Supporting Information for
“Visualization and Manipulation of Meta-stable Polarization Variants in Multiferroic
Materials”
Moonkyu Park, Kwangsoo No* and Seungbum Hong*
S1. In-/Out-of-plane PFM images of BiFeO3 thin films at the as-deposited state
S2. Out-of-plane poling process using biased AFM tip
S3. Calculation of polarization charges at the domain boundaries where the vertical domain
switching initiated
S4. In-/Out-of-plane PFM images of the BiFeO3 thin film at the vertically switched state
S5. Retention characteristics of the vertically poled domains in BiFeO3 thin films
S6. Formation mechanism of intermediate polarization variants
S7. In-plane domain configurations with different sample rotation angles of 10 and 30
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S1. In-/Out-of-plane PFM images of BiFeO3 thin films at the as-deposited state
As discussed in the text, we obtained in-/out-of-plane PFM images of BiFeO3 (BFO)
thin film at the as deposited state while rotating the sample from 0° to 180° with an interval
of 30° between each image as shown in Figs. S1 and S2, respectively. The faceted islands that
are seen in topography are the Fe2O3 phase as described in our previous work.1 In order to
avoid any spurious effects caused by Fe2O3 phase and surface morphology, we chose to
analyze a flat area (dashed box in the PFM images) when acquiring the domain configuration.
The ac modulation voltage applied to Pt coated tip (Micromasch, NCS 14 tip) was 0.7 Vrms at
17 kHz. Scan area was 2 μm  2 μm.
Fig. S1. Topography, in-plane PFM amplitude and phase images over the full scan area at
each angular step from 0o to 180o in <001> BFO thin films at the as-deposited state. Scale
bars in Fig. S1 represent 500 nm.
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Fig. S2 shows the out-of-plane PFM images of the same area as that used for in-plane PFM
images in Fig. S1. The PFM phase images showed a uniform bright contrast indicating
downward polarization direction while rotating the sample from 0° to 180° as expected.
Fig. S2. Topography, and out-of plane PFM amplitude and phase images over the full scan
area at each angular step from 0° to 180° in <001> BFO thin films at the as-deposited state.
Scale bars in Fig. S2 represent 500 nm.
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S2. Out-of-plane poling process using biased AFM tip
Fig. S3 (a) shows the out-of-plane PFM images of the same area as that used for
domain configuration in the text with various external voltage steps (-2 V, -2.3 V, -2.5 V, -2.7
V and -3.5 V). The external voltages were applied to the conductive AFM tip. As shown in
Fig. S3 (a), opposite domains were nucleated at the applied voltage of -2.3 V and domain
growth occurred laterally with increasing the external voltage. Finally, at the applied voltage
of -3.5 V, whole scan area was switched to the opposite direction.
Figs. S3. (a) Out-of plane PFM images after poling with various external voltages and (b)
graph of normalized switched area extracted from PFM phase images versus applied external
voltage.
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S3. Calculation of polarization charges at the domain boundaries where the vertical
domain switching initiated
We calculated the polarization charges of the domain boundaries where the vertical
domain switching initiated. In our previous study, we could quantitatively calculate the
amount of polarization charging at the charged domain boundary using the angle between
polarization direction and boundary.2 In Fig. S4 (a), we denoted the 7 regions where vertical
domain switching initiated. In Figs. S4 and S5, we showed the local in-plane domain map
overlapped with vertically switched region (dark brown region in each figure), schematics of
domain boundaries and polarization direction of the switched region and angle between
boundary and polarization directions at each segment of domain boundaries.
Figs. S4. (a) In-plane domain map of BFO thin film at the as deposited state. Local in-plane
domain maps and collected segments of domain boundaries in the area where vertical domain
switching initiated in regions (b) 1, (c) 2 and (d) 3 shown in (a).
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Figs. S5. Local in-plane domain maps and collected segments of domain boundaries in the
areas where vertical domain switching initiated in regions (a) 4, (b) 5, (c) 6 and (d) 7 shown
in Fig. S4 (a).
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In Table SI, we listed the calculated amount of polarization charges per unit length in
the regions mentioned above. We found that the mean value of amount of polarization
charges per unit length was +0.124 ± 0.304. Positive mean value indicates the boundaries are
on average charged in positive state.
Table SI. Amount of polarization charges per unit length at the each region
Region
Polarization charges per unit length
1
-0.088
2
0.683
3
0
4
0.08
5
0.389
6
-0.017
7
-0.177
Mean value
+0.124 ± 0.304
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S4. In-/Out-of-plane PFM images of the BiFeO3 thin film at the vertically switched state
As discussed in the text, we obtained the in-/out-of-plane PFM images after the
poling process while rotating the sample from 0° to 180° with an interval of 30° between
each image as shown in Figs. S6 and S7, respectively.
Fig. S6. In-plane PFM images over the full scan area at each angular step from 0° to 180° in
<001> BFO thin films after the vertical poling process. Scale bars in Fig. S6 represent 500
nm.
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Fig. S7. Out-of plane PFM images over the full scan area at each angular step from 0° to 180°
in <001> oriented BFO thin films after the vertical poling process. Scale bars in Fig. S7
represent 500 nm.
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S5. Retention characteristics of vertically poled domains in BiFeO3 thin films
We performed the retention experiments on the vertically switched BFO thin films to
check the stability of the switched domains. We obtained the out-of-plane PFM images with
elapsed time after the domain switching. Figs. S8 shows the out-of-plane PFM images at the
as deposited state (Fig. S8(a)), just after the vertically switched state by applying external
voltage of -7 V to tip (Fig. S8(b)), and the evolution of out-of-plane PFM amplitude and
phase images as a function of elapsed time at room temperature (Fig. S8(c)). As shown in Fig.
S8, BFO underwent insignificant retention loss, indicative of excellent retention properties.
No domain reversal was found in the region of interest until 6,685 minutes after the poling.
Fig. S8. Out-of-plane PFM images (a) at the as deposited state, (b) just after vertically
switched state. (c) The evolution of out-of-plane PFM amplitude and phase images as a
function of elapsed time.
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Fig. S9 shows normalized domain area extracted from out-of-plane PFM phase
images (red box region in Fig. S8 (c)) and out-of-plane PFM amplitude values (white box
region in Fig. S8 (c)) with the elapsed time. We found that normalized domain area and
amplitude values were almost constant with elapsed time. We believe that excellent retention
properties of vertically switched state might be associated with the simple and
electrostatically stable domain structure of the BFO thin film as described in the text.
Fig. S9. (a) Normalized domain area extracted from out-of-plane PFM phase signals versus
elapsed time graph and (b) out-of-plane PFM amplitude values versus elapsed time graph
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S6. Formation mechanism of intermediate polarization variants
Fig. S10 illustrates formation mechanism of intermediate polarization in epitaxially
grown BFO thin films. Since BFO has a Curie temperature (1100 K) above the films growth
temperature (953 K), BFO nuclei formed during the initial stages in film growth (stage 1 in
Fig. S10), larger than the critical size to exhibit spontaneous polarization, will have their own
ferroelectric polarization variants when they are deposited on the SRO/STO substrate. As the
film grows first in an island mode, each island with its polarization variant will not interact
with the neighboring ones and grow independently, preserving their polarization variants,
until they touch each other. It is highly likely that in this process many of the polarization
directions in the coalesced islands will not be able to fully switch to form a neutral boundary
with the adjacent regions and so charged domain boundary will form (Stage 3 to Stage 6 in
Fig. S10). We believe that the intermediate polarization variants, which deviate from the
ferroelectric easy axes imposed by the rhombohedral crystal symmetry, are formed to act as
mitigating regions to decrease the electrostatic energy at the charged domain boundaries.
Fig. S10. Schematics of intermediate polarization variants formation during film growth;
Stages 1 and 2: Initial film growth, Stages 3, 4 and 5: Emergence of charged domain
boundaries and stage 6: Formation of intermediate polarization variants.
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S7. In-plane domain configurations with different sample rotation angle of 10 and 30
Fig. S11 shows the in-plane polarization directions obtained by AR-PFM method
with different angular resolutions, i.e. sample rotation angle. We identified the in-plane
polarization directions with the angular resolution of 30° (Fig. S11(a)) and 10° (Fig. S11(b))
in the same region. More elaborated in-plane domain configuration was obtained with high
angular resolution (small sample rotation angle). However, the fact that we can observe the
intermediate polarization variants different from the well-defined crystallographic axis
remained the same regardless of the angular resolution.
Fig. S11. In-plane domain direction of BFO thin film obtained by AR-PFM method with
angular resolution of (a) 30° and (b) 10°.
Regarding the concern whether we can measure the ferroelectric domains smaller
than the tip radius, we believe that AR-PFM and conventional PFM can resolve domains
smaller than the tip radius because the ferroelectric materials show the piezoresponse at the
extremely confined area under the tip. There have been studies about high resolution imaging
of ferroelectric domains smaller than tip radius. Rodriguez et al., reported the high resolution
ferroelectric domain imaging (~3 nm resolution) of ferroelectric materials in liquid
condition.3 In addition, Ko et al., also observed that 25 nm sized ferroelectric domain using
the wedge shaped probe with 300 nm in length,4 which implies that tip radius is the important
factor to determine the spatial resolution, but not the only one.
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References
1
M. Park et al., Appl. Phys. Lett. 97, 112907 (2010).
M. Park, S. Hong, J. Kim, J. Hong, K. No, Appl. Phys. Lett. 99, 142909 (2011).
3
B. Rodriguez et al., Phys. Rev. Lett. 96, 237602 (2006).
4
H. Ko et al., Nano Lett. 11, 1428 (2011).
2
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