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Uniaxial strain controlled ferroelastic domain evolution in BiFeO3 Abdullah Alsubaie, Pankaj Sharma, Jin Hong Lee, Jeong Yong Kim, Chan-Ho Yang, and Jan Seidel ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01711 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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ACS Applied Materials & Interfaces

Uniaxial strain controlled ferroelastic domain evolution in BiFeO3 Abdullah Alsubaie1,2, Pankaj Sharma1,*, Jin Hong Lee3,4, Jeong Yong Kim4, Chan-Ho Yang 4,5

, and Jan Seidel1,*

1

School of Materials Science and Engineering, UNSW Sydney, Sydney NSW 2032, Australia

2

School of Physics, Taif University, Taif, Kingdom of Saudi Arabia

3

Unité Mixte de Physique, CNRS, Thales, Université Paris Sud, Université Paris-Saclay, 91767 Palaiseau, France

4

Department of Physics, KAIST, Yuseong-gu, Daejeon 305-701, Republic of Korea

5

KAIST Institute for the NanoCentury, KAIST, Yuseong-gu, Daejeon 305-701, Republic of Korea *email: [email protected], [email protected] Abstract: We investigate the effect of variable uniaxial tensile strain on the evolution of 71° ferroelastic domains in (001)-oriented epitaxial BiFeO3 (BFO) thin films using piezoresponse force microscopy (PFM). For this purpose, a newly designed bending stage has been employed which allows for tensile bending as wells as in-situ PFM characterization. In-situ PFM imaging reveals polarization-strain correlations at the nanoscale. Specifically, ferroelastic domains with in-plane polarization along the direction of applied tensile strain expand, while the adjoining domains with orthogonal in-plane polarization contract. The switching is mediated by significant domain wall roughening and opposite displacement of the successive walls. Further, the domains with long-range order are more susceptible to an applied external mechanical stimulus compared to the domains, which exhibits short-range periodicity. In addition, the imprint state of film reverses direction under applied tensile strain. Finally, the straininduced changes in the domain structure, and wall motion are fully reversible and revert to their as-grown state upon release of the applied stress. The strain-induced non180° polarization rotation constitutes a route to control connected functionalities such as magnetism via coupled in-plane rotation of the magnetic plane in multiferroic BFO thin films.

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Keywords: uniaxial mechanical strain, ferroelastic switching, domain wall motion, multiferroics, piezoresponse, polarization-strain correlation Introduction Distortion of crystal lattice, especially in thin films, can lead to significant modulation of inherent properties,1, 2 besides leading to the emergence of novel functionalities that do not exist in the strain-free state.3 The approach to effect variable subtle changes in interatomic distances to controllably tune materials properties is known as strain engineering. The ideal systems for strain engineering are heteroepitaxial thin films where lattice distortions in the thin films are induced by their epitaxial growth on crystallographic lattice mismatched underlying substrate. The static elastic deformation imposed by growth on rigid substrates can tune for example electronic properties of oxide thin films ranging from superconductors,4 heterointerfaces between band-insulators,5 to ferroelectrics.6, 7

Ferroelectrics in particular are of special interest because of their technological implications,8, 9 and the inherent coupling they display to the mechanical strain and strain gradients. As a result, functional response of ferroelectrics is greatly influenced by the stress state of the film. Recently, strain/strain gradients have been dynamically regulated at the nanoscale in ferroelectrics using the tip of an atomic force microscope (AFM) as a nanometer-length scale indenter affecting polarization. 10 , 11 , 12 The highly-localized large elastic fields[13] generated around the tip-sample contact area were favorably exploited for instance to write ferroelectric nanodomains10,11 and even to shuffle oxygen vacancies in nonferroelectric oxide thin films.14, 15, 16 Another approach that has been attempted in the past to deliver variable strain is based upon usage of bending techniques by placing a ferroelectric over an evacuated cavity or by deflecting the ferroelectric between a punch and a support structure.17, 18 The advantage here is that it is possible to apply uniaxial/biaxial tensile or compressive stain on the scale of several microns to mm’s compared to nanoscale compressive stress using proximal probe-based approaches. The mechanical bending stress measurements have been performed for various complex oxide materials. 19, 20, 21, 22, 23 For instance from biaxial stress-driven motion of 90° domain walls in polycrystalline ferroelectric Pb(ZrTi)O3 (PZT) thin films19 to uniaxial compressive stress-induced nucleation and ferroelastic domain evolution in bulk BaTiO3 ceramics.24 Therefore, the effect of mechanical stress on domains and domain walls, which governs the functional response of ferroelectrics or multiferroics needs to be elucidated with high-resolution at the nanoscale. Particularly for 2 ACS Paragon Plus Environment

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BiFeO3 (BFO), which is a well-known room temperature multiferroic 25 such studies are currently lacking. In this article, using a custom-designed bending stage compatible with in-situ piezo response force imaging, we investigate the effect of uniaxial mechanical tensile stress on domain microstructure as well as wall motion in multiferroic epitaxial (001)-oriented BFO thin films. In-situ PFM imaging under variable tensile strain conditions reveals polarizationstrain correlation at the nanoscale and demonstrates reversible control of 71° ferroelastic domains. The 71° domain switching involves wall motion, and wall roughening. In addition, local out-of-plane PFM hysteresis loops display a reversal in the imprint state of film under applied uniaxial mechanical tensile strain.

Experiment In this investigation, BiFeO3 thin films were grown with a conductive buffer layer of SrRuO3 (SRO) on (001) SrTiO3 (STO) substrates using pulsed laser deposition (PLD). The resulting samples were thinned down (to approx. 70 μm) by polishing the backside of the STO substrate and were then glued onto a steel wafer on to which a uniaxial mechanical strain was applied. Here, the direction of the STO substrate was aligned along the long axis of the wafer. Bending experiments were conducted with a 3-point bending stage such that an external variable uniaxial stress could be applied to the thin film. The bending can lead to a visible curvature of the sample and accessible strain values of a few percent without breaking the sample. For further details of the bending process and strain calculation we refer to supplementary information (Section S1- S2, Figures S1 (a-c), and S2) and our previous work.22, 26 Several points need to be mentioned for the mechanical situation of the BFO/STO system under investigation before further considerations. Firstly, the STO substrate would be slightly curved upon being thinned down (via polishing) even in the absence of any other external stress; therefore an inhomogeneous stress field would be produced if STO was not isotropically polished. Secondly, the piezoelectric BFO would alter the internal electric field in the course of bending. As a result, a stress would be generated in BFO and contribute additional curvature to STO. This effect is highly related to the domain structures in the thin film, such as stripe domains in BFO.27 Note that piezoelectric characters of BFO can create an additional stress; however, to clearly quantify such effects is not easy and out of our 3 ACS Paragon Plus Environment


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current capability. We therefore have neglected this effect. Finally, the c-axis of monoclinic BFO is treated as approximately normal to the BFO/STO interface considering its small tilting angle (~1o). In this case, both the axis of orthotropy of thin film and substrate are aligned with each other and one axis of orthotropy is normal to the interface. Results and discussion For BFO (001) samples, the in-plane component of polarization alternates by 90° between the neighbouring stripe domains along one of the four equivalent pseudocubic (pc) directions. This gives rise to the typical 4-variant stripe domain microstructure comprising predominantly of 71° domains, i.e. net polarization vector rotates by 71° between the domains. While the traces of domain walls on exposed surface of film run parallel to in-theplane ([100]pc or [010]pc) crystallographic directions, and slopes through the thickness at angle of approximately 45°.28 The wall orientation is a result of the competition between the wall energy and elastic strain energy associated with the ferroelastic domain structure of the film. 29 This balance can likely be tuned via an applied external stress resulting in transformation to a favourable ferroelastic domain variant and displacement of domain walls. Figure 1 illustrates the basic concept for a BFO thin film. A uniaxial tensile strain is applied such that its direction is parallel and perpendicular to the polarization component in-the-plane between the alternating stripe domains respectively. The expectation is that a fully reversible ferroelastic domain wall motion and domain switching can be achieved using applied external mechanical stress. To test our hypothesis, we begin with piezoresponse (PFM) imaging of the BFO (001) thin film to characterize the as-prepared domains.30 The PFM images were acquired with long axis of the cantilever at angle of 45° (along pc direction) to in-the-plane crystallographic directions. Figures 2 (a-c) show in-plane PFM as well as simultaneously acquired topographic image of the BFO thin film. The acquired in-plane PFM maps display the typical 4-variant 71° domain stripes in agreement with previous reports.[28, 30] The out-ofplane PFM map (not shown) shows a uniform response and points to monodomain character of the out-of-plane component of the polarization. Next, the effect of stress on domain structure was investigated using a customdesigned bending stage, which allows in-situ PFM imaging. Figures 2(a-i) show the evolution of domain upon application of a varying uniaxial mechanical tensile strain. The direction of the application of uniaxial tensile mechanical strain is indicated in Fig. 2(d). On application 4 ACS Paragon Plus Environment

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of increasing tensile strain (Figs. 2(a-i)), the stripe domains, which appears with bright contrast in PFM phase image expands at the expense of neighbouring domains that appears with dark contrast. For instance, this behaviour (area with in the yellow ellipse, Figs. 2(c,f,i)) can be seen clearly from PFM images acquired during application of mechanical tensile strain of 1.2%. The degree of expansion (contraction) of bright (dark) domains increases with increasing applied mechanical strain. We envision the following scenario for applied straininduced observed domain evolution (for details see supplementary information, section S3, Figure S3). The applied mechanical stress induces instability between the neighbouring inplane domains, and the energy barrier for transitioning to a polarization state along the direction of the applied mechanical stress decreases. Under applied tensile strain, the in-plane polarization oriented parallel to the strain direction remains (more or less) unaffected, while the polarization oriented orthogonally decreases in magnitude and eventually realigns (switches) along the direction of applied mechanical strain. Usually, polarization switching proceeds via both the domain wall motion and nucleation of new domains of opposite polarity. However, in our case (Figure 2), the switching or the domain evolution takes place via domain wall motion only. This is likely due to low activation energy needed for the displacement of an existing domain wall compared to nucleation and growth of new domains (with polarization orthogonal to those of existing domains) within existing domains. Therefore, the wall front acts as a favourable nucleation site for domains with polarization oriented along the direction of the applied strain. As a result, under an applied mechanical stimulus the domain wall can be visualized to displace from its initial position smoothly unitcell by unit-cell resulting in polarization switching and evolution of domains. Another interesting observation is significant roughening of domain walls with increasing applied mechanical strain, which is in sharp contrast to the strain-free state. The increased roughening can be ascribed either to random nucleation of new domains at the wall driving expansion of domain and domain wall motion31, 32, 33 or the disorder potential causing pinning/depinning transitions along the walls during its propagation through a heterogeneous potential landscape. 34 , 35 , 36, 37 At positions free of any pinning sites, the domain wall moves with considerable ease under an applied external stimulus, while at others it remains pinned unless higher activation energy is supplied. As a result, domain wall acquires an irregular shape because of random continuous sequence of pinning-depinning transitions along the wall propagating wall front. Upon release of the applied tensile mechanical strain (Figs. 2(j-l)), the domain patterns were recorded again to obtain further insight into the evolution of the domain structure. Clearly, the domain structure reverts to the pristine state indicating that the domain 5 ACS Paragon Plus Environment


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wall displacement is reversible when strain is released. There is one aspect of the domain walls that has not yet (i.e., immediately after release of stress) fully recovered, i.e. the domain roughness. The domain walls on release of the stress (Fig. 2j) are rougher compared to their pristine state (Fig. 2c). However, we note that from images acquired at increasing times after release of the applied stress points to domain walls becoming increasingly less rough, leading to restoration of pristine state over a period ranging from several tens of minutes to few hours. To recap, 71° domain switching involving opposite directional displacement of consecutive domain walls in the striped domain structure can be reversibly realized on the scale of several microns via modulating the externally applied uniaxial mechanical stress. These results show a remarkably strong effect of mechanical stress on ferroelastic domain structure. We note that the images were meticulously checked and compensated for any drift between the scans to rule out any extraneous origins of the observed behaviour. In further studies, we tried to establish a correlation between polarization variants (inthe-plane), and expansion/contraction of stripe domains with respect to direction of the uniaxial mechanical tensile strain. To do so, firstly, we assign a net in-plane component of the polarization within each stripe domain using well-established approaches (i.e., PFM imaging at two orthogonal orientations, and on electrical poling of sample with a conductive bottom electrode).28,

30, 38

Figure 3(a) shows such a representative in-plane PFM image in which

direction of polarization component in-the-plane within each stripe domain is labelled. Thereafter, a uniaxial mechanical strain of 1.2% was applied in the direction indicated. As discussed earlier, a clear change in the domain structure is visualized (especially at the areas shown by yellow ellipses). In addition, it is clear that the stripe domains with in-plane polarization pointing along the direction of applied mechanical tensile strain expand, while the adjoining domains with orthogonal polarization variant contract. This is also shown by the overlaid color-coded generated contour maps (Figs. 3(c-e)) of PFM images acquired before (bright domains- green colour; dark domainsblack colour) and on application of mechanical strain of 1.2% (bright domains- red colour; dark domainsblack colour). The contours maps quantify the observed change in the domain structure. Clearly, the area of the striped bright domains is increased (red domain area fraction is 54.2 %) on application 1.2 % tensile strain compared to the strain-free state (green domain area fraction 51.8 %). The bright domain area is increased by nearly 5% on application of the mechanical tensile strain of 1.2%. The compressive stress on the other hand has been shown


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to cause switching of ferroelastic domains in PZT in direction orthogonal to the loading axis.39 Similar behaviour was observed in BaTiO3 single crystals under compressive stress in which 90° (a-c domains) domain switching involving nucleation and expansion of c-domains takes place, in addition to the formation of surface confined sawtooth 180° a-domains (to compensate surface charges).24 In near-morphotropic PZT ceramics consisting of randomly oriented crystallites, a variety of ferroelastic domain evolution processes were recorded such as expansion/contraction of stripe domains, rotation of stripes domains by 90°, coalescence of needle domains with orthogonal stripe domains forming herringbone-like domain structure, and wall motion in herringbone-like domains under an applied compressive load.23 For further investigations, we considered BFO (001) samples that had very wellaligned stripe domains and samples with less perfect stripe domain structures. Figure 4 (a) shows contour map of a sample with long-range well-ordered stripe domains comprising of two variants of the polarization. Application of mechanical tensile strain of 1.2% (Figs. 4(bc)) leads to the evolution of domain structure in agreement with the results presented in Figure 3. For well-ordered stripes (Fig. 4(c)), the domain area changes by 6.4% compared to the pristine state. This is in stark contrast to a sample with less perfect stripe domains, where the corresponding change in the domain structure is comparatively smaller at about 3.1%, i.e. only about half than observed changes for sample with perfect stripe domains. The observed much smaller change of domain structure is likely a result of mechanical constraints or interlock of the more complex domain structure that negatively affects the domain wall motion compared to the sample with well-ordered stripe domains.23 We further investigated out-of-plane PFM hysteresis loops that were recorded under different applied mechanical stresses (Figs. 5(a-c)). The obtained results are presented in Figure 5. Clearly, both the loop shape as well coercive voltages (i.e., where the PFM response acquire a minimum value) show significant variation under rather moderate values of the applied mechanical tensile strain. The maximum change is seen at 1.2% tensile strain. The hysteresis loops display a shift along the bias axis to the right, which increases with the applied strain. Therefore, the positive coercive bias ( V c+ ) increases, whereas the negative coercive voltage ( Vc− ) shows a decrease in magnitude with the applied strain (Figs. 5(d-e)). Consequently, the imprint ( Vc+ − Vc− / 2 ), changes from as-grown negative state, and becomes positive under applied tensile strain (Fig. 5(f)). The direction of the observed rightward shift and positive imprint is consistent with earlier reported results on integrated 7 ACS Paragon Plus Environment


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ferroelectric PZT capacitors under mechanical tensile stress.21 Also, the direction of the observed shift in the local piezoresponse loops is opposite to that reported for ferroelectric thin films (PZT) under uniaxial compressive stress,21, 22 and tip-induced compressive stress and stress gradients (in BaTiO3 ultrathin films).10, 40 Further, the comparatively suppressed piezoresponse during positive half of the applied bias run is consistent with increasing positive imprint with applied tensile strain (Fig. 5). These characteristic changes in the loop shape under tensile strain are also contrary to those observed under compressive stress/elastic gradients in which the acquired piezoresponse becomes highly suppressed during negative half of the applied bias cycle.10 One possible explanation for the observed behavior in our case is that application of a tensile strain can result in a decreasing lattice mismatch between the substrate and film along the direction of the applied strain. This lattice mismatch reduction can likely induce an internal elastic field (such as a built-in flexoelectric field) leading to observation of strongly positive imprinted state of the film with increasing applied tensile stress. The applied modulating external strain therefore presents an avenue to tune the magnitude as well as direction of the built-in field in ferroelectric thin films. Conclusion In summary, using an in-situ bending stage and piezoresponse force microscopy we investigated 71° stripe domain structure and wall motion in bismuth ferrite thin films under variable uniaxial tensile strain conditions. We found that the ferroelastic domains with inplane polarization along the direction of applied tensile strain undergo expansion at the expense of neighbouring domains with orthogonal (in-the-plane) polarization direction. Such that during application of tensile strain significant domain wall roughening takes place and consecutive 71° domain walls moves in opposite direction (45° to the tensile strain direction). The changes in the domain structure, and wall motion were completely reversible and revert to their pristine state on release of the applied mechanical strain. Further, the strain-induced changes were found be highly suppressed for stripe domains with short-range order compared to the micron-long well-aligned stripes possibly due to competing mechanical constraints and interlocked domains with short-range character. Finally, the acquired out-of-plane PFM hysteresis loops reveal control of the imprint, coercive voltages and loop shape on application of varying mechanical tensile strain. These results therefore provide a direct insight in to the polarization-strain couplings at the nanoscale in multiferroic thin films and deliver a general outlook for potential applications, such as control of magnetism, 41 , 42 and domain wall


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conduction.43, 44, 45 Any work in this direction would clearly be out of scope for the current study and will be only addressed in future work. Acknowledgement We acknowledge support by the Australian Research Council through Discovery Grants. This work was also supported by the National Research Foundation of Korea via the Creative Research Initiative Center for Lattice Defectronics (contract no. NRF-2017R1A3B1023686). Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. Strain calculation; applied mechanical strain measurements performed inside SPM; mechanism of domain motion under in-situ external stimuli (PDF).



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Figure Captions

Figure 1: Schematic of the BFO (001) ferroelectric domain structure and wall motion on application of tensile mechanical stress. (a-b) Pristine domain structure of the film. (c-d) On application of uniaxial mechanical tensile strain. Illustrations in (b,d) represent a 2D top view of the film.

Figure 2: AFM topography (a,d,g,j) and the simultaneously acquired high-resolution in-plane PFM amplitude (b,e,h,k) and phase (c,f,i.l) images illustrating the impact of stress on domains. PFM imaging in the pristine state (a-c), on application of tensile strain of 0.5% (df), on application of tensile strain of 1.2% (g-i), and upon reversing back to the strain freestate (j-l). Yellow ellipse marked the area where changes in the domain structure can be seen clearly. The cyan colour dotted-line in (d) indicate the direction of application of uniaxial tensile strain.

Figure 3: (a–b) PFM amplitude images acquired at 0% (a), and 1.2 % (b) mechanical tensile strain respectively. (c-d) Contour generated from PFM images in (a-b) at a tensile strain of 0% (c), and 1.2% (d). (e) Overlay of the counter maps in (c-d) visualizing changes in domain pattern under uniaxial tensile strain. Yellow ellipse marked the area where changes in the domain structure can be seen clearly. Black arrows in (a-b) indicate the direction of in-plane component of the polarization for the periodic striped domain structure.

Figure 4: Domain evolution for BFO (001) samples with different characteristics of the stripe domains. (a-c) Contour map of sample with long-range highly-ordered stripe domains before (a), and after application of tensile strain of 1.2% (b). (c) Overlay of the counter maps in (a-b) visualizing changes in domain pattern under uniaxial tensile strain (46.8%, and 53.2% are the green and red domain areas respectively). (d-f) Contour map of sample with less perfect stripe domain structure before (d), and after application of tensile strain of 1.2% (e). (f) Overlay of the counter maps in (d-e) visualizing changes in domain pattern under uniaxial tensile strain (green and red domain areas are 58.1%, and 61.2%, respectively).

Figure 5: (a-c) Local out-of-plane PFM hysteresis loops under different applied tensile stresses. (d-f) Variation of positive (d), negative coercive voltage (e), and imprint (f) respectively as a function of applied tensile strain. 10 ACS Paragon Plus Environment

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Figures

(c)

Figure 1



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Figure 2


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Figure 3



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Figure 4


(a)

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6

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(a.u.)Amplitude, a.u. Amplitude, a.u. Amplitude, a.u. Piezoresponse

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0.5 0.0

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