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Effect of Nb doping on the morphology and multiferroic behavior of Bi0.9La0.1FeO3 ceramics
Author’s Accepted Manuscript Effect of Nb doping on the morphology and Multiferroic behavior Of Bi0.9La0.1FeO3 ceramics V.A. Khomchenko, J.A. Paixão
www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30120-3 http://dx.doi.org/10.1016/j.matlet.2016.01.120 MLBLUE20248
To appear in: Materials Letters Received date: 7 October 2015 Accepted date: 24 January 2016 Cite this article as: V.A. Khomchenko and J.A. Paixão, Effect of Nb doping on the morphology and Multiferroic behavior Of Bi0.9La0.1FeO3 ceramics, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.01.120 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 Effect of Nb doping on the morphology and multiferroic behavior of Bi0.9La0.1FeO3 ceramics
V. A. Khomchenko and J. A. Paixão
CFisUC, Department of Physics, University of Coimbra, P-3004-516 Coimbra, Portugal
Synthesis and study of the crystal structure, microstructure, local ferroelectric, and magnetic properties of the Bi0.9La0.1Fe1-xNbxO3+x (x≤0.02) perovskites have been carried out to shed light on the conditions underlying the appearance of doping-driven instability of the cycloidal antiferromagnetic order in the polar phase of bismuth ferrite. The light aliovalent substitution has not been found to change the rhombohedral symmetry specific to the parent Bi0.9La0.1FeO3. It has been proven that the doping removes the cycloidal modulation characteristic of the rhombohedral phase to stabilize a weak ferromagnetic ferroelectric state. The antiferromagnetic-weak ferromagnetic transition is accompanied by a dramatic decrease in the average size of crystal grains and ferroelectric domains. The magnetic and morphological transformations can be understood by taking into consideration the charge-compensating mechanism involving the formation of lattice defects in the donor-doped materials. The observation of a correlation between the magnetic and morphological evolution in the Bi0.9La0.1Fe1-xNbxO3+x series suggests that structural defects should be considered as an essential factor controlling the magnetic ground state in BiFeO3-based multiferroics.
Keywords: BiFeO3, Multiferroics, Lattice defects, Spin-cycloid instability, Ferroelectric domains
Corresponding author: Dr. V. A. Khomchenko CFisUC, Department of Physics, University of Coimbra, P-3004-516 Coimbra, Portugal E-mail: [email protected] Phone: +351 239 410 637 Fax: +351 239 829 158
2 1. Introduction
Multiferroic materials have attracted great interest due to their potential applications in memory storage, sensor and spintronic devices [1]. Among the known single-phase magnetic ferroelectrics, BiFeO3 is distinguished by the extremely high transition temperatures (TFE≈1100 K, TAFM≈640 K; FE and AFM stand for ferroelectric and antiferromagnetic, respectively) [2]. Below the Curie point, bulk BiFeO3 adopts a rhombohedrally distorted perovskite structure (space group R3c) and acquires a spontaneous polarization oriented along the threefold axis [3]. The polar atomic displacements are caused by stereochemical activity of the lone pair on Bi [4]. As a result of magnetoelectric coupling [5], the compound exhibits a complex magnetic structure, in which the Fe magnetic moments retain a local canted G-type arrangement while describing the cycloid with a period of ~62 nm [6]. The magnetic and ferroelectric properties of BiFeO3 can be strongly influenced by doping [7]. In particular, suppression of the cycloidal modulation resulting in the formation of a weak ferromagnetic (spin-canted) state occurs in the Bi1-xLnxFeO3 (Ln= lanthanide) ferroelectrics upon Ti4+ for Fe3+ substitution [8-10]. A similar effect is observed in the Nb5+-doped Bi1-xLnxFeO3 [11]. Isovalent (Mn3+, Sc3+) Fe-site substitution, in contrast, does not appear to change magnetic behavior of the Bi1-xLnxFeO3 multiferroics significantly [12-14]. To illustrate the role of aliovalent dopants in the removal of the cycloidal magnetic order, investigation of the crystal
structure,
microstructure,
local
ferroelectric
and
magnetic
properties
of
the
Bi03.9 La03.1Fe13x Nbx5O32x compounds was carried out (Nb5+ has almost exactly the same ionic radius as Fe3+ (0.64 Å and 0.645 Å, respectively) [15] and this similarity was used to minimize the possible influence of a size mismatch-introduced "chemical pressure" [2]; the light isovalent La3+ substitution, in turn, was used to improve the compositional stability of thermodynamically metastable BiFeO3 [2]).
2. Experimental
Ceramic samples of Bi0.9La0.1Fe1-xNbxO3+x (x=0, 0.01, 0.02) were prepared by a conventional solid-state reaction method using the high-purity oxides Bi2O3, La2O3, Fe2O3 and Nb2O5 (Sigma-Aldrich, ≥99%). The reagents were taken in stoichiometric cation ratio, mixed using an agate mortar and pestle, and subsequently pressed into pellets of 10 mm diameter and 2-3 mm thickness at applied load of 1 tonne for 1 min. The synthesis was carried out in air at 940°C for 30 h (annealing at a higher temperature was found to result in partial decomposition of the perovskite phase). X-ray diffraction (XRD) was performed
3 on crushed pellets. XRD patterns were collected over a 2θ range of 15°-100° in steps of 0.01° and exposition of 2 s per step using a Bruker D8 Advance diffractometer with Cu Kα radiation. The data were analyzed by the Rietveld method using the FullProf program [16]. Microstructural characterization was performed on gold-palladium coated fractured surfaces using a VEGA-3 SB (TESCAN) scanning electron microscope (SEM) operated at an accelerating voltage of 30 kV. Local ferroelectric properties were investigated with piezoresponse force microscopy (PFM) using a commercial setup NTEGRA Prima (NT-MDT). The NSG30 probes with a resonance frequency around 300 kHz were used. Domain visualization was performed on mechanically-polished surfaces under an applied AC voltage with an amplitude VAC=5 V and frequency f=100 kHz. Magnetic measurements of ceramic samples were performed with a cryogen-free Physical Properties Measurement System (PPMS DynaCool, Quantum Design).
3. Results and Discussion
XRD measurements carried out for the Bi0.9La0.1Fe1-xNbxO3+x samples annealed at different temperatures showed that the single-phase compounds can be obtained in a rather narrow (x≤0.02) range of Nb concentration (the appearance of a secondary phase isostructural to Bi-Fe-Nb-O pyrochlores (the materials are paramagnetic at room temperature) [17] was detected in the samples with x>0.02). The light Nb doping was not found to change the crystal symmetry of the Bi0.9La0.1FeO3 compound [18]. Indeed, XRD patterns collected for the Bi0.9La0.1Fe1-xNbxO3+x (x=0, 0.01, 0.02) samples were successfully indexed using the hexagonal cell with
a 2a p and c 2 3c p ( a p c p ~4 Å are the parameters of the
primitive pseudocubic perovskite subcell). The observed systematic absences within the diffraction patterns were consistent with the space group R3c. Thus, in contrast to the previous investigation of the Nb-doped Bi0.9La0.1FeO3 [11], our XRD analysis did not reveal any signatures indicative of the necessity to reduce the symmetry to a monoclinic one. An example of the Rietveld refinement of the diffraction data is shown in Fig. 1 (a). With respect to the cubic Pm 3 m lattice characteristic of an ideal ABO3 perovskite (Fig. 1 (b)), structural distortions in the Bi0.9La0.1Fe1-xNbxO3+x samples can be described in terms of a superposition of the antiphase tilting of the adjacent oxygen octahedra (a−a−a− tilt system in Glazer's notation [19]) and polar atomic displacements along the <111>c direction of the parent cubic cell ([001]hex direction in the hexagonal setting) (Fig. 1 (c)). The Nb doping results in expansion of the cell in the basal (001)hex plane and contraction of the cell along the polar [001] hex direction (Fig. 1 (d)). The latter
4 tendency prevails, so a slight decrease (from 372.08 Å3 for x=0 to 371.9 Å3 for x=0.02) of the unit cell volume takes place with increasing Nb content. It is interesting to note that the Ti4+ and Mn3+/4+ doping of Bi1-xLnxFeO3 (Ti and Mn possess a relatively high solid solubility in BiFeO3) gives rise to contraction of the cell both along the a and the c axes [9, 10, 20]. Magnetic measurements of the samples indicate that the Nb doping removes the cycloidal magnetic order specific to the polar phase of the Bi1-xLnxFeO3 perovskites [21-23]. Indeed, while the parent compound shows the field dependence typical of the lightly-doped Bi1-xLnxFeO3 multiferroics (these materials have a very small remanent magnetization and demonstrate a metamagnetic behavior associated with the field-driven suppression of the cycloidal modulation) [21-23], the Nb-substituted samples exhibit the M(H) dependences expected for weak ferromagnets (Fig. 2). Spontaneous magnetization (estimated through a linear extrapolation of the high-field magnetization to H=0) achieves 0.35 emu/g. This value is very close to the maximal spontaneous magnetization observed in the Bi0.9La0.1Fe1-xTixO3+x/2 series at x~0.04 [10]. A high coercivity (Hc~10 kOe) characteristic of the doped samples (Fig. 2) excludes the possibility that the weak ferromagnetic behavior is explained by the presence of ferrimagnetic impurity γ-Fe2O3 (Hc~0.1 kOe). SEM measurements (Fig. 3) confirm the parent and Nb-doped compounds to be single-phase (the backscattered electron microscopy exhibits a uniform contrast incompatible with the presence of any impurities concentrated at grain boundaries). The microstructural study reveals a correlation between the magnetic behavior and the morphology of the Bi0.9La0.1Fe1-xNbxO3+x ceramics. Indeed, the magnetic transformation induced by the doping is found to be accompanied by a drastic (from ~10 μm for x=0 to ~1 μm for x=0.01 and x=0.02) decrease in the average size of crystal grains (Fig. 3). Such a decrease seems to reflect the general trend characteristic of donor-doped BiFeO3 [8-11]. The magnetic and microstructural changes can be understood as associated with the mechanism of compensation of an excess positive charge introduced by the doping. This mechanism is supposed to involve the creation of cation vacancies and exsolution of some portion of the aliovalent dopants into the crystal grain interior of Bi1-xLnxFeO3 [8, 24, 25]. The exsolution is suggested to impede the grain-boundary mobility and restrain the crystal grain growth [8]. Local strain fields generated by lattice defects are expected to suppress coupling between gradients of the magnetic order parameter and stabilize the symmetry-permitted weak ferromagnetic state [26]. Since a ferroelectric domain structure is highly sensitive to the electric and strain fields generated by lattice defects [27], the scenario of the aliovalent doping-induced magnetic/morphological transformation suggests the appearance of correlated changes in the magnetic and local ferroelectric
5 behavior of the samples. The latter was studied with piezoresponse force microscopy which confirmed that the substitution-driven antiferromagnetic-weak ferromagnetic transition in the Bi0.9La0.1Fe1-xNbxO3+x system (Fig. 2) occurs simultaneously with the formation of the new submicrometer-scale ferroelectric/ferroelastic domain configuration (Fig. 4). The decrease in the size of the ferroelectric domains is consistent with the changes observed in the ferroelectric domain structure of the Ti 4+-doped Bi1-xLnxFeO3 [9, 10] and is likely to reflect the common behavior characteristic of donor- and acceptordoped BiFeO3 [28-30]. The results of the work suggest that lattice defects should play a key role in the appearance of spontaneous magnetization in the polar phase of BiFeO 3-based perovskites and propose that the light aliovalent doping can be effectively used to control both the magnetic state and the ferroelectric domain structure of these materials.
Acknowledgements
This work was supported by funds from FEDER (Programa Operacional Factores de Competitividade COMPETE) and from FCT-Fundação para a Ciência e a Tecnologia under the project PEst-C/FIS/UI0036/2014. V.A.K. is grateful to Fundação para a Ciência e a Tecnologia for financial support through the FCT Investigator Programme (project IF/00819/2014). Access to TAIL-UC facility funded under QREN-Mais Centro project ICT_2009_02_012_1890 is gratefully acknowledged.
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10. V.A. Khomchenko, J.A. Paixão, J. Phys. D: Appl. Phys. 48 (2015) 345001. 11. L. Zhai, Y.G. Shi, S.L. Tang, L.Y. Lv, Y.W. Du, J. Phys. D: Appl. Phys. 42 (2009) 165004. 12. I.O. Troyanchuk, A.N. Chobot, O.S. Mantytskaya, N.V. Tereshko, Inorg. Mater. 46 (2010) 424-428. 13. T.D. Rao, A. Kumari, M.K. Niranjan, S. Asthana, Physica B 448 (2014) 267-272. 14. T.D. Rao, S. Asthana, J. Appl. Phys. 116 (2014) 164102. 15. R.D. Shannon, Acta Cryst. A 32 (1976) 751-767. 16. J. Rodríguez-Carvajal, Physica B 192 (1993) 55-69. 17. M.W. Lufaso, T.A. Vanderah, I.M. Pazos, I. Levin, R.S. Roth, J.C. Nino, V. Provenzano, P.K. Schenck, J. Solid State Chem. 179 (2006) 3900-3910. 18. D.A. Rusakov, A.M. Abakumov, K. Yamaura, A.A. Belik, G. Van Tendeloo, E. TakayamaMuromachi, Chem. Mater. 23 (2011) 285-292. 19. A.M. Glazer, Acta Cryst. A 31 (1975) 756-762. 20. S.M. Selbach, T. Tybell, M.-A. Einarsrud, T. Grande, Chem. Mater. 21 (2009) 5176-5186. 21. I.O. Troyanchuk, D.V. Karpinsky, M.V. Bushinsky, V.A. Khomchenko, G.N. Kakazei, J.P. Araujo, M. Tovar, V. Sikolenko, V. Efimov, A.L. Kholkin, Phys. Rev. B 83 (2011) 054109. 22. V.A. Khomchenko, L.C.J. Pereira, J.A. Paixão, J. Phys. D: Appl. Phys. 44 (2011) 185406. 23. V.A. Khomchenko, I.O. Troyanchuk, D.V. Karpinsky, J.A. Paixão, J. Mater. Sci. 47 (2012) 15781581. 24. I.M. Reaney, I. MacLaren, L. Wang, B. Schaffer, A. Craven, K. Kalantari, I. Sterianou, S. Miao, S. Karimi, D.C. Sinclair, Appl. Phys. Lett. 100 (2012) 182902. 25. I. MacLaren, L.Q. Wang, O. Morris, A.J. Craven, R.L. Stamps, B. Schaffer, Q.M. Ramasse, S. Miao, K. Kalantari, I. Sterianou, I.M. Reaney, APL Mater. 1 (2013) 021102. 26. J.A. Schiemer, R.L. Withers, Y. Liu, M.A. Carpenter, Chem. Mater. 25 (2013) 4436-4446. 27. G. Catalan, J. Seidel, R. Ramesh, J.F. Scott, Rev. Mod. Phys. 84 (2012) 119-156. 28. V.A. Khomchenko, J.A. Paixão, J. Mater. Sci. 50 (2015) 7192-7196. 29. V.A. Khomchenko, J.A. Paixão, J. Appl. Phys. 116 (2014) 214105. 30. V.A. Khomchenko, J.A. Paixão, Structural defects as a factor controlling the magnetic properties of pure and Ti-doped Bi1-xCaxFeO3-x/2 multiferroics, J. Phys.: Condens. Matter (accepted for publication).
7
Figures
1.
(a) Observed, calculated and difference XRD patterns obtained for the Bi0.9La0.1Fe0.99Nb0.01O3.01 compound at room temperature [Rp=5.04 %, Rwp=6.99 %, RB=3.00 %]. The low intensity peaks seen at around 20° and 29° are due to the small Kβ component of the radiation leaking through the Kβ filter (Ni foil placed before detector) as "images", at lower angles, of the very strong corresponding Kα peaks. (b) Atomic arrangement in an undistorted perovskite: the A-site (in green) and B-site (in blue) atoms are surrounded by oxygen atoms (in red) forming the AO12 and BO6 (in grey) polyhedra. (c) Atomic arrangement in the "R3c" phase [the representation is based on the refined parameters obtained for the Bi0.9La0.1Fe0.99Nb0.01O3.01 sample: a=b=5.5804(1) Å, c=13.7955(3) Å; Bi/La: 6a (0, 0, 0), Fe/Nb: 6a (0, 0, 0.2247(2)), O: 18 b (0.4415(13), 0.0190(13), 0.9592(4))]. (d) Compositional dependence of the normalized lattice parameters for the Bi0.9La0.1Fe1-xNbxO3+x series.
2.
Field dependences of the magnetization obtained for the Bi0.9La0.1Fe1-xNbxO3+x compounds at room temperature.
3.
Secondary electron microscopy (left column) and backscattered electron microscopy (right column) images of the Bi0.9La0.1Fe1-xNbxO3+x (x=0 and x=0.02) ceramics.
4.
Vertical PFM images of the Bi0.9La0.1Fe1-xNbxO3+x ceramics. An A cos φ signal was recorded. Here, A is the amplitude of the measured vibration proportional to the effective longitudinal piezoelectric coefficient and φ is the phase shift determining the direction of polarization (the bright and dark regions correspond to polarization vectors directed to the free surface and to the bottom electrode, respectively).
8
Figure 1
Figure 2
Figure 3
Figure 4
9
Highlights
Multiferroic behavior of the Bi0.9La0.1Fe1-xNbxO3+x (x≤0.02) ceramics was studied The compounds crystallize with a polar rhombohedral structure Nb doping results in decreasing the size of crystal grains and ferroelectric domains The doping induces antiferromagnetic-weak ferromagnetic transformation The magnetic and morphological changes can be understood as lattice defects-driven
www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30120-3 http://dx.doi.org/10.1016/j.matlet.2016.01.120 MLBLUE20248
To appear in: Materials Letters Received date: 7 October 2015 Accepted date: 24 January 2016 Cite this article as: V.A. Khomchenko and J.A. Paixão, Effect of Nb doping on the morphology and Multiferroic behavior Of Bi0.9La0.1FeO3 ceramics, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.01.120 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 Effect of Nb doping on the morphology and multiferroic behavior of Bi0.9La0.1FeO3 ceramics
V. A. Khomchenko and J. A. Paixão
CFisUC, Department of Physics, University of Coimbra, P-3004-516 Coimbra, Portugal
Synthesis and study of the crystal structure, microstructure, local ferroelectric, and magnetic properties of the Bi0.9La0.1Fe1-xNbxO3+x (x≤0.02) perovskites have been carried out to shed light on the conditions underlying the appearance of doping-driven instability of the cycloidal antiferromagnetic order in the polar phase of bismuth ferrite. The light aliovalent substitution has not been found to change the rhombohedral symmetry specific to the parent Bi0.9La0.1FeO3. It has been proven that the doping removes the cycloidal modulation characteristic of the rhombohedral phase to stabilize a weak ferromagnetic ferroelectric state. The antiferromagnetic-weak ferromagnetic transition is accompanied by a dramatic decrease in the average size of crystal grains and ferroelectric domains. The magnetic and morphological transformations can be understood by taking into consideration the charge-compensating mechanism involving the formation of lattice defects in the donor-doped materials. The observation of a correlation between the magnetic and morphological evolution in the Bi0.9La0.1Fe1-xNbxO3+x series suggests that structural defects should be considered as an essential factor controlling the magnetic ground state in BiFeO3-based multiferroics.
Keywords: BiFeO3, Multiferroics, Lattice defects, Spin-cycloid instability, Ferroelectric domains
Corresponding author: Dr. V. A. Khomchenko CFisUC, Department of Physics, University of Coimbra, P-3004-516 Coimbra, Portugal E-mail: [email protected] Phone: +351 239 410 637 Fax: +351 239 829 158
2 1. Introduction
Multiferroic materials have attracted great interest due to their potential applications in memory storage, sensor and spintronic devices [1]. Among the known single-phase magnetic ferroelectrics, BiFeO3 is distinguished by the extremely high transition temperatures (TFE≈1100 K, TAFM≈640 K; FE and AFM stand for ferroelectric and antiferromagnetic, respectively) [2]. Below the Curie point, bulk BiFeO3 adopts a rhombohedrally distorted perovskite structure (space group R3c) and acquires a spontaneous polarization oriented along the threefold axis [3]. The polar atomic displacements are caused by stereochemical activity of the lone pair on Bi [4]. As a result of magnetoelectric coupling [5], the compound exhibits a complex magnetic structure, in which the Fe magnetic moments retain a local canted G-type arrangement while describing the cycloid with a period of ~62 nm [6]. The magnetic and ferroelectric properties of BiFeO3 can be strongly influenced by doping [7]. In particular, suppression of the cycloidal modulation resulting in the formation of a weak ferromagnetic (spin-canted) state occurs in the Bi1-xLnxFeO3 (Ln= lanthanide) ferroelectrics upon Ti4+ for Fe3+ substitution [8-10]. A similar effect is observed in the Nb5+-doped Bi1-xLnxFeO3 [11]. Isovalent (Mn3+, Sc3+) Fe-site substitution, in contrast, does not appear to change magnetic behavior of the Bi1-xLnxFeO3 multiferroics significantly [12-14]. To illustrate the role of aliovalent dopants in the removal of the cycloidal magnetic order, investigation of the crystal
structure,
microstructure,
local
ferroelectric
and
magnetic
properties
of
the
Bi03.9 La03.1Fe13x Nbx5O32x compounds was carried out (Nb5+ has almost exactly the same ionic radius as Fe3+ (0.64 Å and 0.645 Å, respectively) [15] and this similarity was used to minimize the possible influence of a size mismatch-introduced "chemical pressure" [2]; the light isovalent La3+ substitution, in turn, was used to improve the compositional stability of thermodynamically metastable BiFeO3 [2]).
2. Experimental
Ceramic samples of Bi0.9La0.1Fe1-xNbxO3+x (x=0, 0.01, 0.02) were prepared by a conventional solid-state reaction method using the high-purity oxides Bi2O3, La2O3, Fe2O3 and Nb2O5 (Sigma-Aldrich, ≥99%). The reagents were taken in stoichiometric cation ratio, mixed using an agate mortar and pestle, and subsequently pressed into pellets of 10 mm diameter and 2-3 mm thickness at applied load of 1 tonne for 1 min. The synthesis was carried out in air at 940°C for 30 h (annealing at a higher temperature was found to result in partial decomposition of the perovskite phase). X-ray diffraction (XRD) was performed
3 on crushed pellets. XRD patterns were collected over a 2θ range of 15°-100° in steps of 0.01° and exposition of 2 s per step using a Bruker D8 Advance diffractometer with Cu Kα radiation. The data were analyzed by the Rietveld method using the FullProf program [16]. Microstructural characterization was performed on gold-palladium coated fractured surfaces using a VEGA-3 SB (TESCAN) scanning electron microscope (SEM) operated at an accelerating voltage of 30 kV. Local ferroelectric properties were investigated with piezoresponse force microscopy (PFM) using a commercial setup NTEGRA Prima (NT-MDT). The NSG30 probes with a resonance frequency around 300 kHz were used. Domain visualization was performed on mechanically-polished surfaces under an applied AC voltage with an amplitude VAC=5 V and frequency f=100 kHz. Magnetic measurements of ceramic samples were performed with a cryogen-free Physical Properties Measurement System (PPMS DynaCool, Quantum Design).
3. Results and Discussion
XRD measurements carried out for the Bi0.9La0.1Fe1-xNbxO3+x samples annealed at different temperatures showed that the single-phase compounds can be obtained in a rather narrow (x≤0.02) range of Nb concentration (the appearance of a secondary phase isostructural to Bi-Fe-Nb-O pyrochlores (the materials are paramagnetic at room temperature) [17] was detected in the samples with x>0.02). The light Nb doping was not found to change the crystal symmetry of the Bi0.9La0.1FeO3 compound [18]. Indeed, XRD patterns collected for the Bi0.9La0.1Fe1-xNbxO3+x (x=0, 0.01, 0.02) samples were successfully indexed using the hexagonal cell with
a 2a p and c 2 3c p ( a p c p ~4 Å are the parameters of the
primitive pseudocubic perovskite subcell). The observed systematic absences within the diffraction patterns were consistent with the space group R3c. Thus, in contrast to the previous investigation of the Nb-doped Bi0.9La0.1FeO3 [11], our XRD analysis did not reveal any signatures indicative of the necessity to reduce the symmetry to a monoclinic one. An example of the Rietveld refinement of the diffraction data is shown in Fig. 1 (a). With respect to the cubic Pm 3 m lattice characteristic of an ideal ABO3 perovskite (Fig. 1 (b)), structural distortions in the Bi0.9La0.1Fe1-xNbxO3+x samples can be described in terms of a superposition of the antiphase tilting of the adjacent oxygen octahedra (a−a−a− tilt system in Glazer's notation [19]) and polar atomic displacements along the <111>c direction of the parent cubic cell ([001]hex direction in the hexagonal setting) (Fig. 1 (c)). The Nb doping results in expansion of the cell in the basal (001)hex plane and contraction of the cell along the polar [001] hex direction (Fig. 1 (d)). The latter
4 tendency prevails, so a slight decrease (from 372.08 Å3 for x=0 to 371.9 Å3 for x=0.02) of the unit cell volume takes place with increasing Nb content. It is interesting to note that the Ti4+ and Mn3+/4+ doping of Bi1-xLnxFeO3 (Ti and Mn possess a relatively high solid solubility in BiFeO3) gives rise to contraction of the cell both along the a and the c axes [9, 10, 20]. Magnetic measurements of the samples indicate that the Nb doping removes the cycloidal magnetic order specific to the polar phase of the Bi1-xLnxFeO3 perovskites [21-23]. Indeed, while the parent compound shows the field dependence typical of the lightly-doped Bi1-xLnxFeO3 multiferroics (these materials have a very small remanent magnetization and demonstrate a metamagnetic behavior associated with the field-driven suppression of the cycloidal modulation) [21-23], the Nb-substituted samples exhibit the M(H) dependences expected for weak ferromagnets (Fig. 2). Spontaneous magnetization (estimated through a linear extrapolation of the high-field magnetization to H=0) achieves 0.35 emu/g. This value is very close to the maximal spontaneous magnetization observed in the Bi0.9La0.1Fe1-xTixO3+x/2 series at x~0.04 [10]. A high coercivity (Hc~10 kOe) characteristic of the doped samples (Fig. 2) excludes the possibility that the weak ferromagnetic behavior is explained by the presence of ferrimagnetic impurity γ-Fe2O3 (Hc~0.1 kOe). SEM measurements (Fig. 3) confirm the parent and Nb-doped compounds to be single-phase (the backscattered electron microscopy exhibits a uniform contrast incompatible with the presence of any impurities concentrated at grain boundaries). The microstructural study reveals a correlation between the magnetic behavior and the morphology of the Bi0.9La0.1Fe1-xNbxO3+x ceramics. Indeed, the magnetic transformation induced by the doping is found to be accompanied by a drastic (from ~10 μm for x=0 to ~1 μm for x=0.01 and x=0.02) decrease in the average size of crystal grains (Fig. 3). Such a decrease seems to reflect the general trend characteristic of donor-doped BiFeO3 [8-11]. The magnetic and microstructural changes can be understood as associated with the mechanism of compensation of an excess positive charge introduced by the doping. This mechanism is supposed to involve the creation of cation vacancies and exsolution of some portion of the aliovalent dopants into the crystal grain interior of Bi1-xLnxFeO3 [8, 24, 25]. The exsolution is suggested to impede the grain-boundary mobility and restrain the crystal grain growth [8]. Local strain fields generated by lattice defects are expected to suppress coupling between gradients of the magnetic order parameter and stabilize the symmetry-permitted weak ferromagnetic state [26]. Since a ferroelectric domain structure is highly sensitive to the electric and strain fields generated by lattice defects [27], the scenario of the aliovalent doping-induced magnetic/morphological transformation suggests the appearance of correlated changes in the magnetic and local ferroelectric
5 behavior of the samples. The latter was studied with piezoresponse force microscopy which confirmed that the substitution-driven antiferromagnetic-weak ferromagnetic transition in the Bi0.9La0.1Fe1-xNbxO3+x system (Fig. 2) occurs simultaneously with the formation of the new submicrometer-scale ferroelectric/ferroelastic domain configuration (Fig. 4). The decrease in the size of the ferroelectric domains is consistent with the changes observed in the ferroelectric domain structure of the Ti 4+-doped Bi1-xLnxFeO3 [9, 10] and is likely to reflect the common behavior characteristic of donor- and acceptordoped BiFeO3 [28-30]. The results of the work suggest that lattice defects should play a key role in the appearance of spontaneous magnetization in the polar phase of BiFeO 3-based perovskites and propose that the light aliovalent doping can be effectively used to control both the magnetic state and the ferroelectric domain structure of these materials.
Acknowledgements
This work was supported by funds from FEDER (Programa Operacional Factores de Competitividade COMPETE) and from FCT-Fundação para a Ciência e a Tecnologia under the project PEst-C/FIS/UI0036/2014. V.A.K. is grateful to Fundação para a Ciência e a Tecnologia for financial support through the FCT Investigator Programme (project IF/00819/2014). Access to TAIL-UC facility funded under QREN-Mais Centro project ICT_2009_02_012_1890 is gratefully acknowledged.
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Figures
1.
(a) Observed, calculated and difference XRD patterns obtained for the Bi0.9La0.1Fe0.99Nb0.01O3.01 compound at room temperature [Rp=5.04 %, Rwp=6.99 %, RB=3.00 %]. The low intensity peaks seen at around 20° and 29° are due to the small Kβ component of the radiation leaking through the Kβ filter (Ni foil placed before detector) as "images", at lower angles, of the very strong corresponding Kα peaks. (b) Atomic arrangement in an undistorted perovskite: the A-site (in green) and B-site (in blue) atoms are surrounded by oxygen atoms (in red) forming the AO12 and BO6 (in grey) polyhedra. (c) Atomic arrangement in the "R3c" phase [the representation is based on the refined parameters obtained for the Bi0.9La0.1Fe0.99Nb0.01O3.01 sample: a=b=5.5804(1) Å, c=13.7955(3) Å; Bi/La: 6a (0, 0, 0), Fe/Nb: 6a (0, 0, 0.2247(2)), O: 18 b (0.4415(13), 0.0190(13), 0.9592(4))]. (d) Compositional dependence of the normalized lattice parameters for the Bi0.9La0.1Fe1-xNbxO3+x series.
2.
Field dependences of the magnetization obtained for the Bi0.9La0.1Fe1-xNbxO3+x compounds at room temperature.
3.
Secondary electron microscopy (left column) and backscattered electron microscopy (right column) images of the Bi0.9La0.1Fe1-xNbxO3+x (x=0 and x=0.02) ceramics.
4.
Vertical PFM images of the Bi0.9La0.1Fe1-xNbxO3+x ceramics. An A cos φ signal was recorded. Here, A is the amplitude of the measured vibration proportional to the effective longitudinal piezoelectric coefficient and φ is the phase shift determining the direction of polarization (the bright and dark regions correspond to polarization vectors directed to the free surface and to the bottom electrode, respectively).
8
Figure 1
Figure 2
Figure 3
Figure 4
9
Highlights
Multiferroic behavior of the Bi0.9La0.1Fe1-xNbxO3+x (x≤0.02) ceramics was studied The compounds crystallize with a polar rhombohedral structure Nb doping results in decreasing the size of crystal grains and ferroelectric domains The doping induces antiferromagnetic-weak ferromagnetic transformation The magnetic and morphological changes can be understood as lattice defects-driven