Enhanced Multiferroic Properties and Valence Effect of Ru-Doped

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J. Phys. Chem. C 2010, 114, 6994–6998

Enhanced Multiferroic Properties and Valence Effect of Ru-Doped BiFeO3 Thin Films Feng Yan, Man-On Lai, and Li Lu* Department of Mechanical Engineering, National UniVersity of Singapore, 9, Engineering DriVe 1, Singapore, 117576

Tie-Jun Zhu Department of Materials Science and Engineering, Zhejiang UniVersity, Hangzhou, People’s Republic of China, 310027

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ReceiVed: January 31, 2010; ReVised Manuscript ReceiVed: March 6, 2010

Multiferroic materials are of considerable interest due to the intriguing science and application potential. Effects of the Ru dopant on the structural, electrical, domain structure, and ferromagnetic properties of multiferroic BiFeO3 (BFO) polycrystalline film have been studied. The Ru-doped BFO film (BFRO) possesses a lower electrical conductivity, uniform morphology, larger domain size, and hence fewer domain walls. The BFRO film shows a well-saturated P-E hysteresis loop with an improved remnant polarization close to 99 µC/cm2. The saturated magnetizations of the BFO and BFRO film are 8.69 and 16.53 emu/cm3, respectively, under a maximum magnetic field of 5000 Oe. The improved ferroelectric, ferromagnetic, and dielectric properties of the BFRO film are ascribed to the reduced concentration of defects and defect dipole complex, valence effect of Ru ions, and a different domain behavior. The enhanced magnetic properties of BFRO arise due to the distorted spin cycloid and canting angle of Fe ions in the Ru-doped BFO film. Introduction Multiferroics possess ferroelectricity and ferromagnetism orders at the same time.1–6 These materials have attracted a lot of attention in recent years due to their potential applications in multifunctional devices.5,7,8 Among the most widely studied multiferroic materials, lead-free BiFeO3 (BFO) has been intensively studied due to its rhombohedrally distorted perovskite structure at room temperature, which induces a large spontaneous polarization and weak ferromagnetism, high ferroelectric Curie temperature, TC, of 1123 K, and antiferromagnetic Ne´el temperature, TN, of 647 K.9–11 It has been reported that BFO has a large remnant polarization of 60 µC/cm2 in single crystal12 and thin films,13 and the spontaneous polarization could be up to 91.5 µC/cm2 in theory.14 However, several issues should be solved before realization in devices. The problems are high leakage current density, chemical fluctuation, large coercive field, and inhomogeneous magnetic spin structure.15 Many attempts have been made to enhance the ferroelectric and ferromagnetic properties of the BFO via ion substitution to introduce a “chemical pressure” into the crystal to vary the electronic and crystalline structure. La, Sm, Tb, Gd, and Nd partially substitute Bi and bring the improvement of ferroelectricity and enhanced homogenization of the spin arrangement.16–20 Mn, Sc, Cr, Ti, and Nb ions have been doped into the Fe site to eliminate oxygen vacancies in order to decrease the leakage current and change the overall magnetic spin structure.21–23 Raveau et al.24 demonstrated that, whatever the nature of the antiferromagnetic data of the undoped perovskite, such as Ln0.4Ca0.6MnO3, Pr0.5Sr0.5MnO3, or CaMnO3, ferromagnetism can be induced by Ru doping in a very effective manner, leading to colossal magnetoresistance (CMR) properties. However, few studies have focused on Ru-doped BiFeO3 * To whom correspondence should be addressed. Tel: +65-65167735. Fax: +65 6779 1459. E-mail: [email protected].

thin films, even that the theoretically expected effective moment µeff,Ru4+ ) 2.83 µβ; µβ is the Bohr magneton. Comparing to Cr, Sc, or Nb ions, it is believed that Ru can most effectively stabilize the ferromagnetic state,25 which can be attributed to the 4d metal character of Ru. It is known that the 3d electrons are more localized compared with 4d electrons; therefore, the exchange interaction between Fe-O-Ru could be favored much more than any other transition metals.26 The positive influence of Ru doping on magnetic ordering could be interpreted based on the mixed valence states of Ru because Ru has the ability to exhibit three different oxidation states (Ru3+, Ru4+, and Ru5+), while the low-spin configuration of Ru ions are suitable for a ferromagnetic coupling with Fe ions through the empty eg orbital. Therefore, it is expected that, through the Ru doping in the BiFeO3 thin film, high ferromagnetic properties can be achieved while still keeping the high ferroelectric properties. In this study, we investigate the effects of Ru doping on the crystalline structure, surface topography, domain structure, and ferroelectric and magnetic behavior of the modified BiFeO3 films grown on Pt/TiO2/SiO2/Si substrates. The overall ferroelectric and magnetic properties have been improved using Ru doping, which is close to that of the thin films deposited on the singlecrystal oxide substrate. Furthermore, the possible causes for the enhanced ferroelectric and ferromagnetic characteristics are discussed via defect chemistry and the valence effect. Experimental Methods Pure and Ru-doped BFO thin films with stoichiometric compositions BiFeO3 and BiFe0.95Ru0.05O3 (BFRO) were grown on (111) Pt/TiO2/SiO2/Si substrates using pulsed laser deposition (PLD) at 550 °C in 50 mTorr of oxygen. The laser ablation was carried out at a laser fluence of 2 J/cm2 with a repetition rate of 5 Hz using a KrF excimer laser with a wavelength of 248 nm. Both targets were synthesized by a standard solid-state

10.1021/jp1009127  2010 American Chemical Society Published on Web 03/25/2010

Multiferroic Properties of Ru-Doped BiFeO3 Thin Films

Figure 1. (a) X-ray diffraction spectra of the BFO and BFRO thin films with the inset of magnified patterns showing a diffraction at 2θ ) 32.80° and (b, c) cross-sectional FESEM images of the BFO and BFRO, respectively.

reaction using high purity Bi2O3, Fe2O3, and RuO2 (Aldrich, >99.9%) with 10% excess Bi2O3 to compensate for the loss of Bi during sintering and laser deposition. The as-deposited thin films were cooled to room temperature at about 5 °C min-1. The structure of the films was determined by X-ray diffraction (XRD) with Cu KR radiation using a Shimadzu XRD-7000 diffractometer. Pt top electrodes with a diameter of 100 µm were sputtered on the surface of the films through a shadow metal mask to form metal-insulator-metal (MIM) capacitors. The surface morphology and piezoelectric properties of the BFO and BFRO thin films were examined using piezoresponse force microscopy (PFM) with Pt/Ir-coated Si tips (spring constant of ∼2 N/m, resonance frequency of ∼70 kHz, and the size of the probing tip radius is ∼20 nm). The thickness of the films and the area of the Pt top electrode were examined using a Hitachi S4300 field emission scanning electron microscope (FESEM). The dielectric constant and loss tangent of the films were measured without dc bias using a precision impedance analyzer (Wayne Kerr Electronics 6500B series). X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the surface chemistry of the films using a VG ESCALAB 220i-XL system with a pass energy of 20 eV for high resolution spectrum acquisition with an Al KR (1486.6 eV) X-ray source. The ferroelectric and magnetic properties were characterized using a Radiant Precision Workstation (Radiant Technologies, Median, New York) and a Lakeshore 736 vibrating sample magnetometer (VSM), respectively. Results and Discussion Figure 1a shows the XRD spectra of the BFO and BFRO thin films grown on the Pt/TiO2/SiO2/Si substrate. It is revealed that all the films exhibit a polycrystalline perovskite structure with rhombohedral R3c symmetry27 and are free of impurities, such as Bi2Fe4O9. The rhombohedral distortion is reduced toward the orthorhombic or tetragonal structure with Ru doping deduced from a broadened single peak around 32.7° (FWHM from 0.465° for BFO to 0.513 for BFRO, as shown in the inset in Figure 1a). The out-of-plane lattice parameters calculated from the XRD result are aBFO ) 0.3972 nm and aBFRO ) 0.3960 nm. A slight lattice reduction of the BFRO film is observed in comparison with the BFO film, which is caused by the smaller ion radius of the Ru4+ ion (0.062 nm) than that of Fe2+ (0.078 nm) and Fe3+ (0.065 nm) ions. On comparison with a value of 0.3962 nm for the bulk BFO, it should be noted that the BFRO film is under compressive strain (0.05%) in contrast with a tensile strain (0.25%) for the pure BFO film. The film-substrate misfit strain may impact on the spontaneous polarization of the films, which is via the doping effect on the films rather than from the substrate materials. From the FESEM cross-sectional images of the BFO

J. Phys. Chem. C, Vol. 114, No. 15, 2010 6995 and BFRO thin films, we can see that the BFRO film has a smoother surface and interfaces, and the thicknesses of the BFO and BFRO layers are estimated to be 250 and 260 nm, respectively. Figure 2a,d shows the surface morphologies of the BFO and BFRO thin films, respectively. It can be seen that the grains and root-mean-square (rms) roughness are changed from the order of 100 and 10.7 nm for the BFO film to 300 and 6.4 nm for the BFRO film, indicating the crystallite structure of the films with clearly resolved morphological features, and the Ru dopant can induce a smoother morphology and increase the grains size, correlated to the FESEM results (not shown). The piezoresponse images present a much more complex variation of contrast that reflects the perplexing arrangement of domains in the ferroelectric films.28 The resolved piezoresponse amplitude images of the BFO and BFRO film, as shown in Figure 2b,e, reveals that most domains are limited by the grain boundary. Quite often, the BFRO film shows piezoelectrically inactive regions (>400 pm) than that of the BFO film (>300 pm) under an applied 500 mV amplitude voltage, predicting that the BFRO film has a larger piezoelectricity property corresponding to the 71 and 109° ferroelastic domain types. Figure 2c,f shows the piezoresponse phase images. The majority of the domains of the BFRO film are oriented with the polarization upward (yellow bright), and only a small portion of domains exhibits polarization oriented downward (corresponding to a smaller concentration of the 180° domain walls). In contrast, the BFO phase image shows a ratio close to 6.5:3.5 for upward and downward polarized domains. Furthermore, the BFRO film has a lower volume density of domain walls than that of the BFO film. It is commonly believed that the characteristics of the domains and domain walls directly impact the ferroelectric switching behavior. The energy of the domain walls is proportional to the density of walls, and the conductivity of certain domain walls of the BFO is much higher than that of domains themselves. The 180° domain walls are the most conductive ones.29,30 Therefore, the BFRO film should be expected to have a lower leakage current. Figure 3a shows the leakage current density, J, versus electric field, E, of both films. The BFO film shows a lower leakage current in the electric field, 200 kV/cm. It is known that, in the BFO, Fe ions often exist in a mixed-valence Fe2+/3+ state due to introduction of oxygen vacancies. With the valence fluctuation, holes, h•, in the BFO film will be created in order to compensate for the change in charge at a relatively high oxygen partial pressure, which can be expressed as

Fe3+ ) Fe2+ + h• At a low oxygen partial pressure, the change in the oxidation state will lead to formation of oxygen vacancies according to x ′ 3+ + 5OOx + VO•• Null ) 2BiBi + 2(Fe2+)Fe

As a consequence, the BFO often shows a large leakage current density. By doping tetravalence Ru4+ ions, the valence fluctuation of Fe2+/3+ can be compensated according to x x • ′ 3+ + (Ru4+)Fe Ru4+ ⇒ 2BiBi + (Fe2+)Fe 3+ + 6OO

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Figure 2. (a, d) AFM images (2 × 2 µm2), (b, e) out-of-plane piezoresponse amplitude images, and (c, f) piezoresponse phase images of BFO and BFRO thin films. Yellow (bright) and purple (dark) on the piezoresponse image correspond to positive (upward) and negative (downward) domains, respectively.

Figure 3. (a) Leakage current density, (b) ferroelectric hysteresis loops, (c) dielectric properties of pure, and (d) leakage current and time relationship of the BFO and BFRO thin films measured at room temperature.

Due to the reduction in the concentration of oxygen vacancies caused by doping high valence Ru, the leakage current density is reduced. Figure 3b shows the ferroelectric polarization versus electric field (P-E) curves of the BFO and BFRO films measured at 1 kHz. The P-E loops of the films show well-defined saturate and good symmetry rectangular shapes. The BFO film shows the remnant polarization, Pr, to be as high as 64 µC/cm2, whereas that of the BFRO film is 99 µC/cm2. Meanwhile, the coercive field, Ec, is slightly decreased via Ru doping comparing with that of the BFO film. The enhanced ferroelectric properties of the BFRO film are coincident with their domain structure, as shown in Figure 2f and the compressive film-substrate strain. Figure 3c illustrates the dielectric constant, ε, and dielectric loss, tan δ, of the BFO and BFRO films as a function of

frequency. It is evident that ε and tan δ of the Ru-doped BFO film varied slightly from 210 to 160 and 0.012-0.023 with frequency, respectively. At the frequencies higher than 105 Hz, the increase in tan δ implies that the charge carriers in the films do not have time to respond to the external electric field (6) and the maximum electrical energy is transferred to the oscillation ions.31 The BFRO film has a higher ε than that of the BFO, which can be ascribed to the larger grain size in the BFRO film due to the size effect.32 Figure 3d shows the current-time plots for characteristics of the BFO and BFRO films measured at a positive bias of 5 V. It is clear that the leakage current of the BFO stabilizes immediately, whereas the current relaxation for the BFRO film is very slow and stabilizes after about 80 ms. The time taken to stabilize the leakage current is probably carrier transfer and

Multiferroic Properties of Ru-Doped BiFeO3 Thin Films

Figure 4. Magnetic hysteresis loops of the BFO and BFRO thin films measured at room temperature.

J. Phys. Chem. C, Vol. 114, No. 15, 2010 6997 evidence for the decrease of oxygen vacancies,36 corresponding to the lower leakage current density in the BFRO film. Figure 5b,c shows the Ru XPS spectra in BFRO film; it is found that the Ru 3d3/2 and Ru 3p3/2 peak positions are at the binding energies of 285.1 and 464.0 eV, respectively. The Lorentzian dividing peaks corresponding to 285.1 and 464 eV of Ru suggest the presence of two possible redox couples involving Ru4+/Ru5+ with a ratio of about 1:2 and Ru3+/Ru4+ with a ratio of 1:4, respectively. It is argued that the variation of Ru valence in the BFRO film is presumably due to the comparable oxidation reduction potential (ORP) of Ru4+/Ru5+ (1.07 eV) and Fe2+/Fe3+ (0.77 eV). As a result, Fe3+ gets reduced to the Fe2+ state, while Ru4+ gets oxidized to the Ru5+ state, which is a source of ferromagnetic interaction between Ru and Fe ions in the BFRO film.37 Additionally, part of Fe3+ is also formed as a result of the internal redox reaction

Fe2+ + Ru4+ S Fe3+ + Ru3+ In the oxidizing conditions, the system tends to keep the oxidation state of Ru to 4+ and to reduce the state of iron. The introduction of Ru5+ will induce ferromagnetic coupling and extra Fe2+ can also participate in the enhancement of ferromagnetism via forming an electronic fluctuation along the “Fe2+-O-Ru5+-O-Fe3+” bond.38 Furthermore, an alternative mechanism of the charge compensation of Fe reduction from Fe3+ to Fe2+ is ascribed to that the higher Ru doping induces a • defects. The Ru dopant will distort the small fraction of RuFe long-range cycloidal spin arrangement of BFO and enhance the magnetic moments via the valence effect. Ru ions also promote ferromagnetism by developing ferromagnetic superexchange interactions with surrounding Fe ions. Figure 5. (a) XPS spectra of the Fe 2p lines of the BFO and BFRO films; the insets are the peak-fitting simulations. (b, c) XPS spectra of Ru 3d and 3p showing variable oxidation states for the BFRO film.

domain dynamics dependent, such as the polarization bound charges at the surfaces of the ferroelectric layer, free charge carriers in the conducting electrodes, and domain wall motion.33 For the BFRO film, because it has more complicated defect chemistry and a larger domain and smaller concentration of domain walls than that of BFO film, it will need more time to be stabilized. Figure 4 shows the magnetization hysteresis loops (M-H) of the BFO and BFRO films when a maximum magnetic field of 5000 Oe is applied in-plane to the substrate surface at room temperature, where the saturated magnetization of the BFO and the BFRO is 8.69 and 16.53 emu/cm3. Compared with the BFO film, an increase in magnetization in the BFRO films is observed. The increase in magnetization of the BFRO might be due to either the balance between the antiparallel sublattice magnetization of Fe3+ and a destroyed spin cycloid due to Ru ion substitution29,34 or an increase canting angle.35 To further investigate and understand the role of Ru doping on the electrical and magnetic properties, we examine the chemical state of Fe and Ru ions in the films via XPS analysis, as shown in Figure 5. The Fe 2p3/2 peaks correspond to Fe2+ and Fe3+. By fitting the peaks for the valence state of Fe ions, the ratios of Fe2+/Fe3+ in the BFO and BFRO films are calculated as 1:2.5 and 1:5, respectively, indicating that the presence of Fe2+ ions is less in the BFRO. This could be also

Conclusion In conclusion, the BFO and BFRO films have been fabricated on the (111) Pt/TiO2/SiO2/Si substrate by the pulsed laser deposition system. The films have a pure and polycrystalline phase with uniform surface morphologies. The electrical and magnetic properties are impacted remarkably by the different nanoscale domain structures for both films. The leakage current of the BFRO film decreases near 2 orders of magnitude at a high electric field compared with that of the BFO film as expected. The BFRO film also reveals a high dielectric constant and low dielectric loss. Defect chemistry due to introducing Ru ions is achieved. As the result, a better coupling between ferroelectric and ferromagnetic ordering has been obtained via doping Ru ions into the Fe site of BFO. Acknowledgment. The authors acknowledge the financial support from the National University of Singapore. References and Notes (1) Bibes, M.; Barthelemy, A. Nat. Mater. 2008, 7, 425–426. (2) Scott, J. F. Nat. Mater. 2007, 6, 256–257. (3) Eerenstein, W.; Mathur, N. D.; Scott, J. F. Nat. Mater. 2006, 442, 759–765. (4) Ramesh, R.; Spaldin, N. A. Nat. Mater. 2007, 6, 21–29. (5) Spaldin, N. A.; Fiebig, M. Science 2005, 309, 391–392. (6) Catalan, G. Appl. Phys. Lett. 2006, 88, 102902. (7) Fiebig, M. J. Phys. D 2005, 38, R123. (8) Yamasaki, Y.; Miyasaka, S.; Kaneko, Y.; He, J. P.; Arima, T.; Tokura, Y. Phys. ReV. Lett. 2006, 96, 207204. (9) Nakamura, Y.; Nakashima, S.; Ricinschi, D.; Okuyama, M. Funct. Mater. Lett. 2008, 1, 19.

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