases. As the magnetization is in proportion to the square root of effective permeability, the magnetization also decreases with an increase of BT phase. Fig. 7 shows the ferroelectric hysteresis loops for the composites at room temperature. It can be seen from the figure that the composite samples exhibit typical ferroelectric hysteresis which indicates that the samples are spontaneously polarized. The ferroelectric behavior of the hysteresis loops gradually weakens with the decrease of BT content, which is due to relatively lower electrical resistance of MCZ ferrite. The ferroelectric coercive field (EC) decreases with an increase of BT content (Table 1), which implies that the composites are being easily polarized under the applied electric field. Fig. 8 gives dependence of the ME conversion on dc magnetic field for the composite samples. It can be seen from the figure that the magnetic bias dependence of dE/dH is found to be similar for all the composites. From the figure it is clear that, for MCZBT5, the dE/dH increases with an increase of magnetic field upto 700 mV/

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Fig. 5. SEM pictures of (a) BT, (b) MCZBT3, and (c) MCZBT5 samples.

Table 1 Room temperature data of the composites. Sample name

MCZ MCZBT MCZBT MCZBT MCZBT MCZBT BT

1 2 3 4 5

Ferrite lattice constant (a), Å

8.465 8.443 8.435 8.418 8.271 8.239 –

Ferroelectric lattice constant a, Å

c, Å

– 4.018 4.026 4.014 4.018 4.024 4.022

– 4.021 4.010 4.019 4.013 4.015 4.013

Grain size (Dm), nm

Bulk density g/cm3

MS (emu/g)

HC (Oe)

Ec (kV/cm)

80 98 110 128 136 156 75

5.254 5.085 5.010 4.952 4.742 4.641 4.837

21.18 16.53 12.33 7.51 3.79 0.16 –

11 12 14 11 10 4 –

– 8.78 6.16 4.75 2.56 0.89 –

Table 2 Electrical properties of BaTiO3+Mg0.48Cu0.12Zn0.4Fe2O4 composites at room temperature. Sample name MCZ MCZBT MCZBT MCZBT MCZBT MCZBT BT

1 2 3 4 5

d.c. Resistivity (  109 Ω cm)

ε D (1 MHz) (1 MHz)

mi (1 MHz)

Q (1 MHz)

4.19 4.37 5.92 6.87 5.55 4.06 4.88

58 78 85 95 108 110 180

850 610 540 425 310 – –

0.70 0.96 1.45 2.65 4.56 – –

0.54 0.65 0.72 0.88 0.93 1.19 1.98

cm Oe and decreases with further increase of H and finally saturates at a bias level. The highest ME value was obtained for 1.5 kOe dc field indicating that the magnetostrictive phase has reached a saturation value producing constant electric field in the piezoelectric phase, hence making dE/dH decrease with increase of magnetic field. This shows that the magnetic saturation occurs at low stimulation and the samples are best suited in response to relatively weak magnetic field. The behavior of the magnetic field dependence of ME voltage coefficient is similar to that for the magnetostrictive behavior [22,33]. It is well known that the individual ferrite and ferroelectric materials have no magnetoelectric effect, but, their composites have coupled magnetic–electric effect due to the elastic interaction between the two phases. The magnetoelectric effect is observed as

Fig. 6. VSM data of BaTiO3+Mg0.48Cu0.12Zn0.4Fe2O4 composite samples.

a product property of the composites and it was strongly influenced by the connectivity between the particles and the mole ratio of the two phases. The maximum dE/dH depends nonmonotonically on the volume fraction of MCZ ferrite, since the ME effect is a product property between piezoelectricity and

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mechanical mill. XRD and SEM results show that MCZ phase and BaTiO3 phase coexist in the composite materials. The ferrite phase gradually disappears with the increase of BT phase. The composites exhibit ferroelectric hysteresis loops and magnetic hysteresis loops at room temperature under applied external electric field and magnetic field respectively. The saturation magnetization of the composites decreases with the increase of BT phase. The present composites possess high density and fine microstructure, which indicate the better cofiring properties between BT and MCZ ferrite.

Acknowledgment Authors are thankful to UGC-BSR, New Delhi for sanctioning funds to carry out this work. References Fig. 7. P–E loops of BaTiO3+Mg0.48Cu0.12Zn0.4Fe2O4 composite samples.

Fig. 8. The variation of (dE/dH)H with applied magnetic field for the composite samples.

magnetostriction. The maximum magnetoelectric voltage coefficient of the composites is observed for low fraction of the ferrite (MCZ BT5) sample. With an increase of ferrite content, the value of dE/dH decreases, though the magnetostrictively induced strain of the composites increases with ferrite phase. The decrease in dE/dH is due to a higher concentration of the low resistant ferrite phase, which results in a low piezoelectric constant, and on the other hand provides the conducting path for the charges developed in the ferroelectric phase [33,34]. 4. Conclusions For the first time a series of Mg0.48Cu0.12Zn0.4Fe2O4+BaTiO3 nanocomposites that have ferrimagnetic phase and ferroelectric phase have been prepared successfully using high energy

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