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MR45CH03-Chu ARI 27 December 2014 13:31
a
15
10
5
0
Height (nm)
0.0 0.5 1.0 1.5
–6
–4
–2
0
Displacement (nm)
Distance (µm)
As grown
Switched
Recovered
b
dc
2 µm
50 nm 40 nm
Figure 4
Electric-field-induced R-T phase transition in mixed-phase BiFeO
3
films. (a) High-resolution AFM image of a mixed-phase sample.
The blue outlined area indicates a pure-T phase converted from the mixed phase when subjected to a scanning-probe electric field of
+1,500 kV/cm. The red outlined area indicates a recovery of the mixed phase from a pure-T phase with a probe electric field of
−600 kV/cm. (b) Electric-field-induced surface height change before and after the R-T phase transition. (c,d ) In situ TEM images of
the R-T phase transition induced by (c) mechanical force and (d ) electric field, respectively. The yellow dashed lines outline the R-T
phase boundaries, whereas the red dashed arrows indicate the surface displacements of mixed-phase BiFeO
3
films. Adapted with
permission from Reference 56. Copyright 2011, Nature Publishing Group.
image in Figure 4a. As this figure shows, the application of an electric field can switch stripe-like
mixed-phase regions to flat morphologies, and a reversed electric field can turn the flat regions
back to the mixed-phase state. This reversible process is interpreted to be an electric-field-induced
structural phase transition, accompanied by a significant average surface displacement in this area
of 1 to 2 nm for 100-nm-thick films, i.e., an electric-field-induced strain of 1–2% (Figure 4b).
To confirm that the observations detailed above are the result of a field-induced structural phase
transition and to gain more insight into its atomic-scale origins, in situ TEM experiments were
conducted by electrically and mechanically perturbing the mixed-phase films (Figure 4c,d). The
application of mechanical pressure by using a nanoscale tip resulted in a structural change from
the mixed phase to the pure-R-like phase accompanied by an approximately 5% strain. In contrast,
the application of an electric field by using a conductive nanoscale tip resulted in a structural
change from the mixed phase to the pure-T-like phase with an approximately −5% strain. The
AFM and TEM evidence suggests that achieving a >10% cumulative strain is possible via a
combination of mechanical force and electric field by interconverting the R- and T-like phases.
To reveal more functionality that can be created on the basis of the electric-field control of
lattice, much attention has been paid to the magnetic properties of mixed-phase BFO films because
bulk BFO is a well-known ferroelectric and canted antiferromagnet. Photoemission electron mi-
croscopy (PEEM) imaging with X-ray magnetic circular dichroism (XMCD) analysis is a powerful
method by which to study the magnetic response of a material with nanoscale spatial resolution
3.6 Yang et al.
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