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LSMO composite thin film
Journal Pre-proofs Full Length Article Votage-regulated magnetization reversal in BNTFC/LSMO composite thin film Kaixin Guo, Rongfen Zhang, Min Zhang, Yiliang Hu, Song Yang, Chaoyong Deng PII: DOI: Reference:
S0169-4332(19)33640-2 https://doi.org/10.1016/j.apsusc.2019.144823 APSUSC 144823
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
28 August 2019 5 November 2019 21 November 2019
Please cite this article as: K. Guo, R. Zhang, M. Zhang, Y. Hu, S. Yang, C. Deng, Votage-regulated magnetization reversal in BNTFC/LSMO composite thin film, Applied Surface Science (2019), doi: https:// doi.org/10.1016/j.apsusc.2019.144823
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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.
© 2019 Published by Elsevier B.V.
Votage-regulated magnetization reversal in BNTFC/LSMO composite thin film
Kaixin Guo, Rongfen Zhang, Min Zhang, Yiliang Hu, Song Yang, Chaoyong Deng *
Key laboratory of Electronic Composites of Guizhou Province, College of Big Data and Information Engineering, Guizhou University, Guiyang 550025, China
Email addresses: [email protected] (K. X. Guo), [email protected] (C. Y. Deng)
ABSTRACT In order to investigate the electromagnetic effect in nanoscale, a multiferroic BNTFC/LSMO composite thin film was fabricated on (00l) oriented LAO single crystal substrate employing a PLD system. Structural characterization showed that the LSMO layer is pseudo-cubic but the BNTFC presents a four-layer perovskite structure. Tests of physical properties indicated that such a composite thin film exhibits a good ferroelectricity, the Pr and EC are 5.04 μC·cm-3 and 587.12 kV·cm-1, respectively. And the ferromagnetism of the BNTFC system has been improved effectively with the introduction of LSMO lamina, the MS and HC are promoted to 68.13 emu·g-1 and 0.35 kOe. In the meantime, the dynamic process of the structure and movement of the out-of-plane ferromagnetic domains at different voltages using MFM innovatively was studied. It suggested that an applied voltage does change the distribution of ferromagnetic domains of the sample — voltage-regulated magnetization reversal. Keywords: Electromagnetic effect; Votage-regulated magnetization reversal; Switching of the
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magnetic easy-axis 1. Introduction Single-phase magnetoelectric (ME) multiferroics, exhibiting simultaneously ferroelectric (FE) and ferromagnetic (FM) ordering, have drawn ever-increasing attention in recent years [1] for their significant potential applications in the next-generation novel multifunctional devices, such as new-type solar cells[2], memristors [3], data storage units [4], etc.. Bismuth ferrite (BiFeO3, BFO) with a rhombohedral distorted perovskite structure is one of the representative single-phase multiferroic materials, which has lately received widespread attentions [5] for both its high Curie and Neel temperature. However, applications of BFO are restricted just because of its high leakage currents and a inferior ME coupling effect. A simple and effective way to solve the problems above has developed where magnetic units are implanted in FE matrixes, thus forming new-type layered perovskite (or Aurivillius-type) multiferroics stacked by fluorite-type (Bi2O2)2+ layers and pseudo-perovskite (An-1BnO3n+1)2- lamellas, where n is the number of ABO3 perovskite units per half-cell [6], such as Bi4Ti3O12 (n = 3) [7], Bi5FeTi3O15 (n = 4) [8], Bi6FeMTi3O18 (n = 5 and M= Mn, Fe, Co and Ni) [9], Bi7Fe3Ti3O21 (n = 6) [10], Bi8Fe4Ti3O24 (n = 7) [11]. It indicated that (Bi2O2)2+ layers have a great impact on the ferroelectricity of the system and they act usually as an insulating layer made the resistivity along c-axis is significantly higher than that along a- or b-axis [12, 13]. However, they usually exhibit a relatively weak ferromagnetism at room temperature (RT) [14], recent studies showed that an appropriately introduction of magnetic ions with +3 to +5 oxidation states in the nABO3 perovskite units [6] can improve the ferromagnetism effectively. X.Y. Mao et al. found a magneto-dielectric constant of ~10.5% in Bi5Fe0.5Co0.5Ti3O15 ceramics, the remanent polarization (Pr) and magnetization (Mr) are respectively 7.8 μC·cm−2 and 25.6 memu·g−1 [13]. D. G.
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Porob et al. found that the Pr and Mr of Bi4.2La0.8Fe0.5Co0.5Ti3O15 ceramics are 2.8 μC·cm−2 and 142.2 memu·g−1 [15], and F. J. Yang et al. found a large magnetic response in (Bi4Nd)Ti3(Fe0.5Co0.5)O15 block, the Mr is about 0.58 emu·g−1 which is more than two orders of Bi5Ti3Fe0.5Co0.5O15 [16]. Moreover, composites composed with ferromagnetic oxides, such as Re1-xTxMnO3, Co/Ni1-xZnxFe2O4, etc. can further improve not only their magnetic properties but ME coupling effect of the compounds [17]. Thereinto, the Re1-xTxMnO3 (especially La1-xSrxMnO3) series have become a hot topic in condensed matter physics for their promising properties including a nearly half-metallic and ferromagnetic behavior with an extremely high degree of spin polarization and a colossal magnetoresistance (CMR) [18] since the 1990s. At present, The composite films composed of those comprising manganites and ferroelectrics have arouse ample interest due to their properties such as the tunable antiferromagnetic coupling and exchange bias at the interface [19]. In this work, a multiferroic Bi4NdTi3Fe0.5Co0.5O15/La0.7Sr0.3MnO3 (BNTFC/LSMO) composite thin film was fabricated by a pulsed laser deposition (PLD) system on (00l) oriented LaAlO3 (LAO) single crystal substrate for the similar work functions (~4.8 eV for LSMO, and ~4.7 eV for BNTFC) as well as matched lattice parameters. The phase composition, microstructure, ferroelectric and ferromagnetic properties were investigated at room temperature. Then the dynamic process of the structure and movement of the out-of-plane ferromagnetic domains at different voltages using MFM was studied, providing a novel method to further investigate the electromagnetic effect in nanoscale. 2 EXPERIMENTAL 2.1 Preparation The heteroepitaxial BNTFC/LSMO thin film was fabricated via a PLD system with a focused KrF excimer laser (λ = 248 nm). A LSMO layer with a thickness of ~80 nm was first deposited on (00l)
3
oriented LAO single crystal substrate at 680°C, followed by a 1h annealing process in a flowing O2. Subsequently, BNTFC thin film with a thickness of ~150 nm was deposited on the LSMO thin film at 680°C. The content of bismuthate must exceed 15 mol% to supply its volatilization loss during annealing. 2.2 Characterization The thickness of LSMO and BNTFC was determined by a cross-sectional scanning electron microscope (SEM), the presence of crystalline phases and the structure of the samples were determined by an X-ray diffractometer (XRD, Rigaku D/max-2500 V with Cu Kα monochromatic radiation, λ = 1.5418 Å) at a scanning speed of 2° min-1 in steps of 0.02°. The microstructure was characterized employing a high resolution transmission electron microscopy (TEM, JEM-2100UHR STEM/EDS) and the topography and the dynamic process of domain movements were investigated using an atomic force microscopy (AFM, Bruker Multimode 8) in AFM, piezoelectric force microscopy (PFM) and magnetic force microscopy (MFM) modes respectively, where PFM was performed under a modulated sinusoid AC electrical field of 0.5 V with a SCM-PIT probe (Pt/Ir Coated Si Tips, 1-5 N·m-1, 60-100 kHz, Pt/Ir Reflective Coating), and MFM was detected with a MESP probe (Co/Cr Coated Si Tips, 1-5 N·m-1, 60-100 kHz, Co/Cr Reflective Coating) in a non-contact mode. The ferroelectricity of the samples was analyzed by a model multiferroic 200 V Test System (Radiant Technologies) and the corresponding ferromagnetism were determined in a VSM mode by a physical property measurement system (PPMS, Quantum Design TM). All the measurements were carried out at room temperature. 3 RESULTS AND DISCUSSIONS 3.1 XRD patterns and Microstructure
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Fig. 1. XRD pattern and microstructure of the composite thin film. (a), (b) and (c), (d) AFM and HRTEM images of both BNTFC and LSMO, (e) the cross-section of the sample.
Fig. 1 shows XRD pattern and microstructure of the composite thin film grown on (00l) oriented LAO single crystal substrates with pseudo-cubic structure (a = 3.792 Å). Thereinto (a), (b) and (c), (d) are AFM and HRTEM images of both BNTFC and LSMO, (e) displays the cross-section of the sample. First, it can be seen that only strong (001) and (002) diffraction peaks of LAO substrate and LSMO was observed, confirming that the LSMO thin film is expitaxially grown on the LAO substrate. It is known that LAO and LSMO all represent a pseudo-cubic structure (Figure 1. (d)), the interplanar spacing d between the adjacent (001) plane is equivalent to the pseudo-cubic lattice parameter a. Therefore, the LSMO thin film will suffer a biaxial compressive stress while depositing on LAO substrate, originated from the lattice mismatch ([(aLSMO – aLAO)/aLAO]×100%) between LSMO (aLSMO = 3.874 Å) and LAO (aLAO = 3.793 Å), made the in-plane lattice constant of the LSMO thin film become lower, but the out-of-plane become larger, leading to a strong compressive strain field in the bottom LSMO layer, which will gradually relax as the thickness of the film increases [20, 21]. And meanwhile, the (0010), (0016) and (0018) diffracton peaks of BNTFC lamella displayed
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when 2θ equals to 21.5°, 34.7° and 39.3° respectively are not splited, illustrated that the BNTFC thin film shows an integrated four-layer perovskite structure according to previous studies [15, 16], which can be testified in Figure 1 (c). Second, it is observed that the irregular elliptic crystalline grains of both BNTFC and LSMO layers are arranged uniformly and compactly, the Root-Mean-Square (RMS) roughness are 6.5 and 5.9 respectively. In the meantime, the interfaces of layers are bonded finely and the boundaries are distinguishable, the thicknesses are 150 and 80 nm respectively. And last, HRTEM images of BNTFC and LSMO lamellas showed that the BNTFC thin film does show a four-layer perovskite structure, where three Ti-O layers and one Fe/Co-O layer are sandwiched by two (Bi2O2)2+ layers, and the LSMO exhibits a pseudo-cubic structure (as shown in Figure 1. (d)), eight vertices of which are occupied by La3+ and Sr2+, Mn2+ is located in the centre and O2- are embedded in the six planes. 3.2 Polarization
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Fig. 2. Ferroelectric domain inversion and polarization of BNTFC/LSMO composite thin film. (a) the out-of-plane ferroelectric domains tested at 10 V, (b) amplitude distribution of ferroelectric domains (5μm×5μm), the bright and the dark parts are acquired respectively at -10V and +10V, (c) phase inversion and the corresponding A-V loop, (d) P-E loops of both BNTFC lamina and the composite film at different voltages, the inset is the schematic illustration of the test.
Ferroelectric domain inversion and polarization of BNTFC/LSMO composite thin film tested by AFM in PFM mode are shown in Figure 2. Thereinto, (a) presents the out-of-plane ferroelectric domains tested at 10 V, (b) shows the amplitude distribution of ferroelectric domains (5μm×5μm), the bright and the dark parts are acquired respectively at -10V and +10V, (c) displays phase inversion and the corresponding Amplitude-Voltage (A-V, or the so-called butterfly) loop, and (d) exhibits just the ferroelectric hysteresis (P-E) loops of both BNTFC lamina and the composite film at different voltages, the inset is the schematic illustration of the test. In the test, the voltage applied to the tip equal to the sum of the direct voltage used to control electric domains (10 V in this test) and the alternating voltage used for testing (0.5 V). It is well known that ferroelectric domains are areas that dipoles organized orderly and the directions of spontaneous polarization are the same, the characteristics of which such as genre (e.g. single-/poly-domain, a-/c-domain, 90°/71°/60°/... domain), size and thickness have a great effect on macro-properties of samples, such as ferroelectricity, piezoelectricity, pyroelectricity, etc.. Generally, the directions of demonstrated spontaneous polarization are disorderly when no voltages are applied to the thin film (or the voltages are switched off). Once a certain voltage like 3V or less is applied to the thin film, the electric dipoles begin to redirect along the direction of the applied field, because the voltages are perpendicular to the thin film, the disordered dipoles will turn into vertical orientation (i.e. the
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so-called out-of-plane domains) and form a structure illustrated in Figure 2 (a). With a sharp increase in the number of spontaneous polarization, the ferroelectric phase inversion intensified apparently and the shape and size of ferroelectric domains are growing and expanding, which can be further testified in (c). As is shown in Figure 2 (d), the value of polarization is enlarging with the increasing of the applied field regardless of the polarity, the Pr and the coercive field (EC) of the pure BNTFC are respectively 8.26 μC·cm−2 and 481.67 kV·cm−1 but 5.04 μC·cm−2 and 587.12 kV·cm−1 of the BNTFC/LSMO composite thin film at 25 V but the sample was broken down when the applied voltage is larger than 25 V. It suggested that the BNTFC/LSMO composite thin film we prepared shows a higher breakdown voltage [12-16]. However, many previous experiments have indicated that it is hard to get a good saturated P-E loop for the system of displacement ferroelectrics when n≥4 [6, 8, 12-16]. And the slight inconsistency of the switching voltage between (c) and (d) is attributed to the selected different regions while testing and a lower voltage applied to the thin film. The asymmetry of the P-E loops depends mainly on the asymmetric media between the top and the bottom electrodes (as shown in the inset of Figure 2. (d)), thus forming a series structure of two capacitors, in which the voltage drop of LSMO with a higher resistivity is more pronounced resulted in a higher EC of the BNTFC/LSMO than the pure BNTFC lamina [21]. 3.3 Magnetization
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Fig. 3. Magnetization of both LSMO, BNTFC and BNTFC/LSMO composite thin film tested at 300 K. the inset are M-H loop of BNTFC and drawings of partial enlargement.
As one of the most important characteristics of ferromagnets, Magnetization of both BNTFC, LSMO and BNTFC/LSMO composite thin film tested under room temperatureare are presented in Figure 3, and the inset are M-H loop of BNTFC and drawings of partial enlargement. The magnetism of multiferroic BNTFC thin film derives from both the symmetric and antisymmetric superexchange interactions of Fe3+-O-Fe3+, Co3+-O-Co3+ and Fe3+-O-Co3+ and Dzyaloshinsky-Moriya (DM) interactions due to the spin canting of the Fe (Co) atoms resulting from the titled Fe(Co)O6 octahedra [22]. It can be seen that the saturation magnetization (MS) and coercivity (HC) of BNTFC/LSMO composite thin film are 68.13 emu·g−1 and 0.35 kOe which are relatively smaller than that of LSMO lamina (109.10 emu·g−1 and 0.24 kOe), but much better than the pure BNTFC thin film (380.02 memu·g−1 and 0.37 kOe). The higher MS of LSMO layer mainly comes from the double-exchange interaction between Mn3+ and Mn4+ [23]. But a larger lattice mismatch between BNTFC and LSMO layer led to a larger distortion which made domains reorientated and weakened the intensity of the
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magnetic order, then caused a decrease of MS. It is obvious that the MS of the BNTFC system has been improved at least two orders of magnitude with the introduction of LSMO lamina. 3.4 Voltage-regulated magnetization reversal
Fig. 4. Votage-regulated magnetization reversal of BNTFC/LSMO composite thin film. (a) Schematic illustration of the experimental, (b)~(d) evolution of the out-of-plane ferromagnetic domains measured at different voltages, the arrows represent the order of the tests.
Electromagnetic (EM) coupling is based on the coupling of piezoresponse of ferroelectric phase and magnetostriction of ferromagnetic phase in composites. It is defined as follows [1],
M i (E, H )
F 1 M is 0 ij H j ij E j ijk H j Ei ijk E j Ek . . . H i 2
(1)
where F refers to free energy, M and E are severally magnetization and the applied field, 10
represents a linear tensor of coupling coefficients. Figure 4 shows the votage-regulated magnetization reversal of the composite thin film. Thereinto, (a) displays the schematic illustration of the experimental where two wirings of the power supply are connected respectively to the top and the bottom Pt planar electrodes, (b)~(d) exhibit the evolution of the out-of-plane ferromagnetic domains measured at 0, 5 and 10 V, the arrows represent the order of the tests. It is worth noting that the two wirings of the power supply should be connected respectively to two Pt planar electrodes because the length scales of electric and magnetic field changes are different. It can be seen that the reorientation of magnetic easy-axis is not obvious, the maximal reversal is about 12.9°. Because the variation of the total free energy (ΔF) from OM’ to OM (as shown in the inset of Figure 4. (d)) is defined as follows [25], FM M FM FM Fmc Fshape Fme
(2)
when ΔFM’-M < 0, magnetic easy-axis will rotate from OM’ to OM, the critical electric field requires [25], E3
where K A
KA 0 2td31 d 31
(3)
B Bc K1 K 2 1 2B 2 B 2 0 M S2 1 2 for the composite thin film, and t 1 1 12 . The main 2 c11 2 4 2 3c11 c44
reason for EM coupling effect can be boiled down to dynamic modulation of strain and interface charges and multiferroic exchange bias coupling of BNTFC layer and LSMO lamella [22, 25]. As can be seen from Figure 4, more domains begin to preferentially redirect and the shape of ferromagnetic domains is constantly changing while a certain voltage is applied to the thin film, especially the areas marked with ① to ⑤. The areas shown in (b) and (c) are mutually independent, but ①, ② and ④ combined together and ③ is divided into ③ and ⑤ at last in (d), which
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mean that the applied voltage does change the distribution of ferromagnetic domains－the so-called electromagnetic effect in nanoscale. It indicated that the lone pair electrons of Bi3+ with a (ns)2 structure are unstable, which will mingle easily with the first excited state (ns)1(np)1 and even the p atomic orbit of O2- caused the ionization state loss its symmetry and coordinate with magnetic transition metal ions nearby O2-, and a partial substitution of ferroelectric activator Ti4+ with magnetic transition metal Fe3+ and Co3+ leading to the coexistence of ferroelectricity and ferromagnetism [22]. 4 CONCLUSIONS In conclusion, a novel method was proposed in this paper to investigate the electromagnetic effect in nanoscale. A multiferroic BNTFC/LSMO composite thin film was grown on LAO substrate using a PLD first. The phase composition, microstructure, ferroelectric and ferromagnetic properties were investigated at room temperature. Then the electromagnetic effect in nanoscale was studied by observing the dynamic process of the structure and movement of the out-of-plane ferromagnetic domains at different voltages employing MFM. It suggested that an applied voltage does change the distribution of ferromagnetic domains of the sample and this method is indeed feasible. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of the National Natural Science Foundation of China under Grant Nos. 51762010, 51462003 and 614040330, and we all love our dearest motherland, China. CONFLICT OF INTEREST The authors declare no conflicts of interest. REFERENCES 12
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[20] L. Ranno, A. Llobet, R. Tiron, E. M. Favre-Nicolin, Strain-induced magnetic anisotropy in epitaxial manganite films, Appl. Surf. Sci., 188 (2002) 170-175. [21] M. Dawber, K. M. Rabe, J. F. Scott, Physics of thin-film ferroelectric oxides, Rev. Mod. Phys., 77 (2005) 1083-1130. [22] T. Moriya, Anisotropic Superexchange Interaction and Weak Ferromagnetism, Phys. Rev., 120 (1960) 91. [23] Y. Lu, C. L. Lu, P. Yang, G. C. Han, Uniaxial Magnetic Anisotropy in La0.7Sr0.3MnO3 Thin Films Induced by Multiferroic BiFeO3 with Striped Ferroelectric Domains, Adv. Mater., 22 (2010) 4964-4968. [24] T. R. Gao, X. H. Zhang, W. Ratcliff, S. Maruyama, Electric-Field Induced Reversible Switching of the Magnetic Easy Axis in Co/BiFeO3 on SrTiO3, Nano Lett., 17 (2017) 2825−2832. [25] J. M. Hu, C. W. Nan, Electric-field-induced magnetic easy-axis reorientation in ferromagnetic/ferroelectric layered heterostructures, Phy. Rev. B, 20 (2009) 224416.
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No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
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Highlights A multiferroic BNTFC/LSMO composite thin film was successfully fabricated on (00l) oriented LAO single crystal substrate employing a PLD system. The dynamic process of the movement of the out-of-plane ferromagnetic domains at different voltages was studied employing MFM. A novel method was proposed to investigate the electromagnetic effect in nanoscale using MFM.
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S0169-4332(19)33640-2 https://doi.org/10.1016/j.apsusc.2019.144823 APSUSC 144823
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
28 August 2019 5 November 2019 21 November 2019
Please cite this article as: K. Guo, R. Zhang, M. Zhang, Y. Hu, S. Yang, C. Deng, Votage-regulated magnetization reversal in BNTFC/LSMO composite thin film, Applied Surface Science (2019), doi: https:// doi.org/10.1016/j.apsusc.2019.144823
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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.
© 2019 Published by Elsevier B.V.
Votage-regulated magnetization reversal in BNTFC/LSMO composite thin film
Kaixin Guo, Rongfen Zhang, Min Zhang, Yiliang Hu, Song Yang, Chaoyong Deng *
Key laboratory of Electronic Composites of Guizhou Province, College of Big Data and Information Engineering, Guizhou University, Guiyang 550025, China
Email addresses: [email protected] (K. X. Guo), [email protected] (C. Y. Deng)
ABSTRACT In order to investigate the electromagnetic effect in nanoscale, a multiferroic BNTFC/LSMO composite thin film was fabricated on (00l) oriented LAO single crystal substrate employing a PLD system. Structural characterization showed that the LSMO layer is pseudo-cubic but the BNTFC presents a four-layer perovskite structure. Tests of physical properties indicated that such a composite thin film exhibits a good ferroelectricity, the Pr and EC are 5.04 μC·cm-3 and 587.12 kV·cm-1, respectively. And the ferromagnetism of the BNTFC system has been improved effectively with the introduction of LSMO lamina, the MS and HC are promoted to 68.13 emu·g-1 and 0.35 kOe. In the meantime, the dynamic process of the structure and movement of the out-of-plane ferromagnetic domains at different voltages using MFM innovatively was studied. It suggested that an applied voltage does change the distribution of ferromagnetic domains of the sample — voltage-regulated magnetization reversal. Keywords: Electromagnetic effect; Votage-regulated magnetization reversal; Switching of the
1
magnetic easy-axis 1. Introduction Single-phase magnetoelectric (ME) multiferroics, exhibiting simultaneously ferroelectric (FE) and ferromagnetic (FM) ordering, have drawn ever-increasing attention in recent years [1] for their significant potential applications in the next-generation novel multifunctional devices, such as new-type solar cells[2], memristors [3], data storage units [4], etc.. Bismuth ferrite (BiFeO3, BFO) with a rhombohedral distorted perovskite structure is one of the representative single-phase multiferroic materials, which has lately received widespread attentions [5] for both its high Curie and Neel temperature. However, applications of BFO are restricted just because of its high leakage currents and a inferior ME coupling effect. A simple and effective way to solve the problems above has developed where magnetic units are implanted in FE matrixes, thus forming new-type layered perovskite (or Aurivillius-type) multiferroics stacked by fluorite-type (Bi2O2)2+ layers and pseudo-perovskite (An-1BnO3n+1)2- lamellas, where n is the number of ABO3 perovskite units per half-cell [6], such as Bi4Ti3O12 (n = 3) [7], Bi5FeTi3O15 (n = 4) [8], Bi6FeMTi3O18 (n = 5 and M= Mn, Fe, Co and Ni) [9], Bi7Fe3Ti3O21 (n = 6) [10], Bi8Fe4Ti3O24 (n = 7) [11]. It indicated that (Bi2O2)2+ layers have a great impact on the ferroelectricity of the system and they act usually as an insulating layer made the resistivity along c-axis is significantly higher than that along a- or b-axis [12, 13]. However, they usually exhibit a relatively weak ferromagnetism at room temperature (RT) [14], recent studies showed that an appropriately introduction of magnetic ions with +3 to +5 oxidation states in the nABO3 perovskite units [6] can improve the ferromagnetism effectively. X.Y. Mao et al. found a magneto-dielectric constant of ~10.5% in Bi5Fe0.5Co0.5Ti3O15 ceramics, the remanent polarization (Pr) and magnetization (Mr) are respectively 7.8 μC·cm−2 and 25.6 memu·g−1 [13]. D. G.
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Porob et al. found that the Pr and Mr of Bi4.2La0.8Fe0.5Co0.5Ti3O15 ceramics are 2.8 μC·cm−2 and 142.2 memu·g−1 [15], and F. J. Yang et al. found a large magnetic response in (Bi4Nd)Ti3(Fe0.5Co0.5)O15 block, the Mr is about 0.58 emu·g−1 which is more than two orders of Bi5Ti3Fe0.5Co0.5O15 [16]. Moreover, composites composed with ferromagnetic oxides, such as Re1-xTxMnO3, Co/Ni1-xZnxFe2O4, etc. can further improve not only their magnetic properties but ME coupling effect of the compounds [17]. Thereinto, the Re1-xTxMnO3 (especially La1-xSrxMnO3) series have become a hot topic in condensed matter physics for their promising properties including a nearly half-metallic and ferromagnetic behavior with an extremely high degree of spin polarization and a colossal magnetoresistance (CMR) [18] since the 1990s. At present, The composite films composed of those comprising manganites and ferroelectrics have arouse ample interest due to their properties such as the tunable antiferromagnetic coupling and exchange bias at the interface [19]. In this work, a multiferroic Bi4NdTi3Fe0.5Co0.5O15/La0.7Sr0.3MnO3 (BNTFC/LSMO) composite thin film was fabricated by a pulsed laser deposition (PLD) system on (00l) oriented LaAlO3 (LAO) single crystal substrate for the similar work functions (~4.8 eV for LSMO, and ~4.7 eV for BNTFC) as well as matched lattice parameters. The phase composition, microstructure, ferroelectric and ferromagnetic properties were investigated at room temperature. Then the dynamic process of the structure and movement of the out-of-plane ferromagnetic domains at different voltages using MFM was studied, providing a novel method to further investigate the electromagnetic effect in nanoscale. 2 EXPERIMENTAL 2.1 Preparation The heteroepitaxial BNTFC/LSMO thin film was fabricated via a PLD system with a focused KrF excimer laser (λ = 248 nm). A LSMO layer with a thickness of ~80 nm was first deposited on (00l)
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oriented LAO single crystal substrate at 680°C, followed by a 1h annealing process in a flowing O2. Subsequently, BNTFC thin film with a thickness of ~150 nm was deposited on the LSMO thin film at 680°C. The content of bismuthate must exceed 15 mol% to supply its volatilization loss during annealing. 2.2 Characterization The thickness of LSMO and BNTFC was determined by a cross-sectional scanning electron microscope (SEM), the presence of crystalline phases and the structure of the samples were determined by an X-ray diffractometer (XRD, Rigaku D/max-2500 V with Cu Kα monochromatic radiation, λ = 1.5418 Å) at a scanning speed of 2° min-1 in steps of 0.02°. The microstructure was characterized employing a high resolution transmission electron microscopy (TEM, JEM-2100UHR STEM/EDS) and the topography and the dynamic process of domain movements were investigated using an atomic force microscopy (AFM, Bruker Multimode 8) in AFM, piezoelectric force microscopy (PFM) and magnetic force microscopy (MFM) modes respectively, where PFM was performed under a modulated sinusoid AC electrical field of 0.5 V with a SCM-PIT probe (Pt/Ir Coated Si Tips, 1-5 N·m-1, 60-100 kHz, Pt/Ir Reflective Coating), and MFM was detected with a MESP probe (Co/Cr Coated Si Tips, 1-5 N·m-1, 60-100 kHz, Co/Cr Reflective Coating) in a non-contact mode. The ferroelectricity of the samples was analyzed by a model multiferroic 200 V Test System (Radiant Technologies) and the corresponding ferromagnetism were determined in a VSM mode by a physical property measurement system (PPMS, Quantum Design TM). All the measurements were carried out at room temperature. 3 RESULTS AND DISCUSSIONS 3.1 XRD patterns and Microstructure
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Fig. 1. XRD pattern and microstructure of the composite thin film. (a), (b) and (c), (d) AFM and HRTEM images of both BNTFC and LSMO, (e) the cross-section of the sample.
Fig. 1 shows XRD pattern and microstructure of the composite thin film grown on (00l) oriented LAO single crystal substrates with pseudo-cubic structure (a = 3.792 Å). Thereinto (a), (b) and (c), (d) are AFM and HRTEM images of both BNTFC and LSMO, (e) displays the cross-section of the sample. First, it can be seen that only strong (001) and (002) diffraction peaks of LAO substrate and LSMO was observed, confirming that the LSMO thin film is expitaxially grown on the LAO substrate. It is known that LAO and LSMO all represent a pseudo-cubic structure (Figure 1. (d)), the interplanar spacing d between the adjacent (001) plane is equivalent to the pseudo-cubic lattice parameter a. Therefore, the LSMO thin film will suffer a biaxial compressive stress while depositing on LAO substrate, originated from the lattice mismatch ([(aLSMO – aLAO)/aLAO]×100%) between LSMO (aLSMO = 3.874 Å) and LAO (aLAO = 3.793 Å), made the in-plane lattice constant of the LSMO thin film become lower, but the out-of-plane become larger, leading to a strong compressive strain field in the bottom LSMO layer, which will gradually relax as the thickness of the film increases [20, 21]. And meanwhile, the (0010), (0016) and (0018) diffracton peaks of BNTFC lamella displayed
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when 2θ equals to 21.5°, 34.7° and 39.3° respectively are not splited, illustrated that the BNTFC thin film shows an integrated four-layer perovskite structure according to previous studies [15, 16], which can be testified in Figure 1 (c). Second, it is observed that the irregular elliptic crystalline grains of both BNTFC and LSMO layers are arranged uniformly and compactly, the Root-Mean-Square (RMS) roughness are 6.5 and 5.9 respectively. In the meantime, the interfaces of layers are bonded finely and the boundaries are distinguishable, the thicknesses are 150 and 80 nm respectively. And last, HRTEM images of BNTFC and LSMO lamellas showed that the BNTFC thin film does show a four-layer perovskite structure, where three Ti-O layers and one Fe/Co-O layer are sandwiched by two (Bi2O2)2+ layers, and the LSMO exhibits a pseudo-cubic structure (as shown in Figure 1. (d)), eight vertices of which are occupied by La3+ and Sr2+, Mn2+ is located in the centre and O2- are embedded in the six planes. 3.2 Polarization
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Fig. 2. Ferroelectric domain inversion and polarization of BNTFC/LSMO composite thin film. (a) the out-of-plane ferroelectric domains tested at 10 V, (b) amplitude distribution of ferroelectric domains (5μm×5μm), the bright and the dark parts are acquired respectively at -10V and +10V, (c) phase inversion and the corresponding A-V loop, (d) P-E loops of both BNTFC lamina and the composite film at different voltages, the inset is the schematic illustration of the test.
Ferroelectric domain inversion and polarization of BNTFC/LSMO composite thin film tested by AFM in PFM mode are shown in Figure 2. Thereinto, (a) presents the out-of-plane ferroelectric domains tested at 10 V, (b) shows the amplitude distribution of ferroelectric domains (5μm×5μm), the bright and the dark parts are acquired respectively at -10V and +10V, (c) displays phase inversion and the corresponding Amplitude-Voltage (A-V, or the so-called butterfly) loop, and (d) exhibits just the ferroelectric hysteresis (P-E) loops of both BNTFC lamina and the composite film at different voltages, the inset is the schematic illustration of the test. In the test, the voltage applied to the tip equal to the sum of the direct voltage used to control electric domains (10 V in this test) and the alternating voltage used for testing (0.5 V). It is well known that ferroelectric domains are areas that dipoles organized orderly and the directions of spontaneous polarization are the same, the characteristics of which such as genre (e.g. single-/poly-domain, a-/c-domain, 90°/71°/60°/... domain), size and thickness have a great effect on macro-properties of samples, such as ferroelectricity, piezoelectricity, pyroelectricity, etc.. Generally, the directions of demonstrated spontaneous polarization are disorderly when no voltages are applied to the thin film (or the voltages are switched off). Once a certain voltage like 3V or less is applied to the thin film, the electric dipoles begin to redirect along the direction of the applied field, because the voltages are perpendicular to the thin film, the disordered dipoles will turn into vertical orientation (i.e. the
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so-called out-of-plane domains) and form a structure illustrated in Figure 2 (a). With a sharp increase in the number of spontaneous polarization, the ferroelectric phase inversion intensified apparently and the shape and size of ferroelectric domains are growing and expanding, which can be further testified in (c). As is shown in Figure 2 (d), the value of polarization is enlarging with the increasing of the applied field regardless of the polarity, the Pr and the coercive field (EC) of the pure BNTFC are respectively 8.26 μC·cm−2 and 481.67 kV·cm−1 but 5.04 μC·cm−2 and 587.12 kV·cm−1 of the BNTFC/LSMO composite thin film at 25 V but the sample was broken down when the applied voltage is larger than 25 V. It suggested that the BNTFC/LSMO composite thin film we prepared shows a higher breakdown voltage [12-16]. However, many previous experiments have indicated that it is hard to get a good saturated P-E loop for the system of displacement ferroelectrics when n≥4 [6, 8, 12-16]. And the slight inconsistency of the switching voltage between (c) and (d) is attributed to the selected different regions while testing and a lower voltage applied to the thin film. The asymmetry of the P-E loops depends mainly on the asymmetric media between the top and the bottom electrodes (as shown in the inset of Figure 2. (d)), thus forming a series structure of two capacitors, in which the voltage drop of LSMO with a higher resistivity is more pronounced resulted in a higher EC of the BNTFC/LSMO than the pure BNTFC lamina [21]. 3.3 Magnetization
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Fig. 3. Magnetization of both LSMO, BNTFC and BNTFC/LSMO composite thin film tested at 300 K. the inset are M-H loop of BNTFC and drawings of partial enlargement.
As one of the most important characteristics of ferromagnets, Magnetization of both BNTFC, LSMO and BNTFC/LSMO composite thin film tested under room temperatureare are presented in Figure 3, and the inset are M-H loop of BNTFC and drawings of partial enlargement. The magnetism of multiferroic BNTFC thin film derives from both the symmetric and antisymmetric superexchange interactions of Fe3+-O-Fe3+, Co3+-O-Co3+ and Fe3+-O-Co3+ and Dzyaloshinsky-Moriya (DM) interactions due to the spin canting of the Fe (Co) atoms resulting from the titled Fe(Co)O6 octahedra [22]. It can be seen that the saturation magnetization (MS) and coercivity (HC) of BNTFC/LSMO composite thin film are 68.13 emu·g−1 and 0.35 kOe which are relatively smaller than that of LSMO lamina (109.10 emu·g−1 and 0.24 kOe), but much better than the pure BNTFC thin film (380.02 memu·g−1 and 0.37 kOe). The higher MS of LSMO layer mainly comes from the double-exchange interaction between Mn3+ and Mn4+ [23]. But a larger lattice mismatch between BNTFC and LSMO layer led to a larger distortion which made domains reorientated and weakened the intensity of the
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magnetic order, then caused a decrease of MS. It is obvious that the MS of the BNTFC system has been improved at least two orders of magnitude with the introduction of LSMO lamina. 3.4 Voltage-regulated magnetization reversal
Fig. 4. Votage-regulated magnetization reversal of BNTFC/LSMO composite thin film. (a) Schematic illustration of the experimental, (b)~(d) evolution of the out-of-plane ferromagnetic domains measured at different voltages, the arrows represent the order of the tests.
Electromagnetic (EM) coupling is based on the coupling of piezoresponse of ferroelectric phase and magnetostriction of ferromagnetic phase in composites. It is defined as follows [1],
M i (E, H )
F 1 M is 0 ij H j ij E j ijk H j Ei ijk E j Ek . . . H i 2
(1)
where F refers to free energy, M and E are severally magnetization and the applied field, 10
represents a linear tensor of coupling coefficients. Figure 4 shows the votage-regulated magnetization reversal of the composite thin film. Thereinto, (a) displays the schematic illustration of the experimental where two wirings of the power supply are connected respectively to the top and the bottom Pt planar electrodes, (b)~(d) exhibit the evolution of the out-of-plane ferromagnetic domains measured at 0, 5 and 10 V, the arrows represent the order of the tests. It is worth noting that the two wirings of the power supply should be connected respectively to two Pt planar electrodes because the length scales of electric and magnetic field changes are different. It can be seen that the reorientation of magnetic easy-axis is not obvious, the maximal reversal is about 12.9°. Because the variation of the total free energy (ΔF) from OM’ to OM (as shown in the inset of Figure 4. (d)) is defined as follows [25], FM M FM FM Fmc Fshape Fme
(2)
when ΔFM’-M < 0, magnetic easy-axis will rotate from OM’ to OM, the critical electric field requires [25], E3
where K A
KA 0 2td31 d 31
(3)
B Bc K1 K 2 1 2B 2 B 2 0 M S2 1 2 for the composite thin film, and t 1 1 12 . The main 2 c11 2 4 2 3c11 c44
reason for EM coupling effect can be boiled down to dynamic modulation of strain and interface charges and multiferroic exchange bias coupling of BNTFC layer and LSMO lamella [22, 25]. As can be seen from Figure 4, more domains begin to preferentially redirect and the shape of ferromagnetic domains is constantly changing while a certain voltage is applied to the thin film, especially the areas marked with ① to ⑤. The areas shown in (b) and (c) are mutually independent, but ①, ② and ④ combined together and ③ is divided into ③ and ⑤ at last in (d), which
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mean that the applied voltage does change the distribution of ferromagnetic domains－the so-called electromagnetic effect in nanoscale. It indicated that the lone pair electrons of Bi3+ with a (ns)2 structure are unstable, which will mingle easily with the first excited state (ns)1(np)1 and even the p atomic orbit of O2- caused the ionization state loss its symmetry and coordinate with magnetic transition metal ions nearby O2-, and a partial substitution of ferroelectric activator Ti4+ with magnetic transition metal Fe3+ and Co3+ leading to the coexistence of ferroelectricity and ferromagnetism [22]. 4 CONCLUSIONS In conclusion, a novel method was proposed in this paper to investigate the electromagnetic effect in nanoscale. A multiferroic BNTFC/LSMO composite thin film was grown on LAO substrate using a PLD first. The phase composition, microstructure, ferroelectric and ferromagnetic properties were investigated at room temperature. Then the electromagnetic effect in nanoscale was studied by observing the dynamic process of the structure and movement of the out-of-plane ferromagnetic domains at different voltages employing MFM. It suggested that an applied voltage does change the distribution of ferromagnetic domains of the sample and this method is indeed feasible. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of the National Natural Science Foundation of China under Grant Nos. 51762010, 51462003 and 614040330, and we all love our dearest motherland, China. CONFLICT OF INTEREST The authors declare no conflicts of interest. REFERENCES 12
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No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
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Highlights A multiferroic BNTFC/LSMO composite thin film was successfully fabricated on (00l) oriented LAO single crystal substrate employing a PLD system. The dynamic process of the movement of the out-of-plane ferromagnetic domains at different voltages was studied employing MFM. A novel method was proposed to investigate the electromagnetic effect in nanoscale using MFM.
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