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Tunable multiferroic properties of cerium doped bismuth ferrite
Rozina Patel1*, Pratibha Sawadh2
1Department of Physics, New Arts, Commerce and Science college, Wardha, India 2Department of Physics, Bapurao Deshmukh College of Engineering, Wardha, India
DOI 10.17586/2220-8054-2019-10-3-255-265
Multiferroic Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) nanoparticles were prepared using an auto-combustion method. The effect of cerium substitution on the crystal structure, electrical and magnetic properties of BiFeO3 (Bismuth Ferrite) was studied. X-raydiffraction spectra revealed that Ce substitution increases the lattice parameters. The average particle size estimated from TEM images is less than 50 nanometers. According to the magnetic hysteresis loops, it was found that the enhanced magnetization which results from increasing Ce concentration is attributed to the nanoparticle size and enhanced ferroelectric polarization. The magnetization and ferroelectric polarization were found to exhibit an exotic mutual relationship. The samples were found to exhibit relaxation of dielectric polarization. This rare combination of Ce doped BiFeO3 by auto-combustion method has not been reported before.
Keywords: Multiferroic, nano particles, X-ray diffraction, ferroelectric, ferromagnetic . Received: 9 March 2019 Revised: 24 May 2019
1. Introduction
One of the reasons for fabricating novel materials for attaining rich functionality is to unite different physical properties in single material. Magnetic and Ferroelectric materials are enduring subject of study and have led to notable progress in the development of technology to date. These apparently distinct phenomena can coexist in certain unusual substances, termedas multiferroics which are of key importance, both from research as well as technological perspective.
Multiferroic materials exhibit the phenomena of ferroelectricity (FE), ferromagnetism (FM) and ferroelasticity simultaneously in their singlephase [1-3]. Although the possibility of coupling orinteraction between ferromagnetic and ferroelectric order parameters is known for quite long time, its existence and practical applications were realized only after the discovery of multiferroicity in BiFeO3.
Bismuth ferrite is found to be one such material which demonstrate a coupling between magnetic and ferroelectric order, having Curie temperature TC « 1100 °K and antiferromagnetic temperature TN « 640 °K [4,5]. Bismuth ferrite possesses G-type antiferromagnetic ordering because of its complex cycloidal spin structure with a wavelength of 62 nm with [110] spiral direction and (110) spin rotation plane [6]. However BiFeO3 suffers from several major issues like phase defects such as oxygen vacancies and impurity, high leakage current, spiral spin structure, large difference in transition temperature (TC & TN). The leakage current a rises in BiFeO3 due to its nonstoichiometry. This is particular lybecause of the difficulty in obtaining stoichiometric singlephase BiFeO3 materials. In order to reduce leakage current density efforts have been made by using different fabrication methods. Several techniques have also been recently adopted to eliminate secondary phases and oxygen vacancies [7-11]. Particle size also plays a vital role in controlling the structural and magnetic properties of BiFeO3 [12]. The authors [13] described the necessity to lower the sintering time during the increasein synthesis temperature to confine the formed BiFeO3 crystals within the nanosize range. The current trend is to avoid brute force methods in order to have a better control of stoichiometry, structure, and phase purity of metal oxides. The authors [14]reported cerium-substituted BiFeO3 powders using hydrothermal method revealed the lattice contraction and decrease of grain size. Synthesis of Ce substituted BiFeO3 nanoflakes [15] using sol-gel method has resulted in enhancement of electrical properties. The combustion or fire synthesis is quite simple, fast, and economical. It is possible to control the particle size of solid combustion product to nano-range by using suitable fuels that control the nature of combustion. Urea is documented as an ideal fuel for the combustion synthesis of high temperature oxides [16]. In this work, auto combustion method was used for synthesis as it has not been reported for the synthesis of Ce doped BiFeO3 and urea was chosen as the fuel, which act as a self-catalyst and generates heat as its melting point is very low, which further helps in increasing the reactivity of the samples yielding a good result as compared to solid state process.
Attempts to improve the electrical properties have been made by doping it with rare earth elements such as lanthanum (La), samarium(Sm), gadolinium (Gd) and dysprosium (Dy) etc. [17,18]. Efforts have also been made to
overcome these issues by doping transition metals [19] into BiFeO3, also modifying material by including solid solution of BiFeO3 with BaTiO3 etc. [20,21]. Difficulties also prevail in the practical realization of BiFeO3, resulting from weak ferromagnetism exhibited by BiFeO3, as the spiral spin modulation, superimposed on G-type antiferro-magnetic spin ordering [22,23], cancels out any possible net magnetization. One means for suppressing spiral spin modulation in BiFeO3 is the chemical substitution in the A-sublattice [24-29]. Diamagnetic substitution at A-site in BiFeO3 has been shown to enhance the net magnetization of parent material because of the kind of diamagnetic dopant element [30,31].
In the present work, we have incorporated the compositional variations which have permitted the attainment of improved multiferroic properties. Cerium was chosen to substitute at A-site of BiFeO3 because the ionic radius of Ce3+ is comparable with that of Bi3+. The electronic configuration of Ce3+ also causes hybridization resulting in formation of Ce-O bond [32] leading to a noncentrosymmetrically distorted structure and a suppressed spiral spin structure, probably improving the ferroelectric and magnetic properties. The partial substitution of Bi3+ ions by rare earth ions are reported to improve multiferroic properties of BiFeO3 [33-37].
2. Experimental
The samples with the composition Bi1_xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) were prepared byan auto combustion method using urea as a fuel. The prime basis of auto combustion method depends on the proficiency of high lyexothermic reactions to be self-sustaining and,therefore, energetically efficient. The exothermic reaction is initiated at the ignition temperature, and generate sheat which is displayed in a maximum or combustion temperature (Tcomb), which can volatilize low boiling point impurities, and therefore result in purer products than those produced by more conventional techniques.
The precursor materials used for the synthesis of Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) bythe auto combustion route were analytical reagent grade bismuth nitrate pentahydrate (Bi(NO3)3 5H2O), ferric nitrate nonahydrate (Fe(NO3)3 • 9H2O), Cerium(III) nitratehexahydrate (Ce(NO3)3 • 6H2O) with a purity of more than 99 %. Analytical grade urea in a powder form with purity more than 99 % was used as fuel in the synthesis of the Bi1-xCexFeO3 powder. Appropriate quantities of materials were weighed in microbalance according to the stoichiometry to obtain (0.05, 0.1, 0.15, 0.2) trivalent Ce3+ dopant concentrations to replace trivalent Bi3+. Stoichiometry of there doxmix-ture for combustion is calculated based on the total oxidizing and reducing valencies of oxidizer and fuel. So that the equivalent oxidizer to fuel ratio becomes unity which results in release of maximum heat [38,39]:
0.8Bi(N03)3 • 5H2O + Fe(NO3)3 • 9H2O + 0.2Ce(N03)3 • 6H2O + 5CH4N2O ^ Bio.8Ceo.2FeO3(s) + 8N2(g) + 24H2O(l) + 5CO2(g).
According to propellant chemistry, the valencies of the elements carbon, hydrogen, nitrogen and oxygen are +4, +1, 0 and -2 respectively. The valency of nitrogen is taken as zero because of its conversion to molecular nitrogen during combustion. The valencies of metal depend upon metal ions in that compound. The valencies of the metals bismuth, iron and cerium are +3, +3 and +3 respectively. Thus, amixture of (Bi(NO3)3 • 5H2O), (Fe(NO3)3 • 9H2O), Ce(NO3)3 • 6H2O and urea in an appropriate molar proportion were thorough lymixed by grinding using agate mortar and pestle to form a homogeneous mixture. This homogeneous mixture was then poured into a crucible and was introduced into a 500 °C preheated mufflefurnace, which under goes self-propagating, gas producing combustion reaction to yield voluminous metal oxide in less than 5 minutes, which is a porous and foamy product [40]. The porous powder was ground in a mortar and pestle to obtain a fine powder. The ground powder was again placed into the furnace for sintering at 400 °C for 3 hours.
The samples were then characterized by using various techniques. X-raypowder diffraction data was collected using an XPERT-PRO diffractometer with CuKa radiation (A = 0.15456 nm) at step of 0.02 in the range 2d = 200 to 800. The magnetic properties were measured by a vibrating sample magnetometer (Lakeshore VSM 7410) at room temperature. Dielectric measurements were performed on an impedance analyzer (WeynnKerr6500B) for which the sintered samples were mechanically pressed inh ydraulic press and an Ag paste was applied to both sides of the polished pellets. Ferroelectric hysteresis loops were studied by a P-E loop tracer.
3. Result and discussion
Room temperature powder x-ray diffraction was then carried out on Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2). Rhombohedral perovskite structure is retained even after substituting Ce3+ ions for Bi3+ in BiFeO3. The profile fit for the Rietveld refinement of Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) samples are shown inFig.1(a,b,c,d) respectively. Ce doping causes the peaks to shift toward lower 2d value this indicates that Ce is getting substituted in the BiFeO3 lattice.
(a)
(b)
(c)
(d)
Fig. 1. The fit for the Rietveld refined profile for the Bio.95Ceo.o5FeO3 (a); Bio.9Ceo.1FeO3 (b) Bio.85Ceo.i5FeO3 (c) and Bio.gCeo.2FeO3 (d) samples
The lattice parameters are found to increase with Ce doping because ionic radii of Ce3+ (radius = 1.01 A) is more than that of Bi3+ (radius = 1.03 A) ion [41]. The crystallite sizes of all the samples Bi1_xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) were found to be 16 nm, 19 nm, 22 nm and 27 nm respectively obtained by considering the most intense diffraction peak in the pattern using Scherer's formula.
The stimulated XRD patterns of sample coincide well with the measured XRD pattern with generally small Rvalues as illustrated in Table 1. The lattice parameters and other refined parameter are also tabulated in Table 1.
SEM micrograph of Ce doped samples exhibits finegrained structure with sharp grain boundaries with almost uniform diameter as shown in Fig. 2. The micrographs of the samples showed relatively greater homogeneity in the microstructure. Also the grain size of the samples decreases with increasing cerium in BiFeO3 which is a clear indication of incorporation of cerium into BiFeO3.
Typical TEM images of Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) samples prepared by the auto combustion method is shown in Fig. 3 respectively.
The average particle size estimated from TEM images for Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) were found to be 15 nm, 20 nm, 24 nm and 27 nm respectively, which is in accordance with XRD analysis. The particles are well connected with each other and are found to be approximate lysphericalin shape. The average particle size is found to decrease with Ce doping. Thus we can conclude that the substitution of Bismuth by rare-earth in multiferroic material BiFeO3is accompanied by a significant decrease in particle size [42].
Table 1. Details of Rietveld refined XRD parameters for Bi1-xCexFeO3
Parameters Bio.95Ceo.o5FeO3 Bio.9Ceo.1FeO3 Bio.85Ceo.15 FeO3 Bio.8Ceo.2FeO3
20 range (deg.) 20° to 80° 20° to 80° 20° to 80° 20° to 80°
Step size (deg.) 0.02 0.02 0.02 0.02
Wavelength 1.5406 AA 1.5406 A 1.5406 A 1.5406 A
Space group R3c R3c R3c R3c
a (A) 5.587 A 5.613 A 5.643 A 5.659 A
b (A) 5.587 A 5.613 A 5.643 A 5.659 A
c (A) 13.876 A 13.902 A 13.932 A 13.948 A
Volume (A3) 375.09 A3 379.3 A3 384.19 A3 386.82 A3
Rf 1.7 2.1 2.5 2.8
RBragg 2.95 2.8 2.6 2.35
Rwp 16 15.6 14.5 13.0
Rexp 13.8 14.0 15.6 17.6
Rp 12.4 13.5 14.4 15.4
X2 1.924 1.804 1.423 1.34
Fig. 2. SEM images of Bi1-xCexFeO3 nanoparticles (a) x = 0.05; (b) x = 0.1; (c) x = 0.15 and (d) x = 0.2
Thermal analysis of Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) samples sintered at 400 °C has been carried out with DTA (600 - 900 °C) to study the transition temperatures. DTA results for temperature range (800 - 900 °C) are shown in Fig. 4. For Bi1-xCexFeO3 respectively. A peak is observed for all the samples. Kaczmareket al. [43] have attributed the peak observed in DTA near 830 °C to ferroelectric phase transition (Curie temperature, TC) of bulk BiFeO3. This clear endothermic peak is of chief interest here.
The peak is shifted towards higher temperature with increase in Ce concentration. This increase in TC can be due to the decrease in pressure with increase in the cell volume. In BaTiO3, the various transition temperatures shift down on compression [44]. Similarly, lowering of ferroelectric transition temperature is observed when doped with smaller atoms [45].
The room temperature magnetic hysteresis loops for Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) samples are shown in Fig. 5 recorded at 300 K. It is noted that saturation is achieved in all the samples for an applied field of <10 KOe.
Fig. 3. TEM images of Bi1_xCexFeO3 nanoparticles (a) x = 0.05; (b) x = 0.1; (c) x = 0.15 and (d) x = 0.2
n
se
in f ■
- ■ i
* -n
-»C -
■o o- 111 tia |T1 Ma
T»Hip«*turt I'Cl
FIG. 4. DTA curve of Bi1-xCexFeO3 sintered at 400 °C
All the samples exhibited ferromagnetic ordering at room temperature. Owing to finite size the nanoparticles usually show unusual magnetic behaviors distinct from that of their bulk counterparts. In antiferromagnetic nanoparticles, due to the large surface to volume ratio, the contribution of uncompensated spins at the surface becomes higher.
Neel [46-48] proposed that the finite magnetic moment of nanoparticles of antiferro-magnetic materials is due to the presence of the seun compensated spin sat the surface of the particles. Apart from uncompensated spins at the surface, canting of spins in antiferromagnetic sublattices also plays an important role in the magnetic properties of the nanostructures [49,50]. Ce doping could have increased the canting angle, which in turn has resulted in enhanced magnetic properties. Another reason for the observed ferromagnetism is the suppression of spirals pin structure characteristic of BiFeO3. When the particle size is on the order of or <62 nm, this spiral spin structure changes so as to result in enhancement in magnetic properties. The variation of magnetic parameters and crystallite size is tabulated in Table 2. It is clear that Ce doping has strengthened the magnetic properties of BiFeO3 significantly.
Figure 6 shows P-E hysteresis loops for the Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) as a function of electric field. Ferroelectric parameters at room temperature for Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) samples are given in Table 3. All samples show typical ferroelectric behavior. According to Wang et al. [51], the ferroelectricity of the BiFeO3 samples originated from the displacements of Bi with respect to the FeO6 cages along the (111) plane. Experimental results suggested that partial doping of Ce ions in BiFeO3 acts as donor in oxygen octahedron and force the reduction of oxygen vacancies by restricting the formation of Fe3+ to Fe2+ ions, resulting in a great suppression of leakage current.
It is found that remanent polarization is found to increase with an increase in crystallite size. As the grain boundary is a low permittivity region, it has weak ferroelectricity. Therefore, polarization of grain boundary may be little or
—¡C0.05
-«-*=0.1
-§-1=0.15
f 1 1
-20000 -15000 -10000 -5000 0 5000 10000 15000 20000
Field
fig. 5. The M-H plot of Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) at 300 K Table 2. Details of Magnetic Parameters at Room Temperature for Bi1-xCexFeO3 samples
Sample Remanent Magnetization (emu/g) Saturation Magnetization (emu/g) Crystallite Size (nm)
Bio.95Ceo.o5FeO3 0.5 1.48 16
Bio.9Ceo.1FeO3 0.7 1.89 19
Bio.85Ceo.15FeO3 1.3 2.71 22
Bio.8Ceo.2FeO3 1.6 3.38 27
FIG. 6. Ferroelectric hysteresis loop of Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) at 50 Hz
Table 3. Details of Ferroelectric Parameters at Room Temperature for Bii_xCexFeO3 samples
Sample Electric field, Ec (KV/cm) Remnant polarization Pr (^C/cm2) Crystallite Size (nm)
Bio.95Ceo.o5FeO3 25 3.08 16
Bio.gCeo.1FeO3 38 4.21 19
Bio.85Ceo.15FeO3 46 5.02 22
Bio.8Ceo.2FeO3 63 6.8 27
even nonexistent. On the other hand, space charges in grain boundary exclude polarization charge on grain surface and depletion layer on grain surface can be formed. That results in polarization discontinuity on grain surface to form depolarization field and polarization decreases. The number of grain boundary increases as crystallite size decreases. Consequently, the remanent polarization increases as the crystallite size increases [52]. Also the remanent polarization is found to increase with Ce concentration, hence we may anticipate that replacement of highly volatile Bi3+ with Ce3+ reduces the concentration of oxygen vacancies giving reduced leakage current and thus improves ferroelectric properties.
The measured temperature dependence of dielectric constant e and loss for Bi1_xCexFeO3 (x = 0.05, 0.1, 0.15 and 0.2) samples at 100 Hz frequency are shown in Fig. 7(a,b), respectively. The peak in the dielectric constant is attributed to a transformation from the antiferromagnetic order to the paramagnetic order,indicating an effect of disappearing magnetic order as compared to electric order and attests to magneto-electric coupling in the samples. This type of anomaly near the Neel temperature has also been predicted by several studies [53-58]. Here, the peaks show a diffuse nature. The substitution of cerium shifts the dielectric peak to low temperature region and a diffuse dielectric peak results. The peak shift may be attributed to the slightly larger ionic radii of Ce3+ which replaces Bi3+ due to which tolerance factor decreases which in turn reduces the Neel temperature. Another significant result is peak broadening with increased cerium concentration.
Figure 7(b) shows the variation in dielectric loss tan S for Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) samples. Further as noted in the dielectric constant, anomalies are observed at 390, 380, 374 and 352 °C for x = 0.05, 0.1, 0.15 & 0.2 respectively. In addition, the peaks in dielectric constant e and dielectric loss tan S shifted towards a lower temperature with increased Ce3+ substitution, which indicates a decrease in the antiferromagnetic ordering temperature upon Ce3+ substitution.
Frequency (100 Hz - 1 MHz) dependence of real part of dielectric constant e' and loss for the Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) samples at 300 K is shown in Fig. 8(a,b) respectively. For all samples, the e' decreases with the increase in frequency and is consistent with combined response of orientational relaxation of dipoles and conduction of charge carriers. It may be attributable to the fact that the intra well hopping probability of charge carriers dominates and dipoles are unable to follow the field reversal in such a small interval of time at higher frequencies. It has been observed that e' is increasing with substitution of Ce. Variation in tan S with frequency at 300 K for Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) samples is shown in Fig. 8(a,b).
It is evident that tan S also decreases with an increase infrequency. The increase in the dielectric constant may be attributed to the suppression of oxygen vacancies due to the substitution of Ce3+ at Bi3+ site. Since the substitution of Ce at Bi site is ascribed to compensation of the volatile Bi component in BiFeO3, which causes reduction in the oxygen vacancies, which could have been generated to compensate the positive charge deficiency caused by the volatization of Bi [59,60]. As a result, the number of oxygen vacancies decrease, therefore Bi0.8Ce0.2FeO3 has the highest e value among all the Ce doped compositions.
4. Conclusions
It can be concluded from the present work that auto-combustion synthesis technique can be an advantageous method to prepare single phase cerium-doped BiFeO3. The XRD pattern showed rhombohedrally distorted perovskite structure for cerium-doped BiFeO3. The crystallite size is found to increase with increasing cerium concentration. Magnetization is found to increase considerably in all doped samples due to canting of spins and particle size less than 62 nm, which is less than the periodicity of BiFeO3. Also cerium-doped BiFeO3 sample shows fairly good ferroelectric behavior with increased remanent polarization with increasing crystallite size. Increasing the cerium concentration is found to reduce the N' eel temperature, which is observed from the temperature dependant dielectric
Fig. 7. (a) Temperature dependent dielectric constant at 50 Hz; (b) Dielectric loss of Bi1-xCexFeO3(x = 0.05, 0.1, 0.15, 0.2) at 50 Hz
anomaly. This dielectric anomaly observed in all the samples is a signature of magneto-electric coupling. Dielectric constant also shows strong frequency dependence for all the samples, indicating the usual dielectric dispersion. It is anticipated that appropriate doping at A Site of BiFeO3 can enhance the intrinsic magnets and ferroelectric properties as well as generating novel functionalities.
(a)
(b)
Fig. 8. (a) Real part of Frequency dependent dielectric constant; (b) Dielectric loss of Bi1-xCexFeO3 (x = 0.05, 0.1, 0.15, 0.2) at 300 K
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