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Nanoscale Mechanical Softening of Morphotropic BiFeO3 Yooun Heo, Byong-Kweon Jang, Seung Jin Kim, Chan-Ho Yang, and Jan Seidel* In recent years, substantial efforts have been made to explore novel materials and device concepts based on interfaces and domain walls in oxides.[1–8] BiFeO3 (BFO) has emerged as a promising lead-free multiferroic with copious pivotal developments including electromechanical and magnetoelectric coupling,[9] domain wall conductivity[10] and bias induced semiconductor-insulator transitions.[11] Much attention has been drawn to a morphotropic phase boundary[12,13] where the crystal structure changes abruptly and piezoelectric responses are maximal. In such a transition period, originally found in chemically complicated and engineered lead based oxides, huge responses to external stimuli are often found.[14–16] Piezoelectrics based on this morphotropic phase boundary display large electromechanical properties at phase boundaries, for example, between a rhombohedral and tetragonal phase.[17] The recent discovery of a strain-driven morphotropic phase boundary (MPB) in BFO films grown on (001) LaAlO3 (LAO) has attracted further interest into this multiferroic.[18–24] The compressive strain imposed by the underlying substrate can stabilize a tetragonal-like phase. These highly strained thin films indeed exhibit mixed-phase regions consisting of rhombohedral-like (R) stripe patches embedded into a tetragonal-like (T) matrix with the capacity to selectively switch between the two phases by external electric field.[25–28] Electric-field dependent studies show that a T phase can be reversibly converted into R phase and moreover, a large electric-field-induced strain of over 5% has been reported in BFO film consisting of nanoscale mixture of T and R phases.[29] The mixed phase film exhibits a giant piezoelectric d33 coefficient enhanced by phase boundary motion. Lately, it was identified that a strong magnetic moment exists within the distorted R-phase rather than at the boundaries.[30] Nevertheless, the origin of such intriguing properties and distinct behavior is not fully unveiled. Theoretical approaches via first-principles calculations suggest interesting scenarios Y. Heo, Dr. J. Seidel School of Materials Science and Engineering University of New South Wales Sydney, NSW, 2052, Australia E-mail: [email protected] B.-K. Jang, S. J. Kim Department of Physics Korea Advanced Institute of Science and Technology Daejeon 305-701, Republic of Korea Prof. C.-H. Yang Department of Physics Korea Advanced Institute of Science and Technology Daejeon 305-701, Republic of Korea Prof. C.-H. Yang Institute for the NanoCentury, KAIST Daejeon 305-701, Republic of Korea
DOI: 10.1002/adma.201401958
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to relate aforementioned properties of the mixture of R-T phases with elastic susceptibility as well as misfit strain.[31,32] The strong correlation between T-R interfaces and their enhanced properties serve as a motive to achieve precise control of the R-phase with understanding of its morphology and orientation. In addition, complete control of mixed phase regions can lead to a wide range of technologically notable applications. Very recently, Lu et al. demonstrated that the stress gradient induced by the AFM tip can switch the polarization in a ferroelectric material.[33] Moreover, abnormal Poisson’s ratios and linear compressibility have been observed in perovskite materials.[34] In the present study we explore the mechanism of force induced phase transitions in this morphotropic system. By precisely adjusting the applied force in AFM measurements, we have characterized the phase transformation from T to R phases in detail and achieved full control over the reversible nanoscale phase transformation. Extraordinary soft elastic behavior is observed during the phase transition with values of Young's modulus being two orders of magnitude lower compared to typical ionic solids. Our findings open new pathways to controllably write ferroelectric memory bits by mechanical force in data storage devices. The geometry of our experiment is described in Figure 1. The mechanical force is induced by a tip of an AFM on a minuscule point in T phase areas as described in a schematic diagram in Figure 1a. By controlling the setpoint of the AFM, the stress induced by the tip can be adjusted. Several tip forces have been used, pushing the AFM tip vertically down on our sample to investigate phase switching behavior. Exerting inadequate tip-force results in no change of phases and the area remains the same as the initial condition of T phases. Sufficient tip forces, on the other hand, lead to structural phase transition from T to R phases as illustrated in Figure 1b. In AFM topography scans R phases appear as dark stripes which correspond to a lower height due to their reduced c-axis parameter. As a prototype system to investigate these force induced phase transitions, we have chosen a 35 nm thick BFO sample grown on LAO substrate, consisting mostly of T phase in the asgrown film. To illustrate the force induced phase change behaviour, scanning with a set of tip-forces is performed. Topography images of 3 × 3 µm2 areas where force was applied are shown in Figure 2. The corrugated surface features show atomically flat terraces of the thin film with a step height of 1 unit cell. Figure 2a shows the initial state of the film, consisting of almost pure T phase with very few small stripes of R phase. This characteristic morphology results from the lattice mismatch between the BFO film and LAO substrate.[18] After applying 1100 nN in a 2 × 2 µm2 area marked by a white square in Figure 2a, only a tiny pressure-induced R phase is created locally (Figure 2b). As shown in Figure 2c, scanning the same
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area with 2200 nN is sufficient to induce the phase transitions from bright T phase regions to dark R phase needles. This clearly demonstrates the effect of mechanical switching from the applied tip force. With higher force, the density of R phase becomes larger and its morphology looks more evenly sized and clear. Moreover, it is interesting to note that the dark needles seem to be densely oriented in arrays with the majority of R stripes along the BFO [001] direction. In addition, angle-resolved AFM scans are used to study the impact of directional scans by varying scan angles on phase transformations though no clear trend is observed in orientations or distribution of the stripes created by force-switching (see supplementary Figure S1 and S2). It is also important to point out that mechanical switching induces ferroelectric polarization switching as well as phase transitions at the same time during the loading event via scanning with force and local application of force (see supplementary Figures S3 and S4). Precise control of the phase transition with the ability to accurately locate the emergence of the R phase stripes can open a new pathway and enable applications in storage memory. To further investigate phase transitions induced by mechanical force, we explore force-switching by selectively applying tip-force on locally defined spots of the T phase region
Figure 2. Topography scans of the BFO film after applying force via scanning AFM tip: (a) 0 nN (initial state) (b) 1100 nN (c) 2200 nN. (d) 3300 nN. (e) 4400 nN (f) 5500 nN. The square in a) indicates where force was applied. Image size is 3 × 3 µm2.
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Figure 1. Schematic description of AFM probe induced force switching in highly strained BFO thin films. (a) Pushing the tip against the material surface can induce the T-R phase transition. (b) Cross-sectional illustration of structural phase transition as a consequence of force switching.
without scanning. The duration of applying force is consistently set to one second for these local switching measurements. A topography scan is shown in Figure 3a with all the local R phase areas created due to phase transitions from mechanical tip-force. Unlike the set of scans with different loading forces, local switching by pressing the tip onto the sample surface is found to be viable by a minimal force of 110 nN as proved by minuscule indentations on the bright T phase. With 330 nN tip-force, phase switching is more clearly initiated for four locations except one, as seen by consistent formation of dark R phase stripes shown in the scan. The onset of forceswitching therefore is found to be between 110 nN and 330 nN. Interestingly, the R phase stripes overall look relatively uniform and compressed in a form of fine spots as compared to the needle-like shape formed during tip scanning. We note that R phase stripes created by higher force look slightly deeper and more distinguished as compared to ones created with smaller force. In fact, T-R phase transformations during scanning with force, and locally applying force, show distinct switching onsets, which have occurred at different force ranges: 1100–2200 nN and 110-330 nN respectively. Extra scans with the use of these force ranges at different scan rates are taken to investigate the relation between the tip-surface contact time and force-switching behavior. In Figure 3c, 330nN and 1100 nN tip-forces are applied in scanning on defined scan areas at 10, 1.0 and 0.1 Hz scan rates (per scan line). Scanning with 330 nN leads to phase switching only at a scan rate of 0.1 Hz, which corresponds to a tip-surface contact time of 1 s/100 nm, which is comparable to locally applying force (∼1 s per local switching point). On the other hand, scanning with 1100 nN clearly shows an increasing trend of phase switching as scan rate decreases. Therefore, these scanning results demonstrate the effect of tip-surface contact time, which account for the observed differences in previous force-switching experiments by scanning with force and locally applying force. The detailed morphology of the formed R phase stripes is analysed by taking cross-sectional profiles. In the experiment, a set of several tip-forces is implemented for ten locations and average depth and width of the formed R phase stripes are calculated. Figure 3b shows the average width and depth against force. We note that the R phase stripes formed by switching are correlated to the load of tip-force applied on the surface. As force increases, the width and depth also increase correspondingly as a result of sufficient switching. In addition to the aforementioned phase switching studies, we explore the nanoscale elasticity of this specific morphotropic BFO by extracting several mechanical properties including Young's modulus and transition yield strength. In order to find these values, force-distance spectroscopy is utilized to investigate phase switching behavior during the application of varying force. In this experiment, a minimal loading force is firstly found to ensure that landing of the tip with such force leads to no change of phase transition. By running a force-distance curve with increasing and decreasing tip load force gradually, the onset of phase switching is obtained from a pronounced kink in the force-distance curve (for details see supplementary Figure S4). After calibration of the tip force in relation to
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Figure 3. Phase transition behavior for locally applied force. (a) Topography of the BFO thin film after applying a set of different forces for local T-R phase switching. Image size is 3 × 3 µm2. (b) Cross-section profile analysis of locally created R phase areas by AFM tip-force. Average depth and width of created R phase areas (averaged over 10 data points for every force point) is plotted against tip force. Error bars are also presented to show the variability of the values. (c) Extra set of topography scans after applying 330 nN and 1100 nN at 10, 1.0 and 0.1 Hz scanning rates for varying tip-surface contact time.
the tip position, a depth-force curve is plotted as shown in Figure 4a. The onset force for T-R phase transformation is found to be 564 nN. The successful phase transformation was proven by a topography scan in non-contact mode after running the spectroscopy curve. By analyzing the elastic region of the depth-force curve before the phase transition happens, Young’s modulus E can be calculated by a series of equations in the following way:
E=
σ ∈n
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where σ is the applied stress and ∈ n is the strain. σ is given by the relation: F F = (2) A πr 2 where F is the applied force and A is the contact area on which the force is applied. This contact area can be calculated in first approximation by using the tip radius r.[35,36] Furthermore, the normal strain ∈ n can be directly inferred from the AFM height measurement given that u is the local depth change of the stripe upon phase transition and l is the thickness of the BFO thin film:
σ=
Figure 4. Mechanical properties of BFO film by performing force-distance spectroscopy in AFM. (a) Depth against force plot based on the force-distance curve. (b) Stress against strain plot. Both curves show elastic region used for calculating Young’s modulus and the onset point of T-R transformation as well as plastic deformation region.
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supplementary Figure S5). Scanning with applying a bias of 8V is performed on the surrounding area of the R stripe. After this scan, the R phase bit is completely removed as a result of electrically induced phase transformation back to T phase. Moreover, large fully recoverable strain is observed in our BFO sample with the ability to withstand strains over 4% by mechanical force without cracking and recovering its original phase Figure 5. Complete control of nanoscale phase transformation: Erasing densely written upon heating or applying electric field. ThereR-phase bits by heating. (a) Topography before writing R-phase bits, (b) after writing an array fore, the full control of phase transformations of R-phase areas with 550nN, (c) after heating to 300 °C for 5 minutes. by such robust techniques offers new pathways to write ferroelectric memory bits by u (3) mechanical force in data storage devices.[52] In addition to this, ∈n = l mechanical writing by AFM tip enables highly localized and Calculating E for our given system using values r = 35 nm, controlled phase switching, allowing fabrication of high density u = 0.33 nm and l = 35 nm yields a value for Young’s modulus readable information, which is almost free of obstacles such as of E = 1.54 × 1010 Nm−2. This value is surprisingly low comleakage and dielectric breakdown because no bias is involved. In summary, we have demonstrated a mechanical switching pared to typical values of Young's modulus for bulk BFO in technique to form phase transformed areas in thin films particular[37–39] and ionic solids in general which are typically which might be exploited to yield various soft elastic areas with found to be in the 1011 Nm−2 range due to the properties of the greatly reduced Young's modulus on the nanoscale. It will be constituting ions.[40] In addition, this elastic modulus is even less of great interest to also further investigate new possibilities than one seventh of that of typical PZT films, which is known for the presence of exotic low-dimensional electronic conto be 1.11 × 1011 Nm−2.[41] The stress-strain relationship is also duction[53] using various forms of chemical doping in this plotted as shown in Figure 4b and we can also directly infer the 8 −2 transition yield strength, which is 1.47 × 10 Nm . Both values material.[54,55] Recently it has been shown that due to the are comparable to properties of some polymers such as nylon mechanically susceptible nature of morphotropic phase and PMMA and therefore, this demonstrates an unusually soft boundaries, even small doping values can have a great impact elastic behavior of morphotropic systems on the nanoscale.[42] on phase boundary formation.[56] Since the morphotropic It is noteworthy to compare this extraordinary softening phase boundary is likely to have its peculiar ferroelectric and behavior of morphotropic BFO during phase transformation magnetic orders, which are possibly tied to the anisotropic with superelasticity (SE) of shape memory materials where crystalline structure,[57–59] the present study may contribute the underlying mechanism is stress-induced martensitic to new ferroelastic devices and ferroelectric memories comtransformation.[43,44] Unlike superelastic materials with the bined with electronic, magnetic, and ferroelectric properties in morphotropic systems. ability to fully recover between austenite and martensite phase upon loading and unloading, our morphotropic BFO film undergoes phase switching from T to R phase by loading of Supporting Information the AFM tip force whereas unloading the force results in no back-switching from R to T. Moreover, superelastic materials This material is available from the Wiley Online Library or from the exhibit pronounced hysteresis during forward and backward author. phase transformation while dissipating energy as heat, leading to a wide variety of actuation, energy dampling, and energyAcknowledgements harvesting applications.[45–49] In addition, we explore the complete control of the nanoscale This work was supported by the National Research Foundation of Korea phase transformation by thermal activation and electrical funded by the Ministry of Education, Science, and Technology (contract field. For this we first created defined arrays of force-written nos. 2011-0016133 and NRF-2013S1A2A2035418). We also acknowledge support by the Australian Research Council under grant numbers R phase “bits” in a pure T phase area (Figure 5a and 5b). FT110100523, DP140100463, and DP140102849. After heating the sample to 300 °C for 5 minutes, the written R phase bits are found to be erased completely as seen in Received: May 1, 2014 Figure 4c. Hence, the morphotropic transformation is fully Revised: September 4, 2014 reversible, similar to well-known martensitic transformations Published online: October 18, 2014 in shape memory materials.[50,51] Heating of transformed written R phase areas leads to a full erasure of all previous patterns to produce a “clean state” that exactly resembles the sample area before the writing process. To achieve more strict [1] J. F. Scott, E. K. H. Salje, M. A. Carpenter, Phys. Rev. Lett. 2012, 109, and selective control, electrical field back-switching can be 187601. used to remove only a specific written R phase bit at a certain [2] H. Y. Hwang, Y. Iwasa, M. Kawasaki, B. Keimer, N. Nagaosa, location rather than the whole matrix of the stored bits (see Y. Tokura, Nat. Mater. 2012, 11, 103.
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Nanoscale Mechanical Softening of Morphotropic BiFeO3 Yooun Heo, Byong-Kweon Jang, Seung Jin Kim, Chan-Ho Yang, and Jan Seidel* In recent years, substantial efforts have been made to explore novel materials and device concepts based on interfaces and domain walls in oxides.[1–8] BiFeO3 (BFO) has emerged as a promising lead-free multiferroic with copious pivotal developments including electromechanical and magnetoelectric coupling,[9] domain wall conductivity[10] and bias induced semiconductor-insulator transitions.[11] Much attention has been drawn to a morphotropic phase boundary[12,13] where the crystal structure changes abruptly and piezoelectric responses are maximal. In such a transition period, originally found in chemically complicated and engineered lead based oxides, huge responses to external stimuli are often found.[14–16] Piezoelectrics based on this morphotropic phase boundary display large electromechanical properties at phase boundaries, for example, between a rhombohedral and tetragonal phase.[17] The recent discovery of a strain-driven morphotropic phase boundary (MPB) in BFO films grown on (001) LaAlO3 (LAO) has attracted further interest into this multiferroic.[18–24] The compressive strain imposed by the underlying substrate can stabilize a tetragonal-like phase. These highly strained thin films indeed exhibit mixed-phase regions consisting of rhombohedral-like (R) stripe patches embedded into a tetragonal-like (T) matrix with the capacity to selectively switch between the two phases by external electric field.[25–28] Electric-field dependent studies show that a T phase can be reversibly converted into R phase and moreover, a large electric-field-induced strain of over 5% has been reported in BFO film consisting of nanoscale mixture of T and R phases.[29] The mixed phase film exhibits a giant piezoelectric d33 coefficient enhanced by phase boundary motion. Lately, it was identified that a strong magnetic moment exists within the distorted R-phase rather than at the boundaries.[30] Nevertheless, the origin of such intriguing properties and distinct behavior is not fully unveiled. Theoretical approaches via first-principles calculations suggest interesting scenarios Y. Heo, Dr. J. Seidel School of Materials Science and Engineering University of New South Wales Sydney, NSW, 2052, Australia E-mail: [email protected] B.-K. Jang, S. J. Kim Department of Physics Korea Advanced Institute of Science and Technology Daejeon 305-701, Republic of Korea Prof. C.-H. Yang Department of Physics Korea Advanced Institute of Science and Technology Daejeon 305-701, Republic of Korea Prof. C.-H. Yang Institute for the NanoCentury, KAIST Daejeon 305-701, Republic of Korea
DOI: 10.1002/adma.201401958
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to relate aforementioned properties of the mixture of R-T phases with elastic susceptibility as well as misfit strain.[31,32] The strong correlation between T-R interfaces and their enhanced properties serve as a motive to achieve precise control of the R-phase with understanding of its morphology and orientation. In addition, complete control of mixed phase regions can lead to a wide range of technologically notable applications. Very recently, Lu et al. demonstrated that the stress gradient induced by the AFM tip can switch the polarization in a ferroelectric material.[33] Moreover, abnormal Poisson’s ratios and linear compressibility have been observed in perovskite materials.[34] In the present study we explore the mechanism of force induced phase transitions in this morphotropic system. By precisely adjusting the applied force in AFM measurements, we have characterized the phase transformation from T to R phases in detail and achieved full control over the reversible nanoscale phase transformation. Extraordinary soft elastic behavior is observed during the phase transition with values of Young's modulus being two orders of magnitude lower compared to typical ionic solids. Our findings open new pathways to controllably write ferroelectric memory bits by mechanical force in data storage devices. The geometry of our experiment is described in Figure 1. The mechanical force is induced by a tip of an AFM on a minuscule point in T phase areas as described in a schematic diagram in Figure 1a. By controlling the setpoint of the AFM, the stress induced by the tip can be adjusted. Several tip forces have been used, pushing the AFM tip vertically down on our sample to investigate phase switching behavior. Exerting inadequate tip-force results in no change of phases and the area remains the same as the initial condition of T phases. Sufficient tip forces, on the other hand, lead to structural phase transition from T to R phases as illustrated in Figure 1b. In AFM topography scans R phases appear as dark stripes which correspond to a lower height due to their reduced c-axis parameter. As a prototype system to investigate these force induced phase transitions, we have chosen a 35 nm thick BFO sample grown on LAO substrate, consisting mostly of T phase in the asgrown film. To illustrate the force induced phase change behaviour, scanning with a set of tip-forces is performed. Topography images of 3 × 3 µm2 areas where force was applied are shown in Figure 2. The corrugated surface features show atomically flat terraces of the thin film with a step height of 1 unit cell. Figure 2a shows the initial state of the film, consisting of almost pure T phase with very few small stripes of R phase. This characteristic morphology results from the lattice mismatch between the BFO film and LAO substrate.[18] After applying 1100 nN in a 2 × 2 µm2 area marked by a white square in Figure 2a, only a tiny pressure-induced R phase is created locally (Figure 2b). As shown in Figure 2c, scanning the same
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area with 2200 nN is sufficient to induce the phase transitions from bright T phase regions to dark R phase needles. This clearly demonstrates the effect of mechanical switching from the applied tip force. With higher force, the density of R phase becomes larger and its morphology looks more evenly sized and clear. Moreover, it is interesting to note that the dark needles seem to be densely oriented in arrays with the majority of R stripes along the BFO [001] direction. In addition, angle-resolved AFM scans are used to study the impact of directional scans by varying scan angles on phase transformations though no clear trend is observed in orientations or distribution of the stripes created by force-switching (see supplementary Figure S1 and S2). It is also important to point out that mechanical switching induces ferroelectric polarization switching as well as phase transitions at the same time during the loading event via scanning with force and local application of force (see supplementary Figures S3 and S4). Precise control of the phase transition with the ability to accurately locate the emergence of the R phase stripes can open a new pathway and enable applications in storage memory. To further investigate phase transitions induced by mechanical force, we explore force-switching by selectively applying tip-force on locally defined spots of the T phase region
Figure 2. Topography scans of the BFO film after applying force via scanning AFM tip: (a) 0 nN (initial state) (b) 1100 nN (c) 2200 nN. (d) 3300 nN. (e) 4400 nN (f) 5500 nN. The square in a) indicates where force was applied. Image size is 3 × 3 µm2.
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Figure 1. Schematic description of AFM probe induced force switching in highly strained BFO thin films. (a) Pushing the tip against the material surface can induce the T-R phase transition. (b) Cross-sectional illustration of structural phase transition as a consequence of force switching.
without scanning. The duration of applying force is consistently set to one second for these local switching measurements. A topography scan is shown in Figure 3a with all the local R phase areas created due to phase transitions from mechanical tip-force. Unlike the set of scans with different loading forces, local switching by pressing the tip onto the sample surface is found to be viable by a minimal force of 110 nN as proved by minuscule indentations on the bright T phase. With 330 nN tip-force, phase switching is more clearly initiated for four locations except one, as seen by consistent formation of dark R phase stripes shown in the scan. The onset of forceswitching therefore is found to be between 110 nN and 330 nN. Interestingly, the R phase stripes overall look relatively uniform and compressed in a form of fine spots as compared to the needle-like shape formed during tip scanning. We note that R phase stripes created by higher force look slightly deeper and more distinguished as compared to ones created with smaller force. In fact, T-R phase transformations during scanning with force, and locally applying force, show distinct switching onsets, which have occurred at different force ranges: 1100–2200 nN and 110-330 nN respectively. Extra scans with the use of these force ranges at different scan rates are taken to investigate the relation between the tip-surface contact time and force-switching behavior. In Figure 3c, 330nN and 1100 nN tip-forces are applied in scanning on defined scan areas at 10, 1.0 and 0.1 Hz scan rates (per scan line). Scanning with 330 nN leads to phase switching only at a scan rate of 0.1 Hz, which corresponds to a tip-surface contact time of 1 s/100 nm, which is comparable to locally applying force (∼1 s per local switching point). On the other hand, scanning with 1100 nN clearly shows an increasing trend of phase switching as scan rate decreases. Therefore, these scanning results demonstrate the effect of tip-surface contact time, which account for the observed differences in previous force-switching experiments by scanning with force and locally applying force. The detailed morphology of the formed R phase stripes is analysed by taking cross-sectional profiles. In the experiment, a set of several tip-forces is implemented for ten locations and average depth and width of the formed R phase stripes are calculated. Figure 3b shows the average width and depth against force. We note that the R phase stripes formed by switching are correlated to the load of tip-force applied on the surface. As force increases, the width and depth also increase correspondingly as a result of sufficient switching. In addition to the aforementioned phase switching studies, we explore the nanoscale elasticity of this specific morphotropic BFO by extracting several mechanical properties including Young's modulus and transition yield strength. In order to find these values, force-distance spectroscopy is utilized to investigate phase switching behavior during the application of varying force. In this experiment, a minimal loading force is firstly found to ensure that landing of the tip with such force leads to no change of phase transition. By running a force-distance curve with increasing and decreasing tip load force gradually, the onset of phase switching is obtained from a pronounced kink in the force-distance curve (for details see supplementary Figure S4). After calibration of the tip force in relation to
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Figure 3. Phase transition behavior for locally applied force. (a) Topography of the BFO thin film after applying a set of different forces for local T-R phase switching. Image size is 3 × 3 µm2. (b) Cross-section profile analysis of locally created R phase areas by AFM tip-force. Average depth and width of created R phase areas (averaged over 10 data points for every force point) is plotted against tip force. Error bars are also presented to show the variability of the values. (c) Extra set of topography scans after applying 330 nN and 1100 nN at 10, 1.0 and 0.1 Hz scanning rates for varying tip-surface contact time.
the tip position, a depth-force curve is plotted as shown in Figure 4a. The onset force for T-R phase transformation is found to be 564 nN. The successful phase transformation was proven by a topography scan in non-contact mode after running the spectroscopy curve. By analyzing the elastic region of the depth-force curve before the phase transition happens, Young’s modulus E can be calculated by a series of equations in the following way:
E=
σ ∈n
(1)
where σ is the applied stress and ∈ n is the strain. σ is given by the relation: F F = (2) A πr 2 where F is the applied force and A is the contact area on which the force is applied. This contact area can be calculated in first approximation by using the tip radius r.[35,36] Furthermore, the normal strain ∈ n can be directly inferred from the AFM height measurement given that u is the local depth change of the stripe upon phase transition and l is the thickness of the BFO thin film:
σ=
Figure 4. Mechanical properties of BFO film by performing force-distance spectroscopy in AFM. (a) Depth against force plot based on the force-distance curve. (b) Stress against strain plot. Both curves show elastic region used for calculating Young’s modulus and the onset point of T-R transformation as well as plastic deformation region.
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supplementary Figure S5). Scanning with applying a bias of 8V is performed on the surrounding area of the R stripe. After this scan, the R phase bit is completely removed as a result of electrically induced phase transformation back to T phase. Moreover, large fully recoverable strain is observed in our BFO sample with the ability to withstand strains over 4% by mechanical force without cracking and recovering its original phase Figure 5. Complete control of nanoscale phase transformation: Erasing densely written upon heating or applying electric field. ThereR-phase bits by heating. (a) Topography before writing R-phase bits, (b) after writing an array fore, the full control of phase transformations of R-phase areas with 550nN, (c) after heating to 300 °C for 5 minutes. by such robust techniques offers new pathways to write ferroelectric memory bits by u (3) mechanical force in data storage devices.[52] In addition to this, ∈n = l mechanical writing by AFM tip enables highly localized and Calculating E for our given system using values r = 35 nm, controlled phase switching, allowing fabrication of high density u = 0.33 nm and l = 35 nm yields a value for Young’s modulus readable information, which is almost free of obstacles such as of E = 1.54 × 1010 Nm−2. This value is surprisingly low comleakage and dielectric breakdown because no bias is involved. In summary, we have demonstrated a mechanical switching pared to typical values of Young's modulus for bulk BFO in technique to form phase transformed areas in thin films particular[37–39] and ionic solids in general which are typically which might be exploited to yield various soft elastic areas with found to be in the 1011 Nm−2 range due to the properties of the greatly reduced Young's modulus on the nanoscale. It will be constituting ions.[40] In addition, this elastic modulus is even less of great interest to also further investigate new possibilities than one seventh of that of typical PZT films, which is known for the presence of exotic low-dimensional electronic conto be 1.11 × 1011 Nm−2.[41] The stress-strain relationship is also duction[53] using various forms of chemical doping in this plotted as shown in Figure 4b and we can also directly infer the 8 −2 transition yield strength, which is 1.47 × 10 Nm . Both values material.[54,55] Recently it has been shown that due to the are comparable to properties of some polymers such as nylon mechanically susceptible nature of morphotropic phase and PMMA and therefore, this demonstrates an unusually soft boundaries, even small doping values can have a great impact elastic behavior of morphotropic systems on the nanoscale.[42] on phase boundary formation.[56] Since the morphotropic It is noteworthy to compare this extraordinary softening phase boundary is likely to have its peculiar ferroelectric and behavior of morphotropic BFO during phase transformation magnetic orders, which are possibly tied to the anisotropic with superelasticity (SE) of shape memory materials where crystalline structure,[57–59] the present study may contribute the underlying mechanism is stress-induced martensitic to new ferroelastic devices and ferroelectric memories comtransformation.[43,44] Unlike superelastic materials with the bined with electronic, magnetic, and ferroelectric properties in morphotropic systems. ability to fully recover between austenite and martensite phase upon loading and unloading, our morphotropic BFO film undergoes phase switching from T to R phase by loading of Supporting Information the AFM tip force whereas unloading the force results in no back-switching from R to T. Moreover, superelastic materials This material is available from the Wiley Online Library or from the exhibit pronounced hysteresis during forward and backward author. phase transformation while dissipating energy as heat, leading to a wide variety of actuation, energy dampling, and energyAcknowledgements harvesting applications.[45–49] In addition, we explore the complete control of the nanoscale This work was supported by the National Research Foundation of Korea phase transformation by thermal activation and electrical funded by the Ministry of Education, Science, and Technology (contract field. For this we first created defined arrays of force-written nos. 2011-0016133 and NRF-2013S1A2A2035418). We also acknowledge support by the Australian Research Council under grant numbers R phase “bits” in a pure T phase area (Figure 5a and 5b). FT110100523, DP140100463, and DP140102849. After heating the sample to 300 °C for 5 minutes, the written R phase bits are found to be erased completely as seen in Received: May 1, 2014 Figure 4c. Hence, the morphotropic transformation is fully Revised: September 4, 2014 reversible, similar to well-known martensitic transformations Published online: October 18, 2014 in shape memory materials.[50,51] Heating of transformed written R phase areas leads to a full erasure of all previous patterns to produce a “clean state” that exactly resembles the sample area before the writing process. To achieve more strict [1] J. F. Scott, E. K. H. Salje, M. A. Carpenter, Phys. Rev. Lett. 2012, 109, and selective control, electrical field back-switching can be 187601. used to remove only a specific written R phase bit at a certain [2] H. Y. Hwang, Y. Iwasa, M. Kawasaki, B. Keimer, N. Nagaosa, location rather than the whole matrix of the stored bits (see Y. Tokura, Nat. Mater. 2012, 11, 103.
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