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Magnetic field induced polarization and magnetoelectric effect in Na0.5Bi0.5TiO3–Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite
Accepted Manuscript Magnetic field induced polarization and magnetoelectric effect in Na0.5Bi0.5TiO3 − Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite S.R. Wadgane, S.T. Alone, Asif Karim, Gaurav Vats, Sagar E. Shirsath, R.H. Kadam PII: DOI: Reference:
S0304-8853(18)32337-0 https://doi.org/10.1016/j.jmmm.2018.10.011 MAGMA 64422
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
26 July 2018 15 September 2018 2 October 2018
Please cite this article as: S.R. Wadgane, S.T. Alone, A. Karim, G. Vats, S.E. Shirsath, R.H. Kadam, Magnetic field induced polarization and magnetoelectric effect in Na0.5Bi0.5TiO3 − Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.10.011
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Magnetic field induced polarization and magnetoelectric effect in Na0.5Bi0.5TiO3 Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite S. R. Wadganea, S. T. Aloneb, Asif Karimc, Gaurav Vatsd, Sagar E. Shirsathd,*, R. H. Kadam a,* a
Department of Physics, Materials Science Research Laboratory, Shrikrishna Mahavidyalaya, Gunjoti, Osmanabad 413 613, MS, India b Department of Physics, Rajarshi Shahu College, Pathri 431 111, MS, India c Department of Physics, Sir Sayyed College, Aurangabad 431 002, MS, India d School of Materials Science and Engineering, University of New South Wales, Kensington, Sydney, NSW 2052, Australia.
Abstract: Multiferroic composites of Zn-Cr substituted cobalt ferrite (Co0.75Zn0.25Cr0.2Fe1.8O4) (CZCFO) nanoparticles and lead-free piezoelectric sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) having composition (1x) Na0.5Bi0.5TiO3 + (x)Co0.75Zn0.25Cr0.2Fe1.8O4 (x = 0.0, 0.25, 0.5, 0.75, 1.0) were investigated in this study. Ferromagnetic-CZCFO and ferroelectric-NBT phases were synthesized via sol-gel auto combustion route and solid-state reaction methods respectively. The structural properties of composite materials were studied using X-ray diffraction (XRD) technique. Intense and sharp peaks observed in XRD patterns confirm that both the phases are well crystalline in nature. Two separate phases of CZCFO and NBT are present in composite material without any impurity or chemical reaction among them. The morphology of prepared samples was studied via scanning electron microscopy confirming that composite materials are well-densified and homogenous grain size distribution. The magnetic hysteresis loops of multiferroic composite were investigated by using vibrating sample magnetometer at room temperature. The lead free multiferroic composite exhibiting a large converse magneto-electric (ME) effect at room temperature. The maximum value of ME coefficient (αME = 7.92kV/cm) is obtained at x = 0.50. Furthermore, electrical properties of nanocomposite material were studied by P-E hysteresis loop and dielectric measurements at room temperature. Keywords: Sol-gel method; Magnetization; Magneto-electric effect; Dielectric properties. *corresponding author: [email protected] (RHK); [email protected] (SES) 1
1.
Introduction Multiferroic materials increase the interest of researchers due to their unusual properties
in different fields of modern technology. Magnetoelectric (ME) composite consists of a suitable combination of ferroelectric materials with ferromagnetic material. Magneto-electric effect in these composites is a result of mechanical coupling between piezoelectric (ferroelectric) and magnetostrictive (ferromagnetic) materials. Under applied magnetic field a strain is developed in ferromagnetic (piezomagnetic) material through magnetostriction, which passed to ferroelectric (piezoelectric) material results into stress/strain in it. Thus, extra charges are developed across multiferroic composite when magnetic field is applied which in turn generates induced voltage due to the piezoelectric effect [1]. The ME effect is observable in single phase material as well as composite material, however the observed value of ME coefficient is generally minor preventing their application in various devices. To increase the ME properties ferrite materials can be incorporated into ferroelectric material to enhance the value of saturation magnetization (Ms) and ME voltage coefficient of composite material. The lead-free materials are preferred due to concern of human health and for taking care of environment. Sodium bismuth titanate (NBT) is a lead-free perovskite (ABO3 - type) ferroelectric discovered by Smolenskii et al. in 1960 [2]. Na0.5Bi0.5TiO3 (NBT) exhibits a rhombohedral symmetry at room temperature. It undergoes a series of phase transitions: (i) ferroelectric rhombohedral to antiferroelectric tetragonal around 230 °C (ii) antiferroelectric tetragonal to nonpolar tetragonal around 320 °C, (iii) non-polar tetragonal to cubic around 520 °C [3]. NBT is considered as one of the good candidates for lead-free piezoelectric ceramics due to its large remnant polarization (Pr = 38 µC cm-2) at room temperature and high Curie temperature (Tc = 320 °C). The NBT based composite have great prospect due to this it used in 2
various application. Various composite systems have been reported and they mostly correspond to Co and Ni ferrites with Pb(ZrTi)O3 (PZT) [4–7], BaTiO3 [8–11] or BiFeO3 [12–14] with combinations of both ferrites and piezoelectrics. The CoFe2O4 (CFO) is a type of hard magnetic material and is well-known for its high value of magnetostriction coefficient among all the known spinel ferrites along with a moderate value of saturation magnetization, high coercivity (Hc) and chemical stability [15]. CFO based magneto-electric composites have shown a low magnetoelectric voltage coefficient due to the large magnetic anisotropy and coercivity of CFO which restricts the domain wall motion process [4, 5]. Substitution of Co by Zn at tetrahedral site in CFO is known to reduce magnetic anisotropy and coercivity [16] while slightly reducing the value of magnetostriction coefficient compared to that of CFO. The substitution of Zn2+ ions increases the resistivity of ferrite phase [17], at the same time Zn2+ ions at lower substitution level for Co2+ will also increase the saturation magnetization due to canting effect. It has been reported that the substitution of Cr3+ at Fe3+ site enhance the magnetostriction of CFO [18]. Further, it is well known that the substitutions of Co2+ ions by Zn2+ ions and Fe3+ ions by Cr3+ in CoFe2O4 may manipulate spinal structure, cation distribution between A and B sites and also the magnetic properties [16-18]. Thus, the simultaneous substitution of Zn2+ and Cr3+ for Co2+ and Fe3+ respectively is a good choice for a magnetostrictive phase due to its higher Ms and Hc. In the present work, Zn2+ and Cr3+ substituted Co ferrite with a chemical formula Co0.75Zn0.25Cr0.2Fe1.8O4 has been chosen as a ferromagnetic phase to form composite with Na0.5Bi0.5TiO3. CZCFO and NBT is successfully synthesis by sol-gel method and solid-state reaction method respectively.
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2.
Methods The composite material having a chemical composition of (1x) Na0.5Bi0.5TiO3 +
(x) Co0.75Zn0.25Cr0.2Fe1.8O4 (x = 0.0, 0.25, 0.5, 0.75, 1.0) is synthesized. The ferromagnetic phase consists of Zn-Cr Substituted with CFO is synthesis by sol-gel auto combustion method [19] and ferroelectric phase NBT is obtained by solid state reaction method. The nanoparticle of Zn-Cr substituted cobalt ferrite consist of stating material; cobalt nitrate Co(NO3)2.6H2O, ferric nitrate Fe(NO3)3.9H2O, zinc nitrate hexahydrate Zn(NO3)2.6H2O and chromium nitrate Cr(NO3)2.6H2O. All the nitrates were dissolved in distilled water in their stoichiometric proportion for uniform mixing and then placed on hot plate and magnetically stirred at 80 C. Citric acid is used as fuel agent. In order to maintain the pH=7 value the ammonia is slowly added in solution. The asobtained gel is converted into nanoparticle of Co0.75Zn0.25Cr0.2Fe1.8O4 ferrites in the form of fine powder. Samples was pre-sintered at 600 C for 4 h and allowed to cooled down naturally to room temperature. The ferroelectric phase of sodium bismuth titanate is synthesized by solid state reaction method. The bismuth oxide Bi2O3, sodium carbonate Na2CO3 and titanium oxide TiO3 of > 99% purity (Sigma –Aldrich) were taken as raw materials. The stoichiometric amount of these chemicals was mixed according to their stoichiometry proportion and ground in an agate mortar pestle for 8 h until homogeneous mixture is formed. The mixture of nanoparticle is calcined at 800 C for 4 h. As-calcined material was ground for 6 h followed by final sintering at 1100 C for 4 h to obtain fine powder of NBT. The nanoparticle of Co0.75Zn0.25Cr0.2Fe1.8O4 (hereafter called as CZCFO) and Na0.5Bi0.5TiO3 (hereafter called as NBT) were mixed according to their stoichiometry proportion
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and pelletized (0.1 cm in dimeter and 0.9 mm in thickness) using hydraulic press where PVA was used as a binder. These pellets were finally sintered at 1100 C for 6 h. To study the structural properties of prepared samples they were characterized by x-ray diffractometer. X ray diffraction patterns were recorded at room temperature in the 2θ range of 20 to 80 using Cu-Kα radiation (λ = 1.5404 Å). The microstructures of all the composite samples were studied by scanning electron microscope (SEM). Elemental compositions were estimated using X-ray electron diffraction analysis (EDAX). The magnetic properties of samples are studied by vibrating samples magnetometer (VSM) at room temperature with an applied magnetic field of 0.5 Tesla. The Magnetoelectric coefficient are investigated by magneto dielectric measurement by applying ac magnetic field (Hac = 5Oe) of frequency f =10 KHz, and DC magnetic field in the range of 0 – 7000 Oe. The ferroelectric properties of ME composites were obtained by using P-E hysteresis loop tracer system. Dielectric constant as well as dielectric loss was measured in the frequency range of 20 Hz -10 MHz using frequency dependent LCR-Q meter. 3. Result and Discussion 3.1 X-ray diffraction: X-ray diffraction patterns of all the samples measured at room temperature are shown in Fig. 1. The X-ray diffraction analysis of the sintered multiferroic composite samples shows individual peaks of the constituent parent (CZCFO and NBT) phases. The x-ray diffraction patterns confirm that CZCFO has cubic spinal crystal structure. The XRD peaks of CZCFO for 2θ angle is observed at 30.04 (220), 35.45 (311), 37.06 (222), 43.03 (400), 53.39 (422), 56.39 (511), 62.521 (440), and 73.96 (533). The peaks were indexed as per reference data from ICSD Card No. 22-1086. Similarly, the XRD pattern of ferroelectric NBT shows a single perovskite 5
phase with rhombohedral (R3c) structure and peaks are indexed at 2θ angle are: 22.81 (100), 32.58 (110), 40.24 (111), 46.7 (200), 58.19 (211) and 68.28 (220) (ICSD file no. 280983). No extra peak is appeared in the composite shows that no impurity phases were identified, also no significant chemical reaction occurred among ferroelectric-NBT and ferrite-CZCFO phases. The intensity XRD peaks of NBT decreases with increasing CZCFO ferromagnetic phase in the composite material. The calculated lattice constant (a) of rhombohedral NBT is a = 3.865Å (0.002 Å) and for the ferrite phase is a = 8.416Å (0.002 Å) that slightly changed to 8.395Å with the increase in NBT content in composite. The Debye Scherrer equation is used to calculate the crystalline size (txrd) of composite. The crystallite size of ferroelectric phase decreased from 23 to 18.9 nm and crystalline size of ferrite phase increased from 21.7 to 24.2 nm. The X-ray density (dx) of multiferroic composite linearly increased from 47.95 to 51.83 g/cm3 with the increase in CZCFO phase in composite. The increase in dX can be related to higher molecular weight of CZCFO compare to NBT. The main peak (110) of NBT slightly shifted towards lower 2θ angle, that could be related to the change in lattice parameter and crystalline size.
3.2
Morphological analysis of composite The morphology and microstructure of prepared samples were studied using scanning
electron microscope (SEM). Figure 2 shows SEM micrographs of x = 0.0, 0.50 and 1.0. NBT material shows larger grain size compared to CZCFO and it is observed that material has homogeneous microstructure, dense in nature, however small pores were detected. The concentration of CZCFO is increases in NBT resulted in a decrease in densification and grain size. Further, no distinct grain boundaries were observed. The difference in grain size among CZCFO and NBT samples could be a deciding factor to govern the ME properties as difference 6
in grain size assume to employ stress on each other. The elemental color mapping of NBT (x = 1.0) sample is also shown in Fig. 2 confirming the uniform distribution of Na, Bi, Ti and O elements throughout the specimen. The energy dispersive X-ray diffraction analysis (EDAX) patterns of (1x) NBT + (x) CZCFO at x = 0.0, 0.50 and 1.0 is shown in Fig. 3. These patterns show that only elements of NBT and CZCFO phases are present in the composite material. This shows that prepared sample does not contain any type of impurities. The atomic percentage of all the elements in the composition of x = 0.50 are; oxygen- 34.79%, sodium- 6.76%, titanium9.31, bismuth- 15.84%, chromium- 2.40%, iron- 21.34%, Co-9.52% and zinc- 0.49%.
3.3
Magnetic hysteresis loop measurements Magnetic properties of (1x) NBT + (x) CZCFO nanocomposite materials were studied
by using vibrating sample magnetometer (VSM) with applied magnetic field up to 5000 Oe. The magnetic hysteresis loop of (1x) NBT + (x) CZCFO multiferroic composite measured at room temperature is shown in Fig. 4. The observed value of saturation magnetization (Ms) in CZCFO is 76 emu/g. In present case, it is observed that the saturation magnetization and all other magnetic properties of composite decreased with the decrease in CZCFO phase and increase in NBT phase. The value of Ms, remnant magnetization (Mr), coercivity (Hc) and remanence ratio (R = Mr/Ms) of all the samples are shown in Fig. 4. The maximum value of Hc (109 Oe) is observed for x = 0.5. The observed lower value of Hc is an indication of soft magnetic nature of the composites which may be related to the particle size, ferroelectric nature of NBT and incorporation of Zn in CFO. The Hc of multiferroic material is depends on anisotropy of material.
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3.4
Magnetoelectric coefficient Figure 5 shows the magnetoelectric effect (ME) of (1x)NBT+ (x) CZCFO multiferroic
composite sintered at 1100 C. The ME coefficient of prepared nanocomposite sample were studied at room temperature. The external DC magnetic field ranging from 0 -7000 Oe is applied by using Helmholtz coil to unpoled sample. The stress produced during the sintering process affect the magneto-crystalline anisotropy in composite material resulted into lattice mismatch between ferromagnetic-CZCFO and ferroelectric-NBT phases inducing compressive stress between these two phases. The strain produce in ferrite increases with increasing in DC magnetic field and after that it remains constant. The Magnetoelectric coefficient αME is given by formula:
ME
Voltage developed across pellets(Vout ) H ac Thickness of pellets
where Hac is the constant AC magnetic field applied across the samples. In present work, the value ME coefficient of nanocomposite material increases with increasing the concentration of CZCFO ferromagnetic phase. The maximum value of ME coefficient is observed as; αME = 7.92 mV/cm-Oe for x = 0.50 (i.e. ferroelectric NBT (50%) and ferrite phase CZCFO (50%)). This is due to change in resistivity of both the phases. The ferromagnetic phase CZCFO has low resistivity as compare to the ferroelectric NBT phase.
3.5
Dielectric properties The frequency dependence dielectric constant of (1x)NBT+ (x) CZCFO nanocomposite
measured at room temperature is shown in Fig. 6. It is observed that dielectric constant of multiferroic composite decreased with the increase in frequency which is a typical behavior of spinal ferrite. The observed behaviour can be explained on the basis of interfacial polarization 8
[20]. Presence of Fe3+ (majority) ions and Fe2+ (minority) ions makes CZCFO ferrite material dipolar during the sintering process where Fe2+ ions are usually formed due to partial reduction of Fe3+ ions. The electron exchange interaction between Fe2+ and Fe3+ ions resulted in the local displacement of electrons in the direction of the applied electric field determining the polarization in CZCFO ferrites. The almost constant nature of dielectric constant at higher frequency of electric field related to the fact that; beyond a certain frequency region or electric field the electron exchange among Fe3+ and Fe2+ ions does not able to keep pace with the applied AC electric field. The dielectric constant of ferromagnetic CZCFO phase is very low and is increased with the addition of ferroelectric NBT. The dielectric constant varies with increasing NBT concentration the pure NBT shows the highest value of =116. The frequency dependence dielectric loss tangent (tanδ) of multiferroic composite samples was measured at room temperature (Fig. 6). The dielectric loss is high at low frequency and could be related to the interruption to the passage of electric field through the sample due to the interaction of it with the oscillating majority (Fe3+) charges carriers of the dipoles. The dielectric loss decreased with the increase in frequency. The dielectric loss increases in composite with increasing concentration of ferromagnetic CZCFO phase. In composite the pure CZCFO shows higher value of dielectric loss of 3.75, which may be related to hopping between Fe2+ and Fe3+ ions. The leakage current in composite decreases with increasing NBT concentration because the NBT sample is ferroelectric in nature. A small peaking behavior is observed in the tanδ plots of x = 0.0 -0.5 referring to Debye-type relaxation, where the hopping frequency of the Fe3+ and Fe2+ ions almost matching to the frequency of applied AC electric field [21].
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3.6
Ferroelectric properties Figure 7 shows polarization (P) versus electric field (E) hysteresis loops of representative
composition (x = 0.0, 0.50, 1.0) of (1x)NBT+ (x)CZCFO composites measured at room temperature. The ferroelectric characteristics of these samples were studied without pre-polling. The PE loops of all the samples are observed to be not saturated due to the high conductivity, leakage current and large coercive field of NBT material. The value of polarization at 10 kV/cm decreased from 0.76 to 0.04 C/cm2 with the increase in ferromagnetic CZCFO content in composite because CZCFO has lower resistivity compared to the NBT phase that producing large leakage current in the composite materials. The coercivity increases due to hindering and pinned domain wall motion of ferroelectric region due to existence of ferrite phase in composite [22].
4.
Conclusions The multiferroic composite of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 was
successfully synthesized by employing sol-gel and solid-state methods. The XRD analysis confirms the well crystalline nature of ferroelectric NBT and ferromagnetic CZCFO phases. The XRD
analysis
confirms
the
Na0.5Bi0.5TiO3
has
perovskite
structure
whereas
Co0.75Zn0.25Cr0.2Fe1.8O4 has cubic spinal structure. The intensity of NBT peaks decreases with increasing ferromagnetic CZCFO content in composite. The SEM analysis shows material has crystalline in nature, dense however small pores were observed in composite. The remanance magnetization and saturation magnetization of multiferroic composite increases with increasing CZCFO concentration. The magnetoelectric coefficient of multiferroic composite shows maximum value of 7.92 mV/cm-Oe at x = 0.50. Dielectric constant decreased with the increase in frequency as well as increasing concentration of CZCFO in composite material. The composite material of x = 0.5 shows soft ferromagnetic nature with low loss and possessing 10
moderate ME coefficient making it a suitable candidate for the application in microwave phase shifter.
Acknowledgement: Authors are very thankful to UGC-DAE-CSR for the P-E measurements and SEM-EDAX facility. They are also thankful to Dr. R. K. Kotnala and Dr. Jyoti Shah (National Physical Laboratory, New Delhi) for providing ME measurement facility.
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Figure Captions Fig. 1: XRD pattern of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite sintered at 1100 C, where (a) x = 0.0, (b) x = 0.25, (c) x = 0.50, (d) x = 0.75 and (e) x = 1.0. Fig. 2: (a) SEM micrograph of x = 0.0 and its elemental color mapping where Na, Bi, Ti and O elements are denoted by pink, blue, red and cyan color respectively. (b) and (c) are the SEM micrographs x = 0.50 and x = 1.0 respectively of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite sintered at 1100 C. Fig. 3: EDAX of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite sintered at 1100 C, where for (a) x = 0.0, (b) x = 0.50 and (c) x = 1.0. Fig. 4: Magnetization (M) with applied magnetic field (H) hysteresis curves in the left panel of figure whereas right panel shows the magnetic parameters extracted from MH curves of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite measured at room temperature. Fig. 5: Magnetoelectric coefficient (αME) varies with applied magnetic field of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite for x=0.0, 0.25, 0.50, 0.75 and 1.0 at room temperature. Fig. 6: Variation of dielectric constant () and dielectric loss tangent (tan ) with frequency of (1- x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite. Fig. 7: P-E hysteresis loop of (1- x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite measured at room temperature.
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Research highlights
Nano-composite of Na0.5Bi0.5TiO3 Co0.75Zn0.25Cr0.2Fe1.8O4
Improved dielectric constant
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S0304-8853(18)32337-0 https://doi.org/10.1016/j.jmmm.2018.10.011 MAGMA 64422
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
26 July 2018 15 September 2018 2 October 2018
Please cite this article as: S.R. Wadgane, S.T. Alone, A. Karim, G. Vats, S.E. Shirsath, R.H. Kadam, Magnetic field induced polarization and magnetoelectric effect in Na0.5Bi0.5TiO3 − Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.10.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetic field induced polarization and magnetoelectric effect in Na0.5Bi0.5TiO3 Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite S. R. Wadganea, S. T. Aloneb, Asif Karimc, Gaurav Vatsd, Sagar E. Shirsathd,*, R. H. Kadam a,* a
Department of Physics, Materials Science Research Laboratory, Shrikrishna Mahavidyalaya, Gunjoti, Osmanabad 413 613, MS, India b Department of Physics, Rajarshi Shahu College, Pathri 431 111, MS, India c Department of Physics, Sir Sayyed College, Aurangabad 431 002, MS, India d School of Materials Science and Engineering, University of New South Wales, Kensington, Sydney, NSW 2052, Australia.
Abstract: Multiferroic composites of Zn-Cr substituted cobalt ferrite (Co0.75Zn0.25Cr0.2Fe1.8O4) (CZCFO) nanoparticles and lead-free piezoelectric sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) having composition (1x) Na0.5Bi0.5TiO3 + (x)Co0.75Zn0.25Cr0.2Fe1.8O4 (x = 0.0, 0.25, 0.5, 0.75, 1.0) were investigated in this study. Ferromagnetic-CZCFO and ferroelectric-NBT phases were synthesized via sol-gel auto combustion route and solid-state reaction methods respectively. The structural properties of composite materials were studied using X-ray diffraction (XRD) technique. Intense and sharp peaks observed in XRD patterns confirm that both the phases are well crystalline in nature. Two separate phases of CZCFO and NBT are present in composite material without any impurity or chemical reaction among them. The morphology of prepared samples was studied via scanning electron microscopy confirming that composite materials are well-densified and homogenous grain size distribution. The magnetic hysteresis loops of multiferroic composite were investigated by using vibrating sample magnetometer at room temperature. The lead free multiferroic composite exhibiting a large converse magneto-electric (ME) effect at room temperature. The maximum value of ME coefficient (αME = 7.92kV/cm) is obtained at x = 0.50. Furthermore, electrical properties of nanocomposite material were studied by P-E hysteresis loop and dielectric measurements at room temperature. Keywords: Sol-gel method; Magnetization; Magneto-electric effect; Dielectric properties. *corresponding author: [email protected] (RHK); [email protected] (SES) 1
1.
Introduction Multiferroic materials increase the interest of researchers due to their unusual properties
in different fields of modern technology. Magnetoelectric (ME) composite consists of a suitable combination of ferroelectric materials with ferromagnetic material. Magneto-electric effect in these composites is a result of mechanical coupling between piezoelectric (ferroelectric) and magnetostrictive (ferromagnetic) materials. Under applied magnetic field a strain is developed in ferromagnetic (piezomagnetic) material through magnetostriction, which passed to ferroelectric (piezoelectric) material results into stress/strain in it. Thus, extra charges are developed across multiferroic composite when magnetic field is applied which in turn generates induced voltage due to the piezoelectric effect [1]. The ME effect is observable in single phase material as well as composite material, however the observed value of ME coefficient is generally minor preventing their application in various devices. To increase the ME properties ferrite materials can be incorporated into ferroelectric material to enhance the value of saturation magnetization (Ms) and ME voltage coefficient of composite material. The lead-free materials are preferred due to concern of human health and for taking care of environment. Sodium bismuth titanate (NBT) is a lead-free perovskite (ABO3 - type) ferroelectric discovered by Smolenskii et al. in 1960 [2]. Na0.5Bi0.5TiO3 (NBT) exhibits a rhombohedral symmetry at room temperature. It undergoes a series of phase transitions: (i) ferroelectric rhombohedral to antiferroelectric tetragonal around 230 °C (ii) antiferroelectric tetragonal to nonpolar tetragonal around 320 °C, (iii) non-polar tetragonal to cubic around 520 °C [3]. NBT is considered as one of the good candidates for lead-free piezoelectric ceramics due to its large remnant polarization (Pr = 38 µC cm-2) at room temperature and high Curie temperature (Tc = 320 °C). The NBT based composite have great prospect due to this it used in 2
various application. Various composite systems have been reported and they mostly correspond to Co and Ni ferrites with Pb(ZrTi)O3 (PZT) [4–7], BaTiO3 [8–11] or BiFeO3 [12–14] with combinations of both ferrites and piezoelectrics. The CoFe2O4 (CFO) is a type of hard magnetic material and is well-known for its high value of magnetostriction coefficient among all the known spinel ferrites along with a moderate value of saturation magnetization, high coercivity (Hc) and chemical stability [15]. CFO based magneto-electric composites have shown a low magnetoelectric voltage coefficient due to the large magnetic anisotropy and coercivity of CFO which restricts the domain wall motion process [4, 5]. Substitution of Co by Zn at tetrahedral site in CFO is known to reduce magnetic anisotropy and coercivity [16] while slightly reducing the value of magnetostriction coefficient compared to that of CFO. The substitution of Zn2+ ions increases the resistivity of ferrite phase [17], at the same time Zn2+ ions at lower substitution level for Co2+ will also increase the saturation magnetization due to canting effect. It has been reported that the substitution of Cr3+ at Fe3+ site enhance the magnetostriction of CFO [18]. Further, it is well known that the substitutions of Co2+ ions by Zn2+ ions and Fe3+ ions by Cr3+ in CoFe2O4 may manipulate spinal structure, cation distribution between A and B sites and also the magnetic properties [16-18]. Thus, the simultaneous substitution of Zn2+ and Cr3+ for Co2+ and Fe3+ respectively is a good choice for a magnetostrictive phase due to its higher Ms and Hc. In the present work, Zn2+ and Cr3+ substituted Co ferrite with a chemical formula Co0.75Zn0.25Cr0.2Fe1.8O4 has been chosen as a ferromagnetic phase to form composite with Na0.5Bi0.5TiO3. CZCFO and NBT is successfully synthesis by sol-gel method and solid-state reaction method respectively.
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2.
Methods The composite material having a chemical composition of (1x) Na0.5Bi0.5TiO3 +
(x) Co0.75Zn0.25Cr0.2Fe1.8O4 (x = 0.0, 0.25, 0.5, 0.75, 1.0) is synthesized. The ferromagnetic phase consists of Zn-Cr Substituted with CFO is synthesis by sol-gel auto combustion method [19] and ferroelectric phase NBT is obtained by solid state reaction method. The nanoparticle of Zn-Cr substituted cobalt ferrite consist of stating material; cobalt nitrate Co(NO3)2.6H2O, ferric nitrate Fe(NO3)3.9H2O, zinc nitrate hexahydrate Zn(NO3)2.6H2O and chromium nitrate Cr(NO3)2.6H2O. All the nitrates were dissolved in distilled water in their stoichiometric proportion for uniform mixing and then placed on hot plate and magnetically stirred at 80 C. Citric acid is used as fuel agent. In order to maintain the pH=7 value the ammonia is slowly added in solution. The asobtained gel is converted into nanoparticle of Co0.75Zn0.25Cr0.2Fe1.8O4 ferrites in the form of fine powder. Samples was pre-sintered at 600 C for 4 h and allowed to cooled down naturally to room temperature. The ferroelectric phase of sodium bismuth titanate is synthesized by solid state reaction method. The bismuth oxide Bi2O3, sodium carbonate Na2CO3 and titanium oxide TiO3 of > 99% purity (Sigma –Aldrich) were taken as raw materials. The stoichiometric amount of these chemicals was mixed according to their stoichiometry proportion and ground in an agate mortar pestle for 8 h until homogeneous mixture is formed. The mixture of nanoparticle is calcined at 800 C for 4 h. As-calcined material was ground for 6 h followed by final sintering at 1100 C for 4 h to obtain fine powder of NBT. The nanoparticle of Co0.75Zn0.25Cr0.2Fe1.8O4 (hereafter called as CZCFO) and Na0.5Bi0.5TiO3 (hereafter called as NBT) were mixed according to their stoichiometry proportion
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and pelletized (0.1 cm in dimeter and 0.9 mm in thickness) using hydraulic press where PVA was used as a binder. These pellets were finally sintered at 1100 C for 6 h. To study the structural properties of prepared samples they were characterized by x-ray diffractometer. X ray diffraction patterns were recorded at room temperature in the 2θ range of 20 to 80 using Cu-Kα radiation (λ = 1.5404 Å). The microstructures of all the composite samples were studied by scanning electron microscope (SEM). Elemental compositions were estimated using X-ray electron diffraction analysis (EDAX). The magnetic properties of samples are studied by vibrating samples magnetometer (VSM) at room temperature with an applied magnetic field of 0.5 Tesla. The Magnetoelectric coefficient are investigated by magneto dielectric measurement by applying ac magnetic field (Hac = 5Oe) of frequency f =10 KHz, and DC magnetic field in the range of 0 – 7000 Oe. The ferroelectric properties of ME composites were obtained by using P-E hysteresis loop tracer system. Dielectric constant as well as dielectric loss was measured in the frequency range of 20 Hz -10 MHz using frequency dependent LCR-Q meter. 3. Result and Discussion 3.1 X-ray diffraction: X-ray diffraction patterns of all the samples measured at room temperature are shown in Fig. 1. The X-ray diffraction analysis of the sintered multiferroic composite samples shows individual peaks of the constituent parent (CZCFO and NBT) phases. The x-ray diffraction patterns confirm that CZCFO has cubic spinal crystal structure. The XRD peaks of CZCFO for 2θ angle is observed at 30.04 (220), 35.45 (311), 37.06 (222), 43.03 (400), 53.39 (422), 56.39 (511), 62.521 (440), and 73.96 (533). The peaks were indexed as per reference data from ICSD Card No. 22-1086. Similarly, the XRD pattern of ferroelectric NBT shows a single perovskite 5
phase with rhombohedral (R3c) structure and peaks are indexed at 2θ angle are: 22.81 (100), 32.58 (110), 40.24 (111), 46.7 (200), 58.19 (211) and 68.28 (220) (ICSD file no. 280983). No extra peak is appeared in the composite shows that no impurity phases were identified, also no significant chemical reaction occurred among ferroelectric-NBT and ferrite-CZCFO phases. The intensity XRD peaks of NBT decreases with increasing CZCFO ferromagnetic phase in the composite material. The calculated lattice constant (a) of rhombohedral NBT is a = 3.865Å (0.002 Å) and for the ferrite phase is a = 8.416Å (0.002 Å) that slightly changed to 8.395Å with the increase in NBT content in composite. The Debye Scherrer equation is used to calculate the crystalline size (txrd) of composite. The crystallite size of ferroelectric phase decreased from 23 to 18.9 nm and crystalline size of ferrite phase increased from 21.7 to 24.2 nm. The X-ray density (dx) of multiferroic composite linearly increased from 47.95 to 51.83 g/cm3 with the increase in CZCFO phase in composite. The increase in dX can be related to higher molecular weight of CZCFO compare to NBT. The main peak (110) of NBT slightly shifted towards lower 2θ angle, that could be related to the change in lattice parameter and crystalline size.
3.2
Morphological analysis of composite The morphology and microstructure of prepared samples were studied using scanning
electron microscope (SEM). Figure 2 shows SEM micrographs of x = 0.0, 0.50 and 1.0. NBT material shows larger grain size compared to CZCFO and it is observed that material has homogeneous microstructure, dense in nature, however small pores were detected. The concentration of CZCFO is increases in NBT resulted in a decrease in densification and grain size. Further, no distinct grain boundaries were observed. The difference in grain size among CZCFO and NBT samples could be a deciding factor to govern the ME properties as difference 6
in grain size assume to employ stress on each other. The elemental color mapping of NBT (x = 1.0) sample is also shown in Fig. 2 confirming the uniform distribution of Na, Bi, Ti and O elements throughout the specimen. The energy dispersive X-ray diffraction analysis (EDAX) patterns of (1x) NBT + (x) CZCFO at x = 0.0, 0.50 and 1.0 is shown in Fig. 3. These patterns show that only elements of NBT and CZCFO phases are present in the composite material. This shows that prepared sample does not contain any type of impurities. The atomic percentage of all the elements in the composition of x = 0.50 are; oxygen- 34.79%, sodium- 6.76%, titanium9.31, bismuth- 15.84%, chromium- 2.40%, iron- 21.34%, Co-9.52% and zinc- 0.49%.
3.3
Magnetic hysteresis loop measurements Magnetic properties of (1x) NBT + (x) CZCFO nanocomposite materials were studied
by using vibrating sample magnetometer (VSM) with applied magnetic field up to 5000 Oe. The magnetic hysteresis loop of (1x) NBT + (x) CZCFO multiferroic composite measured at room temperature is shown in Fig. 4. The observed value of saturation magnetization (Ms) in CZCFO is 76 emu/g. In present case, it is observed that the saturation magnetization and all other magnetic properties of composite decreased with the decrease in CZCFO phase and increase in NBT phase. The value of Ms, remnant magnetization (Mr), coercivity (Hc) and remanence ratio (R = Mr/Ms) of all the samples are shown in Fig. 4. The maximum value of Hc (109 Oe) is observed for x = 0.5. The observed lower value of Hc is an indication of soft magnetic nature of the composites which may be related to the particle size, ferroelectric nature of NBT and incorporation of Zn in CFO. The Hc of multiferroic material is depends on anisotropy of material.
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3.4
Magnetoelectric coefficient Figure 5 shows the magnetoelectric effect (ME) of (1x)NBT+ (x) CZCFO multiferroic
composite sintered at 1100 C. The ME coefficient of prepared nanocomposite sample were studied at room temperature. The external DC magnetic field ranging from 0 -7000 Oe is applied by using Helmholtz coil to unpoled sample. The stress produced during the sintering process affect the magneto-crystalline anisotropy in composite material resulted into lattice mismatch between ferromagnetic-CZCFO and ferroelectric-NBT phases inducing compressive stress between these two phases. The strain produce in ferrite increases with increasing in DC magnetic field and after that it remains constant. The Magnetoelectric coefficient αME is given by formula:
ME
Voltage developed across pellets(Vout ) H ac Thickness of pellets
where Hac is the constant AC magnetic field applied across the samples. In present work, the value ME coefficient of nanocomposite material increases with increasing the concentration of CZCFO ferromagnetic phase. The maximum value of ME coefficient is observed as; αME = 7.92 mV/cm-Oe for x = 0.50 (i.e. ferroelectric NBT (50%) and ferrite phase CZCFO (50%)). This is due to change in resistivity of both the phases. The ferromagnetic phase CZCFO has low resistivity as compare to the ferroelectric NBT phase.
3.5
Dielectric properties The frequency dependence dielectric constant of (1x)NBT+ (x) CZCFO nanocomposite
measured at room temperature is shown in Fig. 6. It is observed that dielectric constant of multiferroic composite decreased with the increase in frequency which is a typical behavior of spinal ferrite. The observed behaviour can be explained on the basis of interfacial polarization 8
[20]. Presence of Fe3+ (majority) ions and Fe2+ (minority) ions makes CZCFO ferrite material dipolar during the sintering process where Fe2+ ions are usually formed due to partial reduction of Fe3+ ions. The electron exchange interaction between Fe2+ and Fe3+ ions resulted in the local displacement of electrons in the direction of the applied electric field determining the polarization in CZCFO ferrites. The almost constant nature of dielectric constant at higher frequency of electric field related to the fact that; beyond a certain frequency region or electric field the electron exchange among Fe3+ and Fe2+ ions does not able to keep pace with the applied AC electric field. The dielectric constant of ferromagnetic CZCFO phase is very low and is increased with the addition of ferroelectric NBT. The dielectric constant varies with increasing NBT concentration the pure NBT shows the highest value of =116. The frequency dependence dielectric loss tangent (tanδ) of multiferroic composite samples was measured at room temperature (Fig. 6). The dielectric loss is high at low frequency and could be related to the interruption to the passage of electric field through the sample due to the interaction of it with the oscillating majority (Fe3+) charges carriers of the dipoles. The dielectric loss decreased with the increase in frequency. The dielectric loss increases in composite with increasing concentration of ferromagnetic CZCFO phase. In composite the pure CZCFO shows higher value of dielectric loss of 3.75, which may be related to hopping between Fe2+ and Fe3+ ions. The leakage current in composite decreases with increasing NBT concentration because the NBT sample is ferroelectric in nature. A small peaking behavior is observed in the tanδ plots of x = 0.0 -0.5 referring to Debye-type relaxation, where the hopping frequency of the Fe3+ and Fe2+ ions almost matching to the frequency of applied AC electric field [21].
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3.6
Ferroelectric properties Figure 7 shows polarization (P) versus electric field (E) hysteresis loops of representative
composition (x = 0.0, 0.50, 1.0) of (1x)NBT+ (x)CZCFO composites measured at room temperature. The ferroelectric characteristics of these samples were studied without pre-polling. The PE loops of all the samples are observed to be not saturated due to the high conductivity, leakage current and large coercive field of NBT material. The value of polarization at 10 kV/cm decreased from 0.76 to 0.04 C/cm2 with the increase in ferromagnetic CZCFO content in composite because CZCFO has lower resistivity compared to the NBT phase that producing large leakage current in the composite materials. The coercivity increases due to hindering and pinned domain wall motion of ferroelectric region due to existence of ferrite phase in composite [22].
4.
Conclusions The multiferroic composite of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 was
successfully synthesized by employing sol-gel and solid-state methods. The XRD analysis confirms the well crystalline nature of ferroelectric NBT and ferromagnetic CZCFO phases. The XRD
analysis
confirms
the
Na0.5Bi0.5TiO3
has
perovskite
structure
whereas
Co0.75Zn0.25Cr0.2Fe1.8O4 has cubic spinal structure. The intensity of NBT peaks decreases with increasing ferromagnetic CZCFO content in composite. The SEM analysis shows material has crystalline in nature, dense however small pores were observed in composite. The remanance magnetization and saturation magnetization of multiferroic composite increases with increasing CZCFO concentration. The magnetoelectric coefficient of multiferroic composite shows maximum value of 7.92 mV/cm-Oe at x = 0.50. Dielectric constant decreased with the increase in frequency as well as increasing concentration of CZCFO in composite material. The composite material of x = 0.5 shows soft ferromagnetic nature with low loss and possessing 10
moderate ME coefficient making it a suitable candidate for the application in microwave phase shifter.
Acknowledgement: Authors are very thankful to UGC-DAE-CSR for the P-E measurements and SEM-EDAX facility. They are also thankful to Dr. R. K. Kotnala and Dr. Jyoti Shah (National Physical Laboratory, New Delhi) for providing ME measurement facility.
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Figure Captions Fig. 1: XRD pattern of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite sintered at 1100 C, where (a) x = 0.0, (b) x = 0.25, (c) x = 0.50, (d) x = 0.75 and (e) x = 1.0. Fig. 2: (a) SEM micrograph of x = 0.0 and its elemental color mapping where Na, Bi, Ti and O elements are denoted by pink, blue, red and cyan color respectively. (b) and (c) are the SEM micrographs x = 0.50 and x = 1.0 respectively of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite sintered at 1100 C. Fig. 3: EDAX of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite sintered at 1100 C, where for (a) x = 0.0, (b) x = 0.50 and (c) x = 1.0. Fig. 4: Magnetization (M) with applied magnetic field (H) hysteresis curves in the left panel of figure whereas right panel shows the magnetic parameters extracted from MH curves of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite measured at room temperature. Fig. 5: Magnetoelectric coefficient (αME) varies with applied magnetic field of (1x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite for x=0.0, 0.25, 0.50, 0.75 and 1.0 at room temperature. Fig. 6: Variation of dielectric constant () and dielectric loss tangent (tan ) with frequency of (1- x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite. Fig. 7: P-E hysteresis loop of (1- x) Na0.5Bi0.5TiO3 + (x) Co0.75Zn0.25Cr0.2Fe1.8O4 multiferroic composite measured at room temperature.
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Research highlights
Nano-composite of Na0.5Bi0.5TiO3 Co0.75Zn0.25Cr0.2Fe1.8O4
Improved dielectric constant
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