Synthesis and Characterization of Ba2+, La3+, Zr4+ Co-Modified Multiferroic BiFeO3

The objective of this research project was to reduce the electrical leakage, enhance the magnetic and electric property as well as to stabilize the BiFO3 structure by co-doping La, Ba and Zr. At first the solid state sintering process was done to synthesize the Bi.8LaXBa0.2 − xFe.95 Zr.05O3 (where, x = 0.05, 0.10, 0.15, 0.20). X-ray diffraction data revealed a stabilized R3C structure with the dopants. The amount of zirconia in B-site resulted in the reduction of vacancy which ultimately reduced leakage current. For the higher percentage of Ba2+ the oxygen vacancy increased which led to higher leakage current. Thus for increasing the La3+ up to 15 mole % with decreasing Ba2+ dielectric constant increased For further addition of La3+ (i.e. 20%) the dielectric constant decreased again. From the SEM analysis the coherent grain size was confirmed for the temperature at 950°C. With the increase of La3+ and decrease of Ba2+ the grain size increased. For the composition with 5% of La3+, the p-loop was found to be satisfactory with the remnant polarization of 0.25 (µC/cm2) and the coercivity of 330V. However the p-e loop for the other compositions was found to be unsatisfactory.


1.Introduction
Multiferroics are multifunctional materials which display more than one ferroic order in the same phase. In recent years, multi-ferroics have been drawing the attention of researchers due to their unique behavior of coupling between two or more properties of ferroelectricity, ferromagnetism and ferroelasticity and also because of their potential technological applications in devices in spintronics, information storage, sensing and actuation. There are many magnetic and ferroelectric materials, there are relatively few multi-ferroic materials. BiFeO3 (BFO) is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. Bismuth ferrite is one of the most promising multi-ferroics and at room temperature, bulk BiFeO3 presents a rhombohedrally distorted perovskite structure belonging to the space group R3c. Compared to other multi-ferroics, BFO exhibits a higher ferroelectric Curié temperature (TC ~830 ℃) and a high G-type antiferromagnetic ordering (TN ~370 ℃) temperature. Bulk BiFeO3 suffers from a high leakage current, thought to be the result of vacancies invariably produced due to the volatility of Bi2O3. High leakage current translates into loss of stored charged. However, it is very difficult to observe the ferromagnetic loop of a bulk BiFeO3 sample at room temperature due to its low spiral spin modulation which is caused by defects and nonstoichiometry in the bulk BiFeO3 sample. Hence, it is essential to improve the ferromagnetic properties of BiFeO3 without much disturbing its ferroelectric properties before its actual use in device application. And the formation of oxygen vacancies induces impurity energy level in the band gap and increases the free carrier density by hopping of electrons to these defect levels. The weak magnetization is attributed to the superimposition of a spiral spin structure on BFO antiferromagnetic order.The main purpose of this research was to determine the effects of Ba and La along with co-modification by Zirconia on bismuth ferrite, with a view to improving its dielectric and magnetic properties. Solid state process was the route of choice at first. Sintering was done at 950°C to obtain coherent grain size. The room temperature magnetization improved remarkably compared to pure BFO after co-modification, more work needs to be done before its implementation in real life applications. The increased amount of Zr 4+ resulted in the reduction of leakage current. For the higher percentage of Ba 2+ the oxygen vacancy increased which led to higher leakage current. Thus for increasing the La 3+ up to15 mole % with decreasing Ba 2+ content, dielectric constant increased. For further addition of La 3+ (i.e. 20%) the dielectric constant decreased again.

. E x p e r i m e n t a l
At first, constituent powders were mixed at proper amount depending on the doping level. Next, the mixture was ball milled at 120 rpm in acetone for 18 hrs. Finally, the mixture was pressed into pellets using a hand pressing unit and subsequently sintered. The basic raw materials used are powders of Bi2O3, Fe2O3, La2O3, ZrO2 and BaCO3. These powders were taken according to the target composition of the batch to be prepared. . Four compositions were chosen for this thesis: a) Bi0.8La0.20Fe0.95Zr0.05O3 b) Bi0.8La0.15Ba0.05Fe0.95Zr0.05O3 c) Bi0.8La0.10Ba0.10Fe0.95Zr0.05O3 d) Bi0.8La0.05Ba0.15Fe0.95Zr0.05O3 Then ball milling was done for proper mixing and homogenization of the raw materials. The powders of different compositions were taken into different high density polyethylene (HDPE) pots serving as the milling chamber. Y2O3 stabilized ZrO2 balls, together with acetone, were used as the grinding medium. All the compositions were milled for 18 hours. Then the dried product was taken into a mortar and grinded to form fine powders. After milling, the mixture taken into suspension in acetone and then carefully extracted into a glass beaker (which was ultrasonically cleaned). The beaker was covered with a perforated aluminum foil and kept in a drier at around 100°C. It was important to keep the mixtures in the drier for a sufficiently long time, so that a completely dry powder mixture was obtained. Then the powder was calcined at 800°C for 2 hours. As we used BaCO3, we need to calcine to form BaO. That's why calcination is done. Around 0.6 gm of powder was taken and pressed into disk-like pellets using a manual hydraulic pressing machine. 2 tons of load was applied for 60 seconds. The pressed pellets were then dried in an oven for at least 24 hours to enhance their green strength. Then sintering is done at 950°C to get BiFeO3 pellet of required composition. Then density measurement, SEM Dielectric properties measurement by impedance analyzer , Ferroelectric properties measurements were done.

. R e s u l t s a n d D i s c u s s i o n
3.1 XRD Analysis: XRD analyses of the samples were done at room temperature. The diffraction patterns obtained are shown in Figure 1 for Bi0.8LaxBa0.2-xFe0.95Zr0.05O3 compositions. All the samples were sintered at 950°C. The pattern of x = 0.05 sample showed that BFO structure was indeed formed. The peaks are not identical to pure BFO; there are some differences in peak positions and some split peaks seem to merge upon modification with 20% La. This is in accordance with previous reports [1,2] and is assumed to be the result of space group modification, i.e. the space group is no longer R3c. The diffraction pattern is nearly the same for x = 0.1. And there is no identical 2 nd phase present in any of the sample Figure 1 : Room temperature XRD patterns for Bi0.8LaxBa0.2-xFe0.95Zr0.05O3 sintered at 950° C. Figure 2 shows the peak around 29.5-31.5 degrees more closely for different compositions. It can be readily seen that the peak gradually shifts towards higher theta values with increasing La 3+ content & decreasing Ba 2+ content. Since the theta value (θ) and the planar spacing (d) for a given peak are related by the equation, λ = 2dhkl sin (θhkl) 3 1234567890''"" Where λ is the wavelength, it follows that higher theta values correspond to smaller d values, i.e. lattice spacing. So it can be said with increasing La 3+ content & decreasing Ba 2+ content results in change of the lattice parameter, and this is expected because differences in ionic size. Looking now at the patterns for La-15% Ba-5% & La-20% Ba-0%, we see that the peak in question is quite prominently split indicating that whatever structural modification was set in by La-substitution. The peaks at this position belong to the [100] family of planes in the pseudo-cubic cell. All six [100] planes have the same d-spacing in cubic and rhombohedral symmetry and so this peak splitting indicates that the symmetry has been lowered to at least tetragonal, possibly even lower. Obtaining dense samples is one of the important parts of the optimization process. The best properties will be obtained for those samples that have both high %TD and desirable grain size. Figure 3 shows that Bi0.8La0.05Ba0.15Fe0.95Zr0.05O3 sintered at 900°C contain large amount of porosity. And average grain size is ~ 0.2 μm. It is evident that densification had not occurred to any respectable degree and higher temperature would be required for obtaining proper microstructure. This situation is improved in the case of sintering at 950°C. Porosity decreases sufficiently and the grain size also increased to ~ (0.6-0.7)μm. Literature review reveals that the Ba 2+ ion substitution in BiFeO3 has a strong grain growth inhibiting effect in the microstructure [20]. Hence, sintering temperature at 950°C is seemed to be optimum for Bi0.8La0.05Ba0.15Fe0.95Zr0.05O3. The %TD and grain size achieved in this temperature range was satisfactory.  Figure 4 shows the micrographs of Bi0.8La0.10Ba0.10Fe0.95Zr0.05O3 sintered at various temperatures. Sintering of Bi0.8La0.10Ba0.10Fe0.95Zr0.05O3 at 850°C shows the similar effect of porosity as obtained in the case of Bi0.8La0.05Ba0.15Fe0.95Zr0.05O3 sample sintered at 900°C. Here sintering is not up to the mark. The situation is improved at 950°C where porosity decreases to a significant amount and grain size increases. Average grain size is found to be ~1 μm. Bi0.8La0.15Ba0.05Fe0.95Zr0.05O3 sintered at 900°C shows Figure 5(a) significant amount of porosity which on increasing sintering temperature gives better densification at 950°C. But the grain size here is much larger (~3 μm). And there is some amount of porosity too. Hence, optimum sintering temperature for Bi0.8La0.15Ba0.05Fe0.95Zr0.05O3 should be 950°C. Figure 6 shows the micrographs of Bi0.8La0.20Fe0.95Zr0.05O3 sintered at various temperatures. Sintering of Bi0.8La0.20Fe0.95Zr0.05O3 at 850°C shows the similar effect of porosity as obtained in the case of Bi0.8La0.20Fe0.95Zr0.05O3 sample sintered at 900°C. Here sintering is not up to the mark. The situation is improved at 950°C where porosity decreases to a significant amount and grain size increases. Average grain size is found to be ~1 μm.  La and Ba have important effect on grain growth. La and Ba have dissimilar identity regarding the grain growth effect. Generally, the increased amount of La results in the increase of grain size as well as helps grain growth. As when amount of La is increased the round grain transformed to elongated grain and root mean square surface roughness is decreased as well. On the other hand, Ba impedes or retards the grain growth. From the figure for La (5%) & Ba (15%) in 7(a), As the amount of La is less in compared to other compositions available for La , the grain growth increased effect is least and at the same time the increased percentage of Ba results the impede of grain growth. Thus the overall grain size in the microstructure is smallest.  La-5% Ba-15% La-10% Ba-10% La-15% Ba-5% La-20% Ba-0%

Dielectric study:
The dielectric constant measures the polarizability of a medium. Upon application of an electric field, the medium which polarizes more and does not conduct electricity will be the one to show a higher dielectric constant. Already present polarization, if any, of the material does not contribute to the dielectric constant; it is the change in polarization with electric field which contributes. Thus if a material contains dipoles in its structure, but the dipoles do not change direction or change in magnitude appreciably with applied electric field the dielectric constant will be low.
As we know, the crystal structure of bismuth ferrite is non-centrosymmetric, and the unit cell does have a polarization in the pseudo-cubic [111] direction. The ferroelectric nature of the material allows the dipoles to reverse direction in response to an oscillating electric field. At low frequencies these dipoles can follow the electric field and contribute to the dielectric constant. But as the frequency is increased a point is reached where, the dipoles cannot reorient fast enough and so a drop in dielectric constant is expected. In this range only electronic polarization contributes to the dielectric constant. The reason behind this phenomenon has been described above. Something that's not explained by the above discussion however, is the abnormally high dielectric constant for BLFO at low frequencies. This can be attributed to Maxwell-Wagner type contribution to the dielectric constant. This effect is related to space charge relaxation at the interface. The space charges can arise due to vacancies in the bismuth or oxygen site i.eVO2 + ,VBi 3-. These space charges can follow the electric field at low frequencies but do not have enough time to build up at the interface at high frequencies. Thus that contribution steadily decreases as frequency is increased. At lower frequencies, the net contribution from various polarizations viz. space charges, ionic defects, permanent dipoles, induced atomic and ionic dipoles, contribute dielectric resulting in high values [3]. As frequency increases, different participating mechanisms for dielectric constant decrease gradually and subsequently. In the present scenario, Ba 2+ substitution in Bi 3+ ions increases the oxygen vacancy in the lattice of Bi0.8Ba0.2FeO3 resulting in the increase of space charge [4]. Thus, for Bi0.8Ba0.2FeO3 samples at low frequency show a high value of dielectric constant. The observed dielectric constant decreases rapidly with the frequency, the decrease of dielectric constant with increase in frequency can be explained by a dipole relaxation phenomenon which is due to Maxwell-Wagner type of interfacial polarization contribution [5]. Here, the effect of Ba is absent. So, the leakage is higher. So, the dielectric constant is lower. Figure 9: Frequency dependence of dielectric loss (D-factor) for Bi0.8LaxBa0.2-xFe0.95Zr0.05O3 sintered at 950°C Figure 9 represents the frequency dependence of dielectric loss. Here same results have been found in the frequency range of (0.1-100 kHz). We know that higher dielectric loss means higher leakage current. For Ba-0%, dielectric loss is minimum. Increasing Ba content means increasing dielectric loss. But for Ba-15%, dielectric loss is lower for lower frequencies.

Ferroelectric property analysis
When Ba is doped in the BFO then it results in the creation of oxygen vacancy. As a result of the increase of the oxygen vacancy it generates electrons which ultimately increase leakage current. Thus increased amount of Ba result in the decrease of the remnant polarization. From the composition of La (5%) and Ba (15%) the ploop found was satisfactory. It gives a decent identical curve on the polarization Vs electric field axes. And from that curve the precise data of remnant polarization and the coercively can be found. The comparison with the other composition cannot be done in our research. As the P-Loop obtained from the other variable compositions is not satisfactory as well as improper. As the P-Loop obtained from other compositions are unsatisfactory, this may happen due to the lack of sample preparation, improper surface area calculations or mishandling during technical operations.

Conclusion:
The main purpose of this research was to determine the effects of Ba and La along with co-modification by Zirconia on bismuth ferrite, with a view to improve its dielectric and magnetic properties. Solid state process was the route of choice at first. Sintering was done at 950C to obtain coherent grain size. The room temperature magnetization improved remarkably compared to pure BFO after co-modification, more work needs to be done before its implementation in real life applications. The followings are in brief the highlighted findings from this research work.
Pure BFO produced by solid state process exhibited less amount impurity which was confirmed by XRD analysis. When dopants were used unknown impurities were not found in the XRD analysis. It also found that with the increase of La 3+ the grain size increased. Bi0.8 Lax Ba. 02-x FeO0.95 ZrO0.05 (x=0.05, 0.10, 0.15, 0.20) were successfully produced by solid state methods which was confirmed by XRD analysis that indicated formation of the main BFO structure in all the samples. The increased amount of Zr 4+ resulted in the reduction of leakage current. For the higher percentage of Ba 2+ the oxygen vacancy increased which led to higher leakage current. Thus for increasing the La 3+ up to15 mole % with decreasing Ba 2+ content, dielectric constant increased. For further addition of La 3+ (i.e. 20%) the dielectric constant decreased again. For the compositions with 5% of La 3+ the p-loop was found to be satisfactory with the remnant polarization of 0.25 (µC/cm 2 ) and the coercivity of 330V. However the p-loop for the other compositions was found to be unsatisfactory.