Effect of Ni substitution on the crystal structure and magnetic properties of BiFeO3

Journal of Alloys and Compounds 557 (2013) 120–123

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Effect of Ni substitution on the crystal structure and magnetic properties of BiFeO3 Jianguo Zhao ⇑, Xianghui Zhang, Shijiang Liu, Weiying Zhang, Zhaojun Liu School of Physics and Electronic Information, Luoyang Normal College, Henan, Luoyang 471022, PR China Luoyang Key Laboratory of Laser Spectroscopy Technology, Henan, Luoyang 471022, PR China

a r t i c l e

i n f o

Article history: Received 23 November 2012 Received in revised form 3 January 2013 Accepted 4 January 2013 Available online 9 January 2013 Keywords: Multiferroic BiFeO3 Sol-gel Ferromagnetic

a b s t r a c t In this paper, BiFe1 xNixO3 (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25) nanoparticles were synthesized by a sol– gel process X-ray diffraction (XRD), and Raman technique analysis showed that a rhombohedrally distorted BiFeO3 structure with compressive lattice distortion induced by the Ni substitution at Fe sites. Superconducting QUantum Interference Device (SQUID) results showed that, compared with BiFeO3 prepared under similar conditions, the magnetic properties were significantly enhanced at room temperature. The enhanced ferromagnetism was attributed to the size confinement effect of the nanostructures, the ferromagnetic exchange between the neighboring Fe3+ and Ni3+ ions, and the changing of the Fe–O–Fe bond angle. Superparamagnetism with blocking temperature of 10 K for BiFe0.95Ni0.05O3, 75 K for BiFe0.85Ni0.15O3, 125 K for BiFe0.80Ni0.20O3, and 200 K for BiFe0.75Ni0.25O3 was also observed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Multiferroic materials, exhibiting simultaneously the magnetic and ferroelectric order, have been widely studied in recent years, due to their abundant physics and potential applications in the sensors, data storage, spin valve devices, actuators, ultra-high speed telecommunication devices, and spintronics [1–6]. BiFeO3 is one of several rare single-phase room temperature multiferroic materials and it has great potential for practical applications exhibiting ferroelectricity with high Curie temperature (TC  1103 K), and antiferromagnetic properties below TN  643 K [7,8]. As a result, BiFeO3 has received tremendous attention over the last few decades. However, BiFeO3 has canted G-type antiferromagnetic spin structure with a weak ferromagnetic moment (0.02 lB/Fe), and there is a superimposed cycloidal modulation with a period of about 62 nm, thus the macroscopic magnetization has been averaged to zero [9]. Much effort has been paid to improve the magnetization through cation substitution (A site and B site) in BiFeO3 to get a sizable response to the application of magnetic field. For example, the enhancement in magnetic moment by the structural changes, suppression of spiral spin structure is observed when A site are partially substituted by rare-earth ions in Bi1 xRxFeO3 (R = La, Dy, Eu et al.) induces a spontaneous magnetization [10– 14]. Besides the A-site doping, many literatures are found which are based on B-site doping in BiFeO3 to obtain a collinear magnetic ordering. It was predicted that by substituting such as Mn, Ti, and ⇑ Corresponding author at: School of Physics and Electronic Information, Luoyang Normal College, Henan, Luoyang 471022, PR China. Tel./fax: +86 379 65515016. E-mail address: [email protected] (J. Zhao). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.005

Co ions for the B site in BiFeO3, might not only induce structure modifications to suppress the cycloidal spin structure, but also the ferrimagnetic interaction between Fe and 3d transition metal (TM) ions might further enhance the room temperature ferromagnetism [15–17]. The Ni atom is the leading candidate for enhancing the ferromagnetism because their radii are similar and Ni has been clearly demonstrated to be effective in suppressing the leakage current and in improving the ferroelectricity in BiFeO3, but a systematic magnetic investigation is still rare [18,19]. In contrast to other methods, the sol–gel method is used to speed up the synthesis of complex materials. It is a simple process, which offers a significant saving in time and energy consumption over the traditional methods. This method is employed to obtain improved powder characteristics, more homogeneity and narrow grain size distribution, thereby influencing structural and magnetic properties [20]. In this work, B-site substituted BiFe1 xNixO3 (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25) nanoparticles are prepared by a simple sol– gel method. The structure and magnetic properties of BiFe1 xNixO3 nanoparticles were discussed.

2. Experiments The BiFe1 xNixO3 (x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25) samples are prepared by the sol–gel technique. The chemical reagents were bismuth nitrate (Bi(NO3)3
5H2O), iron nitrate(Fe(NO3)3
9H2O), nickel nitrate (Ni(NO3)3
6H2O), and citric acid. All the chemicals were of analytical grade and were used as received without further purification. Appropriate amounts of Bi(NO3)3
5H2O and Fe(NO3)3
9H2O Ni(NO3)3
6H2O were dissolved in acetic acid, 2-methoxyethanol by constant stirring. After continuous stirring for 2 h, the solution was aged for 1 day, dried at 80 °C for 48 h. The obtained samples were grinded into powders. At last, the powder were annealed for

J. Zhao et al. / Journal of Alloys and Compounds 557 (2013) 120–123 2 h in air at 200, 400, 600 and 750 °C, respectively. A series of samples of Ni-doped BiFeO3 nanoparticles (BiFe1 xNixO3) with x = 0, 0.05, 0.10, 0.15, 0.20, and 0.25 were prepared by adopting the same procedure as mentioned above. The morphology and structure of the obtained samples are characterized by Xray diffraction (XRD) (Bruker D8 Advance X-ray diffractometer) with Cu Ka radiation (k = 1.54 Å), Raman scattering spectra (JY-HR800) using an Ar+ laser (514.5 nm) as the excitation line and high resolution transmission electron microscope (HRTEM, FEI Tecnai™ F30, US) equipped with energy dispersive X-ray spectroscopy (EDX, Oxford Instrument, UK) and a high angle annual dark fieldscanning transmission electron microscopy (HAADF-STEM) attached to the Tecnai. Variable-temperature magnetization measurements under a magnetic field of 1000 Oe (1 Oe = 79.577 A/m) and under both zero-field-cooled (ZFC) and fieldcooled (FC) conditions were performed on a Quantum Design SQUID MPMS XL-7 (SQUID) from 300 K down to 5 K. The dc hysteresis loops were collected on the same SQUID in magnetic fields from 50,000 to 50,000 Oe at 5 and 300 K, respectively.

3. Results and discussion Fig. 1 shows the XRD patterns of the sintered Bi(Fe1 xNix)O3 (x = 0.05, 0.10, 0.15, 0.20, and 0.25) nanoparticles. It can be seen that all the samples are found to crystallize in rhombohedral structure with space group (R3c). The lattice constants a, c and unit-cell volume were calculated, and are 5.580 Å, 13.857 Å and 373.64 Å for BiFeO3, 5.575 Å, 13.880 Å and 373.66 Å for BiFe0.95Ni0.05O3, 5.579 Å, 13.890 Å and 375.13 Å for BiFe0.75Ni0.25O3. As the ion radius of Ni3+ (0.70 Å) is a little larger than that of Fe3+ (0.69 Å), the increase of the lattice constant c and unit-cell volume confirms the substitution of Fe3+ by Ni3+. Small impurity peaks corresponding to Bi25FeO40 were observed at the 2h around 30° marked by stars. An expanded view on the location of (1 1 0) diffraction peaks in the range of 30–35° (inset in Fig. 1) shows that the peak position of the sample obviously shifts toward a lower 2h value in the case of Ni doping. Energy dispersive X-ray spectroscopy (EDS) checked from the obtained products clearly demonstrate the presence of Ni in the prepared products (Fig. 2). According to the above results, it is evident that Ni ions have been effectively incorporated into the crystal structure of BiFeO3. Details of the structural evolution of BiFeO3 with ion substitution can be probed more explicitly through Raman spectra. Fig. 3 shows the Raman spectra of the Bi(Fe1 xNix)O3 (x = 0.05, 0.10, 0.15, 0.20, and 0.25) nanoparticles. As the group-theoretical analysis predicts, there would be 13 Raman-active phonon modes at the C (C = 4A1 + 9E) point [21,22]. It can be seen that the first four intense peaks at 76, 136, 170, and 215 cm 1 manifest E-1, A1-1, A1-2,

Fig. 1. XRD patterns of BiFe1 xNixO3 (x = 0.05, 0.10, 0.15, 0.20, 0.25) nanoparticles annealed at 750 °C for 2 h. Inset: (a) the magnified XRD patterns in the vicinity of 2h around 32°.

121

and A1-3 modes, respectively. The remaining five small peaks at 258, 338, 373, 478, and 534 are assigned to be E-1, E-2, E-3, E-4, and E-5 modes. Since Raman scattering spectra are sensitive to atomic displacements, the evolvement of Raman normal modes with increasing x can provide valuable information about ionic substitution and electric polarization. With increasing of Ni content the intensity of E-1, A1-1, A1-2 and A1-3 modes decreases and the peaks get broadened related to Bi–O vibrations. The shift to higher frequency of A1 modes and E modes with the increasing of Ni content shows that the substitution of Ni on Fe sites induce both the compressive structural distortion on Fe sites and Bi sites. A wide peak around 450 cm 1 is related to stretching and bending of NiO6 octahedera in RNiO3 perovskite, respectively. These results indicate that Ni is being substituted at Fe sites in the BiFeO3 lattice [23,24]. The structural quality of the samples was further probed by electron transmission microscopy (TEM). A typical TEM image of the 15% Ni-doped BiFeO3 nanoparticles is shown in Fig. 4a. The average crystallite size is about 40 nm, close to that estimated from XRD patterns with Scherrer equation. Fig. 4b is a corresponding high-resolution TEM (HRTEM) image of 15% Ni-doped BiFeO3 nanoparticles. The regular spacings of the observed lattice is 0.278 nm, which are consistent with the (1 1 0) crystal planes of rhombohedral BiFeO3 crystal. Fig. 5 shows the M–H curves for BiFe1 xNixO3 powders, measured by SQUID at 300 K. A typical magnetic hysteresis loop was observed, indicating that the Ni doped BiFeO3 nanoparticles show a ferromagnetic order at room temperature. As can be seen, with increasing Ni concentration, the magnetization increases drastically. It is noted that saturation is achieved in all the samples for an applied field of <2000 Oe. Several reasons contribute to the enhancement of ferromagnetism when the Ni is doped in BiFeO3. Firstly, the origin of the magnetic property may be attributed to the size confinement effect of the nanostructures; Secondly, Taking into account the ferromagnetic ordering in BiFe1 xNixO3, the magnetization is calculated to be from 1.6 emu/g for BiFe0.95Ni0.05O3 to 8.5 emu/g for BiFe0.75Ni0.25O3, which is very consistent with our experimental results. Thus we conclude that the ferromagnetic exchange between the neighboring Fe3+ and Ni3+ ions contributes to the enhanced magnetization; Finally, The increase in the magnetization of Ni substituted BiFeO3 may be attributed to the creation of lattice defects due to the Ni ions and therefore change in the Fe–O– Fe bond angle [25,26,18]. The saturation magnetizations (MS) estimated from the magnetization curves is summarized inset Fig. 5a. The saturation magnetization increased linearly from 1.29 emu/g to 8.04 emu/g with increasing Ni content from 0.05 to 0.25. It can be considered that the increment of the saturation magnetization might be attributed to Ni addition. If the added Ni substituted with B site of Fe in BiFeO3, the antiferromagnetic spin configuration of BiFeO3 cannot persist locally due to the differences of the magnetic moment between Fe3+ (4 lB) and Ni3+ (2 lB) at the B-site. A local collapse of the antiferromagnetic spin structure leads to an increase in the total spontaneous magnetization. It is therefore possible that local ferrimagnetic spin structures were formed around the B-site, where an Fe atom was substituted by a Ni ion, and that the total mass value of the magnetic moment increased by increasing the Ni content. The inset in Fig. 5b clearly shows a loop shift to the negative field direction. Such behavior related to the exchange bias effect could be explained using the intuitive model proposed by Meiklejohn and Nogues [27,28]. Fig. 6 shows the M–H curves for BiFe1 xNixO3 powders, measured by SQUID at 5 K. In contrast to 300 K, it can be seen that the saturation magnetizations, coercive field and remanent magnetization are increasing with the decreasing of temperature. Metamagnetism was observed, as shown in the hysteresis loop of

122

J. Zhao et al. / Journal of Alloys and Compounds 557 (2013) 120–123

Fig. 2. The EDS spectroscopy pattern of 15% Ni-doped BiFeO3 nanoparticles.

Fig. 3. Raman spectra of BiFe1 xNixO3 (x = 0.05, 0.10, 0.20, 0.25) nanoparticles annealed at 750 °C for 2 h.

BiFe0.95Ni0.05O3 at 5 K are shown inset in Fig. 6. The first jump of magnetization at 750 Oe saturates at about 0.4 emu/g. At about 1800 Oe, there is a second jumplike increase of magnetization to saturate value of 1.3 emu/g. Qualitatively, the first jump of magnetization can be understood as the rotation of the canted antiferromagnetic arranged neighboring spins to the field direction, and the second jump of magnetization is due to the rotation of the ferrimagnetic spin arrangement of neighboring Fe3+ and Ni3+ to the field direction [29]. Fig. 7 shows the temperature dependent magnetization curves for Bi(Fe1 xNix)O3 ceramics. Zero field cooled (ZFC) and field cooled (FC) temperature dependent magnetization curves were measured under 1000 Oe from 5 K to 300 K with cooling field of 1000 Oe for FC measurements. The ZFC curve for both the samples decreases continuously with lowering of temperature whereas FC data increases with lowering of temperature. Such behavior has been identified to be cluster glass behavior [30]. The ZFC and FC curves bifurcate and a peak related to blocking temperature (TB) can be

Fig. 4. (a) Low resolution, (b) high resolution transmission electron microscopy images of 15% Ni-doped BiFeO3 nanoparticles.

observed in the ZFC curves. The TB is 10 K for BiFe0.95Ni0.05O3, 75 K for BiFe0.85Ni0.15O3, 125 K for BiFe0.80Ni0.20O3, and 200 K for BiFe0.75Ni0.25O3 above which magnetic moments of the superpara-

J. Zhao et al. / Journal of Alloys and Compounds 557 (2013) 120–123

123

magnetic particles move freely owing to thermal fluctuations, while they undergo a transition to a blocked state when T 6 TB [18].

(a)

4. Conclusion

(b)

Fig. 5. Room temperature M–H curves for BiFe1 xNixO3 (x = 0.05, 0.10, 0.15, 0.20, 0.25) nanoparticles. Inset: (a) The variation of saturation magnetizations as a function of Ni doping; (b) The magnified view of the central region.

Ni-doped BiFeO3 nanoparticles were synthesized using a sol–gel process and characterized by X-ray diffraction, transmission electron microscopy, Raman scattering and Superconducting Quantum Interference Device. Structural refinement reveals that the Nidoped BiFeO3 samples were refined to a rhombohedral structure with space group R3c. Magnetic studies of the Ni-doped BiFeO3 nanoparticles were ferromagnetic behavior at room temperature and increased with the increasing of Ni concentration. The enhanced magnetization was attributed to the suppression of the cycloidal spin structure by Ni substitution and the ferrimagnetic exchange interaction between the neighboring Fe3+ and Ni3+ ions. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 11147101) and the Natural Science Foundation of Henan (Grant Nos. 102102210448, 102102210452). The authors are grateful to Prof. Erqing Xie and Dr. Jiecai Fu of Lanzhou University for help with the TEM measurements. References

Fig. 6. M–H curves of BiFe1 xNixO3 (x = 0.05, 0.10, 0.15, 0.20, 0.25) nanoparticles at 5 K. Inset: The magnified view of BiFe0.95Ni0.05O3.

Fig. 7. ZFC/FC curves of BiFe1 xNixO3 (x = 0.05, 0.15, 0.20, 0.25) nanoparticles from 5 K to 300 K.

[1] M.H. Li, D. Berry, J. Das, D. Gray, J.F. Li, D. Viehland, J. Am. Ceram. Soc. 94 (2011) 3738. [2] D.T. Giang, V.N. Thuc, N.H. Duc, J. Magn. Magn. Mater. 324 (2012) 2019. [3] K. Prashanthi, M. Mandal, S.P. Duttagupta, R. Pinto, V.R. Palkar, Sens. Actuat. A: Phys. 166 (2011) 83. [4] R. Nechache, C. Harnagea, F. Rosei, Nanoscale 4 (2012) 5588. [5] J.M. Encarnacion, J.D. Burton, E.Y. Tsymbal, J.P. Velev, NanoLett. 11 (2011) 599. [6] B.D. Huey, R.N. Premnath, S.J. Lee, N.A. Polomoff, J. Am. Ceram. Soc. 95 (2012) 1147. [7] J.B. Neaton, C. Ederer, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, Phys. Rev. B 71 (2005) 014113. [8] A. Kumar, D. Varshney, Ceram. Int. 38 (2012) 3935. [9] J. Liu, M.Y. Li, Z.Q. Hu, L. Pei, J. Wang, X.L. Liu, X.Z. Zhao, Appl. Phys. A 102 (2011) 713. [10] K. Prashanthi, B.A. Chalke, R.D. Bapat, S.C. Purandare, V.R. Palkar, Thin Solid Films 518 (2010) 5866. [11] G.S. Lotey, N.K. Verma, J. Nanopart. Res. 14 (2012) 742. [12] P. Thakuria, P.A. Joy, Appl. Phys. Lett. 97 (2010) 162504. [13] K. Chakrabarti, K. Das, B. Sarkar, S.K. De, J. Appl. Phys. 110 (2011) 103905. [14] A. Lahmar, S. Habouti, M. Dietze, C.H. Solterbeck, M. Es-Souni, Appl. Phys. Lett. 94 (2009) 012903. [15] H. Naganuma, J. Miura, S. Okamura, Appl. Phys. Lett. 93 (2008) 052901. [16] B.L. Choudhary, S. Kumar, A. Krishnamurthy, B.K. Srivastava, AIP Conf. Proc. 1372 (2011) 83. [17] Y. Wang, C.W. Nan, Appl. Phys. Lett. 89 (2006) 052903. [18] Y.R. Dai, Q.Y. Xu, X.H. Zheng, S.J. Yuan, Y. Zhai, M.X. Xu, Physica B 407 (2012) 560. [19] Y.H. Wang, X.D. Qi, Proc. Eng. 36 (2012) 455. [20] A.T. Raghavender, N.H. Hong, J. Magn. 16 (2011) 19. [21] .P. Hermet, M. Goffinet, J. Kreisil, P. Ghosez, Phys. Rev. B 75 (2007) 220102(R). [22] C. Beekman, A.A. Reijnders, Y.S. Oh, S.W. Cheong, K.S. Burch, Phys. Rev. B 86 (2012) 020403(R). [23] M. Weber, S. Pignard, J. Kreisel, Appl. Phys. Lett. 97 (2010) 031915. [24] C. Girardot, J. Kreise, S. Pignard, N. Caillault, F. Weiss, Phys. Rev. B 78 (2008) 104104. [25] D. Lee, M.G. Kim, S. Ryu, H.M. Jang, S.G. Lee, Appl. Phys. Lett. 86 (2005) 222903. [26] S. Layek, S. Das, H.C. Verma, AIP Conf. Proc. 1349 (2011) 351. [27] W.H. Meiklejohn, J. Appl. Phys. 33 (1962) 1328. [28] J. Nogues, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [29] Q.Y. Xu, H.F. Zai, D. Wu, T. Qiu, M.X. Xu, Appl. Phys. Lett. 95 (2009) 112510. [30] Q.Y. Xu, S.Q. Zhou, D. Wu, M. Uhlarz, Y.K. Tang, K. Potzger, M.X. Xu, H. Schmidt, J. Appl. Phys. 107 (2010) 093920.