Dielectrics are nonconducting materials that can be polarized through applied electric field. When a dielectric material is placed between two conductor plates of capacitor, the polarization of the dielectric will increase the capacitance by a factor κ where κ is a dimensionless factor called dielectric constant [1]. The information of the dielectric constant is one of the most important parameter when designing capacitors or in other cases where a material might be expected to introduce capacitance into a circuit. For example, when a material with high dielectric constant is placed in an electric field, the magnitude of the field will be reduced. This is usually used to increase the capacitance of a particular capacitor.
Barium titanate (BaTiO3) is one of the dielectric materials that contribute to many applications in electronic devices, such as capacitor and positive temperature coefficient resistance (PTC), which are used for temperature control [19]. Recently, synthesis of multiferroic materials had been in research of interest because of their potential application in practical electronic devices [11].
Multiferroic materials exhibit ferroelectric and ferromagnetic properties simultaneously. Coupling between these two orders provides distinctive technique of switching electric polarization using magnetic field and magnetic polarization in the existence of electric field [12]. There are various probable compounds of composites where magnetostrictive materials such as ferrites and manganates; which are insulator, combined with piezoelectric materials like BaTiO3, Pb(Zr,Ti)O3 (PZT), Rochellesalt and PbTiO3. Another option is a multilayer structure consisting of magnetostrictive and piezoelectric layers [17]. Study done by Jungho Ryu et.al [16] used TbDyFe2 (Terfenol-D) as the magnetostrictive materials whereas PZT as the piezoelectric materials.
Figure 1: Magnetoelectric lamina composite using Terfenol-D and PZT disks [16].
When an applied magnetic field is introduced to the composites which contain the magnetostrictive and piezoelectric materials, the magnetostrictive materials will be strained. This strain will induce a stress in the piezoelectric that consequently will produce electric field. This effect can also happen oppositely where applied electric field that acts on the piezoelectric materials, producing strain which will be transferred to the magnetostrictive materials. As a result, the magnetic permeability of the materials is changed [16]. This magnetoelectric effect (ME) is very important property induced by coupling between electrical and magnetic dipoles [14]. Such material that posses this property and has attracted a great deal of attention is bismuth ferrite (BiFeO3) [11].
There is another option where magnetoelectric coupling in thin films can be examined. That way is by magnetocapacitance measurements where the change of dielectric permittivity under magnetic field is to be measured. A.Kumar et al. [14] defined magnetocapacitance (MC) as
Results showed that magnetocapacitance increased with increasing in magnetic field of BiFe1-xCrxO3 (x=0.04, 0.06 and 0.08) at room temperature. This results can proved the existence of strong magnetoelectric coupling in the system [14].
By variation of applied magnetic field, the dielectric constant (É›) changes can be induced by not only magnetoelectric coupling effect but also other factors such as the magnetostriction effect, which happens due to the change in lattice parameters.
At room temperature, bismuth ferrite (BiFeO3) show ferroelectric properties with the structural symmetry of a rhombohedrally distorted ABO3 perovskite structure [13]. It posses a a polarization ordering with a high Curie temperature Tc of 1103K and a spin (antiferromagnetic) ordering of a G type with a magnetic transition temperature TN of 643K [14]. The Bi lone electron pair are the factors contributing to the ferroelectric polarization in the compounds while the partially filled 3d orbitals of the Fe3+ ions originate the G-type antiferromagnetic order [15].
D.-C Jia et al. [18] reported that BiFeO3 exhibit weak ferromagnetism nature at room temperature based on its nonlinearity with the remanent magnetization of 0.004 emu/g and coercive field of 145 Oe based on its M-H curve.
Figure 2: Magnetic property of BiFeO3 powders measured at room temperature [18].
Meanwhile, CaCu3Ti4O12 (CCTO) is a perovskite compound with significantly high dielectric constant about 104 at room temperature [25]. However, its considerably high dielectric loss had become a barrier for usage of CCTO in technical use [26]. To prevent this problem, researchers had found many ways to reduce dielectric loss of CCTO by manipulating the internal barrier layer capacitor (IBLC) model in CCTO. One of the most compromising techniques is by adding glass i.e TeO2, B2O3, PbO. Other than that, cation doping (Zr4+, Fe3+, Nb5+, Mn2+) has also been investigated.
Process whereby impurities are added to the crystal structure of metal titanate is called doping. This process can be done by various methods such as by conventional ceramic processing [8], metal-organic deposition process [7], as well as by a sol-gel spin coating process [9]. Addition of the elemental dopant will change the structural and electrical properties of the metal titanate. For example, a pure barium titanate is an isulator. However, its perovskite crystal structure can easily accommodate by other ions and it will significantly change its dielectric properties [10]. Study done by Taeyun Kim et al. [5] shows that Ca-doped lead zirconate titanate (PZT) had significantly lowered the temperature coefficient of capacitance. Besides that, when a pure barium titanate is doped with small amount of metals, it can exhibit semiconducting properties. Meanwhile, the structure of the final titanate products can be inspected by X-Ray diffraction [7]. Other than that, transmission electron microscopy (TEM) also can be used to give a real space image on the distribution of particles, their surface and shape [6].
1.1 Problem statement
There are reports on investigating the magnetoelectric coupling in nanocomposites of ferrites (CoFe2O4, CuFe2O4 and ZnFe2O4) with BiFeO3. Xian-Ming Liu et al. [11] had used a hard magnetic material which is CoFe2O4 and it requires high magnetic field to be polarized. In contrary, Ravinder Tadi et al. [12] used soft magnetic material, magnesium ferrite (MgFe2O4) with BaTiO3 where the control of electric polarization can be done with application of low magnetic field. It would be of great technical potential to synthesize MgFe2O4 and BiFeO3 composite miltiferroic materials, in which electric polarization can be controlled at low magnetic fields. However, there are no studies done on MgFe2O4-BiFeO3.
BiFeO3 has antiferromagnetic behavior at room temperature. This property change only to weak ferromagnetic state by addition of other perovskite such as BaTiO3 [24] and PbTiO3 [23]. However, study done by S. Kazhugasalamoorthy et al. [27] proves that by substituting Mn, La content in BiFeO3, the saturation magnetization of the samples were improved, and the ferromagnetic property was enhanced. CCTO enormously high dielectric contstant which is desirable in most applications. Therefore, it is interesting to study the multiferroic behavior and magnetocapacitance of of La doped BiFeO. Nevertheless, there are no reports on BiFe1-xLaxO3-CCTO.
High sintering temperature leads to secondary phase impurities which leads to leakage current. Sintering need to optimized to reduce the secondary phase. Mechanochemical synthesis can produce nano powders with high activation energy and can lead to lower sintering temperatures. No studies in MgFe2O4-BiFeO3 and CCTO-BiFeO3 composites on sintering behavior of mechanochemically synthesized nano powders.
1.2 Objective:
To synthesize and study the structural, magnetic and magnetoelectric coupling of BiFeO3-MgFe2O4 nanocomposites.
To investigate multiferroic behavior of La doped BiFeO3 with CCTO by sol-gel method using magnetocapacitance measurement.
To optimize and study micro structure of the nanocomposites by FESEM and phase analysis by XRD pattern.
1.3 Significance of study
The development of composites materials has been a subject of demanding research today. The occurrence of the multiferroic materials which exhibit both ferroelectric and ferromagnetic characteristic like BiFeO3 had open various opportunity to manipulate the magnetization and electrical polarization of the material to be use in devices such as transducers and sensors.
CHAPTER 2
2.0 LITERATURE REVIEW
There are many studies that have been done on investigating the effect on structural and electrical properties by the elemental doping on metal titanates. From Hong-Wen Wang et al. [7] found out that when (Ba0.7Sr0.3)TiO3 thin films with various gold (Au) concentrations were prepared via a metal-organic deposition process, the X-ray diffraction shows enhanced crystallization as well as expanded lattice constants for the gold-doped BST films. Thermal analysis reveals that the gold dopant induces more complete decomposition of precursor for the doped films than those of undoped ones. The leakage current density of BST films is greatly reduced by the gold dopant over a range of biases (1-5V).
On the other hand, Hongtao Yu et al. [8] reported that addition of SrTiO3 onto CaCuTiO3 where the volume ratio of 6/4, it exhibit a high dielectric constant of about 2000 and low dielectric loss with good temperature stability at certain frequency.
Subsequently, M.C. Kao et al. [9] indicate the electrical properties of Mg-Al
co-doped Ba0.5Sr0.5TiO3(BST) films. It shows that the leakage current of Mg-Al doped BST films are lower than those of Mg-doped specimens.
Composite material, which contains of at least two chemically distinct materials have been widely investigated for electric and magnetic applications. The dielectric constant variation with frequency of the composites with the composition of xCuFe2O4-(1-x)BiFeO3 [21] showed the dielectric constant decreases with an increase in frequency demonstrating dispersion in a lower frequency range and remains comparatively constant at frequency beyond 10kHz. A similar behavior is also gained from other report for different composites [22].
Another report on multiferroic ceramics was done by Felicia Prihor Gheorghiu et al. [23] illustrating the antiferromagnetic behavior of BiFeO3 which indicates by linear M(H) dependence is turned into a weak ferromagnetic state by addition of BaTiO3. This similar properties also can be observed in compounds of BiFeO3(1-x)-PbTiO3 [24] which explained the result in large magnetization of the materials is due to the occupancy of Ti4+ in B site, leading to destruction of the canting spin structure.
Study done by R. Grossinger et al. [17] investigates the magnetoelectric effect of BaTiO3-CoFe2O4 composites.
Figure 3: Set-up for measuring the magnetoelectric coefficient at room temperature [17].
A DC-field is applied defining a certain working point. Additionally, a small AC-field produced by Helmholtz coils is superimposed. The ME voltage is measured using a lock-in amplifier. The signal on the sample is either connected first to a charge sensitive amplifier (measuring the charge) or connected directly as voltage to the high impedance (R= 100MΩ, C= 25pF) input of the lock-in amplifier. The ME coefficient is gained using equation
Where V, d and H are the ME voltage, the effective thickness of the sample and the amplitude of the applied magnetic AC-field, respectively.
Meanwhile, magnetostriction was measured in varying magnetic fields from 0 to 11,000G for the different composites (piezomagnetic phase of lithium ferrite and the piezoelectric phase of barium titanate) was reported by Sarah et al. [20]. Strain gages were set up on the surface of the pellets and a p-3500 strain indicator was used to measure the magnetostriction directly in micro strains.
Figure 4: Variation of magnetostriction as afunction of magnetic field. (All values of magnetostriction are negative and of the order of 10-6) [20].
The negative value of magnetostriction indicates that there is contraction in lithium ferrite under the influence of magnetic field. It is also shown that magnetostriction decreased with increasing barium titanate. The existence of a nonmagnetic phase of the boundaries of the piezomagnetic phase is the cause for the decrease in magnetostriction with addition of barium titanate [20].
CHAPTER 3
MethodOLOGY
3.1 Chemical used
Bi(NO3)35H2O
Mg(NO3).6H2O
Fe(NO3).9H2O
Citric acid
Ethylene glycol
3.2 Sample preparation
The nanocomposites is prepared by wet chemical solution method with x varying from 0.1 to 0.6. Ferric nitrate Fe(NO3).9H2O, magnesium nitrate Mg(NO3).6H2O, bismuth nitrate Bi(NO3)35H2O, citric acid and ethylene glycol are used in appropriate molar proportion. The precursor solution is dried at 80oC for 5 hours to obtain (1-x)BiFeO3-(x)MgFe2O4 xerogel powders. The xerogel initially started to swell and filled the beaker producing foamy precursor. Then, the xyrogel powders are ground and the resulting powders are annealed at various temperatures ranging from 500 to 800oC to obtain desired phase.
3.3 Characterization
Structural characterization of the powders is carried out using X-ray diffraction (XRD). Transmission electron microscopy (TEM) is used to observe particle morphology and average particle size. Magnetic measurement is also characterized by certain set up [17].
Meanwhile, La-modified BiFeO3 powders (Bi0.5La0.5FeO3) is synthesize by ferrioxalate precursor route.
1. Bismuth (III) nitrate pentahydrate [Bi(NO3)3. H2O], iron (III) nitrate nanohydrate [Fe(NO3)3.9H2O] are used as the precursor.
2. These precursors of required composition are dissolved in nitric acid.
3. Oxalic acid then is added to the solution under constant stirring.
4. The solution is then heated on a hot plate under continuous stirring to its boiling temperature to get the powder.
5. These fine powders is dried at 150oC and then sintered at different temperature for 1 hour in air after cooling by furnace.
Bi(NO3)3. H2O + La(NO3)3.6H2O + Fe(NO3)3.9H2O
Adding
HOOCCOOH.2H2O + HNO3
Stirring and heating
Yellow mass powder
Sintering
Bi1-xLaxFeO3 powder
Sample measurement
Phase of compound measurement
(X-ray diffractometer)
Microstructure of the samples measurement
(Scanning Electron Microscope)
Magnetic characterization
