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Nanoscale structural modulation and the enhanced room-temperature multiferroic properties Shujie Sun,a Yan Huang,a Guopeng Wang,a Jianlin Wang,a Zhengping Fu,ac Ranran Peng,*ac Randy J. Knized and Yalin Lu*abcd 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Availability of a single-phase and room temperature functional multiferroic material is still a big challenge though it is very important to both fundamental physics and application development. Layered Aurivillius oxide materials, one of the most promising candidates, recently have been attracting considerable interests. In this work, we focused on investigating the nanoscale structural evolution of the six-layer Bi7Fe3-xCoxTi3O21, when substituting excessive Co. Existence of the nanoscale structural modulation (NSM), occurred at the boundaries when changing the material gradually from the originallydesigned six-layer nanoscale architectures down to five and then four when increasing the Co content, induces a previously unidentified 'analogous' morphotropic transformation (AMT) effect. The AMT’s net contribution to the enhanced room temperature intrinsic multiferroic properties had been confirmed by quantifying and deducting the contribution from the existed impurity phase using the Derivative thermomagneto-gravimetry measurements (DTMG). Significantly, this new AMT effect may be caused by a possible coupling contribution from the co-existed NSM phases, indicating a potential way to realize the room temperature functioning multiferroic materials.
Introduction Except BiFeO3 which is unfortunately antiferromagnetic (AFM) at the room temperature (RT)1, a single-phase multiferroic material that could present a noticeable magnetoelectric (ME) coupling at the RT is still hardly to be available. Single-phase multiferroic materials are important for potential applications in multi-state memory, field sensing, quantum controlling devices and so on2-7. Recently, layer-structured bismuth-containing Aurivillius complex oxides8, 9 have been investigated as one of the candidates potentially able to become a single-phase multiferroic material exhibiting a simultaneous co-existence of ferroelectric (FE) and ferromagnetic (FM) orders10. However, intrinsic multiferroics directly from the single phase in the material system have not been proven so far. Normally such layered Aurivillius complex oxides, which can be in a general formula of Bin+1Fen-3Ti3O3n+3 (here n denotes an integer corresponding to the number of the sandwiched perovskite layers), are alternatively stacked with fluorite-type (Bi2O2)2+ layers and perovskite-type (Bin-1Fen-3Ti3O3n+1)2- slabs11. Previous researches show that such materials in common could present unusual dielectric behaviours, much elevated Curie temperature, and small yet detectable FM occurring usually at temperatures far below the RT, indicating for such materials a fairly large distance away from practical applications in reality10, 12, 13. Fortunately, a ground-breaking work was performed in the four-layer Aurivillius compound of Bi5Fe0.5Co0.5Ti3O15 (n = 4) that is as a simple building block in the entire layered material system, in which a new way of partially cobalt (Co)-substitution This journal is © The Royal Society of Chemistry [year]
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in iron (Fe) sites of the nanoscale structural units was adopted and a remarkable coexistence of FE and FM at the RT was found, owing to a possible exchanging among oxygen-connected Fe and Co ions and to the realigned spin structure caused by the substitution14-16. This phenomenon was further verified in a subsequent study in a five-layer Bi6Fe2-xCoxTi3O18 (n = 5) thin films17, which testified the effectiveness of the Fe/Co cosubstitution method. However, FM in two studied materials is still weak at the RT. Apparently, along this direction future questions will undoubtedly point to finding new ways able to boast FE, FM and then most importantly ME coupling at or even well above the RT, aiming directly at practical applications. Straightforwardly for researchers, therefore, it should be reasonable to look into new Aurivillius systems with different layer numbers, using various A- or B-site modifications or codoping of different magnetic ions including Fe3+, Co3+, Mn3+, Ni3+, et al18-21, developing new material theories able to understand the resulting enhancement or coupling mechanisms, understanding the impurity phase contribution, and finally, finding new techniques able to further enhance the ME interaction at least at the RT. Normally, morphotropic phase boundary (MPB) is defined to separate rhombohedral (R) and tetragonal (T) regions in a phase diagram of two dielectric phases. It was found that compositions at MPB could exhibit much enhanced polarization-related properties, including dielectric, piezoelectric, and electro-optic (EO), etc., owing to the easy switching of spontaneous polarizations between the co-existed and coupled phases at [journal], [year], [vol], 00–00 | 1
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MPB22, 23. Well-known past examples of utilizing the MPB effect include PbZrxTi1-xO3 as a piezoelectric ceramics24, lanthanummodified (1-x)[Pb(Mg1/3Nb2/3)O3]-x[PbTiO3] (PMN-PT) as a transparent EO ceramics25, 26, and an alternative PMN-PT R/T phase stacking as an enhanced EO single crystalline film27. In the layered Aurivillius materials to be discussed here, that to answer the question of what will happen if two phases with different layer numbers structurally modulated together at the nanoscale will be very instructive. If the nanoscale structural modulation (NSM) happens and if it functions well, it would be reasonably to refer this as a new type of morphotropic transformation, 'analogous' to the before-mentioned MPB (AMT). Hence, in light of above discussions, in this work we investigated the six-layer layer-structured Bi7Fe3-xCoxTi3O21 (BFCT-x, 0 x 1.0) system by excessively increasing the Co content. We observed a gradual layer number reduction as increasing the Co content, and an apparent AMT effect at the transformation boundaries mainly caused by the existence of the NSM phases, when carefully investigating their spontaneous polarization and magnetization properties at the RT. Allowed by fully deducting the impurity phase’s contribution, we concluded that this intrinsic AMT/NSM effect could indeed become an alternative way to enhance the multiferroic properties at the RT.
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Fabrication of the BFCT-x samples followed the procedures of pre-sintering for powders and then hot-press sintering of the pellets. X-ray diffraction (XRD) spectra of the BFCT-x powders pre-sintered at 1023 K and the BFCT-x samples hot-pressed at 1153 K are shown in Fig. 1a and b, respectively. In XRD spectra for the pre-sintered powders, all diffraction peaks were identified belonging to the Aurivillius phase when x < 0.3. When x 0.3, additional peaks originated from the BiFe1-CoO3 ( = 0.3~0.5) secondary phase are observed (in Fig. 1a), which could be referred from the transient phase between tetragonal and cubic phases in Bi-Fe-Co-O phase diagram28. With increasing the Co concentration, characteristic peak intensity of the secondary BiFe-Co-O phase increases, indicates the increase of its relative quantity. This Bi-Fe-Co-O phase was reported to only exist in BiCo1-xFexO3 when x is from 0.5 to 0.7 (the ratio of iron and cobalt is approximately 2:1), and will decompose above 1123 K. Therefore, our subsequent hot-press sintering at 1153 K will decompose this intermediate Bi-Fe-Co-O phase, most probably into Bi2O3 (fully evaporated at this temperature due to its lower melting point at 1090 K) and CoFe2O4 spinel phase as the residuals.
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Polycrystalline BFCT-x powders were prepared by the Pechini’s method. Procedures of the preparation were as follows: Firstly, appropriate amounts of Ti(OC4H9)4, Bi(NO3)3•5H2O, Fe(NO3)3•9H2O and Co(CH3COO)2•6H2O were dissolved into HNO3 as precursors. C6H8O7•H2O and C10H16N2O8 were then added to the solution in molar ratios of 1:0.7:1 with respect to the metal ions. Secondly, NH4OH was dropped into above solution to reach a pH = 6~7, then the precursors were heated until combusted inside the crucible. Thirdly, the formed gels were preburned at 1023 K for 2 h in order to remove organic residues. Finally, the obtained powders were cold-pressed into pellets and then hot-pressed at 1153 K for 3 h, in oxygen and argon mixture atmosphere (O2/Ar = 1/4) under a pressure of 12.56 MPa. After that, pellets of the samples were cooled slowly down to the RT. The crystalline structures were investigated by powder X-Ray diffraction (XRD) with the Cu-Kα radiation (D/Max-gA, Japan). The high-resolution transmission electron microscopy (HRTEM) images were obtained using a field emission electron microscope (JEOL JEM-2010, Japan). Magnetic properties of the resulting samples were characterized by a vibrating samples magnetometer (VSM) (EV-7, ADE Co., USA). Derivative thermo-magnetogravimetric (DTMG) measurements are realized by Thermogravimetric Analysis and by applying a magnetic field of 200 Oe (TGA Q5000IR, USA). For ferroelectric measurements, the pellets are polished to a thickness of about 0.2 mm and then Ag was evaporated on both sides as electrodes, the measurements were conducted using a Precision LC ferroelectric analyzer (Radiant Technology product, USA). Dielectric measurements were measured using a dielectric spectrometer at the RT (Novocontrol Technologies, Germany).
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Fig. 1 (a) X-ray diffraction patterns of BFCT-x powders pre-sintered at 1023 K. BiFe1-CoO3 ( = 0.3~0.5) were specifically synthesized at 1023 K for reference. This impurity phase is marked by “”. (b) X-ray diffraction patterns of BFCT-x ceramics hot-pressed at 1153 K, for 3 h in oxygen and argon mixture atmosphere (O2/Ar = 1/4) under a pressure of 12.56 MPa.
In Figure 1b, qualitative XRD analyses support the expected Aurivillius structure in all samples. Yet, some large peak position shifts were observed when increasing the Co content. This may imply that the Co substitution in Bi7Fe3Ti3O21 (n = 6 as originally designed) could lead to a reduction in layer number29. The Rietveld refinements of these XRD patterns are shown in This journal is © The Royal Society of Chemistry [year]
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supplementary Fig. S1. Those spectra were refined in the orthorhombic lattice with a space group of F2mm (x = 0.3), F2mm (x = 0.5) and A21am (x = 0.75 and 1.0), respectively. The refinement suggests that the observed XRD peaks could be well indexed in accordance with the standard powder diffraction data from the six-layer Bi7Fe3Ti3O21 (JCPDS, No.54-1044)30 when x 0.3 and from the four-layer Bi5FeTi3O15 (JCPDS, No.82-0063) when 0.75 x 1.0. When 0.4 x 0.7, the XRD peaks match well to those from the five-layer Bi6Fe2-xCoxTi3O1817. The c-axis lattice parameters are then identified to be 5.74 nm for x = 0.3,
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4.93 nm for x = 0.5, 4.11 nm for x = 0.75 and 4.11 nm for x = 1.0, respectively. Detailed indexing of the peaks is shown in supplementary Fig. S2. As a result, our refinement results greatly support the layer-number reduction suggestion. Importantly, the refinement results also suggest an existence of minor CoFe2O4 with a space group of Fd-3m in the samples with x 0.75. For BFCT-0.75 and BFCT-1.0, their impurity phase amount, according to the refinements, is below the XRD instrumental resolution.
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Fig. 2 HRTEM images of those samples: (a) BFCT-0.3, (b) BFCT-0.5, (c) BFCT-0.7 and (d) BFCT-1.0; (e-h) Magnified HRTEM images of blue boxes in “(a-d)”.
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HRTEM images and magnified HRTEM images of BFCT-x (x = 0.3, 0.5, 0.7, 1.0) samples are shown in Fig. 2a-d and Fig. 2eh, respectively, in which bright spots represent the location of bismuth atoms. The Aurivillius microstructure can be well identified by the sandwiched perovskite layers30. c-axis can be measured by the lattice periodicity, which is approximately to be 5.76 nm for x = 0.3, 5.07 nm for x = 0.5, 5.04 nm for x = 0.7 and 4.19 nm for x = 1.0, respectively. These values are consistent with the above XRD and the Rietveld refinement results, confirming the perovskite layer number reduction, when increasing the Co content. Most importantly, this co-existence of two phases can be clearly seen in HRTEM images of x = 0.4 and x = 0.75 samples. The former indicates the six- and five-layer coexistence, and the latter presents the five- and four-layer coexistence, as shown in Fig. 3a and b, respectively. Note that here the co-existence is actually a NSM and is not simply a mixture of two phases with different layer numbers. Hence, according to the XRD and HRTEM results, schematic illustration of the structure evolution of BFCT-x with an increasing in Co content in BFCT-x is assumed in Fig. 4.
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Fig. 3 HRTEM images of (a) BFCT-0.4 and (b) BFCT-0.75. The red “4”, “5” and “6” stand for the four-, five- and six-layer perovskite structures, respectively, indicating the modulated phases at the nanoscale (NSM).
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Fig. 4 Schematic illustration of the evolution route in the BFCT-x fabrication
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P-E loops of BFCT-x samples at 100 Hz and the RT are shown in supplementary Fig. S3, and dependence of lattice constant on the Co content x, the pulsed remanent polarization (P = switched polarization - non-switched polarization) at 100 Hz and 120 kV/cm, and dielectric constant (’) with the frequency of 1 kHz are shown in Fig. 5a-c, respectively, all measured at the RT. Variation of the lattice constant as x is distinct in phases with six-, five-, and four-layer numbers. For six- and five-layer structures, the closeness of a and b supports the unchanged F2mm space group, and a further split of the two in the four-layer structure supports the F2mm to A21am phase transition (in Fig. 5a). Changing of P-E loops with x is consisted with the variation of lattice constants a and b, indicating that the polarization was originated from the structural distortion. When x 0.75, P has improved significantly, comparing with those in x < 0.75 (in Fig. 5b), and this may be attributed to the phase transition of F2mmA21am. That P is different with the remanent polarizations (2Pr) from the P-E Loops implies a leakage current contribution, which is also clearly seen in P-E Loops. Very importantly, P and dielectric anomalies are clearly observable at x = 0.4 and 0.75 boundaries (in Fig. 5b and c). This definitely relates to the changing of layer number inside their Aurivillius microstructure, which leads to a co-existence of two phases with two layer numbers. HRTEM images show that this co-existence is a NSM type, rather than a simple two-phase mixture. Coupling between modulated structures was used to enhance the EO effect via the modulated spontaneous polarization switching27. Therefore it is reasonable to assume a similar enhancement to FE and dielectric properties, because that the nanoscale modulated structures can be tightly correlated to both ferroelectric domain and space charge distribution. Hysteresis loops of magnetization (M) in BFCT-x (x 0) ceramics at the RT are shown in supplementary Fig. S4. The linear field dependence of M in BFCT-0 indicates its AFM nature, as shown in supplementary Fig. S5, similar to those previously obtained results13, 20. However, plots of magnetic moment vs. H for rest of the samples with a partial Co substitution clearly indicate the presence of a large FM moment at the RT which is consistent with the previous studies too14, 17. Hence, it will be natural to infer that the main origin of the enhancement in FM in This journal is © The Royal Society of Chemistry [year]
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the BFCT-x samples (x ≠ 0) should be similar to that in previously studied Bi5Fe0.5Co0.5Ti3O15 (n = 4)14 and in Bi6Fe217 xCoxTi3O18 (n = 5) . The proposed mechanisms mainly suggested that FM originates from the exchanging interaction among Fe3+ and Co3+ or Fe3+ and Co2+ via bonding with oxygen and the spin canting via anti-symmetric Dzyaloshinskii-Moriya (DM) interaction, and the mechanisms should be intrinsic to the Cosubstituted layer structures31, 32.
Fig. 5 The doping level of Co (x) dependence of (a) lattice constants, (b) remanent polarizations (2Pr) from the P-E Loops and the pulsed remanent polarization P at 100 Hz and 120 kV/cm, and (c) dielectric constant (’) and dielectric loss (tan) at the RT with the frequency of 1
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kHz. The colour wide background lines stand for the area that could have the co-existed NSM phases.
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Fig.6 The doping level of Co (x) dependence of (a) remnant magnetization (2Mr) and (b) saturation magnetization (Ms) of BFCT-x. Open green circle is the experimental data of the samples. According to our DTMG experimental results, CoFe2O4 phase does not exist in the samples (x < 0.4), almost linearly increasing from 0.5 wt% at x = 0.4 to 3.6 wt% at x = 0.75, and then stay closely stable at 3.6 wt% in the samples (x=0.9 and 1.0). Solid blue ball is the final values of 2Mr or Ms after deducting the corresponding contribution of CoFe2O4 from the experimental data. The red line is error bar caused by calculation (0.5 wt%). The color wide background lines stand for the area that could have the co-existed NSM phases.
However, more information can be derived from the dependence of remnant magnetization (2Mr) and saturation magnetization (Ms) of the BFCT-x samples on the Co content, which was plotted using open green circle in Fig. 6a and b. For 0 < x < 0.75, 2Mr and Ms increase as the Co content increases, except a small enhancement occurring at the x~0.4 boundary. A sharp enhancement happens to the x~0.75 boundary too. After the enhancement peak, 2Mr and Ms reduce as the Co content increases. At least two contributions should account for the magnetization: 1) from the impurity phase of CoFe2O4; 2) intrinsically from the layered materials themselves. To fully interpret the variation of 2Mr and Ms, the DTMG33 was used to measure the dependence of a sample’s weight under a magnetic field over a varying temperature range. For samples BFCT-x (x = 0.3~1.0), CoFe2O4 and ZrO2-13wt%CoFe2O4 (the last two samples were specifically synthesized for the quantitative estimation), the results are in Fig. 7. For pure CoFe2O4 its FM-toparamagnetic (PM) transition peak occurs at a temperature ~720 K in the derivative weight (dW/dT) curve, and this peak can be seen in samples BFCT-x (x = 0.4~0.75), confirming the existence of the impurity phase, though it is minor. Also for BFCT-0.3, BFCT-0.5, BFCT-0.6, BFCT-0.7, occurring of only one peak at the temperature around 755.1 K, 741.5 K, 731.7 K and 697.5 K, respectively, indicates a net contribution from the FM-to-PM phase transition from the only existed six-, five-, five- and fiveThis journal is © The Royal Society of Chemistry [year]
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layer phase. For BFCT-0.4 and BFCT-0.75, two additional peaks appear at 748.4 K and 739.6 K, and 688.1 K and 712.6 K, respectively. The two peaks are coming from the co-existed and nanoscale structurally modulated five- and six-layer phases in BFCT-0.4 and four- and five-layer phases in BFCT-0.75. For BFCT-x (x > 0.75), one peak at around 680.6 K for BFCT-0.9 and 677 K for BFCT-1.0 diverged from the pure CoFe2O4 peak position is coming from the impurity spinel phase Co 1+Fe2-O4, causing the further lowered transition temperature by the increase in Co content34, 35. The other peak occurring at 651.4 K for BFCT-0.9 and 649 K for BFCT-1.0 indicates the net contribution from the four-layer phase. In short, the analysis of DTMG measurements is very consisted with above XRD and HRTEM results and further proves the intrinsic magnetism from the Aurivillius phase. According to the dW/dT curves from pure CoFe2O4 and the predesigned ZrO2-13wt% CoFe2O4 (in Fig. 7), and the resolution of the DTMG measurement (the accuracy of the measurement experimentally determined is approximately 0.5wt%), relative quantities of CoFe2O4 impurity in BFCT-x samples can be accordingly estimated through curve fitting, as shown in supplementary Fig. S6. According to above DTMG experimental results, FM contributions from the intrinsic layered materials themselves can be got by considering the measured pure CoFe2O4’s 2Mr = 53 emu/g and Ms = 79.5 emu/g36 and its relative content, then deducting the contribution of the CoFe2O4 impurity phase from the experimental data. Solid blue ball signed in Fig. 6a and b is the final FM results after the impurity deduction. This net FM enhancement should be intrinsic to the Fe and Co co-substituted layered structures, explainable by the previous exchanging and DM mechanisms. Interestingly, the intrinsic FM response significantly increases from four- to five-layer phases (about four-hundred times higher), and it becomes flat when from five- to six-layer. Four-layer BFCT-x (x 0.9) have intrinsic 2Mr and Ms close to those previously reported in the four-layer Bi5Fe0.5Co0.5Ti3O15 (2Mr = 7.8 memu/g)14. The five-layer samples, BFCT-0.5, BFCT-0.6 and BFCT-0.7, have much enhanced intrinsic FM response when comparing to the four-layer samples, and the average FM from the three is similar to the previously reported in Bi6FeCoTi3O18 (2Mr = 0.5 emu/g)37. This similarity actually testifies the accuracy of the way used to deduct the CoFe2O4 impurity phase contribution. Inside the range of each single phase, variation of the intrinsic FM enhancement can be explained by the increasing in Co content which changes the relative Fe/Co ratio. This again confirms the significance of the Fe/Co co-substitution technique used to boost the intrinsic multiferroic properties at the RT, as firstly suggested in Bi5Fe0.5Co0.5Ti3O1514. Two FM anomalies at x = 0.4 and x = 0.75 boundaries were also clearly observed in Fig. 6a and b, in which the later one is more significant. This definitely also relates to the changing of layer number inside their nanoscale Aurivillius structure, which leads to a NSM type with two layer numbers. This NSM type is tightly correlated to both exchanging among oxygen-bonded magnetic ions and the spin canting. The space group corresponding to six-, five- and four-layer structures is F2mm, F2mm and A21am, respectively. Layer number reduction across the second AMT boundary accompanies a nanoscale structural
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multiferroic enhancement was attributed to the new AMT effect when selecting Co contents close to the two AMT boundaries. The AMT/NSM modulation may become an alternative new way to boost the RT multiferroic properties. Recently, similar NSM phenomena were also found in others high layer-structural Aurivillius phases in our group, and this is a continuing effort and the results will be published separately.
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We thank the financially support from the National Basic Research Program of China (2012CB922001) and the Natural Science Foundation of China (51072193). We are grateful to X. Y. Mao and Prof. X. B. Chen (Yangzhou University) and Y. W. Ding (USTC) for discussions of the measurements. S. J. Sun acknowledges fellowship support from the college of Physical Science and Technology, Yangzhou University. The work is also partially supported by Defence Threat Reduction Agency under grants HDTRA1-10-1-0001 and HDTRA122221.
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Notes and references a
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CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China b Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China c Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China d Laser Optics Research Center, US Air Force Academy, Colorado 80840, USA * Corresponding Authors: [email protected],.
1. 2. 3. 4. 5. 6. 7.
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Fig. 7 Weight loss and DTMG curves of BFCT-x (x = 0.3~1.0), CoFe2O4 and ZrO2-13wt%CoFe2O4. The DTMG measurement was performed in the same conditions of nitrogen atmosphere at a heating rate of 20 K/min with a 200 Oe applied magnetic field. Under the magnetic field, weight loss from the contribution of thermal active has been automatically deducted.
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In summary, using the Aurivillius BFCT-x, the NSM phases were confirmed by XRD measurements, Rietveld refinements, HRTEM, and DTMG. Enhanced intrinsic multiferroic properties from the NSM phases had been proved by quantifying and deducting the contribution from the existed impurity phase. This 6 | Journal Name, [year], [vol], 00–00
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M. M. Shirolkar, C. Hao, X. Dong, T. Guo, L. Zhang, M. Li and H. Wang, Nanoscale, 2014, 6, 4735. H. Schmid, Ferroelectrics, 1994, 162, 317-338. N. A. Hill, J. Phys. Chem. B, 2000, 104, 6694-6709. M. Fiebig, J. Phys. D:Appl. Phys., 2005, 38, R123-R152. N. A. Spaldin and M. Fiebig, Science, 2005, 309, 391-392. W. Eerenstein, N. D. Mathur and J. F. Scott, Nature, 2006, 442, 759765. N. A. Spaldin, S. W. Cheong and R. Ramesh, Phys. Today, 2010, 63, 38-43. B. Aurivillius, Arki. Kemi, 1948, 26, 1-6. B. Aurivillius, Arki. Kemi, 1950, 1, 463-480. R. S. Singh, T. Bhimasankaram, G. S. Kumar and S. V. Suryanarayana, Solid State Commun., 1994, 91, 567-569. Ismailza.Ig, Nesteren.Vi, F. A. Mirishli and P. G. Rustamov, Soviet Phys. Crystal., 1967, 12, 400-&. S. K. Kim, M. Miyayama and H. Yanagida, Mater. Res. Bull., 1996, 31, 121-131. A. Srinivas, M. M. Kumar, S. V. Suryanarayana and T. Bhimasankaram, Mater. Res. Bull., 1999, 34, 989-996. X. Mao, W. Wang, X. Chen and Y. Lu, Appl. Phys. Lett., 2009, 95, 082901. X. Mao, H. Sun, W. Wang, Y. Lu and X. Chen, Solid State Commun., 2012, 152, 483-487.
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phase transition from F2mm to A21am, additional to the simple structural modulation within the same space group at the first boundary. This difference may explain the significance of the second boundary. Overall, the results here at the boundaries are important due to three reasons: 1) the 2Mr is at the emu/g level, which is indeed very large in existing single phase multiferroics; 2) it occurs remarkably at the RT; 3) co-existence of the NSM phases is technically feasible (here via Co doping, for example), and it is also reasonable to refer them as a new multiferroics.
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16. X. Mao, W. Wang, H. Sun, Y. Lu and X. Chen, Integ. Ferroelectrics, 2012, 132, 16-21. 17. Z. Liu, J. Yang, X. W. Tang, L. H. Yin, X. B. Zhu, J. M. Dai and Y. P. Sun, Appl. Phys. Lett., 2012, 101, 122402. 18. B. Yuan, J. Yang, J. Chen, X. Z. Zuo, L. H. Yin, X. W. Tang, X. B. Zhu, J. M. Dai, W. H. Song and Y. P. Sun, Appl. Phys. Lett., 2014, 104, 062413. 19. Z. Lei, Y. Huang, M. Liu, W. Ge, Y. Ling, R. Peng, X. Mao, X. Chen and Y. Lu, J. Alloys Compd., 2014, 600, 168-171. 20. S. Sun, Y. Ling, R. Peng, M. Liu, X. Mao, X. Chen, R. J. Knize and Y. Lu, RSC Adv., 2013, 3, 18567-18572. 21. X. Mao, H. Sun, W. Wang, X. Chen and Y. Lu, Appl. Phys. Lett., 2013, 102, 072904. 22. M. Ahart, M. Somayazulu, R. E. Cohen, P. Ganesh, P. Dera, H.-K. Mao, R. J. Hemley, Y. Ren, P. Liermann and Z. Wu, Nature, 2008, 451, 545-U542. 23. R. J. Zeches, M. D. Rossell, J. X. Zhang, A. J. Hatt, Q. He, C. H. Yang, A. Kumar, C. H. Wang, A. Melville, C. Adamo, G. Sheng, Y. H. Chu, J. F. Ihlefeld, R. Erni, C. Ederer, V. Gopalan, L. Q. Chen, D. G. Schlom, N. A. Spaldin, L. W. Martin and R. Ramesh, Science, 2009, 326, 977-980. 24. R. Guo, L. E. Cross, S. E. Park, B. Noheda, D. E. Cox and G. Shirane, Phys. Rev. Lett., 2000, 84, 5423-5426. 25. Y. L. Lu, G. H. Jin, M. Cronin-Golomb, S. W. Liu, H. Jiang, F. L. Wang, J. Zhao, S. Q. Wang and A. J. Drehman, Appl. Phys. Lett., 1998, 72, 2927-2929. 26. J. J. Zheng, Y. L. Lu, X. S. Chen, M. Cronin-Golomb and J. Zhao, Appl. Phys. Lett., 1999, 75, 3470-3472. 27. Y. L. Lu, J. J. Zheng, M. C. Golomb, F. L. Wang, H. A. Jiang and J. Zhao, Appl. Phy. Lett., 1999, 74, 3764-3766. 28. M. Azuma, S. Niitaka, N. Hayashi, K. Oka, M. Takano, H. Funakubo and Y. Shimakawa, Jpn. J. Appl. Phys., 2008, 47, 7579-7581. 29. M. I. Morozov and V. V. Gusarov, Inorg. Mater., 2002, 38, 723-729. 30. M. Krzhizhanovskaya, S. Filatov, V. Gusarov, P. Paufler, R. Bubnova, M. Morozov and D. C. Meyer, Z. Anorg. Allg. Chem., 2005, 631, 1603-1608. 31. I. Dzyaloshinsky, J. Phys. Chem. Solids, 1958, 4, 241-255. 32. D. N. Astrov, Soviet Phys. Jetp-Ussr, 1960, 11, 708-709. 33. R. Moskalewicz and W. Zych, Phys. Status Solidi A, 1986, 97, K43K48. 34. I. P. Muthuselvam and R. N. Bhowmik, Solid State Sci., 2009, 11, 719-725. 35. S. R. Liu, D. H. Ji, J. Xu, Z. Z. Li, G. D. Tang, R. R. Bian, W. H. Qi, Z. F. Shang and X. Y. Zhang, J. Alloys Compd., 2013, 581, 616-624. 36. X. M. Liu, S. Y. Fu and L. P. Zhu, J. Solid State Chem., 2007, 180, 461-466. 37. J. Yang, L. H. Yin, Z. Liu, X. B. Zhu, W. H. Song, J. M. Dai, Z. R. Yang and Y. P. Sun, Appl. Phys. Lett., 2012, 101, 012402.
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Nanoscale structural modulation and the enhanced room-temperature multiferroic properties Shujie Sun,a Yan Huang,a Guopeng Wang,a Jianlin Wang,a Zhengping Fu,ac Ranran Peng,*ac Randy J. Knized and Yalin Lu*abcd 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Availability of a single-phase and room temperature functional multiferroic material is still a big challenge though it is very important to both fundamental physics and application development. Layered Aurivillius oxide materials, one of the most promising candidates, recently have been attracting considerable interests. In this work, we focused on investigating the nanoscale structural evolution of the six-layer Bi7Fe3-xCoxTi3O21, when substituting excessive Co. Existence of the nanoscale structural modulation (NSM), occurred at the boundaries when changing the material gradually from the originallydesigned six-layer nanoscale architectures down to five and then four when increasing the Co content, induces a previously unidentified 'analogous' morphotropic transformation (AMT) effect. The AMT’s net contribution to the enhanced room temperature intrinsic multiferroic properties had been confirmed by quantifying and deducting the contribution from the existed impurity phase using the Derivative thermomagneto-gravimetry measurements (DTMG). Significantly, this new AMT effect may be caused by a possible coupling contribution from the co-existed NSM phases, indicating a potential way to realize the room temperature functioning multiferroic materials.
Introduction Except BiFeO3 which is unfortunately antiferromagnetic (AFM) at the room temperature (RT)1, a single-phase multiferroic material that could present a noticeable magnetoelectric (ME) coupling at the RT is still hardly to be available. Single-phase multiferroic materials are important for potential applications in multi-state memory, field sensing, quantum controlling devices and so on2-7. Recently, layer-structured bismuth-containing Aurivillius complex oxides8, 9 have been investigated as one of the candidates potentially able to become a single-phase multiferroic material exhibiting a simultaneous co-existence of ferroelectric (FE) and ferromagnetic (FM) orders10. However, intrinsic multiferroics directly from the single phase in the material system have not been proven so far. Normally such layered Aurivillius complex oxides, which can be in a general formula of Bin+1Fen-3Ti3O3n+3 (here n denotes an integer corresponding to the number of the sandwiched perovskite layers), are alternatively stacked with fluorite-type (Bi2O2)2+ layers and perovskite-type (Bin-1Fen-3Ti3O3n+1)2- slabs11. Previous researches show that such materials in common could present unusual dielectric behaviours, much elevated Curie temperature, and small yet detectable FM occurring usually at temperatures far below the RT, indicating for such materials a fairly large distance away from practical applications in reality10, 12, 13. Fortunately, a ground-breaking work was performed in the four-layer Aurivillius compound of Bi5Fe0.5Co0.5Ti3O15 (n = 4) that is as a simple building block in the entire layered material system, in which a new way of partially cobalt (Co)-substitution This journal is © The Royal Society of Chemistry [year]
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in iron (Fe) sites of the nanoscale structural units was adopted and a remarkable coexistence of FE and FM at the RT was found, owing to a possible exchanging among oxygen-connected Fe and Co ions and to the realigned spin structure caused by the substitution14-16. This phenomenon was further verified in a subsequent study in a five-layer Bi6Fe2-xCoxTi3O18 (n = 5) thin films17, which testified the effectiveness of the Fe/Co cosubstitution method. However, FM in two studied materials is still weak at the RT. Apparently, along this direction future questions will undoubtedly point to finding new ways able to boast FE, FM and then most importantly ME coupling at or even well above the RT, aiming directly at practical applications. Straightforwardly for researchers, therefore, it should be reasonable to look into new Aurivillius systems with different layer numbers, using various A- or B-site modifications or codoping of different magnetic ions including Fe3+, Co3+, Mn3+, Ni3+, et al18-21, developing new material theories able to understand the resulting enhancement or coupling mechanisms, understanding the impurity phase contribution, and finally, finding new techniques able to further enhance the ME interaction at least at the RT. Normally, morphotropic phase boundary (MPB) is defined to separate rhombohedral (R) and tetragonal (T) regions in a phase diagram of two dielectric phases. It was found that compositions at MPB could exhibit much enhanced polarization-related properties, including dielectric, piezoelectric, and electro-optic (EO), etc., owing to the easy switching of spontaneous polarizations between the co-existed and coupled phases at [journal], [year], [vol], 00–00 | 1
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MPB22, 23. Well-known past examples of utilizing the MPB effect include PbZrxTi1-xO3 as a piezoelectric ceramics24, lanthanummodified (1-x)[Pb(Mg1/3Nb2/3)O3]-x[PbTiO3] (PMN-PT) as a transparent EO ceramics25, 26, and an alternative PMN-PT R/T phase stacking as an enhanced EO single crystalline film27. In the layered Aurivillius materials to be discussed here, that to answer the question of what will happen if two phases with different layer numbers structurally modulated together at the nanoscale will be very instructive. If the nanoscale structural modulation (NSM) happens and if it functions well, it would be reasonably to refer this as a new type of morphotropic transformation, 'analogous' to the before-mentioned MPB (AMT). Hence, in light of above discussions, in this work we investigated the six-layer layer-structured Bi7Fe3-xCoxTi3O21 (BFCT-x, 0 x 1.0) system by excessively increasing the Co content. We observed a gradual layer number reduction as increasing the Co content, and an apparent AMT effect at the transformation boundaries mainly caused by the existence of the NSM phases, when carefully investigating their spontaneous polarization and magnetization properties at the RT. Allowed by fully deducting the impurity phase’s contribution, we concluded that this intrinsic AMT/NSM effect could indeed become an alternative way to enhance the multiferroic properties at the RT.
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Fabrication of the BFCT-x samples followed the procedures of pre-sintering for powders and then hot-press sintering of the pellets. X-ray diffraction (XRD) spectra of the BFCT-x powders pre-sintered at 1023 K and the BFCT-x samples hot-pressed at 1153 K are shown in Fig. 1a and b, respectively. In XRD spectra for the pre-sintered powders, all diffraction peaks were identified belonging to the Aurivillius phase when x < 0.3. When x 0.3, additional peaks originated from the BiFe1-CoO3 ( = 0.3~0.5) secondary phase are observed (in Fig. 1a), which could be referred from the transient phase between tetragonal and cubic phases in Bi-Fe-Co-O phase diagram28. With increasing the Co concentration, characteristic peak intensity of the secondary BiFe-Co-O phase increases, indicates the increase of its relative quantity. This Bi-Fe-Co-O phase was reported to only exist in BiCo1-xFexO3 when x is from 0.5 to 0.7 (the ratio of iron and cobalt is approximately 2:1), and will decompose above 1123 K. Therefore, our subsequent hot-press sintering at 1153 K will decompose this intermediate Bi-Fe-Co-O phase, most probably into Bi2O3 (fully evaporated at this temperature due to its lower melting point at 1090 K) and CoFe2O4 spinel phase as the residuals.
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Polycrystalline BFCT-x powders were prepared by the Pechini’s method. Procedures of the preparation were as follows: Firstly, appropriate amounts of Ti(OC4H9)4, Bi(NO3)3•5H2O, Fe(NO3)3•9H2O and Co(CH3COO)2•6H2O were dissolved into HNO3 as precursors. C6H8O7•H2O and C10H16N2O8 were then added to the solution in molar ratios of 1:0.7:1 with respect to the metal ions. Secondly, NH4OH was dropped into above solution to reach a pH = 6~7, then the precursors were heated until combusted inside the crucible. Thirdly, the formed gels were preburned at 1023 K for 2 h in order to remove organic residues. Finally, the obtained powders were cold-pressed into pellets and then hot-pressed at 1153 K for 3 h, in oxygen and argon mixture atmosphere (O2/Ar = 1/4) under a pressure of 12.56 MPa. After that, pellets of the samples were cooled slowly down to the RT. The crystalline structures were investigated by powder X-Ray diffraction (XRD) with the Cu-Kα radiation (D/Max-gA, Japan). The high-resolution transmission electron microscopy (HRTEM) images were obtained using a field emission electron microscope (JEOL JEM-2010, Japan). Magnetic properties of the resulting samples were characterized by a vibrating samples magnetometer (VSM) (EV-7, ADE Co., USA). Derivative thermo-magnetogravimetric (DTMG) measurements are realized by Thermogravimetric Analysis and by applying a magnetic field of 200 Oe (TGA Q5000IR, USA). For ferroelectric measurements, the pellets are polished to a thickness of about 0.2 mm and then Ag was evaporated on both sides as electrodes, the measurements were conducted using a Precision LC ferroelectric analyzer (Radiant Technology product, USA). Dielectric measurements were measured using a dielectric spectrometer at the RT (Novocontrol Technologies, Germany).
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Fig. 1 (a) X-ray diffraction patterns of BFCT-x powders pre-sintered at 1023 K. BiFe1-CoO3 ( = 0.3~0.5) were specifically synthesized at 1023 K for reference. This impurity phase is marked by “”. (b) X-ray diffraction patterns of BFCT-x ceramics hot-pressed at 1153 K, for 3 h in oxygen and argon mixture atmosphere (O2/Ar = 1/4) under a pressure of 12.56 MPa.
In Figure 1b, qualitative XRD analyses support the expected Aurivillius structure in all samples. Yet, some large peak position shifts were observed when increasing the Co content. This may imply that the Co substitution in Bi7Fe3Ti3O21 (n = 6 as originally designed) could lead to a reduction in layer number29. The Rietveld refinements of these XRD patterns are shown in This journal is © The Royal Society of Chemistry [year]
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supplementary Fig. S1. Those spectra were refined in the orthorhombic lattice with a space group of F2mm (x = 0.3), F2mm (x = 0.5) and A21am (x = 0.75 and 1.0), respectively. The refinement suggests that the observed XRD peaks could be well indexed in accordance with the standard powder diffraction data from the six-layer Bi7Fe3Ti3O21 (JCPDS, No.54-1044)30 when x 0.3 and from the four-layer Bi5FeTi3O15 (JCPDS, No.82-0063) when 0.75 x 1.0. When 0.4 x 0.7, the XRD peaks match well to those from the five-layer Bi6Fe2-xCoxTi3O1817. The c-axis lattice parameters are then identified to be 5.74 nm for x = 0.3,
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4.93 nm for x = 0.5, 4.11 nm for x = 0.75 and 4.11 nm for x = 1.0, respectively. Detailed indexing of the peaks is shown in supplementary Fig. S2. As a result, our refinement results greatly support the layer-number reduction suggestion. Importantly, the refinement results also suggest an existence of minor CoFe2O4 with a space group of Fd-3m in the samples with x 0.75. For BFCT-0.75 and BFCT-1.0, their impurity phase amount, according to the refinements, is below the XRD instrumental resolution.
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Fig. 2 HRTEM images of those samples: (a) BFCT-0.3, (b) BFCT-0.5, (c) BFCT-0.7 and (d) BFCT-1.0; (e-h) Magnified HRTEM images of blue boxes in “(a-d)”.
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HRTEM images and magnified HRTEM images of BFCT-x (x = 0.3, 0.5, 0.7, 1.0) samples are shown in Fig. 2a-d and Fig. 2eh, respectively, in which bright spots represent the location of bismuth atoms. The Aurivillius microstructure can be well identified by the sandwiched perovskite layers30. c-axis can be measured by the lattice periodicity, which is approximately to be 5.76 nm for x = 0.3, 5.07 nm for x = 0.5, 5.04 nm for x = 0.7 and 4.19 nm for x = 1.0, respectively. These values are consistent with the above XRD and the Rietveld refinement results, confirming the perovskite layer number reduction, when increasing the Co content. Most importantly, this co-existence of two phases can be clearly seen in HRTEM images of x = 0.4 and x = 0.75 samples. The former indicates the six- and five-layer coexistence, and the latter presents the five- and four-layer coexistence, as shown in Fig. 3a and b, respectively. Note that here the co-existence is actually a NSM and is not simply a mixture of two phases with different layer numbers. Hence, according to the XRD and HRTEM results, schematic illustration of the structure evolution of BFCT-x with an increasing in Co content in BFCT-x is assumed in Fig. 4.
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Fig. 3 HRTEM images of (a) BFCT-0.4 and (b) BFCT-0.75. The red “4”, “5” and “6” stand for the four-, five- and six-layer perovskite structures, respectively, indicating the modulated phases at the nanoscale (NSM).
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Fig. 4 Schematic illustration of the evolution route in the BFCT-x fabrication
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P-E loops of BFCT-x samples at 100 Hz and the RT are shown in supplementary Fig. S3, and dependence of lattice constant on the Co content x, the pulsed remanent polarization (P = switched polarization - non-switched polarization) at 100 Hz and 120 kV/cm, and dielectric constant (’) with the frequency of 1 kHz are shown in Fig. 5a-c, respectively, all measured at the RT. Variation of the lattice constant as x is distinct in phases with six-, five-, and four-layer numbers. For six- and five-layer structures, the closeness of a and b supports the unchanged F2mm space group, and a further split of the two in the four-layer structure supports the F2mm to A21am phase transition (in Fig. 5a). Changing of P-E loops with x is consisted with the variation of lattice constants a and b, indicating that the polarization was originated from the structural distortion. When x 0.75, P has improved significantly, comparing with those in x < 0.75 (in Fig. 5b), and this may be attributed to the phase transition of F2mmA21am. That P is different with the remanent polarizations (2Pr) from the P-E Loops implies a leakage current contribution, which is also clearly seen in P-E Loops. Very importantly, P and dielectric anomalies are clearly observable at x = 0.4 and 0.75 boundaries (in Fig. 5b and c). This definitely relates to the changing of layer number inside their Aurivillius microstructure, which leads to a co-existence of two phases with two layer numbers. HRTEM images show that this co-existence is a NSM type, rather than a simple two-phase mixture. Coupling between modulated structures was used to enhance the EO effect via the modulated spontaneous polarization switching27. Therefore it is reasonable to assume a similar enhancement to FE and dielectric properties, because that the nanoscale modulated structures can be tightly correlated to both ferroelectric domain and space charge distribution. Hysteresis loops of magnetization (M) in BFCT-x (x 0) ceramics at the RT are shown in supplementary Fig. S4. The linear field dependence of M in BFCT-0 indicates its AFM nature, as shown in supplementary Fig. S5, similar to those previously obtained results13, 20. However, plots of magnetic moment vs. H for rest of the samples with a partial Co substitution clearly indicate the presence of a large FM moment at the RT which is consistent with the previous studies too14, 17. Hence, it will be natural to infer that the main origin of the enhancement in FM in This journal is © The Royal Society of Chemistry [year]
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the BFCT-x samples (x ≠ 0) should be similar to that in previously studied Bi5Fe0.5Co0.5Ti3O15 (n = 4)14 and in Bi6Fe217 xCoxTi3O18 (n = 5) . The proposed mechanisms mainly suggested that FM originates from the exchanging interaction among Fe3+ and Co3+ or Fe3+ and Co2+ via bonding with oxygen and the spin canting via anti-symmetric Dzyaloshinskii-Moriya (DM) interaction, and the mechanisms should be intrinsic to the Cosubstituted layer structures31, 32.
Fig. 5 The doping level of Co (x) dependence of (a) lattice constants, (b) remanent polarizations (2Pr) from the P-E Loops and the pulsed remanent polarization P at 100 Hz and 120 kV/cm, and (c) dielectric constant (’) and dielectric loss (tan) at the RT with the frequency of 1
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kHz. The colour wide background lines stand for the area that could have the co-existed NSM phases.
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Fig.6 The doping level of Co (x) dependence of (a) remnant magnetization (2Mr) and (b) saturation magnetization (Ms) of BFCT-x. Open green circle is the experimental data of the samples. According to our DTMG experimental results, CoFe2O4 phase does not exist in the samples (x < 0.4), almost linearly increasing from 0.5 wt% at x = 0.4 to 3.6 wt% at x = 0.75, and then stay closely stable at 3.6 wt% in the samples (x=0.9 and 1.0). Solid blue ball is the final values of 2Mr or Ms after deducting the corresponding contribution of CoFe2O4 from the experimental data. The red line is error bar caused by calculation (0.5 wt%). The color wide background lines stand for the area that could have the co-existed NSM phases.
However, more information can be derived from the dependence of remnant magnetization (2Mr) and saturation magnetization (Ms) of the BFCT-x samples on the Co content, which was plotted using open green circle in Fig. 6a and b. For 0 < x < 0.75, 2Mr and Ms increase as the Co content increases, except a small enhancement occurring at the x~0.4 boundary. A sharp enhancement happens to the x~0.75 boundary too. After the enhancement peak, 2Mr and Ms reduce as the Co content increases. At least two contributions should account for the magnetization: 1) from the impurity phase of CoFe2O4; 2) intrinsically from the layered materials themselves. To fully interpret the variation of 2Mr and Ms, the DTMG33 was used to measure the dependence of a sample’s weight under a magnetic field over a varying temperature range. For samples BFCT-x (x = 0.3~1.0), CoFe2O4 and ZrO2-13wt%CoFe2O4 (the last two samples were specifically synthesized for the quantitative estimation), the results are in Fig. 7. For pure CoFe2O4 its FM-toparamagnetic (PM) transition peak occurs at a temperature ~720 K in the derivative weight (dW/dT) curve, and this peak can be seen in samples BFCT-x (x = 0.4~0.75), confirming the existence of the impurity phase, though it is minor. Also for BFCT-0.3, BFCT-0.5, BFCT-0.6, BFCT-0.7, occurring of only one peak at the temperature around 755.1 K, 741.5 K, 731.7 K and 697.5 K, respectively, indicates a net contribution from the FM-to-PM phase transition from the only existed six-, five-, five- and fiveThis journal is © The Royal Society of Chemistry [year]
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layer phase. For BFCT-0.4 and BFCT-0.75, two additional peaks appear at 748.4 K and 739.6 K, and 688.1 K and 712.6 K, respectively. The two peaks are coming from the co-existed and nanoscale structurally modulated five- and six-layer phases in BFCT-0.4 and four- and five-layer phases in BFCT-0.75. For BFCT-x (x > 0.75), one peak at around 680.6 K for BFCT-0.9 and 677 K for BFCT-1.0 diverged from the pure CoFe2O4 peak position is coming from the impurity spinel phase Co 1+Fe2-O4, causing the further lowered transition temperature by the increase in Co content34, 35. The other peak occurring at 651.4 K for BFCT-0.9 and 649 K for BFCT-1.0 indicates the net contribution from the four-layer phase. In short, the analysis of DTMG measurements is very consisted with above XRD and HRTEM results and further proves the intrinsic magnetism from the Aurivillius phase. According to the dW/dT curves from pure CoFe2O4 and the predesigned ZrO2-13wt% CoFe2O4 (in Fig. 7), and the resolution of the DTMG measurement (the accuracy of the measurement experimentally determined is approximately 0.5wt%), relative quantities of CoFe2O4 impurity in BFCT-x samples can be accordingly estimated through curve fitting, as shown in supplementary Fig. S6. According to above DTMG experimental results, FM contributions from the intrinsic layered materials themselves can be got by considering the measured pure CoFe2O4’s 2Mr = 53 emu/g and Ms = 79.5 emu/g36 and its relative content, then deducting the contribution of the CoFe2O4 impurity phase from the experimental data. Solid blue ball signed in Fig. 6a and b is the final FM results after the impurity deduction. This net FM enhancement should be intrinsic to the Fe and Co co-substituted layered structures, explainable by the previous exchanging and DM mechanisms. Interestingly, the intrinsic FM response significantly increases from four- to five-layer phases (about four-hundred times higher), and it becomes flat when from five- to six-layer. Four-layer BFCT-x (x 0.9) have intrinsic 2Mr and Ms close to those previously reported in the four-layer Bi5Fe0.5Co0.5Ti3O15 (2Mr = 7.8 memu/g)14. The five-layer samples, BFCT-0.5, BFCT-0.6 and BFCT-0.7, have much enhanced intrinsic FM response when comparing to the four-layer samples, and the average FM from the three is similar to the previously reported in Bi6FeCoTi3O18 (2Mr = 0.5 emu/g)37. This similarity actually testifies the accuracy of the way used to deduct the CoFe2O4 impurity phase contribution. Inside the range of each single phase, variation of the intrinsic FM enhancement can be explained by the increasing in Co content which changes the relative Fe/Co ratio. This again confirms the significance of the Fe/Co co-substitution technique used to boost the intrinsic multiferroic properties at the RT, as firstly suggested in Bi5Fe0.5Co0.5Ti3O1514. Two FM anomalies at x = 0.4 and x = 0.75 boundaries were also clearly observed in Fig. 6a and b, in which the later one is more significant. This definitely also relates to the changing of layer number inside their nanoscale Aurivillius structure, which leads to a NSM type with two layer numbers. This NSM type is tightly correlated to both exchanging among oxygen-bonded magnetic ions and the spin canting. The space group corresponding to six-, five- and four-layer structures is F2mm, F2mm and A21am, respectively. Layer number reduction across the second AMT boundary accompanies a nanoscale structural
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multiferroic enhancement was attributed to the new AMT effect when selecting Co contents close to the two AMT boundaries. The AMT/NSM modulation may become an alternative new way to boost the RT multiferroic properties. Recently, similar NSM phenomena were also found in others high layer-structural Aurivillius phases in our group, and this is a continuing effort and the results will be published separately.
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We thank the financially support from the National Basic Research Program of China (2012CB922001) and the Natural Science Foundation of China (51072193). We are grateful to X. Y. Mao and Prof. X. B. Chen (Yangzhou University) and Y. W. Ding (USTC) for discussions of the measurements. S. J. Sun acknowledges fellowship support from the college of Physical Science and Technology, Yangzhou University. The work is also partially supported by Defence Threat Reduction Agency under grants HDTRA1-10-1-0001 and HDTRA122221.
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CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China b Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China c Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China d Laser Optics Research Center, US Air Force Academy, Colorado 80840, USA * Corresponding Authors: [email protected],.
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Fig. 7 Weight loss and DTMG curves of BFCT-x (x = 0.3~1.0), CoFe2O4 and ZrO2-13wt%CoFe2O4. The DTMG measurement was performed in the same conditions of nitrogen atmosphere at a heating rate of 20 K/min with a 200 Oe applied magnetic field. Under the magnetic field, weight loss from the contribution of thermal active has been automatically deducted.
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In summary, using the Aurivillius BFCT-x, the NSM phases were confirmed by XRD measurements, Rietveld refinements, HRTEM, and DTMG. Enhanced intrinsic multiferroic properties from the NSM phases had been proved by quantifying and deducting the contribution from the existed impurity phase. This 6 | Journal Name, [year], [vol], 00–00
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phase transition from F2mm to A21am, additional to the simple structural modulation within the same space group at the first boundary. This difference may explain the significance of the second boundary. Overall, the results here at the boundaries are important due to three reasons: 1) the 2Mr is at the emu/g level, which is indeed very large in existing single phase multiferroics; 2) it occurs remarkably at the RT; 3) co-existence of the NSM phases is technically feasible (here via Co doping, for example), and it is also reasonable to refer them as a new multiferroics.
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