ARTICLE IN PRESS. Journal of Magnetism and Magnetic Materials

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1 Journal of Magnetism and Magnetic Materials (8) 5 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: Effect of Sc substitution on the structure, electrical, and magnetic properties of multiferroic BiFeO thin films grown by a sol gel process S.R. Shannigrahi a,, A. Huang a, D. Tripathy b, A.O. Adeyeye b a Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), Research Link, Singapore 76, Singapore b Department of Electrical and Computer Engineering, Information Storage Materials Laboratory, National University of Singapore, Singapore 7576, Singapore article info Article history: Received 8 September 7 Received in revised form April 8 Available online April 8 Keywords: Ferroelectric Dielectric Magnetic Multiferroic abstract Multiferroic BiFeO (BFO), Bi x Sc x FeO (BSF), and BiFe x Sc x O (BFS) (x ¼. mol%) thin films are prepared on Pt/Ti/SiO /Si substrates using a sol gel technique. The effect of Sc substitution along with the annealing ambient (N and O ) on the structure, electrical, and magnetic properties of the films are reported. X-ray diffraction (XRD) analysis reveals that the films can be prepared with the single-phase perovskite structure by annealing at 7 C for min either in O or N ambient. The unit cell volume increases on the substitution of Sc, which are 6.9, 6.5, and 6.57 (Å) for BFO, BSF, and BFS, respectively. X-ray photoelectron spectroscopy (XPS) study reveals that the chemical environments of Bi and Fe are different in BFO, BSF, BFS films. Similarly, XPS spectra for Scp lines in BSF and BFS also have different peak positions; this indicates Sc doping has certain chemical impact on BSF and BFS films. Systematic studies of Sc substitution along with the effect of annealing ambient on the dielectric constant (e) and dielectric loss (tan d), leakage current, remnant polarization (P r ), coercive field (E c ), and magnetic properties of the films are carried out. The room temperature values of e and tan d at khz for BFO and BFS films annealed in N ambient are (8;.5) and (5;.), respectively. The comparative value of leakage current for the BFO and BFS films at an applied field strength of 5 kv/cm are.997 and.87 5 A/cm, respectively. Room temperature value of coercive magnetization for BFS films has one order small compared to that of the BFO films; this indicates BFS films are magnetically soft and more suitable for potential device applications. Finally, among the studied compositions, the BFS films annealed in N ambient show the best property. & 8 Elsevier B.V. All rights reserved.. Introduction In a certain range of temperature multiferroic materials have coupled ferroelectric and magnetic order parameters. The coupling of the electric and the magnetic polarizations could employ these materials in novel device applications [ ]. BiFeO (BFO) is one of such material that belongs to the perovskite (ABO ) class with a rhombohedrally distorted cell having the polar Rc space group. BFO is of particular interest in terms of practical applications such as the sensor techniques, microelectronics, and magnetic memory system because of its high electrical (T C ¼ K) and magnetic (T N ¼ 6 K) ordering temperature [5]. However, at room temperature BFO suffers from the high dc conductivity that relates to oxygen nonstoichiometry and oxidation states of Fe [6,7], which can be present as Fe + and Fe +.In addition to this, BFO films have very high magnetic coercive field, which make them difficult to use in device applications [6 ]. In Corresponding author. Tel.: ; fax: address: santi-s@imre.a-star.edu.sg (S.R. Shannigrahi). this respect doping and processing conditions have been reported to play a decisive role on the dc conductivity of BFO. Several research groups have tried A-site (Bi-site) doping using lanthanides such as La and Tb to enhance ferroelectric reliability and modify its specially inhomogeneous spin-modulated incommensurate structure [9,]. A few have focused on the effect of B-site (Fe-site) doping [6,]. Moreover, no systematic study has been reported on the effect of same dopant both in the A and B-sites along with the annealing ambient (O and N ). In this article we report the effect of Sc (. mol%) doping on the A and B-sites of the BFO thin films annealed in O and N ambient through comparing their structure, electrical, dielectric, ferroelectric, and magnetic properties.. Experimental details The polycrystalline thin films of BiFeO (BFO), Bi x Sc x FeO (BSF), and BiFe x Sc x O (BFS) (x ¼. mol%) films were prepared on Pt/Ti/SiO /Si substrates using a sol gel technique. To produce BFO sol, Bi acetate and Fe acetylacetonate were dissolved in -885/$ - see front matter & 8 Elsevier B.V. All rights reserved. doi:.6/j.jmmm.8..9

2 6 S.R. Shannigrahi et al. / Journal of Magnetism and Magnetic Materials (8) 5 Int. (a.u.) () () () Pt() () Pt() () () () () BFS:N BFS:O BSF:N BSF:O BFO:N de-ionized water and acetic acid mixed in : volume ratio. The final concentration of the precursor was.5 mol/l. Bi excess of mol% was added to the solution to compensate for bismuth loss during high-temperature annealing. Sc nitrate was used as an additional precursor to prepare BSF and BFS sols. Each layer of ascoated film was dried on a hot plate at 5 C and, subsequently, at 5 C for 5 min, and were intermittently annealed in a preheated hot plate in air at 58 C for 5 min after each third-layer deposition. The number of coating and heating cycles was repeated for several times to have a desired thickness. The films were finally annealed at 7 C for min separately in O and N ambient. The final films were named with an extension of O and N, which indicate the respective annealing ambient. The crystalline quality was investigated using an X-ray diffraction (XRD) system (D8-ADVANCE) (Bruker AXS GmbH, Karlsruhe, Germany) with Cu Ka radiation (l ¼.58 nm) in the Bragg angle range of pyp6. The surface morphology Table Comparison of cell parameters of BFO, BSF, and BFS films annealed in N ambient BFO:O Sample Rhombohedral 5 θ (degree) Fig.. XRD spectra of BFO:O, BFO:N, BSF:O, BSF:N, BFS:O, and BFS:N films deposited on Pt/Ti/SiO /Si substrates. 6 a (Å) a () V (Å) BFO BSF BFS BFS:N BSF:N BFO:N.5m.5 μm.5 μm BFS:O BSF:O BFO:O.5 μm.5 μm.5 μm BFO:N BFS:N BSF:N.5 μm.5 μm.5 μm BFS:O BFO:O.5 μm BSF:O.5 μm.5 μm Fig.. FE-SEM micrographs for surface morphology and cross-sectional view of the BFO:O, BFO:N, BSF:O, BSF:N, BFS:O, and BFS:N films.

3 S.R. Shannigrahi et al. / Journal of Magnetism and Magnetic Materials (8) 5 7 and thickness of the films were characterized using the field emission scanning electron microscopy (FE-SEM) (JSM 67F, JEOL Ltd., Japan). A vibrating sample magnetometer (VSM) system (GMW) was used to measure the field dependence magnetization of the samples. Using X-ray photoelectron spectroscopy (XPS) (VG ESCALAB -XL Imaging System, England) the XPS spectra for Bi, Fe in BFO, BSF, BFS and for Sc in BSF and BFS films were investigated. XPS profiles of the samples were obtained using Al Ka source (86.6 ev). The CS peak was used as the reference standard []. For electrical characterization, gold was sputtered through a shadow mask to get round electrodes of.-mm diameter. Polarization electric field (P E) hysteresis loop was carried out using a standard ferroelectric testing unit RT66A (Radiant Technologies) that was linked to a high voltage interface. The room temperature leakage current property of the films was measured using an electrometer (Keithley 657A) system. Impedance analyzer (HP9A) was used to determine the dielectric constant (e) and loss tangent (tan d) of the films.. Results and discussion Fig. compares the XRD patterns of the BFO, BSF, and BFS films annealed in O and N ambient. All the films show the formation of solely perovskite phase, and no traces of Bi O or Bi 6 Fe O 57 could be found. All the reflection peaks were indexed and lattice parameters were determined. Compared to pure BFO, no shift in the major peak position was observed. From this, we conclude that the basic crystal structure of pure BFO has not been affected by the incorporation of the Sc + ions and Sc is not remaining - BFS:N BFS:O - ε (x ) 9 6 tan δ (x - ) J (A/cm ) BSF:O BSF:N -6 BSF:N - BSF:O - ε (x ) tan δ (x - ) J (A/cm ) - - BSF:O -5 BSF:N -6 BFO:N - BFO:O 6 - ε (x ) tan δ (x - ) J (A/cm ) BFO-O BFO-N Frequency (Hz) Fig.. Room temperature dielectric properties of the BFO:O, BFO:N, BSF:O, BSF:N, BFS:O, and BFS:N films. Fig.. I V characteristics of BFO:O, BFO:N, BSF:O, BSF:N, BFS:O, and BFS:N films.

4 8 S.R. Shannigrahi et al. / Journal of Magnetism and Magnetic Materials (8) 5 isolated in the matrix of the compound but enters the lattice. But a minor shift is observed in the peak positions with two additional peaks for BSF and BFS films, which indicates a small change in the lattice parameters. The lattice parameters of the BFO, BSF, and BFS films annealed in N ambient are tabulated in Table. Both BSF and BFS films belong to the rhombohedral crystal system at room temperature. Lee et al. made a similar observation for Ti substitution of BFO films prepared using pulsed laser deposition (PLD) technique []. Sc + leads to an increase in the unit cell volume from 6.9 (Å) for BFO to 6.5 and 6.57 (Å), respectively, as observed for BSF and BFS unit cells. The increase in unit cell volume with Sc substitution for Fe sites is expected since the ionic radius of Sc + (.885 Å) is higher than that of Fe + (.78 Å). Increase of volume for Sc substitution to Bi sites is yet to be understood as the ionic radius of Sc + is smaller than that of Bi + (.96 Å). Fig. shows the results of the microstructure and cross-section of the films depend on the annealing conditions. The deposition conditions adopted lead to uniform and crack-free films, regardless of the annealing ambient chosen. But, annealing in N ambient promotes a rather dense morphology, whereas annealing in O ambient leads to slightly porous morphology. The annealing ambient was found to dramatically affect the electrical properties of the films []. Comparing the surface morphologies of the studied films, the BFS films annealed in N ambient exhibit the most dense surface morphology with finer grains of about -nm size. The thickness was estimated from the cross-section, which were 7, 65, and 65 nm, respectively for BFO, BSF, and BFS films. But no significant difference in cross-sectional view was observed for BSF and BFS films. Fig. illustrates the frequency (f) dependence of e and tan d for the BFO, BSF, and BFS films annealed in O and N ambient. The values of e and tan d for BFO and BFS films (annealed in N ambient) measured at khz frequency are 8;.5 and 5;., respectively. The higher value of dielectric constant is caused from the better densification in the BFS films, which can be supported by the surface morphology observation (Fig. ). All the samples show decreasing trends in e and tan d, with increasing frequency from to 5 Hz. As it is known that at low frequency, any charged defects present in the sample are able to follow the applied electric field, resulting in the increased e and tan d values. However, at elevated frequency, the weak dependence of e and tan d on f are generally observed. But if we compare the values of e and tan d at both ends we can conclude that in comparison to BFO films, the amounts of charge defects are less in BFS films. Once again this implies that the BFS films annealed in 6 P (μc/cm ) - BFS:O P (μc/cm ) - - BFS:N P (μc/cm ).. -. BSF:O P (μc/cm ).. -. BSF:N BFO:O BFO:N P (μc/cm ) P (μc/cm ) Fig. 5. Comparison of room temperature polarization electric field (P E) hysteresis loops of the BFO:O, BFO:N, BSF:O, BSF:N, BFS:O, and BFS:N films.

5 S.R. Shannigrahi et al. / Journal of Magnetism and Magnetic Materials (8) 5 9 N ambient can reduce the charge defects. The improved dielectric constant probably originates from the better densification of the system due to Sc modifier and N ambient. The dc leakage current is one of the most vital characteristics for any multiferroic material to be used in devices. In the case of BFO films, high leakage current was reported due to the presence of Fe + species, oxygen vacancies, charge defects, and nonstoichiometry in its perovskite structure [,5]. Recently improvements in the polarization and dc leakage current in Cr-modified BFO thin films have been reported [7]. The existing conduction mechanisms were unable to explain the current conduction mechanism in such oxide films. Fig. shows the variation of current density (J) as a function of electric field (E) for all the BFO and modified BFO films. The data show that the leakage current density of BFS:N films is significantly decreased compared to the BFO thin films at all applied field. The value of leakage current density at an applied field of 5 kv/cm for BFS:N films annealed in N is.87 5 A/cm, which is the lowest among the studied films. This result demonstrates that the enhanced electrical resistance of Sc modified together with N annealing ambient plays a critical role. The conductivity in BFO is rather a complex phenomenon and is still to be addressed in more detail. Generally, it was assumed that the conduction mechanism of BFO was related to the hopping of electrons from Fe + to Fe + when oxygen vacancies are present in the lattice, which act as a bridge between Fe + and Fe + and play an important role in the electronic conduction. The leakage current density of BFS:N films is lower than that of the BFO:O films. This is possibly due to the reduction of defects and stabilization of Fe + state upon substitution of Sc + ions. The room temperature P E hysteresis loops of BFO, BSF, and BFS films annealed at 7 C for min in O and N ambient are shown in Fig. 5. All the polarization measurements were done at 5 Hz. The BFO:N and BFS:N films result in a higher P s and smaller E c compared to the BFO:O and BFS:O films. The maximum remnant polarization of. mc/cm was observed for BFS:N films. Fig. 6 shows the variation of magnetization (M) with applied magnetic field (H) at room temperature for BFO as well as for BSF films in their in-plane orientation. The magnetization for pure BFS:O BFS:N BSF:O BSF:N BFO:O BFO:N Fig. 6. Room temperature magnetization magnetic (M H) hysteresis loops of the BFO:O, BFO:N, BSF:O, BSF:N, BFS:O, and BFS:N films in their in-plane orientation.

6 S.R. Shannigrahi et al. / Journal of Magnetism and Magnetic Materials (8) 5 Scp / BFS BSF Scp / The XPS profiles of the Sc (p / and p / ) in BFS and BSF thin films, Bi (f 7/ and f 5/ ) and Fe p / in BFO, BFS, and BSF thin films are shown in Fig. 7. We found that the chemical environments of Bi, Fe, and Sc have been changed in BFO, BSF, and BFS thin films. The obvious shifts in the Fep /, Scp /, and Bif 5/ spectra of BSF and BFS as compared to those of the BFO film show that the Sc doping in the Bi and Fe sites have some influence on the binding states of Fe O and Bi O bonds [6,7]. The difference in the shifts in BSF and BFS samples suggests that the Sc is substituted preferentially at the Bi and Fe sites of BSF and BFS, respectively Bif7/ 5 Bif 5/. Conclusions Cts (Arb. units) BFS BSF BFO Fep / We have synthesized Sc-modified BFO multiferroic thin films on platinized silicon substrates using a sol gel deposition technique. The films exhibited both ferroelectric and magnetic properties at room temperature. Based on the results described above, the following conclusions may be inferred: (i) BFS:N films show the most dense morphology, with least leakage current compared to the other studied films. It was also observed through XPS analysis that the chemical state of Sc associated with BFS and BSF films is different. Moreover, BFS:N films also show the least value of magnetic coercive field and are magnetically the softest materials among the studied films. The further improvement of the electrical properties lies in the optimization of the dopant concentrations. 7. Fe + Fe+ BFS BSF Acknowledgments We would also like to thank Dr. Han Gu Chang of DSI for some experimental work to carry out by his help. References B.E. (ev) BFO 76 Fig. 7. XPS spectra of the Fep, Scp and Bif lines for BFO, BSF, and BFS thin films. The dotted lines represent the fitting of the peaks. BFO is small and varies linearly with field as expected for an antiferromagnetic material. Magnetization improves with the addition of Sc. Thus ferroelectric and magnetic orders coexist in this compound at room temperature. The increase in magnetization of BFO is not much significant. We believe, the prime source of magnetism is due to the super exchange between the Fe + and Fe + ions. From the observed improved leakage currents we also believe that the amount of Fe + ions reduced [,6]. [] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 99 () 79. [] K.Y. Yun, M. Noda, M. Okuyama, Appl. Phys. Lett. 8 () 98. [] V.R. Palkar, J. John, R. Pinto, Appl. Phys. Lett. 8 () 68. [] L. Lian, N.R. Sottos, J. Appl. Phys. 87 () 9. [5] F. Kubel, H. Schmid, Acta Cryst. B 6 (99) 698. [6] Y. Wang, C.-W. Nan, Appl. Phys. Lett. 89 (6) 59. [7] J.K. Kim, S.S. Kim, W.-J. Kim, A.S. Bhalla, R. Guo, Appl. Phys. Lett. 88 (6) 9. [8] S. Lakovlev, C.H. Solterbeck, M. Kuhnke, M. Es-Souni, J. Appl. Phys. 97 (5) 99. [9] Y.H. Lee, J.M. Wu, Y.L. Chueh, L.J. Chou, Appl. Phys. Lett. 87 (5) 79. [] D. Lee, M.G. Kim, S. Ryu, H.M. Jang, Appl. Phys. Lett. 86 (5) 9. [] V.R. Palkar, K.G. Kumara, S.K. Malik, Appl. Phys. Lett. 8 () 856. [] X. Qi, J. Dho, R. Tomov, M.G. Blamire, J.L. MacManus-Driscoll, Appl. Phys. Lett. 86 (5) 69. [] C.D. Wagner, W.M. Riggs, L.E. Davis, F.J. Moulder, Handbook of X-ray Photoelectron Spectroscopy, Chapman & Hall, London, 99. [] J.K. Kim, S.S. Kim, W.J. Kim, Integrated Ferro. 76 (5). [5] Y. Wang, Q.H. Jiang, H.C. He, C.W. Nan, Appl. Phys. Lett. 88 (6) 5. [6] W. Eeresntein, Science 7 (5) 9. [7] S.R. Shannigrahi, A. Huang, N. Chandrasekhar, D. Tripathy, A.O. Adeyeye, Appl. Phys. Lett. 9 (7) 9.