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.
Changes may still occur before final publication online and in print
Annu. Rev. Mater. Res. 2015.45. Downloaded from www.annualreviews.org
Access provided by University of Nebraska - Lincoln on 02/02/15. For personal use only.
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