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Revealing the flexoelectricity in the mixed-phase regions of epitaxial BiFeO3 thin films.

Cheng CE, Liu HJ, Dinelli F, Chen YC, Chang CS, Chien FS, Chu YH - Sci Rep (2015)

Bottom Line: Understanding the elastic response on the nanoscale phase boundaries of multiferroics is an essential issue in order to explain their exotic behaviour.Significantly, the correlation between elastic modulation and piezoresponse across the mixed-phase regions manifests that the flexoelectric effect results in the enhancement of the piezoresponse at the phase boundaries and in the MI regions.This accounts for the giant electromechanical effect in strained mixed-phase BiFeO3 films.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, 30010, Taiwan [2] Department of Applied Physics, Tunghai University, Taichung, 40704, Taiwan.

ABSTRACT
Understanding the elastic response on the nanoscale phase boundaries of multiferroics is an essential issue in order to explain their exotic behaviour. Mixed-phase BiFeO3 films, epitaxially grown on LaAlO3 (001) substrates, have been investigated by means of scanning probe microscopy to characterize the elastic and piezoelectric responses in the mixed-phase region of rhombohedral-like monoclinic (MI) and tilted tetragonal-like monoclinic (MII,tilt) phases. Ultrasonic force microscopy reveal that the regions with low/high stiffness values topologically coincide with the MI/MII,tilt phases. X-ray diffraction strain analysis confirms that the MI phase is more compliant than the MII,tilt one. Significantly, the correlation between elastic modulation and piezoresponse across the mixed-phase regions manifests that the flexoelectric effect results in the enhancement of the piezoresponse at the phase boundaries and in the MI regions. This accounts for the giant electromechanical effect in strained mixed-phase BiFeO3 films.

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(a) Asymmetric θ-2θ diffraction peak to the surface normal of the MI phase extracted from the RSM image (Fig. 3) at H = 0.041 r.l.u. The peak can be better fitted by an asymmetric Gaussian function (red), while a symmetric Gaussian function (blue) is reported only to visualize the asymmetry of the peak. (b) Schematic illustration of the strain gradient of the MI phase near the interface (yellow line) between the MI (brown) and MII (green) phases.
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f5: (a) Asymmetric θ-2θ diffraction peak to the surface normal of the MI phase extracted from the RSM image (Fig. 3) at H = 0.041 r.l.u. The peak can be better fitted by an asymmetric Gaussian function (red), while a symmetric Gaussian function (blue) is reported only to visualize the asymmetry of the peak. (b) Schematic illustration of the strain gradient of the MI phase near the interface (yellow line) between the MI (brown) and MII (green) phases.

Mentions: To estimate the strain magnitude in the mixed-phase MI phase near the MI and MII,tilt interface, a θ-2θ scan with respect to the surface normal of the MI phase (Fig. 5a) is extracted from the RSM image (Fig. 3) at H = 0.041 r.l.u. It shows an asymmetric diffraction peak, indicating that the lattice constant c is not uniform in MI phase. The peak is fitted by an asymmetric Gaussian function and the half widths at half maximum are σh = 6.63×10−3 r.l.u. on the high Qz side and σl = 7.89×10−3 r.l.u. on the low Qz side. σh presents the contributions of the normal distribution of lattice constant and the grain-size broadening. The larger σl indicates there is an additional component (σsg) due to strain gradient at phase boundaries. Assuming a combination of peak widths by , σsg = 4.29×10−3 r.l.u. is obtained, corresponding to a strain of +0.47%. There is a strain in the compliant MI phase at the boundary. To accommodate the lattice mismatch between MI and MII,tilt phases (Fig. 5b), the more compliant MI phase exhibits an expansion of the out-of-plane lattice constant by ΔcMI, whereas ΔcMI become its maximum at the interface. The strain relaxes with the distance apart from the phase boundary. Such a strain gradient supports the findings by UFM and PFM that the flexoeletric effect accounts for the extraordinary polarization in mixed-phase BFO thin films. The enhancement of polarization in the mixed-phase regions is about 55 μC/cm2 from Ref. 22, the region with strain gradient is 17 nm in width (half width of MI regions), and the volume fraction of the MI phase is 50% according to topography. Therefore the flexoelectric coefficient of the MI phase is estimated to be ≈ 1.4 μC/m by Eq. (1), which is comparable to that of perovskite ceramics in paraelectric phases13.


Revealing the flexoelectricity in the mixed-phase regions of epitaxial BiFeO3 thin films.

Cheng CE, Liu HJ, Dinelli F, Chen YC, Chang CS, Chien FS, Chu YH - Sci Rep (2015)

(a) Asymmetric θ-2θ diffraction peak to the surface normal of the MI phase extracted from the RSM image (Fig. 3) at H = 0.041 r.l.u. The peak can be better fitted by an asymmetric Gaussian function (red), while a symmetric Gaussian function (blue) is reported only to visualize the asymmetry of the peak. (b) Schematic illustration of the strain gradient of the MI phase near the interface (yellow line) between the MI (brown) and MII (green) phases.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4308693&req=5

f5: (a) Asymmetric θ-2θ diffraction peak to the surface normal of the MI phase extracted from the RSM image (Fig. 3) at H = 0.041 r.l.u. The peak can be better fitted by an asymmetric Gaussian function (red), while a symmetric Gaussian function (blue) is reported only to visualize the asymmetry of the peak. (b) Schematic illustration of the strain gradient of the MI phase near the interface (yellow line) between the MI (brown) and MII (green) phases.
Mentions: To estimate the strain magnitude in the mixed-phase MI phase near the MI and MII,tilt interface, a θ-2θ scan with respect to the surface normal of the MI phase (Fig. 5a) is extracted from the RSM image (Fig. 3) at H = 0.041 r.l.u. It shows an asymmetric diffraction peak, indicating that the lattice constant c is not uniform in MI phase. The peak is fitted by an asymmetric Gaussian function and the half widths at half maximum are σh = 6.63×10−3 r.l.u. on the high Qz side and σl = 7.89×10−3 r.l.u. on the low Qz side. σh presents the contributions of the normal distribution of lattice constant and the grain-size broadening. The larger σl indicates there is an additional component (σsg) due to strain gradient at phase boundaries. Assuming a combination of peak widths by , σsg = 4.29×10−3 r.l.u. is obtained, corresponding to a strain of +0.47%. There is a strain in the compliant MI phase at the boundary. To accommodate the lattice mismatch between MI and MII,tilt phases (Fig. 5b), the more compliant MI phase exhibits an expansion of the out-of-plane lattice constant by ΔcMI, whereas ΔcMI become its maximum at the interface. The strain relaxes with the distance apart from the phase boundary. Such a strain gradient supports the findings by UFM and PFM that the flexoeletric effect accounts for the extraordinary polarization in mixed-phase BFO thin films. The enhancement of polarization in the mixed-phase regions is about 55 μC/cm2 from Ref. 22, the region with strain gradient is 17 nm in width (half width of MI regions), and the volume fraction of the MI phase is 50% according to topography. Therefore the flexoelectric coefficient of the MI phase is estimated to be ≈ 1.4 μC/m by Eq. (1), which is comparable to that of perovskite ceramics in paraelectric phases13.

Bottom Line: Understanding the elastic response on the nanoscale phase boundaries of multiferroics is an essential issue in order to explain their exotic behaviour.Significantly, the correlation between elastic modulation and piezoresponse across the mixed-phase regions manifests that the flexoelectric effect results in the enhancement of the piezoresponse at the phase boundaries and in the MI regions.This accounts for the giant electromechanical effect in strained mixed-phase BiFeO3 films.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, 30010, Taiwan [2] Department of Applied Physics, Tunghai University, Taichung, 40704, Taiwan.

ABSTRACT
Understanding the elastic response on the nanoscale phase boundaries of multiferroics is an essential issue in order to explain their exotic behaviour. Mixed-phase BiFeO3 films, epitaxially grown on LaAlO3 (001) substrates, have been investigated by means of scanning probe microscopy to characterize the elastic and piezoelectric responses in the mixed-phase region of rhombohedral-like monoclinic (MI) and tilted tetragonal-like monoclinic (MII,tilt) phases. Ultrasonic force microscopy reveal that the regions with low/high stiffness values topologically coincide with the MI/MII,tilt phases. X-ray diffraction strain analysis confirms that the MI phase is more compliant than the MII,tilt one. Significantly, the correlation between elastic modulation and piezoresponse across the mixed-phase regions manifests that the flexoelectric effect results in the enhancement of the piezoresponse at the phase boundaries and in the MI regions. This accounts for the giant electromechanical effect in strained mixed-phase BiFeO3 films.

No MeSH data available.


Related in: MedlinePlus