Structural, Dielectric, Ferroelectric and Magnetic Properties of Bi0.80A0.20FeO3 (A=Pr,Y) Multiferroics Vikash Singh, Subhash Sharma, R. K. Dwivedi, Manoj Kumar, R. K. Kotnala, N. C. Mehra & R. P. Tandon Journal of Superconductivity and Novel Magnetism Incorporating Novel Magnetism ISSN 1557-1939 Volume 26 Number 3 J Supercond Nov Magn (2013) 26:657-661 DOI 10.1007/s10948-012-1775-y 1 23 Your article is protected by copyright and all rights are held exclusively by Springer Science+Business Media, LLC. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your work, please use the accepted author’s version for posting to your own website or your institution’s repository. You may further deposit the accepted author’s version on a funder’s repository at a funder’s request, provided it is not made publicly available until 12 months after publication. 1 23 Author's personal copy J Supercond Nov Magn (2013) 26:657–661 DOI 10.1007/s10948-012-1775-y O R I G I N A L PA P E R Structural, Dielectric, Ferroelectric and Magnetic Properties of Bi0.80A0.20FeO3 (A = Pr, Y) Multiferroics Vikash Singh · Subhash Sharma · R.K. Dwivedi · Manoj Kumar · R.K. Kotnala · N.C. Mehra · R.P. Tandon Received: 5 September 2012 / Accepted: 28 September 2012 / Published online: 1 November 2012 © Springer Science+Business Media, LLC 2012 Abstract Here we studied the effect of homovalent Pr3+ and Y3+ substitution on the crystal structure, dielectric, electronic polarization and magnetic properties of the BiFeO3 multiferroic ceramic. The samples were synthesized by the conventional solid-state reaction method. Pure phase formation of Pr doped BiFeO3 (BFO) has been obtained, while Y3+ doped BFO has shown a few impurity peaks. It has shown that the crystal structure of the compounds is described within the space group R3c. Pr3+ modified BFO has shown an anomaly in the εr vs. T plot around and a Néel temperature ‘T N ’ ∼ 370 ◦ C. P –E hysteresis loops have shown higher value of remnant polarization for Pr3+ modified BFO. Magnetic properties of ceramics are determined by the ionic radius of the substituting element. Experimental results propose that the increase in the radius of A-site ion leads to effective suppression of the spiral spin structure of BiFeO3 , resulting in the appearance of net magnetization. Keywords Dielectric properties · Ferroelectric properties · Magnetic properties · Multiferroics materials and bismuth ferrites V. Singh () · S. Sharma · R.K. Dwivedi · M. Kumar Department of Physics and Materials Science and Engineering, Jaypee Institute of Information Technology, Noida 201307, India e-mail: vikas21jiit.in@gmail.com R.K. Kotnala National Physical Laboratory (CSIR), K.S. Krishna Marge, Pusa, New Delhi 110012, India N.C. Mehra · R.P. Tandon Department of Physics & Astrophysics, University of Delhi, Delhi 110007, India 1 Introduction Presently the multiferroic materials have received a great attention due to exciting physics and potential applications in the sensor, data storage and spintronics. Multiferroic materials are those materials which have more than one ferroic properties like ferroelectric, ferromagnetic and/or ferroelastic in the same materials. These materials are very rare because localized ‘d’ electrons of transition metal, responsible for magnetism are not compatible with the requirement of empty d orbitals for ferroelectricity [1–3]. BiFeO3 (BFO) is one of the well known multiferroic materials which show G-type antiferromagnetic behavior below Néel Temperature (T N ) ∼ 370 ◦ C and ferroelectric behavior below Curie temperature (T C ) ∼ 830 ◦ C [4]. In terms of practical applications this property is very good because of coexistence of ferroelectricity and magnetism simultaneously. However, problems of secondary phase due to bismuth volatilization and low resistivity have fixed the practical application of BFO ceramics to electronic devices. Ferroelectricity in BFO comes from the long-range ordering of dipolar moments on Bi-site with the existence of Bi lone pair and hybridization between Bi 6s and O 2p orbitals [1]. In last few years, various processing techniques have been used to synthesize BFO. Bi and/or Fe sites substitution by rare-earth elements in BFO has come out as an approach to suppress the formation of secondary phases and improved ferroelectric and magnetic properties by destroying the spatial modulated spiral spin structures [5–11]. Zhang et al. [6] and Das et al. [7] recommended that La3+ doping for Bi3+ reduces impurity phases and destroys the spiral spin structure. The effect of rare-earth dopants in BiFeO3 has been suggested for the enhancement of ferroelectric and magnetic properties and also decreasing the Néel temperature T N from 370 ◦ C [12]. Ion substitution effects on the magnetic and ferroelectric properties of BFO is not clearly understood so far and hence Author's personal copy 658 J Supercond Nov Magn (2013) 26:657–661 research work is still required to understand the multifunctional properties for Bi-site substitution in this system. It is therefore worthwhile to study Y3+ and Pr3+ doped BFO system completely. Primarily, in order to get pure phase formation, Y3+ and Pr3+ doped sample with x = 0.20 have been thought to be studied. For this reason in this report, we are reporting our studies on the structure, dielectric, ferroelectric and magnetic properties of typical composition Bi0.80 Y0.20 FeO3 (BYFO) and Bi0.80 Pr0.20 FeO3 (BPFO). To the best of our knowledge, Pr3+ and Y3+ doped BFO for composition x = 0.20 has not been studied in detail so far. 2 Experimental Details In this study Bi0.80 Y0.20 FeO3 (BYFO) and Bi0.80 Pr0.20 FeO3 (BPFO) ceramics were synthesized by conventional solidstate reaction method using high purity powder of bismuth oxide (Bi2 O3 ∼ 99.99 %, Aldrich), iron oxide (Fe2 O3 ∼ 99.99 %, Aldrich), praseodymium oxide (Pr2 O3 ) and yttrium oxide (Y2 O3 ∼ 99.9 %, Aldrich) as starting raw materials. After appropriate weighing, grinding, mixing in acetone medium and drying, the mixed powders were calcined at 760 ◦ C for 2 h. The obtained powders were again ground for 1/2 hour using 0.2 wt% PVA binders and the mixtures were pressed uniaxially into small cylindrical pellets with a diameter of 10 mm. The samples in the form of cylindrical disc were put into air atmosphere in a furnace at temperature 830 ◦ C for sintering. The X-ray diffraction patterns of the calcined powders were taken at room temperature using an X-ray Powder Diffractometer (Bruker D8 Advance) with Cu Kα radiation (λ ∼ 1.5418 Å) in the range of 2θ form 20◦ –60◦ . Temperature dependent dielectric measurements from room temperature to 550 ◦ C were performed at few selected frequencies using an automated LCR Meter (HIOKI 3532-50 Hi Tester). The electric field controlled polarization (P –E hysteresis loops) was measured at room temperature by the modified Sawyer–Tower circuit (Automatic P –E loop tracer system, Marine India Pvt. Ltd). Temperature dependent magnetization was measured from 30 ◦ C to 450 ◦ C and the M–H loop measurements for these samples were done at room temperature using vibrating sample magnetometer (model 7305, Lakeshore). Table 1 Lattice parameters, volume, remanent polarization and remanent magnetization of BiFeO3 (BFO), Bi0.80 Y0.20 FeO3 (BYFO) and Bi0.80 Pr0.20 FeO3 (BPFO) samples 3 Results and Discussion The substitution effect of different ionic size elements Y3+ (1.01 Å) and Pr3+ (1.12 Å) on the crystallization of BiFeO3 (BFO) samples were identified by X-Ray Diffraction. Figure 1 shows the room temperature X-Ray Diffraction patterns for the composition with x = 0.20 for Y3+ and Pr doped BiFeO3 samples abbreviated as BYFO and BPFO, respectively. XRD pattern of BFO and BYFO show few impurity phases of Bi2 Fe4 O9 (marked as ∗ in Fig. 1) which was also observed by Yuan et al. [8], whereas BPFO samples show characteristics peaks with no signature of impurity phase. The occurrence of impurity peaks in BFO and BYFO samples may be according to the following reaction: Bi2 O3 + Fe2 O3 → (1/2)Bi2 Fe4 O9 + (1/2)Bi2 O3 Y3+ and Pr3+ are smaller, as compared to Bi3+ (∼1.17 Å) therefore, we have observed the decrease in lattice parameters and hence unit cell volumes for both the ions, but cell parameter for Pr3+ doped BFO are found to be more as compared to Y3+ doped BFO, which can be explained because of smaller Y3+ as compared to Pr3+ [13] (see in Table 1). Figure 2 shows the temperature dependence of the dielectric constant and dielectric loss for BYFO and BPFO samples at the frequencies of 50 kHz and 100 kHz. It was clearly Fig. 1 X-ray diffraction patters for the BiFeO3 , Bi0.80 Y0.20 FeO3 and Bi0.80 Pr0.20 FeO3 a = b (Å) c (Å) V (Å3 ) Remanent polarization (P r ) µC/cm2 Remanent magnetization (M r ) Emu/g m BFO 5.580 13.876 374.15 0.121 0.0021 BYFO 5.5652 13.705 367.59 0.133 0.038 BPFO 5.584 13.848 373.93 0.25 0.057 Sample Author's personal copy J Supercond Nov Magn (2013) 26:657–661 659 Fig. 2 Dielectric constant and loss versus temperature plot (a) Bi0.80 Y0.20 FeO3 and (b) Bi0.80 Pr0.20 FeO3 Fig. 3 SEM microstructure of samples (a) Bi0.80 Y0.20 FeO3 and (b) Bi0.80 Pr0.20 FeO3 observed from the ε r vs. T plot of BYFO that a typical frequency dependent dielectric anomaly occurs near 310 ◦ C and dielectric loss was very low at the peak position (nearly 0.6). The dielectric constant vs. temperature plot of BYFO sample exhibits a small anomaly at around 310 ◦ C (which is far below the Néel temperature), which is consistent with earlier reports [14]. This anomaly attributed to a transient interaction between oxygen ion vacancies and the Fe3+ /Fe2+ redox couple. Replacement of Bi3+ by Y3+ modifies the dielectric characteristics of BFO, resulting in vanishing of the anomaly and substantial reduction of tan δ. Pr3+ modified BFO has shown a dielectric anomaly near the Néel temperature (T N ) ∼ 370 ◦ C and at this temperature dielectric loss is quite large as compared to BYFO sample. Dielectric anomaly observed around magnetic transition temperature, indicating magnetoelectric coupling in these samples. This type of dielectric anomaly is predicted by the Landau– Devonshire theory of phase transition in magnetoelectrically ordered systems as an influence of vanishing magnetic order on the electric order [15]. In both samples, a strong increase of the dielectric characteristics is observed with increasing temperature or decreasing frequency. The behavior can be explained in the following way [16, 17]: the defect–related dipoles are able to follow the alternating field at low frequencies, providing high values of ε r . The increase of the dielectric constant and loss factor with increasing temperature is related to thermally induced enhancement of the hopping conduction. The replacement of some volatile Bi3+ with non-volatile Pr3+ and Y3+ may prevent oxygen ion vacancies causing stabilization of the Fe3+ /Fe2+ couple–oxygen vacancy interaction [18–20]. Due to Bi evaporation during sintering in BFO as a result of Bi3+ loss, it is difficult to control oxygen loss. In order to maintain charge neutrality over all in the sample, this oxygen loss is leading to generation of oxygen vacancies. This occurs as per the following reactions: Bi2 O3 → 2Bi3+ + 3O2− 2+ 2Bi3+ + 3O2− → 2Bi (gas) + 3/2O2 (gas) + 2V3− Bi + 3VO 4Fe3+ + O2− → 3Fe2+ + 3O2 (gas) + 6V2+ O Thus, the defects may be responsible for high dielectric loss. Figure 3 shows the Scanning electron micrographs of BYFO and BPFO ceramics. SEM micrograph of BYFO shows spherical grain growth. The average grain size of this sample is in the range of 2–3 µm. SEM of BPFO sample shows agglomeration. It appears that nuclei aggregate into clusters. The higher dielectric constant in BYFO sample may be attributed to the better quality microstructure and small grain size. The poor quality of microstructure for Pr3+ doped BFO could be responsible for the lower dielectric constant. Author's personal copy 660 Figure 4 shows the variation of dielectric polarization (P ) with applied electric field (E) for BYFO and BPFO systems at room temperature. The samples are highly conductive at room temperature and only partial reversal of the polarization takes place quite similar to that observed by Pradhan et al. [21]. The relatively high conductivity of BiFeO3 is known to be attributed to the variable oxidation states of Fe ions (Fe3+ /Fe2+ ) which require oxygen vacancies for charge compensation. Also, during synthesis the slow heating rate and long sintering time will enable the equilibrium concentration of the oxygen vacancies at high temperature to be reached and will result in the high oxygen vacancy concentration in the synthesized product. So the presence of Fe2+ ions and oxygen deficiency leads to high conductivity. No saturated polarization hysteresis loop has been observed. For BYFO sample the value of remnant polarization is 0.036 µC/cm2 . Remnant polarization of BPFO sample is 0.44 µC/cm2 . The value of remnant polarization (P r ) is higher for BPFO sample. The low value of P r in pure and Y3+ doped BFO samples may be attributed to the presence of minor impurity phases. The unsaturated state of polarization in P –E curve may be due to leakage currents develop because of oxygen vacancies and other possible defects. The magnetic hysteresis (M–H ) loops of BFO, BYFO and BPFO samples have been measured using VSM at room Fig. 4 Ferroelectric hysteresis loop of BiFeO3 , Bi0.80 Y0.20 FeO3 and Bi0.80 Pr0.20 FeO3 Fig. 5 (a) Room temperature M–H loops (b) M–T curves for Bi0.80 Y0.20 FeO3 and Bi0.80 Pr0.20 FeO3 J Supercond Nov Magn (2013) 26:657–661 temperature (see Fig. 5a). All the samples show non linear magnetization loops representing weak ferromagnetic behavior. The ferromagnetic nature is more visible for BPFO samples and lesser for BFO and BYFO samples. BFO is reported to be a G-type antiferromagnetic at room temperature [4]. The crystal structure of BFO allows the appearance of weak ferromagnetic arising from the canting of the antiferromagnetic sublattices [4]. The appearance of remanent magnetization of BPFO and BYFO are attributed to the suppression of the spiral spin structure by Pr3+ and Y3+ doping at Bi-site [6]. But the spiral spin structure of BFO is not completely destroyed by doping. The relatively higher value of remnant magnetization (M r ) for the BPFO sample has been obtained as compared to BYFO sample. This relative high value of M r for BPFO sample may occur due to smaller bond angle of Fe–O–Fe bond in the lattice and hence significant improvement in the value of magnetization [22, 23]. Fe–O–Fe angle and Fe–O distances are changed by Pr3+ and Y3+ doping. As the super exchange interaction is responsive to bond angles and bond distances, the spiral spin structure might be destroyed by doping, which leads to the release of latent magnetization and the enhancement of remanent magnetization. The data are tabulated in Table 1. However, this value of M r is very low for both samples may be attributed to the rhombohedrally distorted perovskite structure with space group R3c, in which both ferroelectricity due to lone pair of Bi ion and weak ferromagnetic ordering due to the canting of spin moments [22]. Plots of M vs. T for BYFO and BPFO above room temperature are shown in Fig. 5b and inset of 5b. The temperature dependence of magnetization of the samples was measured at a magnetic field of 500 Oe from room temperature to 450 ◦ C to determine the magnetic ordering. The magnetization of both samples shows a sharp decrease at around of 383 ◦ C for Pr3+ and Y3+ (Y3+ see in inset), which is considered to be the magnetic transition temperature of both samples. This transition temperature is close to the spin ordering temperature of BFO (370 ◦ C [4]). This result indicates that the addition of Pr3+ and Y3+ does not affect the magnetic transition temperature too effectively. The antiferromagnetic Author's personal copy J Supercond Nov Magn (2013) 26:657–661 interaction seems enhanced in BPFO since the increase of magnetization with temperature is faster than BYFO. Pr3+ and Y3+ content, the lattice volume decreases and the overlapping Fe-orbitals in Fe–O–Fe bond increases. Eventually, this leads to the enhancement of the antiferromagnetic superexchange [24]. The occurrence of the low value of magnetization in BYFO sample may be attributed to d0 character of yttrium [2]. 4 Conclusion A typical composition with x = 0.20 in Bi1−x Yx FeO3 (BYFO) and Bi1−x Prx FeO3 (BPFO) has been successfully synthesized by solid-state ceramic method. BPFO sample has shown complete single phase formation, whereas BYFO sample has shown single phase formation with minor presence of impurity phase and their crystal structure is described by R3c space group. Low value of lattice constants in BYFO sample may be attributed to small ionic radii of Y3+ as compared to Pr3+ . Spherical grains of size 2–3 µm have been observed in the microstructure of BYFO. The dielectric constant anomaly observed at temperature 370 ◦ C in BPFO sample may be due to the magnetic transition temperature. However, this anomaly clearly appears for BYFO sample at reduced at around temperature (310 ◦ C). This sample has shown typical frequency dependence behavior, which is not reported so far. BPFO sample has shown high value of remnant polarization (P r ) as well as remnant magnetization (M r ) as compared to BYFO sample. Magnetization measurements showed that the magnetic state of the sample is determined by the ionic radius of the substituting elements, and dopants with the biggest ionic radius effectively suppress the spiral spin structure of BFO. The M–H hysteresis loops exhibit a weak ferromagnetic behavior of the samples. Acknowledgements We are thankful to Defense Research and Development Organization (DRDO), Government of India, New Delhi for financial support to this work through research project (No.: ERIP/ER/0803744/M/01/1246). One of the authors, Vikash Singh is also thankful to Jaypee Institute of Information Technology for teaching assistance ship. 661 References 1. Hill, N.A.: J. Phys. Chem. B 104, 669 (2000) 2. Eerenstein, W., Mathur, N.D., Scott, J.F.: Nature 442, 759 (2006) 3. Chae, S.C., Murugavel, P., Lee, J.H., Ryu, H.J., Noh, T.W., Bae, C., Park, J.G.: J. Korean Phys. Soc. 47, S345 (2005) 4. Kim, D.H., Lee, H.N., Biegalski, M.D., Christen, H.M.: Appl. Phys. Lett. 92, 012911 (2008) 5. Nalwa, K.S., Grag, A., Upadhyaya, A.: Mater. Lett. 62, 878 (2008) 6. Zhang, S.T., Lu, M.H., Wu, D., Chen, Y.F., Ming, N.B.: Appl. Phys. Lett. 88, 162901 (2008) 7. Das, S.R., Choudhary, R.N.P., Bhattacharya, P., Katiyar, R.S., Dutta, P., Manivannan, A., Seehra, M.S.: J. Appl. Phys. 101, 034104 (2007) 8. 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