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Modulation of magnetic, ferroelectric and leakage properties by HoFeO3 substitution in multiferroic 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions
Ceramics International 46 (2020) 212–217
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Modulation of magnetic, ferroelectric and leakage properties by HoFeO3 substitution in multiferroic 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions
T
Chiranjib Chakrabartia, Qingshan Fua, Xinghan Chena, Yang Qiub, Songliu Yuana,∗, Canglong Lia a b
School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, PR China Key Laboratory of Microelectronics and Energy of Henan Province, School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang, 464000, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Multiferroic Ferromagnetic Ferroelectric Bismuth ferrite
An investigation on the structural, magnetic, electrical properties of HoFeO3-substituted 0.7BiFeO30.3Ba0.8Ca0.2TiO3 solid solutions synthesized using conventional solid state reaction method was carried out. The structural study confirms that an additional orthorhombic (Pbnm) phase of HoFeO3 appears in the ceramic matrix and the presence of the aforementioned phase significantly influences the magnetic characteristics in the solid solutions. A detailed high temperature dielectric study suggests that the oxygen vacancies can be effectively controlled by the appropriate amount of substitution, successively regulating the ferroelectric as well as the leakage properties in the ceramics. Furthermore, the foremost remnant polarization and the feeble dielectric loss is achieved when the substitution content is x~0.2. Therefore, an appropriate amount of HoFeO3 substitution can be an effective way to modulate the multiferroic properties in the present ceramics.
1. Introduction Since the past decade, a very rare class of materials, known as multiferroic materials, have attracted enormous attention due to their application possibilities in future generation data storage, sensors, spintronic devices, etc. Generally, two different ferroic orders, such as ferromagnetism and ferroelectricity, simultaneously occur in multiferroics [1]. However, most of the available multiferroics having a very low operating temperature. One of the extensively studied (ABO3) type room temperature multiferroic ferroelectric perovskite is BiFeO3 (BFO), which is a G-type antiferromagnet with a high Néel temperature (TN) ~ 643 K as well as a high Curie temperature (Tc) ~ 1100 K [2]. The bulk form of BFO suffers from some basic challenges such as poor resistivity, long range cycloidal order, generation of impurity phases, etc. [3–5]. To improve the magnetic, ferroelectric and resistive properties of BFO, different measures have been taken, including rare earth or transition metal elements substitution in Bi or Fe-site, the formation of binary solid solutions with other ABO3 –type perovskites, such as, BaTiO3, Ba0.75Ca0.25TiO3, Bi0.5Na0.5TiO3, etc. [6–9]. Among them, improvements of the ferroelectric properties in BiFeO3–BaTiO3 ceramics were reported earlier, although, poor ferromagnetic property and poor resistive nature makes them less suitable for practical applications [10]. Recently, Zhang et al. reported enhanced electrical as well as
∗
magnetic (Mr = 0.06 emu g−1) properties by introducing a rare earth orthoferrite LaFeO3 in 0.7BiFeO3-0.3BaTiO3 ceramics [12]. Improvement of magnetic and electric properties in different BiFeO3 based ceramics by various rare earth orthoferrites (ReFeO3) had been reported earlier as well [2,10]. HoFeO3 is a distorted perovskite which holds a G-type antiferromagnetic structure with a high Néel temperature (TN) ~ 653 K [11]. Therefore, a suitable amount of HoFeO3 addition in BiFeO3 based ceramics can be advantageous for the modulation of multiferroic properties. However, to the best of our knowledge, the effect of HoFeO3 addition in similar ceramics was not reported before. In this work, 0.7 ((1-x)BiFeO3-xHoFeO3)-0.3Ba0.8Ca0.2TiO3 (BFOBCTO) ceramics (x = 0–0.3) were prepared by conventional solid state reaction method and the composition (x) dependent magnetic, ferroelectric and leakage properties are systematically studied. 2. Experimental details A conventional solid state reaction process was used to synthesize multiferroic 0.7 ((1-x)BiFeO3-xHoFeO3)-0.3Ba0.8Ca0.2TiO3 solid solutions of x = 0–0.3. Stoichiometric amounts of analytical grade Bi2O3, Ho2O3, Fe2O3, BaCO3, CaCO3 and TiO2 were hand grinded using a mortar and pestle for 5 h. The mixtures were placed into an oven and then calcined at 1073 K for 3 h. Thereafter, the calcined powders were ground for further 3 h after adding 2% wt. PVA. Finally, the ground
Corresponding author. E-mail address: [email protected] (S. Yuan).
https://doi.org/10.1016/j.ceramint.2019.08.250 Received 26 June 2019; Received in revised form 3 August 2019; Accepted 26 August 2019 Available online 26 August 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Ceramics International 46 (2020) 212–217
C. Chakrabarti, et al.
Fig. 1. X-Ray Diffraction patterns measured at room temperature in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions (a) refined versions of XRD peaks appearing for x = 0 (b) x = 0.2 (c) and variations in lattice parameters as well as cell volume with respect to x (d).
x. The XRD refinement results indicate that the additional peak well fitted with orthorhombic Pbnm phase of HoFeO3 (COD ID: 2107086) and the goodness of fit indicator (s) is ~1.7 (i.e. < 2) for all of the substituted samples, showing a superior match between the observed and determined values. The appearance of nonpolar RFeO3-type orthorhombic phases, such as, LaFeO3 and SmFeO3, were previously reported in other rare earth substituted similar ceramics and were verified by the refinement results [2,10,12]. In addition, thermal (TG-DTA) analysis of the ceramics is confirming the appearance of an additional phase near 1068 K (see Fig. S1 in the supplementary information section), which can be attributed to the formation of HoFeO3 phase in the ceramics. As shown in Fig. 1(d), with an increase in the substitution level, the lattice parameters, as well as the cell volume (v), declines ensuring a structural deformation in the ceramics. Since the ionic radius of Ho3+ is smaller than Bi3+, the decreasing trend in lattice parameters, as well as in the cell volume of R3c phase, can be attributed to the ionic radius difference between the partially substituted Bi3+ (1.14 Å) ions and comparatively smaller size of Ho3+ (1.015 Å) ions [13]. The declining tendency in lattice parameters and the unit cell volume of R3c phase were previously reported in rare-earth doped BFO ceramics [16,17]. For further increase in the x i.e. up to x = 0.3, unit cell volume and lattice appears to be slightly increased, which might be because of the increased strain at the rhombohedral and orthorhombic phase boundary, as has been reported before in case of Ho doped BFO ceramics [18]. The room temperature hysteresis (M-H) loops, are shown in Fig. 2, for pristine and HFO-substituted samples at the maximum field of ± 30 kOe. The MH loop of the x = 0 sample shows a very weak ferromagnetic response at room temperature. When HFO is included in the BFO-BCTO matrix, magnetic nature of the samples are considerably
mixtures were pelletized (8 mm diameter) and sintered at 1273 K for 3 h. To measure the electrical properties of the samples, silver paste was applied to both sides of the pallets. Philips Panalytical X’ pert diffractometer (XRD) with Cu-Kα radiation (λ = 1.5406 Å) was deployed for the structural analysis of the ceramics at room temperature. TG-DTA analysis was done using a Diamond TG/DTA Analyzer (PerkinElmer Instruments). To perform the room temperature magnetization hysteresis (M-H) study, a physical property measurement system (PPMS, Quantum Design) was used. Ferroelectric, leakage and dielectric properties were investigated by employing a ferro-electric tester (Precision Premier II, Radiant Technology) as well as a Precision Impedance Analyzer (WAYNEKERR-6500B), respectively. 3. Results and discussions The room temperature XRD patterns as well as the phase evolution for 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions with different (x = 0–0.3) HoFeO3 contents, in the range of 2θ = 20° −80°, are analyzed using a Panalytical X-pert pro software, as depicted in Fig. 1(a–d).The corresponding Reitvled refinement fitting, for the x = 0 and the x = 0.2 samples were performed using a Full prof suite software (shown in Fig. 1(b) and (c), respectively). The sample of x = 0 can be well fitted with the perovskite tetragonal P4/mmm and the rhombohedral R3c symmetry, which is very much consistent with the earlier reports in similar ceramics, where it was identified as rhombohedral (R)-tetragonal (T) morphotropic phase boundary (MPB) [9]. However, with increasing x, an additional phase along with the R and T phase appears in the sample near 2θ~32° (when x = 0.05 or higher) and the intensity of the diffraction peak gradually increases with the increase in 213
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sample. In addition, the substitution of comparatively smaller ionic radii of Ho3+ ions in BFO manipulates the lattice structure and the upshot of which is the suppression of space modulated spiral spin structure in BFO and hence the magnetization increases. Fig. 3(a–d) shows the temperature dependence of dielectric constant (εr) for the x = 0, 0.1, 0.2 and 0.3 samples at a frequency range 10 kHz −2 MHz. At a low frequency, dielectric anomalies were observed, which is marked with blue arrows and the measured dielectric anomalies for a relatively lower frequency of 10 kHz were found 576 K, 555 K, 547 K and 549 K for the x = 0, 0.1, 0.2, 0.3 samples, respectively. With increasing x, Tm shifts toward lower temperature; probably due to the dilution of polar coupling by the partial substitution of the less polarisable Ho3+ in Bi3+, which was reported previously in substituted BiFeO3 based ceramics [19,20]. Moreover, the anomaly peak (Tm) shifts toward the higher temperature with the increase in frequency and the sharp decrease in the magnitude of the dielectric maxima was observed, indicating relaxor like frequency dispersion. Similar relaxor-type behavior was reported earlier in other BFO based solid solutions, which may be attributed to the charge imbalance at the A and B site of BFO due to the substitution [2]. So as to explore the relaxation mechanism in the x = 0–0.3 samples, the variation of dielectric modulus M//with temperature is investigated, as shown in Fig. 4(a–d). The dielectric relaxation at high temperature is fitted and represented in Fig. 5 according to the well-known Arrhenius law:
Fig. 2. Room temperature hysteresis curves measured in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions and remnant magnetization and coercivity as functions of x (inset).
effected, as represented in the inset of Fig. 2, indicating a variation of remnant magnetization (Mr) and coercivity (Hc) with respect to the substitution content (x). Interestingly, with increasing x to 0.3, the Mr increases monotonically, whereas, the Hc value climbs to the maximum value near x = 0.1 (Mr~0.08) and thereafter decreases sharply with the further increase in x. The variation of Mr and Hc is very much consistent with the previous report on BiFeO3–x (0.5CaTiO3–0.5SmFeO3) ceramics [2]. Therefore, HFO addition in the sample significantly changes the magnetic nature in the ceramics. Highly ferromagnetic nature of Ho probably brings new exchange interactions in the ceramics, such as Ho–Ho and Ho–Fe –type, which is supplementary to the conventional Fe–Fe interaction in the pristine
f = f0 exp (-Ea/kBT) Where Ea is the activation energy, f0 is a prefactor, kB is the Boltzmann constant and T is the temperature in K. The obtained values of Ea are matched well with the previous reports on similar BFO based ceramics, indicating middle temperature dielectric relaxation process in the solid solutions [2]. For the x = 0 sample, Ea ~0.53 and the value of activation energy considerably increases in the HFO substituted samples
Fig. 3. Temperature dependence of dielectric constant measured under different frequencies in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions: x = 0 (a), 0.1 (b), 0.2 (c) and 0.3 (d). 214
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Fig. 4. Temperature dependence of dielectric modulus (M//) measured under different frequencies in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions: x = 0 (a), 0.1 (b), 0.2 (c) and 0.3 (d).
dielectric polarization, hence, at a lower frequency range (5 kHz and 10 kHz) a huge hump is observed for x = 0 sample near the transition temperature (Tm) and this hump diminished with the increase in HFO substitution, stipulating the decrease of resistivity in the ceramics. Temperature dependence of the dielectric loss (tan δ) of the samples (x = 0–0.3) at a frequency of 10 kHz- 1 MHz are displayed in Fig. 6(a–d). The dielectric loss is negligibly small at a lower temperature region while a sharp increase in the tan δ values are observed at higher temperature region (see inset of Fig. 6 (d)), indicating the enhancement of conductivity with the increase in temperature, which is ascribed to the contribution of the highly energized charge carriers at that region. The value of tan δ decreases with the increase in x. The overall dielectric loss is found to be minimum at x ~0.2. Thus, the overall improvements of the lossy characteristics in the samples can be renovated by an appropriate amount of HFO substitution. The room temperature ferroelectric hysteresis loops, for x = 0–0.3 samples, with applied electric field 20 kV cm−1 were measured and shown in Fig. 7(a). The observed PE loop for the x = 0 sample exhibits an unsaturated loop showing high electrical leakage in the ceramics (see inset of Fig. 7(a)) [15]. Hence, higher electric fields were not imposed in the samples. With the increase in the substitution level, PE loops become more and more regular in shape, while the value of remnant polarization Pr being maximum in the x = 0.2 (Pr ~0.22 μC cm−2) sample, which can be ascribed to the suppression of oxygen vacancy by HFO substitution in the present ceramics. Thermally generated oxygen vacancy associated dipoles work as a key factor for the domain wall pinning in BFO, leading to the suppression of intrinsic polarization and rise in the leakage current. Therefore, the ferroelectric domain pinning weakened by the substitution of HFO in BFO. This technique has been demonstrated earlier to enhance polarization and reduce leakage current in a similar ceramics [8]. However, when x is increased, the nonpolar (Pbnm) structure becomes more dominant, as
Fig. 5. Arrhenius fitting of dielectric modulus (M//) in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions for x = 0–0.3.
(0.63–0.7) ev, which may be attributed to the trapping of oxygen ion vacancies by the Ho3+ substitution, reported earlier in Ca2+ substituted BFO [14]. Whereas, a slight increment of the lattice parameters for x = 0.3 (Fig. 1(d)), sequentially yielding a less compact crystal structure, leading to an enhancement in the mobility of the charge carriers, subsequently an anomaly in Ea is observed. It is well established fact that due to the volatization of Bi at high temperature sintering process, point charge defect mechanisms, such as, the oxygen vacancy manufactured by the Fe+2-Fe+3 charge imbalance is believed to be the contributing factor for decreasing the resistivity in BFO based ceramics [8], which successively over manipulates the high temperature 215
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Fig. 6. Temperature dependence of dielectric loss (tan δ) measured under different frequencies in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions: x = 0 (a), 0.1 (b), 0.2 (c) and 0.3 (d).
Fig. 7. Ferroelectric hysteresis loop (a) and leakage current (b) measured at room temperature in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions.
ferroelectric as well as the resistive performance in the ceramics.
indicated by the XRD, which might be a reason for a punier value of remnant polarization (Pr) in the substituted samples. The overall phenomenon is very similar to a recent report on LaFeO3 substituted BaTiO3–BiFO3 ceramics [10]. To investigate further details, room temperature leakage current density was measured for x = 0–0.3, at a frequency of 10 kHz, as shown in Fig. 7(b). With increasing x from 0 to 0.3, leakage current study shows a significant decrease in the current density (J), being minimum in the sample of x~0.2, which can be interpreted as a suppression of oxygen vacancy caused by Ho-substitution, while the minimal increment in the leakage current density (J) was observed in the x = 0.3 sample is probably emerged due to the lattice deformation triggered by the structural transformation, as indicated previously in XRD section [5]. Therefore, the substitution of HFO in the ceramics enhances
4. Conclusion HFO-substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions were synthesized using conventional solid state reaction method. XRD result confirms the appearance of an additional orthorhombic Pbnm phase of HoFeO3, along with the R and T phase in the ceramic matrix. Magnetic nature of the samples is considerably tuned and enhanced by the substitution of BFO-BCTO matrix with HFO, perhaps due to the combined effect of additional exchange interactions introduced by highly ferromagnetic Ho substitution and the destruction of the space modulated spiral spin structure. High temperature dielectric, as well as the leakage study, suggest that the oxygen vacancy can be significantly suppressed 216
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by the HFO substitution in the ceramics, which in turn greatly modifies the resistive and the ferroelectric properties in the ceramics. The strongest remnant polarization and the feeble dielectric loss is achieved when the substitution content is x~0.2. Our overall results demonstrate that the substitution of the 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solution with an appropriate amount of RFeO3, in present case HoFeO3, can be an effective way to regulate the multiferroic properties in the ceramics.
[7]
[8]
[9]
Acknowledgement This work was financially assisted by the National Natural Science Foundation of China (Grant Nos. 11474111 and 11604281). We are pleased to the members of the Analysis Center of Huazhong University of Science and Technology for their cooperation in various measurements.
[10]
Appendix A. Supplementary data
[13]
[11] [12]
[14]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.08.250.
[15]
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Gd-doped BiFeO3:BaTiO3 (3:2) multiferroic ceramic materials, J. Mater. Sci. Mater. Electron. 30 (2019) 2154. Z.M. Tian, Y.S. Zhang, S.L. Yuan, M.S. Wu, C.H. Wang, Z.Z. Ma, S.X. Huo, H.N. Duan, Enhanced multiferroic properties and tunable magnetic behavior in multiferroic BiFeO3–Bi0.5Na0.5TiO3 solid solutions, Mater. Sci. Eng. B 177 (2012) 74–78. Gebru Zerihun, Shuai Huang, Gaoshang Gong, Songliu Yuan, Influence of Eu doping on the magnetoelectric and dielectric properties of BiFeO3–Bi0.5Na0.5TiO3 ceramics, Ceram. Int. 41 (2015) 6589–6595. X.N. Zhu, M.S. Rahman, Y.J. Wu, X.Q. Liu, Y.H. Huang, G. Liu, R. Guo, X.M. Chen, A.S. Bhalla, Enhanced ferroelectricity, piezoelectricity and ferromagnetism in (Ba0.75Ca0.25)TiO3 modified BiFeO3 multiferroic ceramics, J. Alloy. Comp. 658 (2016) 973–980. Xiwei Qi, Min Zhang, Xiaoyan Zhang, Yaohang Gu, Hongen Zhu, Weicheng Yang, Ying Li, Compositional dependence of ferromagnetic and magnetoelectric effect properties in BaTiO3– BiFeO3–LaFeO3 solid solutions, RSC Adv. 7 (2017) 51801. T. Chatterji, M. Meven, P.J. Brown, Temperature evolution of magnetic structure of HoFeO3 by single crystal neutron diffraction, AIP Adv. (2017) 7 045106. Min Zhang, Xiaoyan Zhang, Xiwei Qi, Hongen Zhu, Ying Li, Yaohang Gu, Enhanced ferroelectric, magnetic and magnetoelectric properties of multiferroic BiFeO3–BaTiO3–LaFeO3 ceramics, Ceram. Int. 44 (2018) 21269–21276. Pittala Suresh, P.D. Babu, S. Srinath, Effect of Ho substitution on structure and magnetic properties of BiFeO3, J. Appl. Phys. 115 (2014) 17D905. Nahum Maso, Anthony R. West, Electrical properties of Ca-doped BiFeO3 ceramics: from p-type semiconduction to oxide-ion conduction, Chem. Mater. 24 (2012) 2127–2132. Z.X. Cheng, A.H. Li, X.L. Wang, S.X. Dou, K. Ozawa, H. Kimura, S.J. Zhang, T.R. Shrout, Structure, ferroelectric properties, and magnetic properties of the Ladoped bismuth ferrite, J. Appl. Phys. 103 (2008) 07E507. P.C. Sati, M. Kumar, S. Chhoker, M. Jewariya, Inuence of Eu substitution on structural, magnetic, optical and dielectric properties of BiFeO3 multiferroic ceramics, Ceram. Int. 41 (2015) 2389–2398. Y. Wang, C.W. Nan, Effect of Tb doping on electric and magnetic behavior of BiFeO3 thin films, J. Appl. Phys. 103 (2008) 024103. Bojan Stojadinović, Zorana Dohčević-Mitrović, Dimitrije Stepanenko, Milena Rosić, Ivan Petronijević, Nikola Tasić, Nikola Ilić, Branko Matović, Biljana Stojanović, Dielectric and ferroelectric properties of Ho-doped BiFeO3 nanopowders across the structural phase transition, Ceram. Int. 43 (2017) 16531–16538. Dawei Wang, Amir Khesro, Shunsuke Murakami, Antonio Feteira, Quanliang Zhao, M. Ian, Reaney, Temperature dependent, large electromechanical strain in Nddoped BiFeO3-BaTiO3 lead-free ceramics, J. Eur. Ceram. Soc. 37 (2016) 1857–1860. Q. Zhou, C. Zhou, H. Yang, G. Chen, W. Li, H. Wang, Dielectric ferroelectric, and piezoelectric properties of Bi(Ni½Ti½)O3-modified BiFeO3-BaTiO3 ceramics with high curie temperature, J. Am. Ceram. Soc. 95 (2012) 3889–3893.
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Modulation of magnetic, ferroelectric and leakage properties by HoFeO3 substitution in multiferroic 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions
T
Chiranjib Chakrabartia, Qingshan Fua, Xinghan Chena, Yang Qiub, Songliu Yuana,∗, Canglong Lia a b
School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, PR China Key Laboratory of Microelectronics and Energy of Henan Province, School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang, 464000, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Multiferroic Ferromagnetic Ferroelectric Bismuth ferrite
An investigation on the structural, magnetic, electrical properties of HoFeO3-substituted 0.7BiFeO30.3Ba0.8Ca0.2TiO3 solid solutions synthesized using conventional solid state reaction method was carried out. The structural study confirms that an additional orthorhombic (Pbnm) phase of HoFeO3 appears in the ceramic matrix and the presence of the aforementioned phase significantly influences the magnetic characteristics in the solid solutions. A detailed high temperature dielectric study suggests that the oxygen vacancies can be effectively controlled by the appropriate amount of substitution, successively regulating the ferroelectric as well as the leakage properties in the ceramics. Furthermore, the foremost remnant polarization and the feeble dielectric loss is achieved when the substitution content is x~0.2. Therefore, an appropriate amount of HoFeO3 substitution can be an effective way to modulate the multiferroic properties in the present ceramics.
1. Introduction Since the past decade, a very rare class of materials, known as multiferroic materials, have attracted enormous attention due to their application possibilities in future generation data storage, sensors, spintronic devices, etc. Generally, two different ferroic orders, such as ferromagnetism and ferroelectricity, simultaneously occur in multiferroics [1]. However, most of the available multiferroics having a very low operating temperature. One of the extensively studied (ABO3) type room temperature multiferroic ferroelectric perovskite is BiFeO3 (BFO), which is a G-type antiferromagnet with a high Néel temperature (TN) ~ 643 K as well as a high Curie temperature (Tc) ~ 1100 K [2]. The bulk form of BFO suffers from some basic challenges such as poor resistivity, long range cycloidal order, generation of impurity phases, etc. [3–5]. To improve the magnetic, ferroelectric and resistive properties of BFO, different measures have been taken, including rare earth or transition metal elements substitution in Bi or Fe-site, the formation of binary solid solutions with other ABO3 –type perovskites, such as, BaTiO3, Ba0.75Ca0.25TiO3, Bi0.5Na0.5TiO3, etc. [6–9]. Among them, improvements of the ferroelectric properties in BiFeO3–BaTiO3 ceramics were reported earlier, although, poor ferromagnetic property and poor resistive nature makes them less suitable for practical applications [10]. Recently, Zhang et al. reported enhanced electrical as well as
∗
magnetic (Mr = 0.06 emu g−1) properties by introducing a rare earth orthoferrite LaFeO3 in 0.7BiFeO3-0.3BaTiO3 ceramics [12]. Improvement of magnetic and electric properties in different BiFeO3 based ceramics by various rare earth orthoferrites (ReFeO3) had been reported earlier as well [2,10]. HoFeO3 is a distorted perovskite which holds a G-type antiferromagnetic structure with a high Néel temperature (TN) ~ 653 K [11]. Therefore, a suitable amount of HoFeO3 addition in BiFeO3 based ceramics can be advantageous for the modulation of multiferroic properties. However, to the best of our knowledge, the effect of HoFeO3 addition in similar ceramics was not reported before. In this work, 0.7 ((1-x)BiFeO3-xHoFeO3)-0.3Ba0.8Ca0.2TiO3 (BFOBCTO) ceramics (x = 0–0.3) were prepared by conventional solid state reaction method and the composition (x) dependent magnetic, ferroelectric and leakage properties are systematically studied. 2. Experimental details A conventional solid state reaction process was used to synthesize multiferroic 0.7 ((1-x)BiFeO3-xHoFeO3)-0.3Ba0.8Ca0.2TiO3 solid solutions of x = 0–0.3. Stoichiometric amounts of analytical grade Bi2O3, Ho2O3, Fe2O3, BaCO3, CaCO3 and TiO2 were hand grinded using a mortar and pestle for 5 h. The mixtures were placed into an oven and then calcined at 1073 K for 3 h. Thereafter, the calcined powders were ground for further 3 h after adding 2% wt. PVA. Finally, the ground
Corresponding author. E-mail address: [email protected] (S. Yuan).
https://doi.org/10.1016/j.ceramint.2019.08.250 Received 26 June 2019; Received in revised form 3 August 2019; Accepted 26 August 2019 Available online 26 August 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Ceramics International 46 (2020) 212–217
C. Chakrabarti, et al.
Fig. 1. X-Ray Diffraction patterns measured at room temperature in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions (a) refined versions of XRD peaks appearing for x = 0 (b) x = 0.2 (c) and variations in lattice parameters as well as cell volume with respect to x (d).
x. The XRD refinement results indicate that the additional peak well fitted with orthorhombic Pbnm phase of HoFeO3 (COD ID: 2107086) and the goodness of fit indicator (s) is ~1.7 (i.e. < 2) for all of the substituted samples, showing a superior match between the observed and determined values. The appearance of nonpolar RFeO3-type orthorhombic phases, such as, LaFeO3 and SmFeO3, were previously reported in other rare earth substituted similar ceramics and were verified by the refinement results [2,10,12]. In addition, thermal (TG-DTA) analysis of the ceramics is confirming the appearance of an additional phase near 1068 K (see Fig. S1 in the supplementary information section), which can be attributed to the formation of HoFeO3 phase in the ceramics. As shown in Fig. 1(d), with an increase in the substitution level, the lattice parameters, as well as the cell volume (v), declines ensuring a structural deformation in the ceramics. Since the ionic radius of Ho3+ is smaller than Bi3+, the decreasing trend in lattice parameters, as well as in the cell volume of R3c phase, can be attributed to the ionic radius difference between the partially substituted Bi3+ (1.14 Å) ions and comparatively smaller size of Ho3+ (1.015 Å) ions [13]. The declining tendency in lattice parameters and the unit cell volume of R3c phase were previously reported in rare-earth doped BFO ceramics [16,17]. For further increase in the x i.e. up to x = 0.3, unit cell volume and lattice appears to be slightly increased, which might be because of the increased strain at the rhombohedral and orthorhombic phase boundary, as has been reported before in case of Ho doped BFO ceramics [18]. The room temperature hysteresis (M-H) loops, are shown in Fig. 2, for pristine and HFO-substituted samples at the maximum field of ± 30 kOe. The MH loop of the x = 0 sample shows a very weak ferromagnetic response at room temperature. When HFO is included in the BFO-BCTO matrix, magnetic nature of the samples are considerably
mixtures were pelletized (8 mm diameter) and sintered at 1273 K for 3 h. To measure the electrical properties of the samples, silver paste was applied to both sides of the pallets. Philips Panalytical X’ pert diffractometer (XRD) with Cu-Kα radiation (λ = 1.5406 Å) was deployed for the structural analysis of the ceramics at room temperature. TG-DTA analysis was done using a Diamond TG/DTA Analyzer (PerkinElmer Instruments). To perform the room temperature magnetization hysteresis (M-H) study, a physical property measurement system (PPMS, Quantum Design) was used. Ferroelectric, leakage and dielectric properties were investigated by employing a ferro-electric tester (Precision Premier II, Radiant Technology) as well as a Precision Impedance Analyzer (WAYNEKERR-6500B), respectively. 3. Results and discussions The room temperature XRD patterns as well as the phase evolution for 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions with different (x = 0–0.3) HoFeO3 contents, in the range of 2θ = 20° −80°, are analyzed using a Panalytical X-pert pro software, as depicted in Fig. 1(a–d).The corresponding Reitvled refinement fitting, for the x = 0 and the x = 0.2 samples were performed using a Full prof suite software (shown in Fig. 1(b) and (c), respectively). The sample of x = 0 can be well fitted with the perovskite tetragonal P4/mmm and the rhombohedral R3c symmetry, which is very much consistent with the earlier reports in similar ceramics, where it was identified as rhombohedral (R)-tetragonal (T) morphotropic phase boundary (MPB) [9]. However, with increasing x, an additional phase along with the R and T phase appears in the sample near 2θ~32° (when x = 0.05 or higher) and the intensity of the diffraction peak gradually increases with the increase in 213
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sample. In addition, the substitution of comparatively smaller ionic radii of Ho3+ ions in BFO manipulates the lattice structure and the upshot of which is the suppression of space modulated spiral spin structure in BFO and hence the magnetization increases. Fig. 3(a–d) shows the temperature dependence of dielectric constant (εr) for the x = 0, 0.1, 0.2 and 0.3 samples at a frequency range 10 kHz −2 MHz. At a low frequency, dielectric anomalies were observed, which is marked with blue arrows and the measured dielectric anomalies for a relatively lower frequency of 10 kHz were found 576 K, 555 K, 547 K and 549 K for the x = 0, 0.1, 0.2, 0.3 samples, respectively. With increasing x, Tm shifts toward lower temperature; probably due to the dilution of polar coupling by the partial substitution of the less polarisable Ho3+ in Bi3+, which was reported previously in substituted BiFeO3 based ceramics [19,20]. Moreover, the anomaly peak (Tm) shifts toward the higher temperature with the increase in frequency and the sharp decrease in the magnitude of the dielectric maxima was observed, indicating relaxor like frequency dispersion. Similar relaxor-type behavior was reported earlier in other BFO based solid solutions, which may be attributed to the charge imbalance at the A and B site of BFO due to the substitution [2]. So as to explore the relaxation mechanism in the x = 0–0.3 samples, the variation of dielectric modulus M//with temperature is investigated, as shown in Fig. 4(a–d). The dielectric relaxation at high temperature is fitted and represented in Fig. 5 according to the well-known Arrhenius law:
Fig. 2. Room temperature hysteresis curves measured in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions and remnant magnetization and coercivity as functions of x (inset).
effected, as represented in the inset of Fig. 2, indicating a variation of remnant magnetization (Mr) and coercivity (Hc) with respect to the substitution content (x). Interestingly, with increasing x to 0.3, the Mr increases monotonically, whereas, the Hc value climbs to the maximum value near x = 0.1 (Mr~0.08) and thereafter decreases sharply with the further increase in x. The variation of Mr and Hc is very much consistent with the previous report on BiFeO3–x (0.5CaTiO3–0.5SmFeO3) ceramics [2]. Therefore, HFO addition in the sample significantly changes the magnetic nature in the ceramics. Highly ferromagnetic nature of Ho probably brings new exchange interactions in the ceramics, such as Ho–Ho and Ho–Fe –type, which is supplementary to the conventional Fe–Fe interaction in the pristine
f = f0 exp (-Ea/kBT) Where Ea is the activation energy, f0 is a prefactor, kB is the Boltzmann constant and T is the temperature in K. The obtained values of Ea are matched well with the previous reports on similar BFO based ceramics, indicating middle temperature dielectric relaxation process in the solid solutions [2]. For the x = 0 sample, Ea ~0.53 and the value of activation energy considerably increases in the HFO substituted samples
Fig. 3. Temperature dependence of dielectric constant measured under different frequencies in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions: x = 0 (a), 0.1 (b), 0.2 (c) and 0.3 (d). 214
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Fig. 4. Temperature dependence of dielectric modulus (M//) measured under different frequencies in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions: x = 0 (a), 0.1 (b), 0.2 (c) and 0.3 (d).
dielectric polarization, hence, at a lower frequency range (5 kHz and 10 kHz) a huge hump is observed for x = 0 sample near the transition temperature (Tm) and this hump diminished with the increase in HFO substitution, stipulating the decrease of resistivity in the ceramics. Temperature dependence of the dielectric loss (tan δ) of the samples (x = 0–0.3) at a frequency of 10 kHz- 1 MHz are displayed in Fig. 6(a–d). The dielectric loss is negligibly small at a lower temperature region while a sharp increase in the tan δ values are observed at higher temperature region (see inset of Fig. 6 (d)), indicating the enhancement of conductivity with the increase in temperature, which is ascribed to the contribution of the highly energized charge carriers at that region. The value of tan δ decreases with the increase in x. The overall dielectric loss is found to be minimum at x ~0.2. Thus, the overall improvements of the lossy characteristics in the samples can be renovated by an appropriate amount of HFO substitution. The room temperature ferroelectric hysteresis loops, for x = 0–0.3 samples, with applied electric field 20 kV cm−1 were measured and shown in Fig. 7(a). The observed PE loop for the x = 0 sample exhibits an unsaturated loop showing high electrical leakage in the ceramics (see inset of Fig. 7(a)) [15]. Hence, higher electric fields were not imposed in the samples. With the increase in the substitution level, PE loops become more and more regular in shape, while the value of remnant polarization Pr being maximum in the x = 0.2 (Pr ~0.22 μC cm−2) sample, which can be ascribed to the suppression of oxygen vacancy by HFO substitution in the present ceramics. Thermally generated oxygen vacancy associated dipoles work as a key factor for the domain wall pinning in BFO, leading to the suppression of intrinsic polarization and rise in the leakage current. Therefore, the ferroelectric domain pinning weakened by the substitution of HFO in BFO. This technique has been demonstrated earlier to enhance polarization and reduce leakage current in a similar ceramics [8]. However, when x is increased, the nonpolar (Pbnm) structure becomes more dominant, as
Fig. 5. Arrhenius fitting of dielectric modulus (M//) in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions for x = 0–0.3.
(0.63–0.7) ev, which may be attributed to the trapping of oxygen ion vacancies by the Ho3+ substitution, reported earlier in Ca2+ substituted BFO [14]. Whereas, a slight increment of the lattice parameters for x = 0.3 (Fig. 1(d)), sequentially yielding a less compact crystal structure, leading to an enhancement in the mobility of the charge carriers, subsequently an anomaly in Ea is observed. It is well established fact that due to the volatization of Bi at high temperature sintering process, point charge defect mechanisms, such as, the oxygen vacancy manufactured by the Fe+2-Fe+3 charge imbalance is believed to be the contributing factor for decreasing the resistivity in BFO based ceramics [8], which successively over manipulates the high temperature 215
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Fig. 6. Temperature dependence of dielectric loss (tan δ) measured under different frequencies in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions: x = 0 (a), 0.1 (b), 0.2 (c) and 0.3 (d).
Fig. 7. Ferroelectric hysteresis loop (a) and leakage current (b) measured at room temperature in HoFeO3 substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions.
ferroelectric as well as the resistive performance in the ceramics.
indicated by the XRD, which might be a reason for a punier value of remnant polarization (Pr) in the substituted samples. The overall phenomenon is very similar to a recent report on LaFeO3 substituted BaTiO3–BiFO3 ceramics [10]. To investigate further details, room temperature leakage current density was measured for x = 0–0.3, at a frequency of 10 kHz, as shown in Fig. 7(b). With increasing x from 0 to 0.3, leakage current study shows a significant decrease in the current density (J), being minimum in the sample of x~0.2, which can be interpreted as a suppression of oxygen vacancy caused by Ho-substitution, while the minimal increment in the leakage current density (J) was observed in the x = 0.3 sample is probably emerged due to the lattice deformation triggered by the structural transformation, as indicated previously in XRD section [5]. Therefore, the substitution of HFO in the ceramics enhances
4. Conclusion HFO-substituted 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions were synthesized using conventional solid state reaction method. XRD result confirms the appearance of an additional orthorhombic Pbnm phase of HoFeO3, along with the R and T phase in the ceramic matrix. Magnetic nature of the samples is considerably tuned and enhanced by the substitution of BFO-BCTO matrix with HFO, perhaps due to the combined effect of additional exchange interactions introduced by highly ferromagnetic Ho substitution and the destruction of the space modulated spiral spin structure. High temperature dielectric, as well as the leakage study, suggest that the oxygen vacancy can be significantly suppressed 216
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by the HFO substitution in the ceramics, which in turn greatly modifies the resistive and the ferroelectric properties in the ceramics. The strongest remnant polarization and the feeble dielectric loss is achieved when the substitution content is x~0.2. Our overall results demonstrate that the substitution of the 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solution with an appropriate amount of RFeO3, in present case HoFeO3, can be an effective way to regulate the multiferroic properties in the ceramics.
[7]
[8]
[9]
Acknowledgement This work was financially assisted by the National Natural Science Foundation of China (Grant Nos. 11474111 and 11604281). We are pleased to the members of the Analysis Center of Huazhong University of Science and Technology for their cooperation in various measurements.
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.08.250.
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