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and [110]. 110. Structure and Lattice Dynamics of Heterostructures. Based on Bismuth Ferrite and Barium Strontium Titanate on Magnesium Oxide Substrates.
ISSN 10637834, Physics of the Solid State, 2010, Vol. 52, No. 7, pp. 1432–1438. © Pleiades Publishing, Ltd., 2010. Original Russian Text © Yu.I. Golovko, V.M. Mukhortov, O.A. Bunina, I.N. Zakharchenko, A.S. Anokhin, V.B. Shirokov, Yu.I. Yuzyuk, 2010, published in Fizika Tverdogo Tela, 2010, Vol. 52, No. 7, pp. 1336–1341.

MAGNETISM AND FERROELECTRICITY

Structure and Lattice Dynamics of Heterostructures Based on Bismuth Ferrite and Barium Strontium Titanate on Magnesium Oxide Substrates Yu. I. Golovkoa, V. M. Mukhortova, O. A. Buninab, I. N. Zakharchenkob, A. S. Anokhinb, V. B. Shirokovb, and Yu. I. Yuzyukb, * a

Southern Scientific Center, Russian Academy of Sciences, ul. Chekhova 41, RostovonDon, 344006 Russia b Southern Federal University, ul. Bolshaya Sadovaya 105/42, RostovonDon, 344006 Russia * email: [email protected] Received November 12, 2009

Abstract—Bismuth ferrite films doped with neodymium on MgO singlecrystal substrates with an epitaxial barium strontium titanate thin (1–2 nm) sublayer have been prepared by rf sputtering of ceramic targets at an elevated oxygen partial pressure and at temperatures below the ferroelectric and magnetic transition temper atures. It has been revealed using Xray diffraction and Raman scattering spectroscopy that, in these bismuth ferrite films, a new phase (not observed in bulk samples) is formed. The symmetry of this phase is monoclinic, the unit cell contains two formula units, and the spontaneous polarization vector deviates from the [111]cub direction and can have different components along the x, y, and z axes. DOI: 10.1134/S1063783410070188

1. INTRODUCTION Experimental data accumulated from the begin ning of the preparation of the first multiferroics [1, 2], i.e., crystalline solids in which there coexist at least two of the three order parameters (magnetic, electri cal, or strain), have made it possible to design and syn thesized materials that, under normal conditions, exhibit strong magnetoelectric properties. Bismuth ferrite BiFeO3 (BFO) is the only material with both ferroelectric and antiferromagnetic orderings already at room temperature, which is very important for practical applications of multiferroics. Below the ferroelectric transition temperature (Tc = 1083 K), the crystal structure of a BFO single crystal is described by space group R3c. The rhombo hedral unit cell contains two formula units and, at room temperature, has parameters a = 0.562 nm and α = 59.38° [3]. The spontaneous polarization is ori ented in the [111] direction of the pseudocubic perovs kite unit cell. An antiferromagnetic ordering of the G type arises below the temperature TN = 643 K, so that the magnetic moments of iron ions retain locally anti parallel orientation and rotate over a spiral oriented along the [ 101 ] direction and the period of cycloid is equal to 62 nm [4]. The presence of this cycloid results in the fact that, on average over the volume, the linear magnetoelectric effect and spontaneous magnetiza tion are equal to zero [5, 6].

The spin cycloid can be suppressed by applying a strong magnetic field [7] or replacing bismuth ions by rareearth ions. The addition of neodymium ions favors the suppression of the cycloidal structure and can lead to an increase in the remanent polarization and saturation magnetization and to a decrease in the ferroelectric coercive field and leakage currents [8]. High values of the electric polarization, magnetoelec tric effect, and giant magnetocapacitance have been achieved in BiFeO3 thin films due to the strong epitax ial stresses that destroy the cycloidal magnetic order ing [10]. These effects open wide prospects for practi cal applications of multiferroics in controlled micro wave devices and memory elements of a new generation. A new effect, i.e., the control of the prop erties of an inverted p–n junction by the electric field, which was recently revealed in bismuth ferrite films doped with calcium [11], indicates that structures based on multiferroic films can be widely used in solidstate electronics. The most promising method for optimizing the functional properties of the bismuth ferrite consists in varying the epitaxial stresses in films due to the choice of the substrate or the preparation of a buffer layer between the film and substrate. In our earlier works [12–14], we synthesized (Bi0.98Nd0.02)FeO3 (BNFO) films in which the [001] axis was parallel to the [001] axis of the MgO substrate and the [100] and [010] axes of the film were oriented along the [ 110 ] and [110]

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ϕ 2θ ω

ϕ

2θc

500 nm ω

Fig. 1. Microrelief of the surface of the BNFO/BST/MgO film.

directions of the substrate, respectively. This paper reports on the results of the preparation and investiga tion of BNFO films on MgO singlecrystal substrates with Ba0.8Sr0.2TiO3 (BST) singlecrystal sublayers. 2. FILM PREPARATION AND STRUCTURAL INVESTIGATIONS Multilayer heterostructures BNFO/BST on (001) cuts of MgO singlecrystals were prepared on two Plasma 50 SÉ rf sputtering systems, where stoichio metric ceramic disks 50 mm in diameter were used as targets. The main difference of the used sputtering technique from the other known analogs consists in using a highcurrent rf discharge. The supplied rf power (80 W/cm2), high pressure of oxygen (0.6 Torr), and special geometry of electrodes made it possible to sputter the oxide at the cluster level with the subse quent formation of dynamically stable nanoparticles of the complex oxide in a plasma, which served as a vapor phase for the deposited film. Our rf sputtering technique was previously used with advantage for pro ducing epitaxial barium strontium titanate films [15]. The image of the microrelief of the surface region (5 × 5 μm; thickness, 50 nm) of the BNFO/BST/MgO film, which was obtained on an Integra atomicforce microscope, is displayed in Fig. 1. The measurements were performed in a semicontact mode with the use of an NSG11 conventional silicon cantilever. The rough ness of the film was 7.5 nm. The surface microrelief is typical of BFO films [16]. The structural perfection of the films, the unit cell parameters in the direction of the normal to the sub strate plane and in the substrate plane, and orienta tional relationships between the film and substrate at room temperature were determined using Xray dif fraction on a Rigaku Ultima IV diffractometer (high PHYSICS OF THE SOLID STATE

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Fig. 2. Geometry of the recording of Xray diffraction pat terns on a Ultima IV diffractometer for (a) asymmetric reflections and (b) grazing incidence of Xrays.

resolution configuration for the study of thin films, germanium monochromator in the primary beam, Cu K α1 radiation). Xray structural investigations were carried out using θ–2θ, 2θ–ω, and ϕ scan modes, grazing incidence of the Xray beam, and measure ments of symmetric and asymmetric Bragg reflections. The geometries of the measurements of asymmetric reflections and the measurements for the grazing inci dence of Xrays are shown in Figs. 2a and 2b, respec tively. The composition of the prepared films was checked on a COMEBAXmicro analyzer. The BFO single crystal served as a reference sample. The ceramic target used for depositing the BNFO films has a rhombohedrally distorted perovskite unit cell with the parameters (in the hexagonal setting) a = b = 0.588 nm and c = 1.390 nm (at room temperature), which coincide with the lattice parameters of bismuth ferrite [17]. The main features of the deposition of the BNFO film as compared to the BST film manifest themselves in the influence of the substrate temperature on the synthesis and crystallization of the film. In the barium strontium titanate structures, an increase in the tem perature from 723 to 1023 K resulted in the following sequence of the structural perfection of the films: Xray amorphous films–polycrystalline films–tex tured films–singlecrystal films. However, in the structures based on BNFO, the opposite sequence was observed with an increase in the substrate temperature from 613 to 823 K: singlecrystal films, textured films, polycrystalline films, and films consisting of an oxide mixture. The threshold temperature of the substrate during the sputtering of BNFO onto the MgO(100)

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210 (001)

(113)BNFO

300 (004)

Intensity, arb. units

200

MgO

140

100 0 98

100

102 2θ, deg

104

(002) MgO

70

(003) 0

20

40

60 2θ, deg

(004) 80

(113)MgO

100

Fig. 3. Xray diffraction pattern (2θ–ω scan mode) of the BNFO singlecrystal film on the MgO(100) substrate with the BST sublayer. The inset shows the profile of the (004) reflection.

substrate for the singlecrystal growth of the film was equal to 613 K, which is lower than the temperatures TN and TC for the bismuth ferrite. The thickness of the BST singlecrystal sublayer before the deposition of the BNFO film was varied from 2 to 100 nm. The influence of the sublayer man ifested itself in a change in the orientation of the BNFO film with respect to the crystallographic direc tions of the magnesium oxide as compared to the dep osition onto the substrate without sublayer. The paral lel orientation of the axes of the BNFO film and the MgO substrate in the composition plane was observed for all the films under investigation irrespective of the thickness of the BST singlecrystal sublayer. The Xray diffraction data obtained using the 2θ– ω scan mode in the angle range 5°–120° (scan step, 0.04°) for the BNFO singlecrystal film 90 nm thick on the Mg(100) substrate with the BST sublayer (3 nm thick) are presented in Fig. 3. In the range of the (002) and (004) reflections of the substrate, the intensity was attenuated in a ratio of 1/800 with the use of an atten uator. The Xray diffraction pattern contains only the (00l) reflections of the primitive perovskite unit cell of BNFO and (002) and (004) reflections of the sub strate. This indicates that the [001] axis of the film is parallel to the [001] axis of the MgO substrate. No impurity phases were revealed. The vertical misorien tation with respect to the normal to the substrate according to the data obtained from the rocking curve of the (002) reflection is approximately equal to 2°. The parameter c of the primitive perovskite unit cell is larger than that for the bulk material and equal to 0.3987 ± 0.0001 nm.

0

90

180 ϕ, deg

270

360

Fig. 4. Xray diffraction patterns (ϕ scan mode) of the (113) reflection for the BNFO film and the MgO substrate in the range of ϕ angles from 0 to 360°.

In order to prove the heteroepitaxial growth and to determine the azimuthal misorientation of the film and the orientational relationships between the film and substrate, we used the ϕ scans of the (113) and (103) pseudocubic reflections of the film and sub strate. With the aim of providing the reflecting position of the (hkl) plane, the angle ω (Fig. 2a) was set equal to the difference between the angle of reflection θ and the angle α between the normals to the substrate plane and the family of {hkl} planes. The use of this approach makes it possible to record the reflections only from the crystallographic planes for which the inequality θ > α is satisfied. Figure 4 shows the Xray diffraction patterns of the (113) reflections of the film and sub strate with a variation in the angle ϕ from 0 to 360°. The Xray diffraction pattern contains four reflections spaced at 90°; in this case, the angular positions of the reflections of the film and substrate coincide with each other. The positions of the [100] and [010] axes corre spond to the angles ϕ equal to 0 and 90°, respectively. Consequently, the film is characterized by only one azimuthal orientation of the film with respect to the substrate: the [001] axis is parallel to the [001] axis of

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(100)

(110)

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Intensity, arb. units

4

3

2

1

0 20

22

24

26

30

32

34 44 2θ, deg

46

48 64

66

68

70

Fig. 5. Profiles of the (hk0) reflections obtained in the 2θχ–ϕ scan mode for grazing incidence of Xrays.

the MgO substrate and the other two axes [100] and [010] of the film are aligned with the [100] and [010] axes of the substrate, respectively. This result differs from that obtained in the deposition of the BNFO film onto the MgO substrate without BST sublayer where the film in the azimuthal plane is rotated by an angle of 45° with respect to the substrate [14]. The hk0 reflections of the film were recorded using the measurements in the horizontal plane in the graz ing incidence geometry. In this case, the primary beam is incident on the film surface at the angle ω = 0.3° close to a critical angle (Fig. 2b) and the reflection is recorded at the angle θD = 0.3° with respect to the sample surface. In this geometry of the measurements, the reflections of the substrate are not recorded. The reflecting positions of the hk0 planes were provided using ϕ scan with a fixed detector, and then the reflec tions were recorded with correlated rotation of the detector in the horizontal plane and the rotation of the sample around the vertical axis (2θχ–ϕ scan mode). The Xray diffraction patterns of the (100), (110), (200), and (220) reflections are shown in Fig. 5. The results obtained in the 2θχ–ϕ scan mode confirm that the angle between the [100] and [010] axes is equal to 90°. The parameters of the primitive perovskite unit cell in the substrate plane a = b = 0.3950 ± 3 nm are smaller than those for the bulk material. This indicates that twodimensional compressive stresses occur in the film in the substrate plane. The scanning of the separated regions of the recip rocal space revealed the presence of additional super structure reflections, which were indexed as (135) and (117) under the assumption that the perovskite unit cell is doubled. Consequently, the unit cell of the BNFO film is tetragonal (even though the monoclinic PHYSICS OF THE SOLID STATE

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unit cell is not ruled out because the angle between the [001] axis and the (001) plane was not determined in our work) with the parameters C = 2c = 0.7974 nm and A = B = 2a = 0.7900 nm. The choice between the monoclinic and tetragonal unit cells calls for further precise investigation, because the Xray diffraction data obtained in our work are insufficient for this pur pose. 3. RAMAN SCATTERING SPECTRA The Raman scattering spectra were excited by the polarized radiation of an argon laser (λ = 514.5 nm) and recorded on a Renishaw singlepass spectrometer equipped with a NExT (NearExcitation Tunable) fil ter for the analysis of the lowfrequency spectral range. The excitation radiation was focused on the sample with the use of a Leica optical microscope. The diam eter of the focused beam on the sample was equal to 2 μm. The polarized Raman spectra of the BNFO/BST/MgO heterostructure in Fig. 6 were obtained for the samples accurately oriented accord ing to the crystallographic axes of the MgO substrate so that X || [100], Y || [010], and Z || [001]. The thick nesses of the BNFO film and the BST sublayer were equal to 250 and 6 nm, respectively. The Raman spec tra were recorded both in the normal backscattering geometry when the wave vectors of the exciting and scattered light are directed normal to the film surface along the Z axis and in the geometry of backscattering from the end of the film when the wave vectors of the incident and scattered light are parallel to the Y axis and the polarization of incident/scattered light is par allel to the X or Z axis. It is important to emphasize that the spectrum of the BST thin film is not recorded

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Z(XX)Z Z(YY)Z Y(ZZ)Y Y(XZ)Y Z(XY)Z 0

200

400 600 Wave number, cm–1

800

Fig. 6. Polarized Raman scattering spectra of the BNFO/BST/MgO heterostructure. Arrows indicate weak lines at frequencies of 275, 352, 473, 531, and 625 cm–1 in the Z(XX)Z spectrum and at frequencies of 224, 268, and 667 cm–1 in the Z(XY)Z spectrum.

as a result of its small thickness and the MgO crystal has not Ramanactive lines in the frequency range below 800 cm–1. Therefore, the observed spectra are attributed to the BNFO film. The frequencies of the lines in the polarized Raman spectra of the BNFO/BST/MgO heterostruc ture do not coincide with the frequencies of the lines in the spectra of the BFO single crystal from the (001)cub plane at room temperature [18]. It should be noted that the halfwidth of the lines in the spectrum of the BNFO/BST/MgO film is considerably larger than that in the spectrum of the BFO single crystal. Therefore, the lines at frequencies higher than 200 cm–1 overlap strongly. The line broadening is most likely associated with the local structural distortions upon replacement of Bi by Nd, as was previously noted by Singh et al. [19]. The separation of the spec tra into constituting profiles in terms of the model of additive oscillators allowed us to determine the fre quencies of the lines in the spectra of the BNFO/BST/MgO heterostructure. The spectra of the diagonal scattering geometries Z(YY)Z, Z(XX)Z, and Y(ZZ)Y, which are almost equivalent, contain lines at frequencies of 74, 144, 174, 228, 275, 352, 473, 531, and 625 cm–1. The spectra of the offdiagonal scatter ing geometries Y(XZ)Y and Z(XY)Z involve lines at fre quencies of 76, 143, 175, 224, 268, and 667 cm–1. The frequencies of the lines and their separation in polar izations in the spectra of the BNFO/BST/MgO het

erostructure differ substantially from the experimental results obtained for BFO and NBFO films on other substrates. In the Raman spectra of rhombohedral BNFO polycrystalline films on Pt/TiO2/SiO2/Si sub strates [12], undoped BFO films on LaNiO3/SrTiO3 substrates [20], and rhombohedral BFO polycrystal line films grown by the sol–gel method on the surface of a BST thin film (30 nm) preliminarily deposited onto a silicon substrate coated by platinum (BST/Pt/TiO2/SuO2/Si) [21], no polarization depen dences were observed in view of the polycrystalline structure of the films. In the polarized spectra of the tetragonal and rhombohedral BFO films [19, 22], the most intense lines at 136, 168, and 212 cm–1 were always observed in the diagonal scattering geometries and less intense lines in the range 250–595 cm–1 were observed in the offdiagonal scattering geometries. The polarization characteristics of the Raman spectra exclude the rhombohedral symmetry of the BNFO/BST/MgO heterostructure, which is in agree ment with the Xray diffraction data obtained. The presence of the spectrum in the Z(XY)Z scattering geometry (Fig. 6) uniquely indicates that the NBFO film is not a tetragonal cdomain film. The occurrence of the lines at 143, 175, and 224 cm–1 both in the diag onal and offdiagonal scattering geometries suggests a lowering of the symmetry. It is evident that the sym metry of the unit cell of the NBFO film on the BST/MgO substrate differs from that on the SrTiO3 substrate. This is evidenced by the following facts. The lowest frequency line at 76 cm–1 characterized by sym metry E in the rhombohedral phase appears both in the diagonal and offdiagonal scattering geometries, which can be associated with the removal of the degeneracy of this mode. The lines at frequencies of 143, 175, and 224 cm–1 are assigned to totally symmet ric vibrations of the rhombohedral phase of the single crystal, and their appearance in the spectra of the BNFO film for offdiagonal components of the Raman scattering activity tensor can occur either due to the lowering of the symmetry to orthorhombic or monoclinic or due the presence of a and c domains of the tetragonal phase. The last variant is inconsistent with the aforementioned Xray diffraction data. Therefore, although the unit cell parameters in the substrate plane are identical, the symmetry of the unit cell should be lower than tetragonal; i.e., there occurs an orthorhombic or monoclinic phase with two for mula units in the unit cell so that the degree of mono clinic distortion is small because the spectra of the diagonal components of the Raman scattering activity tensor are similar to each other and the spectra of the offdiagonal components within the limits of experi mental error consist of the lines at identical frequen cies and differ only in intensities.

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4. DISCUSSION OF THE RESULTS It is well known that the lowsymmetry rhombohe dral phase of the BFO single crystal has a distorted perovskite structure, which is described by two three component order parameters R25 and F1u, where the order parameter R25 characterizes antiphase rotations of octahedra and the order parameter F1u corresponds to the polarization. The rotations of octahedra are identical around each of three axes with a rotation angle of the order of 12° [23–25] already at the ferro electric transition temperature (1083 K), which is accompanied by the appearance of the spontaneous polarization along the body diagonal. The symmetry of the lowsymmetry phase is described by space group R3c, and the order parameter has the form (ϕ, ϕ, ϕ, p, p, p). The symmetry analysis of structural distortions induced by the rotation of oxygen octahedra and polar displacements of cations in perovskite films on the (100) surface of a cubic substrate was performed in our earlier work [26], where we revealed more than 30 pos sible phases of which 19 phases contain two formula units in the unit cell and are ferroelectric. Among these phases, one phase is tetragonal I4cm, one phase is trigonal C1, and the other phases are orthorhombic or monoclinic. It should be noted that, in the case of the epitaxial growth on the (100) surface of a cubic substrate, the rhombohedral symmetry of the film is forbidden. In our case, the epitaxial BNFO thin films are deposited at temperatures substantially lower than the temperature TC on the cubic substrate with the prelim inarily grown BST thin film, which is in the paraelec tric tetragonal phase at the deposition temperature of the BNFO film [27]. The tetragonal symmetry of the substrate surface and the intermediate BST layer leads to the growth of the BNFO film with the pseudotetrag onal unit cell, for which the symmetry is lower than tetragonal. The structural distortions, which arise in the epitaxial BNFO film and involve both antiphase rotations of oxygen octahedra and displacements of Bi ions with respect to these octahedra, can lead to the formation of the orthorhombic or monoclinic phase, in which the spontaneous polarization vector deviates from the [111]cub direction and can have different components along the x, y, and z axes. Among possible ferroelectric phases noted in [26], it is necessary to reject four phases with the preferred direction of the spontaneous polarization along the z axis, because, in this case, there should arise a cdomain structure that is inconsistent with the Raman scattering data. According to the Curie principle, the symmetry of the film should be a subgroup of the symmetry group of the bulk bismuth ferrite and tetragonal cubic surface of PHYSICS OF THE SOLID STATE

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the substrate. The fulfillment of these conditions leads to the symmetry Cc of the film with the order parame ter (ϕ1, ϕ1, ϕ2, p1, p1, p2), i.e., with the polarization at an angle with respect to the film surface. The refine ment of the symmetry of BNFO films on MgO and BST/MgO singlecrystal substrates calls for further precise structural investigation. 5. CONCLUSIONS Thus, the bismuth ferrite films on Mg(001) single crystal substrates with barium strontium titanate nanosized sublayers have been prepared by rf cathode sputtering at an elevated oxygen partial pressure. In the epitaxial BNFO films grown at temperatures below the ferroelectric and magnetic transition tempera tures, there arise compressive stresses in the substrate plane, which lead to the formation of the pseudotet ragonal structure with double unit cell parameters a, b, and c in the film in contrast to bulk samples. The anal ysis of the polarized Raman spectra measured in dif ferent scattering geometries has made it possible to exclude the tetragonal symmetry of the unit cell. A combined analysis of the experimental results and grouptheoretical calculations has allowed us to make the inference that a new phase that is not observed in bulk samples is formed in the BNFO films on the MgO substrates with the BST sublayers. The symmetry of this phase is monoclinic, the unit cell contains two for mula units, and the spontaneous polarization vector deviates from the [111]cub direction and can have dif ferent components along the x, y, and z axes. The structure of these films should be refined, and their magnetic properties call for further investigation. ACKNOWLEDGMENTS This study was supported by the Russian Founda tion for Basic Research (project nos. 080213511 ofi ts, 090200254a, and 090200666a).

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