Direct Observation of Magnetic Field Induced Ferroelectric Domain

Dec 10, 2015 - The self-assembly growth mechanism has been studied, which demonstrates that the perovskite (BFO) matrix segments the connectivity of s...
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Direct Observation of Magnetic Field Induced Ferroelectric Domain Evolution in Self-Assembled Quasi (0-3) BiFeO3−CoFe2O4 Thin Films Linglong Li,† Lu Lu,‡ Dawei Zhang,† Ran Su,† Guang Yang,*,‡ Junyi Zhai,§ and Yaodong Yang*,† †

Multi-disciplinary Materials Research Center, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China ‡ Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China § Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China S Supporting Information *

ABSTRACT: Strain-mediated magnetoelectric (ME) coupling effect is expected in self-assembly heterostructures engineered by ferroelectric and ferromagnetic materials, contributing to the enhanced overall magnetoelectric effect. Microstructures as well as the connectivity configuration are considered to play a significant role in achieving efficient magnetoelectric properties. Different from the conventional (1-3) and (2-2) type composite films, we fabricate BiFeO3− CoFe2O4 (BFO−CFO) composite thin films with a novel quasi (0-3) type connectivity via a dual-target pulsed laser deposition process. The self-assembly growth mechanism has been studied, which demonstrates that the perovskite (BFO) matrix segments the connectivity of spinel (CFO) resulting in a quasi (0-3) composite. Direct observation of ferroelectric domain wall motion under external magnetic fields proves a strong magnetoelectric coupling effect in these (0-3) thin films. Our preliminary findings reveal the promising application potential of this new structure as multiferroic domain wall devices. KEYWORDS: epitaxial growth, self-assemble, nanocomposites, piezoresponse force microscopy, domain wall motion

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INTRODUCTION Magnetoelectric (ME) materials demonstrate electric and magnetic orderings simultaneously, providing significant platforms to explore the complexity and new paradigms for sensors and transducer studies.1−4 Most of these materials for practical applications have composite forms, especially in combinations of ferroelectric perovskites and ferromagnetic spinel.5−7 Hence the magnetoelectric effect is induced by strong elastic coupling, rather than spin−orbit interaction in single-phase multiferroics and has been widely investigated in the past decade.8−11 The bismuth ferrite and cobalt ferrite composite thin films are of particular interest for their well-defined lattice matching and practical properties. BiFeO3 (BFO), with a pseudocubic lattice constant of 3.96 Å, exhibits strong ferroelectricity, while CoFe2O4 (CFO), with a cubic lattice constant of 8.38 Å, is a room-temperature ferromagnet. Compatible in the structural aspect due to an approximate 2-fold relationship of one another in lattice constants, they are deposited as epitaxial thin films on SrTiO3 (STO) substrate via pulsed laser deposition.12 So far, a number of multiferroic ME composites of ferroelectric and magnetic share three different configurations: (2-2) laminate composite, (0-−3) particulate composite, and (1-3) fiber/rod composite.13 The (2-2) type sandwich structure © 2015 American Chemical Society

can be obtained via layer-by-layer deposition and the (1-3) type vertical structure can be approached during spontaneous phase separation (involving one composite target), while (0-3) type composite is rarely obtained in thin film formats. Only very recently, Li and co-workers used a complicated process to obtain a quasi (0-3) structure on (001)-oriented substrates.14 If (0-3) type composite can be fabricated in a simpler method, it may be possible to reduce the clamping effect (better than the (2-2) type) and reduce the leakage current (better than the (13) structure) at the same time. Thin films with new composite configuration will also improve our understanding of the interaction between two different materials and the film growth mechanism. Considering the difference of the surface energy and wetting property between perovskite and spinel, morphologies and crystallographic anisotropy are intensively dependent on the orientation of STO substrates.15,16 Generally, {100} planes have the lowest surface energy in most perovskites, so it leads to a cubic equilibrium shape with {100} facets. Dominant Received: September 30, 2015 Accepted: December 10, 2015 Published: December 10, 2015 442

DOI: 10.1021/acsami.5b09265 ACS Appl. Mater. Interfaces 2016, 8, 442−448

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type composite thin films. This direct observation provided concrete evidence of magnetoelectric effect showing the magnetic field induced ferroelectric domain evolution. Local magnetoelectricity in both single-phase multiferroics and magnetoelectric composites has been extensively investigated before.19−21 Magnetic field induced ferroelectric switching phenomena have been observed by convincing mappings through advanced in situ piezoresponse force microscopy (PFM) technology.19,20 The changes of ferroelectricity both in phase and amplitude demonstrate the strain-mediated ME coupling effect in composite multiferroics. The local ferroelectric domain evolution under external magnetic field is of particular interest. However, more details such as domain wall motion and statistic piezoelectricity changes under magnetic fields are still unclear and under further exploration.

perovskite phase covers most part of the (100)-oriented substrate as the matrix, while the restricted spinel phase forms the nanorods along the (001) direction. Meanwhile, spinel phases have the lowest surface energy in {111} planes, reflected in an equilibrium shape of octahedron bounded by {111} facets.5,17 Therefore, the spinel phase is dominant on the (111)-oriented substrate and forms the matrix with (1-3) type nanostructures, while the perovskite phase forms the nanorods. However, it becomes complicated on the (110)-oriented STO substrate because the wetting abilities of both BFO and CFO phases are close, which results in forming a maze pattern.18 The competition between BFO and CFO gives rise to variable growth behavior and morphologies on STO substrates. Particularly, the sequence of nucleation and growth mode of BFO and CFO at the beginning of the deposition process still remains unexplored on the (110)-oriented substrate. To clarify the dynamic competition between two phases on the (110) substrate and develop a (0-3) type composite, we deposited a BFO−CFO composite via fast switching dual-target PLD process on SrRuO3 buffered SrTiO3 substrate. The deposition approach is illustrated in Figure 1. First the laser is

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EXPERIMENTAL SECTION

BiFeO3−CoFe2O4 composite thin films were deposited on (110)oriented SrTiO3 substrates with SrRuO3 buffer layer by pulsed laser deposition. This SRO layer with a thickness of 50 nm was first deposited at 660 °C with 150 mTorr oxygen pressure. For BFO−CFO thin film, the spot size of the laser was about 2 mm2, which was focused on the surface of the target with the energy density of 3 J·cm−2 and a frequency of 1 Hz. The distance between the substrate and the target was 6 cm, and the base vacuum of the chamber was 10−5 Torr, while the oxygen pressure was 90 mTorr. The X-ray diffraction experiment was performed by PANalytical X’Pert Pro diffractometer. Piezoresponse force microscopy (PFM) images were taken by atomic force microscopy (AFM) under the dual AC resonance tracking (DART) mode (Cypher, Asylum Research).22 The average value and standard deviation value of piezoelectric amplitude signals were provided by the standard AFM software (offered by Asylum Research). Magnetic force microscopy (MFM) images were acquired by the cobalt−chromium coated tips (after magnetizing). Scanning transmission electron microscopy (STEM) images and energy dispersive Xray spectroscopy (EDS) mappings were acquired by the probe spherical aberration corrected JEOL-ARM200F microscope. A FEI Helios 600i focused ion beam (FIB) was used to prepare the STEM samples. Low-temperature magnetic moment versus magnetic field curve was achieved by physical property measurement system (PPMS) under vibrating sample magnetometer (VSM) module.

Figure 1. Schematic illustration showing the fast switching deposition process and the self-assembly growth of quasi (0−3) nanocomposite.

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focused on the CFO target for the first 500 pulses, and then switched to the BFO target for another 500 pulses; the same procedure repeats once before finishing. Though four portions of CFO/BFO/CFO/BFO (500 pulses per time) arrive at the substrate successively, experimental characterization proves the self-assembly growth of these films. Layer-by-layer deposition involving two single-phase targets usually leads to a (2-2) type film. However, when these two targets are switched at a very high frequency, the time is not long enough for either of the two kinds of materials to fully cover the whole substrate before switching to the other one; thus the layer-by-layer growth will be prevented. In such a case, this film growth process is quite similar to using one mixed target (BFO−CFO) to grow the (13) type self-assembly thin films.17 Even when the CFO is deposited initially, it does not fully wet and cover the substrate. Two partitions of BFO and CFO form one layer at the same time and compete with each other (based on the different surface energies) at the beginning of deposition. During phase separation, recrystallization and grain coarsening occur, and they grow into quasi (0-3) type composite thin films in a selfassembly method. We also investigated the intrinsic ME coupling effect of our samples and found the magnetic field induced ferroelectric domain wall motion phenomenon in these scarce quasi (0-3)

RESULTS AND DISCUSSION Self-Assembly in (0−3) Type Nanocomposite. The Xray diffraction line scan exhibits the high degree of crystallographic orientation between different layers in the thin film. The result in Figure 2 shows the diffraction peaks that can be assigned to CFO (220), BFO (110), buffer layer SRO (110), and STO (110) from the substrate, respectively. It indicates the

Figure 2. X-ray diffraction scan of the preferred orientation of CFO (220) and BFO (110) layers on STO (110) substrate. Inset: the SEM image of the thin film with stripe features. 443

DOI: 10.1021/acsami.5b09265 ACS Appl. Mater. Interfaces 2016, 8, 442−448

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Figure 3. Piezoresponse force microscopy images, magnetic force microscopy images, and EDS analysis: (a and c) morphologies of two different scanning modes, PFM and MFM respectively; (b) amplitude image of PFM; (d) phase image of MFM. Insets are higher magnification images of the selected representative grains. (e) EDS mapping of the cross-sectional view; (f and g) element mappings of bismuth (purple) and cobalt (yellow), respectively; (h) Bi and Co line scans of the marked red line in panel e.

of ferroelectric and ferromagnetic phases in nanoparticle formation and the spontaneous separation behavior in the growth process. From these top view images, we can ensure that these ferroelectric and ferromagnetic phases have quite strong in-plane connections. As supporting evidence to show the magnetic phase in the composite, the magnetic anisotropy is measured by a vibrating sample magnetometer module of PPMS at 50 K (Supporting Information Figure S1). The out-ofplane M−H curve shows a lower coercive field (about 70 G) than the in-plane signal (about 200 G). Phase distributions and microstructures inside the thin films are probed further by transmission electron microscopy with energy dispersive X-ray spectroscopy. Figure 3e shows the full view of the cross-section sample. The thin film with a thickness of about 30 nm (verified by EDS Fe element mapping in Supporting Information Figure S2c) is covered by the protective Pt layer which is produced during FIB sample preparation. Panels f and g of Figure 3 are bismuth and cobalt mappings, respectively. Line scans of the red line in Figure 3e provide the direct evidence of the corresponding element segment. From the EDS analysis it is clear that both Bi (white arrows) and Co (red arrows) are well-separated in the thin film. Interfaces and self-assembly growth mechanism are studied by spherical aberration corrected transmission electron microscope with high angle annular dark field (HAADF) detector. Figure 4a shows the HAADF image of the interface which

epitaxial growth relationship as well as the phase separation. The scanning electron microscopy image shows the featured morphology of stripe structures at the surface as an inset in Figure 2. The morphology is similar to previously reported vertical self-assembly (1-3) nanocomposite structures.17,18 In order to further clarify what these stripe-like structures are and how these two phases distribute in the composite thin films, PFM and MFM are employed. The SPM equipped with a PFM module, in which polarization and electromechanical activity are coupled together intrinsically, has the sensitivity extended to picometer-scale surface displacement and allows studying domain structures and polarization dynamics under external fields. Panels a and c of Figure 3 are images scanned by PFM and MFM modes from the same area, respectively. Figure 3b shows the piezoresponse amplitude signal. The red and blue regions represent the different piezoresponse amplitude values in which the red regions stand for grains with higher piezoresponse referring to the ferroelectric phase (BFO, marked with white circles), and the blue regions stand for weak piezoelectric grains being considered as the ferromagnetic phase in this case (CFO, marked with white squares). A similar conclusion can also be made by MFM mapping in Figure 3d that ferromagnetic grains (white squares) show the magneticphase signal with clear converse colors, while ferroelectric grains (white circles) only have a middle ground in yellow color. Specifically, the inset in Figure 3a shows the tight growth 444

DOI: 10.1021/acsami.5b09265 ACS Appl. Mater. Interfaces 2016, 8, 442−448

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region. The CFO phase varying from 15 to 50 nm in triangle shapes form hut-shaped islands, and the BFO grains have a higher surface topography; thus, the quasi(0-3) nanostructures are obtained such that CFO grains are located inside the BFO matrix (supported by a lower magnification image in Supporting Information Figure S3). Three characteristic interfaces between BFO and CFO are shown in Figure 4b−d, respectively. Figure 4b shows a BFO grain settled inside the CFO grain. Because of the clamping effect from both SRO and spinel CFO, the BFO core forms a pyramid shape. To release the residual inhomogeneous strain between these two different phases during crystalline growth, large amounts of defects are generated along the interface revealing the dislocation in the CFO shell (see the arrow in Figure 4b). As shown in Figure 4c, a well-ordered epitaxial interface with atomic resolution reveals the relationship between BFO (110) and CFO (220) planes. The (001) and (110) planes are marked, and illustration in the Figure 4c inset shows the atomic epitaxial relationship between the two phases. The details of the bottom part of a CFO grain on the top of the SRO are presented in Figure 4d. Two or more layers of BFO with clear perovskite structure exist between CFO and SRO. In the deposition sequence, the CFO plasma is excited by the first 500 pulses. However, BFO occupies the first two atom layers on the top of the SRO. Since BFO has a perovskite similar to the SRO layer, it has a higher wetting ability to form lamellas or nanosheets with a layer-by-layer growth mode. CFO in less contact with the SRO layer would form particles in an island growth mode.17,25 Even when the BFO was deposited after CFO, it covered most of the substrate and segments the connectivity of the CFO phase. Therefore, the CFO nanoparticles grew on the BFO rather than the SRO and form the quasi (0-3) nanostructures. A long pulse interval (about 1 s) and fast switching between the targets are important issues to break the layer-by-layer growth. Since the

Figure 4. HAADF images in cross-sectional view of the details of selfassembly thin film growth behavior: (a) full view of the cross-section sample; (b−d) higher magnification HAADF images of selected red, blue, green squares in panel a. Three types of crystallographic interfaces between BFO, CFO, and SRO layers have been exposed.

indicates the self-assembly growth behavior in BFO−CFO composite on SRO along the (110) direction. In the HAADF image, the contrast is proportional to the Z2 (Z is the atomic number).23,24 Therefore, the dark triangle area is CFO rich while the bright area is BFO rich, both of which are on the SRO

Figure 5. PFM images taken before magnetization (column a), after applying 9000 G (column b), and with reverse magnetic fields (column c). The directions of the magnetic induction lines are shown as two insets at the lower right corners. Top raw (M) is morphology, and bottom raw (A) is piezoresponse amplitude. “Avg” is short for average, which is the mean value of the whole image, and “Sdev” is standard deviation of the piezoresponse amplitude. 445

DOI: 10.1021/acsami.5b09265 ACS Appl. Mater. Interfaces 2016, 8, 442−448

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Figure 6. Local PFM images revealing the ferroelectric domain wall motion induced by an external magnetic field: morphology of the area scanned after the removal of a 9000 G magnetic field (a) and a reverse magnetic field (b); (c and d) related amplitude images, respectively; (e and f) line scans of the black lines marked in panels c and d.

BFO−CFO thin film before and after magnetization by different DC magnetic fields. Column a in Figure 5 includes the PFM images of the original thin film. After being magnetized under a 9000 G out-of-plane magnetic field for 10 min, the same sample was reloaded at the same position for the second PFM scanning (shown in column b in Figure 5). Then it was magnetized under a reverse −9000 G out-of-plane magnetic field for 10 min, and we got the third PFM results (shown in column c in Figure 5). Red cycles in morphology mark the same region of the sample during three scans. From Figure 5a to Figure 5b, the average piezoelectric amplitude changes from 118 to 119 pm, while the Sdev (standard deviation of the piezoelectric amplitude) varies from 60.2 to 103.5 pm. Magnetic field intensifies the divergence of the piezoresponse, represented as distinct contrasts in yellow and blue in Figure 5b-A. The average piezoresponse amplitude value is 74.4 pm, and its standard deviation is 73.1 pm in Figure 5c, which indicates that reverse magnetic field decreases this difference as well as the average piezoresponse amplitude. Compared to Figure 5b, this anisotropic magnetoelectric property may be attributed to the clamping effect from the substrate.

substrates used in the PLD process were heated to a high temperature during deposition, atoms would retain their mobility even when the laser was turned off. If the film is thin enough (such as in this case), the atoms would reorganize to reach a new stability. The final structure was formed during cooling the substrates to room temperature, during which the spontaneous phase separation and self-assembly growth occurred and is the likely reason why fast switching layer-bylayer deposition generates quasi (0-3) films. Compared to using one composite target only, one advantage of our dual-target method is that we can control the ratio between two different materials via simply adjusting the laser pulses, rather than making new targets. Magnetoelectric Coupling: Magnetic Field Induced Piezoelectricity Change and Domain Wall Motion. Epitaxial growth relationship leads to a strong strain-mediated interaction between two phases. As a combination of both piezoelectric and magnetostrictive materials, this BFO−CFO composite exhibits magnetoelectric coupling effect.13 Compared to the 2−2 composite configuration, this quasi (0-3) heterostructure can enhance the ferroelectricity by tolerating less clamping effect from the substrate for its lateral geometry.26 Figure 5 summarizes the change of piezoelectricity in this 446

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ACS Applied Materials & Interfaces Notes

Detailed local magnetoelectric effects are presented in Figure 6. Panels a and b of Figure 6 show the morphology of two scans, and their corresponding phase and amplitude mappings are shown in Supporting Information Figure S4. Blue and red squares mark the same local regions. Panels c and d of Figure 6 are amplitude signals of these two selected regions, in which ferroelectric grains are identified by dashed lines. Furthermore, images of amplitude show the ferroelectric grain, especially the ferroelectric domain wall inside the BFO grain (marked by the black line). The details of the black line scan are shown in Figure 6e,f, respectively. Shaded areas with the lowest amplitude value and saltation in phase signal reveal the ferroelectric domain walls. After alternating the direction of the applied magnetic field, we can determine the ferroelectric domain wall motion in the BFO grain. The diameter of the domain decreases from 20 to 10 nm. Another case of magnetic field induced ferroelectric domain wall motion is shown in Supporting Information Figure S5. This difference occurs without applying any external electric field but only with the reversal of DC magnetic field. As the strain-mediated ME composite material, the CFO grains around BFO grains are considered as the origin of strain for their magnetostrictive property.13,27,28 The deformation of CFO grains induced by external magnetic field acts on ferroelectric BFO grains and further triggers the ferroelectric domain motion.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China through a 973-Project under Grant No. 2012CB619401, the Natural Science Foundation of China (Grant Nos. 11204233, 51431007, 51321003, and 51202180), and fundamental research funds for the central universities and MOE innovation team (Grant No. IRT13034) both from the Ministry of Education.

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CONCLUSIONS In conclusion, we designed a fast switching dual-targets PLD process (with two single-phase materials BFO and CFO) to obtain heteroepitaxial self-assembly thin films with quasi (0-3) nanostructures. Different wetting abilities of these two phases result in the layer-by-layer growth of BFO and island growth of CFO. BFO matrix segments the connectivity of the CFO phase, and CFO grains grow on the BFO rather than SRO in their ripening process to form the quasi (0-3) nanostructures. The magnetoelectric effect of the ferroelectric grains (BFO) and ferromagnetic grain (CFO) is detected by the piezoresponse amplitude mappings directly. The difference of piezoelectric amplitudes between BFO and nonferroelectric background is enhanced by the external magnetic fields. After applying a reverse magnetic field, the domain shrinks, which shows the field induced ferroelectric domain wall evolution. This work identifies that subtle and microscopic dynamics of the polarization structure at the domain wall is at the origin of the strain-mediated ME effect and paves the way toward understanding of perovskite- and spinel-based self-assembly growth together with their local magnetoelectric effect. We believe that it will provide important information for the future development of multiferroic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09265. M−H curve, full EDS data, TEM image, and corresponding PFM-phase and amplitude images (PDF)

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ABBREVIATIONS BFO, BiFeO3 CFO, CoFe2O4 DART, dual AC resonance tracking EDS, energy dispersive X-ray spectroscopy FIB, focused ion beam HAADF, high angle annular dark field MFM, magnetic force microscopy PFM, piezoresponse force microscopy PLD, pulsed laser deposition PPMS, physical property measurement system STEM, scanning transmission electron microscopy STO, SrTiO3 SRO, SrRuO3 VSM, vibrating sample magnetometer

AUTHOR INFORMATION

Corresponding Authors

*(G.Y.) E-mail: [email protected]. *(Y.Y.) E-mail: [email protected]. 447

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