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Multiferroic and magnetoelectric properties of nanostructured BaFe0.01Ti0.99O3 thin films obtained under polyethylene glycol conditions
Solid State Communications 178 (2014) 11–15
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Multiferroic and magnetoelectric properties of nanostructured BaFe0.01Ti0.99O3 thin films obtained under polyethylene glycol conditions Kuldeep Chand Verma a,n, Jasneet Kaur a, N.S. Negi b, R.K. Kotnala c a
Akal School of Physics, Eternal University, Baru Sahib, Himachal Pradesh-173101, India Department of Physics, Himachal Pradesh University, Shimla 171005, India c National Physical Laboratory, New Delhi 110012, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 August 2013 Received in revised form 22 September 2013 Accepted 15 October 2013 by P. Chaddah Available online 25 October 2013
Multiferroic BaFe0.01Ti0.99O3 (BFT) thin films were prepared by sol–gel combined metallo-organic decomposition method using spin-coating technique with polyethylene glycol (PEG) conditions [BFT coating solution was prepared by adding PEG (in volume) 3% (BFT3) and 5% (BFT5)]. X-ray diffraction confirms the formation of polycrystalline BFT of tetragonal phase. Atomic force and scanning electron microscopy shows nanoparticles in BFT3 and nanorods in BFT5 sample. The coexistence of ferromagnetism and ferroelectricity show more enhancement in case of BFT nanorods than nanoparticles. The observed ferromagnetism depends upon the oxygen vacancy and ferroelectric polarization on nanostructures, tetragonal phase and epitaxial strain. The magnetoelectric coefficient as the function of applied dc magnetizing field under ac magnetic field 10 Oe and frequency 847 Hz is measured. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Multiferroic B. Spin coating C. Nanostructure D. Magnetoelectric coupling
1. Introduction One-dimensional (1-D) multiferroics have attracted much attention because of the dependence of their properties, such as ferromagnetic, ferroelectric and magnetoelectric, on their dimensionality and size [1–3]. Such studies are essential for realizing nanoscale devices for a wide range of applications, including spintronics, memory, transducers, sensors, etc. [1–3]. Various types of multiferroics such as Ba(FexTi1 x)O3 [3], BiFeO3 [4], TbMn2O5 [5], BaTiO3– CoFe2O4 [6], 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 [7], Ni47.4Mn32.1Ga20.5/ PZT [8] etc. exist. Generally, the magnetoelectric coupling between polarization and magnetization of BiFeO3 and TbMn2O5 exists weak or remains far below room temperature which hinders their practical applications. However, some perovskites based multiferroic BaTiO3 and PbTiO3 [3,6,7] show room temperature multiferroic properties and large magnetoelectric (ME) coupling coefficients. The coupling interaction between ferroelectric and ferromagnetic phases in the multiferroic nanostructures is an elastic interaction [9]. The survey of reported work about multiferroic shows that there is little study about 1-D multiferroic systems (nanorods, nanowires, nanobelts,
n
Corresponding author. Tel.: þ 91 9418941286; fax: þ 91 1799276006. E-mail addresses: [email protected], [email protected] (K. Chand Verma). 0038-1098/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2013.10.020
nanotubes etc.) which may result in improvement of the coexistence of ferroelectricity and ferromagnetism, and ME coefficient. There are different ways to fabricate a 1-D multiferroic system such as synthesizing methods, adjust appropriate stoichiometric ratio, concentration of polymer as a surfactant etc. The role of surfactant is extensibly usable for 1-D nanostructures using polymers of polyvinyl alcohol and polyethylene glycol (PEG) [3,10]. It is well known that PEG, as a sort of nonionized surfactant, has molecular formula of H–(O–CH2–CH2)n–OH. The monomer of PEG was apt to exist with chain structures in water (hydrophilic site) and there existed a large quantity of activated oxygen in PEG molecular chains (hydrophobic site), resulting in strong interactions between PEG molecules and metal ions to form metal ions– PEG 1D chain structures. With decreasing particle size in BaTiO3 perovskite lattice, an increase in cubic lattice parameter has been observed at room temperature [11] which results in crystal structure transformation from tetragonal (ferroelectric phase) to cubic (paraelectric phase). The critical size of BaTiO3 exists between 25 and 200 nm [10]. Therefore the existence of physical properties of BaTiO3 with small crystalline size for stability of tetragonal phase and reducing value of critical size below 25 nm the doping with transition metal ions such as Fe3 þ is necessary for multiferroic and ME parameters. Frey et al. [12] reported that when the grains size of polycrystalline BaTiO3 tetragonal phase is reduced into nano scale, the enhancement in electrical polarization occurs because the twinning behaviour of
12
K. Chand Verma et al. / Solid State Communications 178 (2014) 11–15
polycrystals which reduce the bulk strain energy contributes to the resulting twin structure. As the grain size reduced, the density of twins required to substantially relieve the transformation stress within the grain interior, and minimize the residual stress near grain boundaries, increased more than for single crystal. Choi et al. [13] observed epitaxial strain can be introduced to the thin film of BaTiO3 which leads to an increase in Curie temperature (TC) by nearly 500 1C and produces a remnant polarization of at least 250% higher than bulk BaTiO3 single crystals. To figure out the magnetism origin in the doped system, different approaches can be implemented. The reported work of Fe doped BaTiO3 [3,14] and PbTiO3 [15] multiferroic show room temperature ferromagnetism at low Fe concentration (0–4 mol% of Fe). The ferromagnetism of these multiferroic systems is intrinsic in which the electrons trapped in the oxygen vacancy via the F-centre will overlap the d shells of both neighbouring iron. At a low Fe concentration, the 3d5 orbitals have unoccupied minority spin orbitals, resulting in weak ferromagnetism. However, the higher Fe concentration leads to antiferromagnetic superexchange interaction between Fe ions via oxygen. The ME coupling in transition metal doped perovskite is a product of piezoelectric and piezomagnetic in which elastic strain field puts in relationships the electric polarization in the piezoelectric phase with the magnetization arising in the piezomagnetic component giving rise to both the direct and converse magneto-electric effects [16]. The ME coupling has a wide range of applications in magnetic field probes, electric packaging, hydrophones, medical ultrasonic imaging, wireless powering of MEMS, sensors and actuators. In perovskite based multiferroic nanostructures, surface magnetism interacts with electric dipoles of nano grains more effectively to induce ME coupling than bulk particles [17]. In this paper, we report the structural, microstructural, ferromagnetic, ferroelectric properties and ME coefficient of BaFe0.01Ti0.99O3 (BFT) nanostructural thin films. BFT thin films were prepared by sol–gel combined metallo organic decomposition method under PEG conditions [BFT solution prepared by adding PEG (in volume) 3% (BFT3) and 5% (BFT5)].
perspex free from any metallic impurity. Extreme care was taken to clean the holder ultrasonically to remove any magnetic material traces. The same measurement procedure was repeated for empty sample holder, and magnetization data of the holder were subtracted from the measured magnetic signal of the sample. VSM has magnetization sensitivity of the order of 10 6 emu. The ME effect of BFT was determined by dynamic method using ME coefficient measurement setup. The ac magnetic field of 10 Oe is applied to remove charge collection at the electrode of samples otherwise it will give spurious voltage.
3. Result and discussion Fig. 1 shows the XRD pattern of prepared BFT3 and BFT5 thin films on Si substrate exhibiting that the products are of good crystallinity. All diffraction peaks can be assigned to tetragonal BaTiO3 phase in both the films. The diffraction pattern shows presence of tetragonal BaTiO3 structure [18] by splitting of cubic into tetragonal by 200/002 reflection nearly at about 451. Using the CHEKCELL lattice constant refinement program, the values of lattice parameters are a(Å)¼ 3.997 and 3.991, and c(Å) ¼4.021 and 4.029, and c/a ¼1.0060 and 1.0095 respectively, for BFT3 and BFT5, which are very close to the reported values pertaining to this tetragonal structure (JCPDS data no. 05-0626) [18]. The broadening of the full width at half maxima of the diffraction peaks indicates the formation of nanocrystalline products. To see the effect of PEG concentration in BFT films to change the shape of nanostructure, the AFM images of the films (Fig. 2(a) and (b)) are given. Both the BFT films show nano size grains. However BFT5 show rod like structure. Moreover, the film images are seen to be crack free, dense and the grains are uniformly distributed in all images. The value of root-mean-square roughness is 1.5 and average grains size is 17 nm for BFT3 films. While for BFT5 nanorods, the average diameter is 45 nm and length
2. Experimental detail The precursors solution of BFT thin films was prepared using (C7H15COO)2Ba, iron 3-ethylhexanoate (C7H15COO)3Fe and tetra-nbutyl orthotitanate in xylene. The precursors solution was mixed in the molar ratio of Ba:Fe:Ti::1:0.01:0.99 to get a coating solution. The solution was refluxed at 110 1C with constant stirring for 10 h for homogeneous mixing and has to be added 3% PEG (in volume) for BFT3 film. Similarly another BFT coating solution was prepared by adding 5% PEG (in volume) for BFT5. The solutions were coated on substrates of Pt/Ti/SiO2/Si for electrical and on Si for magnetic measurements using spin-coating technique with 4500 rpm for 60 s, and annealed at 600 1C for 3 h. The final film thickness after three coatings is 400 nm. The phase structure of BFT films was analyzed by X-ray diffraction (XRD) using X-Pert PRO system. The microstructure of BFT is analyzed by atomic force microscopy (AFM) using VECCO DI CP-II system and scanning electron microscopy (SEM) by using JEOL JSM6100 system. Electrical measurements were made on a metal–insulator–metal (MIM) capacitor configuration, with Pt as both the top and bottom electrodes. Pt dots 0.5 mm in diameter were deposited as top electrodes on the film surface through a shadow mask by radio frequency sputtering. Polarization under the influence of applied electric field was measured using a Radiant Technologies ferroelectric test system. Magnetization (M–H) of the BFT thin films was measured at room temperature using Lakeshore 7304 vibrating sample magnetometer (VSM) using a sample holder of high-purity
Fig. 1. (Colour online) XRD pattern of BFT thin films.
K. Chand Verma et al. / Solid State Communications 178 (2014) 11–15
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Fig. 2. (Colour online) AFM (inset show SEM) images of BFT thin films.
Fig. 3. (Colour online) Possible growth mechanism of BFT nanostructures involving metal ions and PEG as surfactant.
200 nm, and root-mean-square roughness 2.1 nm. For more confirmation of nanostructures, the insets of Fig. 2(a) and (b) are the SEM images show nanoparticles in BFT3 and rod like nanostructure in BFT5 whose dimensions are consistent with the measurement of AFM images. The surface of AFM and SEM images of BFT films show some nano gaps between the grains which is an indication of the existence of small porosity. In understanding the growth mechanism of nanoparticles and nanorods, the surfactant PEG plays a crucial role and proposed growth mechanism can be explained in terms of crystal growth [3,10] as shown in Fig. 3. Generally, nanorods are formed at lower heating temperature below 200 1C such as in hydrothermal based processes [19] and nanoparticles are formed at higher heating. But in the present work, we have synthesized nanorods at 600 1C by increasing the concentration of PEG. During heating, BFT precursor with PEG involves typically two sites namely hydrophobic (Hb) and hydrophilic (Hp) of n clusters. The Hb keeps materials separate due to repulsive forces and slows down the reaction process. Secondly, the Hp keeps the metal intact and retards the growth process. When the concentration of PEG is enhanced from 3% to 5%, there is increase in length of Hp layer which assists in the growth of metal ions into 1D nanostructure like rods due to heating. This allows the BFT nanocrystals to grow continuously in the confined reverse micelles causing the formation of nanorods with heating. Moreover, when the concentration of PEG is 3%, isotropic growth becomes dominant, which causes the formation of spherical BFT3 nanocrystals. As more clusters of Hp of the PEG present with M þ ions, the heating is responsible for growing spherical particles into 1D nanorods. Fig. 4 shows the coexistence of ferromagnetism and ferroelectricity of BFT3 and BFT5 thin films by measuring magnetization versus magnetizing field (M–H) and polarization versus electric
field (P–E) hysteresis curves at room temperature. According to Fig. 4, the values of saturation magnetization (Ms) are 1.73 and 5.89 emu/cm3, remanent magnetization (Mr) are 0.48 and 2.27 emu/cm3, and coercive field (Hc) are 76.23 and 119.25 Oe respectively, for BFT3 and BFT5 thin films. The existence of strong ferromagnetism is explained in two ways: partially filled inner shells (d- or f-levels) and formation of nanostructures. An F-centre exchange (FCE) mechanism describes the ferromagnetism of present BFT films, which is similar to Fe-doped TiO2 given in our previous work [15,20]. Such a mechanism anticipates that the Fe3 þ –V2o –Fe3 þ group will be common in the structure and an electron trapped in the oxygen vacancy makes an F-centre, where the electron occupies an orbital (pz) which overlaps the d2z orbitals of the d shells of both iron neighbours. For Fe3 þ , 3d5, the trapped electron will spin down, and the two iron neighbours will spin up. The exchange interaction between the two irons via the F-centre leads to the resulting ferromagnetism. The formation of nanorod like structure in BFT5 results in large magnetization than BFT3 films may be caused by effective exchange interactions between the unpaired electrons spins, originating from the surface defects such as oxygen vacancy clusters instead of single neutral oxygen vacancies associated with the nanoparticles [21]. The P–E hysteresis loops measured at room temperature on poled BFT thin films on Pt/Ti/SiO2/Si using MIM configuration with 50 Hz the frequency of polarization are presented in Fig. 4 (insets). The maximum polarization, Pmax ¼6.93 and 11.01 mC/cm2, remanent polarization, Pr ¼ 2.48 and 4.91 mC/cm2, and coercive field, Ec ¼18.43 and 24.13 kV/cm, respectively, are observed for BFT3 and BFT5 thin films. These Pr values of BFT3 and BFT5 thin films show improvement which is similar to the reported work of polycrystalline BaTiO3 thin films [22,23] and are explained by different factors. Firstly the surface defects by large value of
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K. Chand Verma et al. / Solid State Communications 178 (2014) 11–15
Fig. 4. (Colour online) Ferromagnetism of BFT3 and BFT5 thin films. Inset shows the respective ferroelectricity.
introduced by the lattice mismatch of BFT thin films of tetragonal phase with substrate of cubic phase. The coexistence of ferroelectricity and ferromagnetism in present BFT films gives rise to a ME effect [9]. The magnetoelectric coupling coefficient of samples has been determined by the dynamic magnetic field method [27]. A buffer and high-pass filter circuit to reduce the background voltage noise are employed in the measurement set-up to detect the induced ME voltage across the sample thickness. The ME voltage coefficient is given by αE ¼Vout/Hac t, where t is the sample thickness and Vout is the induced ME voltage. The variations in ME coefficient with Hdc at ac magnetic field frequency of 847 Hz and ac magnetizing field of 10 Oe for both BFT thin film are shown in Fig. 5. It has been found that the BFT5 has the larger value of αE as compared to BFT3 because the value of ferroelectricity and ferromagnetism is higher than BFT3. This indicates that the ME coupling improves in BFT5 because nanorods like structure of large surface defects than nanoparticles [17]. The maximum value of αE is found to be 16.16 and 45.61 mV/cm Oe, respectively, for BFT3 and BFT5. Moreover, the value of αE of both the film increases and then starts to decrease at a particular value of Hdc. This trend of αE is attributed to the piezomagnetic phase, since its magnetostriction increases with the field until saturation. The saturated magnetostrictive strain is transferred to the piezoelectric phase and a constant electrical signal is generated in spite of the increasing magnetic field [28]. As a result, the αE decreases at high magnetic field. It has also been reported by Nan et al. [9] that the magnetic field induced electric polarization of the multiferroic nanostructured film is closely related to the magnetostrictive behaviour of the ferromagnetic phase.
4. Conclusion
Fig. 5. (Colour online) ME voltage coefficient (αE) with Hdc under the influence of Hac ¼ 10 Oe and frequency of 847 Hz.
surface to volume ratio of nanostructures which is higher in nanorods than nanoparticles [1–3,24]. Secondly, these BFT thin films are polycrystalline of tetragonal phase and nano grains contribute to twin structure in which stress resides near grain boundaries which could lead to an enhanced polarization [12]. By this assumption, it will lead to an easily controllable depolarization field and long-range interactions which support the development of homogeneous spontaneous polarization. The length factors used to describe the interaction between polar units in ferroelectric materials are the correlation lengths parallel and perpendicular to the polarization vector [25]; charge defects might serve to stabilize polar micro-domains in chemically prepared ferroelectric samples and elastic constraints forms strain energetic and its stress relieving twinning mechanism. Finally, the epitaxial strain growth result in large ferroelectric polarization [12,26] is
The nanostructures of BFT thin films have been prepared by sol–gel combined metallo organic decomposition method with PEG conditions. XRD pattern show polycrystalline tetragonal phase of both BFT films. The AFM and SEM analysis measures the average particles size of 17 nm for BFT3 while BFT5 have nanorod like structure of diameter 45 nm and length 200 nm. The growth reaction mechanism shows that the increase in length of hydrophilic layer of PEG is responsible for the formation of nanorods. The values of Ms are 1.73 and 5.89 emu/ cm3, respectively, measured for BFT3 and BFT5 thin films. The value of Pmax ¼ 6.93 and 11.01 mC/cm2, respectively, is measured for BFT3 and BFT5 thin films which is explained on the basis of large surface to volume ratio of nanostructures, tetragonal phase of nano grains contributes energetic strain and epitaxial strain by the mismatch of lattice parameters. Both BFT thin films show good ME coupling and the calculated value of αE is 16.16 and 45.61 mV/cm Oe, respectively, for BFT3 and BFT5. References [1] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. [2] B. Li, C. Wang, G. Dou, CrystEngComm 15 (2013) 2147. [3] J. Kaur, R.K. Kotnala, K.C. Verma, Mater. Lett. 65 (2011) 3160. [4] V.A Reddy, N.P. Pathak, R. Nath, J. Alloys Compd. 543 (2012) 206. [5] N. Hur, S. Park, P.A. Sharma, J.S. Ahn, S. Guha, S.W. Cheong, Nature (London) 429 (2004) 392. [6] K. Raidongia, A. Nag, A. Sundaresan, C.N.R. Rao, Appl. Phys. Lett. 97 (2010) 062904. [7] D.R. Patil, Y. Chai, R.C. Kambale, B.G. Jeon, K. Yoo, J. Ryu, W.H. Yoon, D.S. Park, D.Y. Jeong, S.G. Lee, J. Lee, J.H. Nam, J.H. Cho, B.I. Kim, K.H. Kim, Appl. Phys. Lett. 102 (2013) 062909. [8] R.H. Wang, M.Q. Zhao, G.J. Zhang, G. Liu, J Mater. Sci. 45 (2010) 4490. [9] C.W. Nan, G. Liu, Y. Lin, Phys. Rev. Lett. 94 (2005) 197203. [10] K. Pal, U.N. Maiti, T.P. Majumder, S.C. Debnath, Appl. Surf. Sci. 258 (2011) 163. [11] U.A. Joshi, S. Yoon, S. Baik, J.S. Lee, J. Phys. Chem. B 110 (2006) 12249. [12] M.H. Frey, D.A. Payne, Phys. Rev. B 54 (1996) 3158.
K. Chand Verma et al. / Solid State Communications 178 (2014) 11–15 [13] K.J. Choi, M. Biegalski, Y.L. Li, A. Sharan, J. Schubert, R. Uecker, P. Reiche, Y.B. Chen, X.Q. Pan, V. Gopalan, L.Q. Chen, D.G. Schlom, C.B. Eom, Science 306 (2004) 1005. [14] X.K. Wei, Y.T. Su, Y. Sui, Q.H. Zhang, Y. Yao, C.Q. Jin, R.C. Yu, J. Appl. Phys. 110 (2011) 114112. [15] K.C. Verma, R.K. Kotnala, N.S. Negi, Appl. Phys. Lett. 92 (2008) 152902. [16] A. Milazzo, C. Orlando, Compos. Struct. 94 (2012) 3710. [17] C.A.F. Vaz, J. Phys.: Condens. Matter 24 (2012) 333201. [18] Y. Isohamaa, N. Nakajima, H. Maruyama, Y. Tezuka, T. Iwazumi, J. Electron Spectros. Relat. Phenom. 184 (2011) 207. [19] K.C. Verma, V. Gupta, J. Kaur, R.K. Kotnala, J. Alloys Compd. 578 (2013) 5. [20] K.C. Verma, J. Singh, M. Ram, D.K. Sharma, A. Sharma, R.K Kotnala, Phys. Scr. 86 (2012) 025704.
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[21] S. Banerjee, M. Mandal, N. Gayathri, M. Sardar, Appl. Phys. Lett. 91 (2007) 182501. [22] Y. Tanaka, T. Harigai, H. Adachi, N. Sakamoto, N. Wakiya, H. Suzuki, E. Fujii, Appl. Phys. Lett. 102 (2013) 192901. [23] Y.H. Jeon, E.A. Patterson, D.P. Cann, B.J. Gibbons, Mater. Lett. 106 (2013) 63. [24] S. Baruah, M.A. Mahmood, M.T.Z. Myint, T. Bora, J. Dutta, Beilstein J. Nanotechnol. 1 (2010) 14. [25] H. Ikawa, Ceram. Trans. 32 (1993) 19. [26] C. Ederer, N.A. Spaldin, Phys. Rev. Lett. 95 (2005) 257601. [27] M. Kumar, A. Srinivas, S.V. Suryanarayana, G.S. Kumar, T. Bhimasankaram, Bull. Mater. Sci. 21 (3) (1998) 251. [28] G.V. Duong, R. Groessinger, M. Schoenhart, D.B. Basques, J. Magn. Magn. Mater. 316 (2007) 390.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Multiferroic and magnetoelectric properties of nanostructured BaFe0.01Ti0.99O3 thin films obtained under polyethylene glycol conditions Kuldeep Chand Verma a,n, Jasneet Kaur a, N.S. Negi b, R.K. Kotnala c a
Akal School of Physics, Eternal University, Baru Sahib, Himachal Pradesh-173101, India Department of Physics, Himachal Pradesh University, Shimla 171005, India c National Physical Laboratory, New Delhi 110012, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 August 2013 Received in revised form 22 September 2013 Accepted 15 October 2013 by P. Chaddah Available online 25 October 2013
Multiferroic BaFe0.01Ti0.99O3 (BFT) thin films were prepared by sol–gel combined metallo-organic decomposition method using spin-coating technique with polyethylene glycol (PEG) conditions [BFT coating solution was prepared by adding PEG (in volume) 3% (BFT3) and 5% (BFT5)]. X-ray diffraction confirms the formation of polycrystalline BFT of tetragonal phase. Atomic force and scanning electron microscopy shows nanoparticles in BFT3 and nanorods in BFT5 sample. The coexistence of ferromagnetism and ferroelectricity show more enhancement in case of BFT nanorods than nanoparticles. The observed ferromagnetism depends upon the oxygen vacancy and ferroelectric polarization on nanostructures, tetragonal phase and epitaxial strain. The magnetoelectric coefficient as the function of applied dc magnetizing field under ac magnetic field 10 Oe and frequency 847 Hz is measured. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Multiferroic B. Spin coating C. Nanostructure D. Magnetoelectric coupling
1. Introduction One-dimensional (1-D) multiferroics have attracted much attention because of the dependence of their properties, such as ferromagnetic, ferroelectric and magnetoelectric, on their dimensionality and size [1–3]. Such studies are essential for realizing nanoscale devices for a wide range of applications, including spintronics, memory, transducers, sensors, etc. [1–3]. Various types of multiferroics such as Ba(FexTi1 x)O3 [3], BiFeO3 [4], TbMn2O5 [5], BaTiO3– CoFe2O4 [6], 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 [7], Ni47.4Mn32.1Ga20.5/ PZT [8] etc. exist. Generally, the magnetoelectric coupling between polarization and magnetization of BiFeO3 and TbMn2O5 exists weak or remains far below room temperature which hinders their practical applications. However, some perovskites based multiferroic BaTiO3 and PbTiO3 [3,6,7] show room temperature multiferroic properties and large magnetoelectric (ME) coupling coefficients. The coupling interaction between ferroelectric and ferromagnetic phases in the multiferroic nanostructures is an elastic interaction [9]. The survey of reported work about multiferroic shows that there is little study about 1-D multiferroic systems (nanorods, nanowires, nanobelts,
n
Corresponding author. Tel.: þ 91 9418941286; fax: þ 91 1799276006. E-mail addresses: [email protected], [email protected] (K. Chand Verma). 0038-1098/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2013.10.020
nanotubes etc.) which may result in improvement of the coexistence of ferroelectricity and ferromagnetism, and ME coefficient. There are different ways to fabricate a 1-D multiferroic system such as synthesizing methods, adjust appropriate stoichiometric ratio, concentration of polymer as a surfactant etc. The role of surfactant is extensibly usable for 1-D nanostructures using polymers of polyvinyl alcohol and polyethylene glycol (PEG) [3,10]. It is well known that PEG, as a sort of nonionized surfactant, has molecular formula of H–(O–CH2–CH2)n–OH. The monomer of PEG was apt to exist with chain structures in water (hydrophilic site) and there existed a large quantity of activated oxygen in PEG molecular chains (hydrophobic site), resulting in strong interactions between PEG molecules and metal ions to form metal ions– PEG 1D chain structures. With decreasing particle size in BaTiO3 perovskite lattice, an increase in cubic lattice parameter has been observed at room temperature [11] which results in crystal structure transformation from tetragonal (ferroelectric phase) to cubic (paraelectric phase). The critical size of BaTiO3 exists between 25 and 200 nm [10]. Therefore the existence of physical properties of BaTiO3 with small crystalline size for stability of tetragonal phase and reducing value of critical size below 25 nm the doping with transition metal ions such as Fe3 þ is necessary for multiferroic and ME parameters. Frey et al. [12] reported that when the grains size of polycrystalline BaTiO3 tetragonal phase is reduced into nano scale, the enhancement in electrical polarization occurs because the twinning behaviour of
12
K. Chand Verma et al. / Solid State Communications 178 (2014) 11–15
polycrystals which reduce the bulk strain energy contributes to the resulting twin structure. As the grain size reduced, the density of twins required to substantially relieve the transformation stress within the grain interior, and minimize the residual stress near grain boundaries, increased more than for single crystal. Choi et al. [13] observed epitaxial strain can be introduced to the thin film of BaTiO3 which leads to an increase in Curie temperature (TC) by nearly 500 1C and produces a remnant polarization of at least 250% higher than bulk BaTiO3 single crystals. To figure out the magnetism origin in the doped system, different approaches can be implemented. The reported work of Fe doped BaTiO3 [3,14] and PbTiO3 [15] multiferroic show room temperature ferromagnetism at low Fe concentration (0–4 mol% of Fe). The ferromagnetism of these multiferroic systems is intrinsic in which the electrons trapped in the oxygen vacancy via the F-centre will overlap the d shells of both neighbouring iron. At a low Fe concentration, the 3d5 orbitals have unoccupied minority spin orbitals, resulting in weak ferromagnetism. However, the higher Fe concentration leads to antiferromagnetic superexchange interaction between Fe ions via oxygen. The ME coupling in transition metal doped perovskite is a product of piezoelectric and piezomagnetic in which elastic strain field puts in relationships the electric polarization in the piezoelectric phase with the magnetization arising in the piezomagnetic component giving rise to both the direct and converse magneto-electric effects [16]. The ME coupling has a wide range of applications in magnetic field probes, electric packaging, hydrophones, medical ultrasonic imaging, wireless powering of MEMS, sensors and actuators. In perovskite based multiferroic nanostructures, surface magnetism interacts with electric dipoles of nano grains more effectively to induce ME coupling than bulk particles [17]. In this paper, we report the structural, microstructural, ferromagnetic, ferroelectric properties and ME coefficient of BaFe0.01Ti0.99O3 (BFT) nanostructural thin films. BFT thin films were prepared by sol–gel combined metallo organic decomposition method under PEG conditions [BFT solution prepared by adding PEG (in volume) 3% (BFT3) and 5% (BFT5)].
perspex free from any metallic impurity. Extreme care was taken to clean the holder ultrasonically to remove any magnetic material traces. The same measurement procedure was repeated for empty sample holder, and magnetization data of the holder were subtracted from the measured magnetic signal of the sample. VSM has magnetization sensitivity of the order of 10 6 emu. The ME effect of BFT was determined by dynamic method using ME coefficient measurement setup. The ac magnetic field of 10 Oe is applied to remove charge collection at the electrode of samples otherwise it will give spurious voltage.
3. Result and discussion Fig. 1 shows the XRD pattern of prepared BFT3 and BFT5 thin films on Si substrate exhibiting that the products are of good crystallinity. All diffraction peaks can be assigned to tetragonal BaTiO3 phase in both the films. The diffraction pattern shows presence of tetragonal BaTiO3 structure [18] by splitting of cubic into tetragonal by 200/002 reflection nearly at about 451. Using the CHEKCELL lattice constant refinement program, the values of lattice parameters are a(Å)¼ 3.997 and 3.991, and c(Å) ¼4.021 and 4.029, and c/a ¼1.0060 and 1.0095 respectively, for BFT3 and BFT5, which are very close to the reported values pertaining to this tetragonal structure (JCPDS data no. 05-0626) [18]. The broadening of the full width at half maxima of the diffraction peaks indicates the formation of nanocrystalline products. To see the effect of PEG concentration in BFT films to change the shape of nanostructure, the AFM images of the films (Fig. 2(a) and (b)) are given. Both the BFT films show nano size grains. However BFT5 show rod like structure. Moreover, the film images are seen to be crack free, dense and the grains are uniformly distributed in all images. The value of root-mean-square roughness is 1.5 and average grains size is 17 nm for BFT3 films. While for BFT5 nanorods, the average diameter is 45 nm and length
2. Experimental detail The precursors solution of BFT thin films was prepared using (C7H15COO)2Ba, iron 3-ethylhexanoate (C7H15COO)3Fe and tetra-nbutyl orthotitanate in xylene. The precursors solution was mixed in the molar ratio of Ba:Fe:Ti::1:0.01:0.99 to get a coating solution. The solution was refluxed at 110 1C with constant stirring for 10 h for homogeneous mixing and has to be added 3% PEG (in volume) for BFT3 film. Similarly another BFT coating solution was prepared by adding 5% PEG (in volume) for BFT5. The solutions were coated on substrates of Pt/Ti/SiO2/Si for electrical and on Si for magnetic measurements using spin-coating technique with 4500 rpm for 60 s, and annealed at 600 1C for 3 h. The final film thickness after three coatings is 400 nm. The phase structure of BFT films was analyzed by X-ray diffraction (XRD) using X-Pert PRO system. The microstructure of BFT is analyzed by atomic force microscopy (AFM) using VECCO DI CP-II system and scanning electron microscopy (SEM) by using JEOL JSM6100 system. Electrical measurements were made on a metal–insulator–metal (MIM) capacitor configuration, with Pt as both the top and bottom electrodes. Pt dots 0.5 mm in diameter were deposited as top electrodes on the film surface through a shadow mask by radio frequency sputtering. Polarization under the influence of applied electric field was measured using a Radiant Technologies ferroelectric test system. Magnetization (M–H) of the BFT thin films was measured at room temperature using Lakeshore 7304 vibrating sample magnetometer (VSM) using a sample holder of high-purity
Fig. 1. (Colour online) XRD pattern of BFT thin films.
K. Chand Verma et al. / Solid State Communications 178 (2014) 11–15
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Fig. 2. (Colour online) AFM (inset show SEM) images of BFT thin films.
Fig. 3. (Colour online) Possible growth mechanism of BFT nanostructures involving metal ions and PEG as surfactant.
200 nm, and root-mean-square roughness 2.1 nm. For more confirmation of nanostructures, the insets of Fig. 2(a) and (b) are the SEM images show nanoparticles in BFT3 and rod like nanostructure in BFT5 whose dimensions are consistent with the measurement of AFM images. The surface of AFM and SEM images of BFT films show some nano gaps between the grains which is an indication of the existence of small porosity. In understanding the growth mechanism of nanoparticles and nanorods, the surfactant PEG plays a crucial role and proposed growth mechanism can be explained in terms of crystal growth [3,10] as shown in Fig. 3. Generally, nanorods are formed at lower heating temperature below 200 1C such as in hydrothermal based processes [19] and nanoparticles are formed at higher heating. But in the present work, we have synthesized nanorods at 600 1C by increasing the concentration of PEG. During heating, BFT precursor with PEG involves typically two sites namely hydrophobic (Hb) and hydrophilic (Hp) of n clusters. The Hb keeps materials separate due to repulsive forces and slows down the reaction process. Secondly, the Hp keeps the metal intact and retards the growth process. When the concentration of PEG is enhanced from 3% to 5%, there is increase in length of Hp layer which assists in the growth of metal ions into 1D nanostructure like rods due to heating. This allows the BFT nanocrystals to grow continuously in the confined reverse micelles causing the formation of nanorods with heating. Moreover, when the concentration of PEG is 3%, isotropic growth becomes dominant, which causes the formation of spherical BFT3 nanocrystals. As more clusters of Hp of the PEG present with M þ ions, the heating is responsible for growing spherical particles into 1D nanorods. Fig. 4 shows the coexistence of ferromagnetism and ferroelectricity of BFT3 and BFT5 thin films by measuring magnetization versus magnetizing field (M–H) and polarization versus electric
field (P–E) hysteresis curves at room temperature. According to Fig. 4, the values of saturation magnetization (Ms) are 1.73 and 5.89 emu/cm3, remanent magnetization (Mr) are 0.48 and 2.27 emu/cm3, and coercive field (Hc) are 76.23 and 119.25 Oe respectively, for BFT3 and BFT5 thin films. The existence of strong ferromagnetism is explained in two ways: partially filled inner shells (d- or f-levels) and formation of nanostructures. An F-centre exchange (FCE) mechanism describes the ferromagnetism of present BFT films, which is similar to Fe-doped TiO2 given in our previous work [15,20]. Such a mechanism anticipates that the Fe3 þ –V2o –Fe3 þ group will be common in the structure and an electron trapped in the oxygen vacancy makes an F-centre, where the electron occupies an orbital (pz) which overlaps the d2z orbitals of the d shells of both iron neighbours. For Fe3 þ , 3d5, the trapped electron will spin down, and the two iron neighbours will spin up. The exchange interaction between the two irons via the F-centre leads to the resulting ferromagnetism. The formation of nanorod like structure in BFT5 results in large magnetization than BFT3 films may be caused by effective exchange interactions between the unpaired electrons spins, originating from the surface defects such as oxygen vacancy clusters instead of single neutral oxygen vacancies associated with the nanoparticles [21]. The P–E hysteresis loops measured at room temperature on poled BFT thin films on Pt/Ti/SiO2/Si using MIM configuration with 50 Hz the frequency of polarization are presented in Fig. 4 (insets). The maximum polarization, Pmax ¼6.93 and 11.01 mC/cm2, remanent polarization, Pr ¼ 2.48 and 4.91 mC/cm2, and coercive field, Ec ¼18.43 and 24.13 kV/cm, respectively, are observed for BFT3 and BFT5 thin films. These Pr values of BFT3 and BFT5 thin films show improvement which is similar to the reported work of polycrystalline BaTiO3 thin films [22,23] and are explained by different factors. Firstly the surface defects by large value of
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K. Chand Verma et al. / Solid State Communications 178 (2014) 11–15
Fig. 4. (Colour online) Ferromagnetism of BFT3 and BFT5 thin films. Inset shows the respective ferroelectricity.
introduced by the lattice mismatch of BFT thin films of tetragonal phase with substrate of cubic phase. The coexistence of ferroelectricity and ferromagnetism in present BFT films gives rise to a ME effect [9]. The magnetoelectric coupling coefficient of samples has been determined by the dynamic magnetic field method [27]. A buffer and high-pass filter circuit to reduce the background voltage noise are employed in the measurement set-up to detect the induced ME voltage across the sample thickness. The ME voltage coefficient is given by αE ¼Vout/Hac t, where t is the sample thickness and Vout is the induced ME voltage. The variations in ME coefficient with Hdc at ac magnetic field frequency of 847 Hz and ac magnetizing field of 10 Oe for both BFT thin film are shown in Fig. 5. It has been found that the BFT5 has the larger value of αE as compared to BFT3 because the value of ferroelectricity and ferromagnetism is higher than BFT3. This indicates that the ME coupling improves in BFT5 because nanorods like structure of large surface defects than nanoparticles [17]. The maximum value of αE is found to be 16.16 and 45.61 mV/cm Oe, respectively, for BFT3 and BFT5. Moreover, the value of αE of both the film increases and then starts to decrease at a particular value of Hdc. This trend of αE is attributed to the piezomagnetic phase, since its magnetostriction increases with the field until saturation. The saturated magnetostrictive strain is transferred to the piezoelectric phase and a constant electrical signal is generated in spite of the increasing magnetic field [28]. As a result, the αE decreases at high magnetic field. It has also been reported by Nan et al. [9] that the magnetic field induced electric polarization of the multiferroic nanostructured film is closely related to the magnetostrictive behaviour of the ferromagnetic phase.
4. Conclusion
Fig. 5. (Colour online) ME voltage coefficient (αE) with Hdc under the influence of Hac ¼ 10 Oe and frequency of 847 Hz.
surface to volume ratio of nanostructures which is higher in nanorods than nanoparticles [1–3,24]. Secondly, these BFT thin films are polycrystalline of tetragonal phase and nano grains contribute to twin structure in which stress resides near grain boundaries which could lead to an enhanced polarization [12]. By this assumption, it will lead to an easily controllable depolarization field and long-range interactions which support the development of homogeneous spontaneous polarization. The length factors used to describe the interaction between polar units in ferroelectric materials are the correlation lengths parallel and perpendicular to the polarization vector [25]; charge defects might serve to stabilize polar micro-domains in chemically prepared ferroelectric samples and elastic constraints forms strain energetic and its stress relieving twinning mechanism. Finally, the epitaxial strain growth result in large ferroelectric polarization [12,26] is
The nanostructures of BFT thin films have been prepared by sol–gel combined metallo organic decomposition method with PEG conditions. XRD pattern show polycrystalline tetragonal phase of both BFT films. The AFM and SEM analysis measures the average particles size of 17 nm for BFT3 while BFT5 have nanorod like structure of diameter 45 nm and length 200 nm. The growth reaction mechanism shows that the increase in length of hydrophilic layer of PEG is responsible for the formation of nanorods. The values of Ms are 1.73 and 5.89 emu/ cm3, respectively, measured for BFT3 and BFT5 thin films. The value of Pmax ¼ 6.93 and 11.01 mC/cm2, respectively, is measured for BFT3 and BFT5 thin films which is explained on the basis of large surface to volume ratio of nanostructures, tetragonal phase of nano grains contributes energetic strain and epitaxial strain by the mismatch of lattice parameters. Both BFT thin films show good ME coupling and the calculated value of αE is 16.16 and 45.61 mV/cm Oe, respectively, for BFT3 and BFT5. References [1] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. [2] B. Li, C. Wang, G. Dou, CrystEngComm 15 (2013) 2147. [3] J. Kaur, R.K. Kotnala, K.C. Verma, Mater. Lett. 65 (2011) 3160. [4] V.A Reddy, N.P. Pathak, R. Nath, J. Alloys Compd. 543 (2012) 206. [5] N. Hur, S. Park, P.A. Sharma, J.S. Ahn, S. Guha, S.W. Cheong, Nature (London) 429 (2004) 392. [6] K. Raidongia, A. Nag, A. Sundaresan, C.N.R. Rao, Appl. Phys. Lett. 97 (2010) 062904. [7] D.R. Patil, Y. Chai, R.C. Kambale, B.G. Jeon, K. Yoo, J. Ryu, W.H. Yoon, D.S. Park, D.Y. Jeong, S.G. Lee, J. Lee, J.H. Nam, J.H. Cho, B.I. Kim, K.H. Kim, Appl. Phys. Lett. 102 (2013) 062909. [8] R.H. Wang, M.Q. Zhao, G.J. Zhang, G. Liu, J Mater. Sci. 45 (2010) 4490. [9] C.W. Nan, G. Liu, Y. Lin, Phys. Rev. Lett. 94 (2005) 197203. [10] K. Pal, U.N. Maiti, T.P. Majumder, S.C. Debnath, Appl. Surf. Sci. 258 (2011) 163. [11] U.A. Joshi, S. Yoon, S. Baik, J.S. Lee, J. Phys. Chem. B 110 (2006) 12249. [12] M.H. Frey, D.A. Payne, Phys. Rev. B 54 (1996) 3158.
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