Dielectric and piezoelectric nonlinearities in oriented PYbN-PT thin films
Open Access
- Author:
- Bassiri Gharb, Nazanin
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 27, 2005
- Committee Members:
- Susan E Trolier Mckinstry, Committee Chair/Co-Chair
Clive A Randall, Committee Member
Thomas R Shrout, Committee Member
Srinivas A Tadigadapa, Committee Member
Dragan Damjanovic, Committee Member - Keywords:
- ferroelectric
thin film
nonlinearities
extrinsic contributions
domain walls - Abstract:
- This work provides a comprehensive study of the ac field amplitude dependence of the dielectric constant, quantifying the extrinsic contributions of the domain walls and phase boundaries in two different orientations of PYbN-PT thin films. From the Rayleigh parameters obtained it was determined that {100} films had a higher concentration of mobile interfaces. The Rayleigh parameters decreased logarithmically with frequency. Comparison of the irreversible Rayleigh parameters’ dependence on frequency confirmed a higher concentration of mobile interfaces in {100} oriented PYbN-PT thin films. Frequency dependent Rayleigh parameters were used successfully to predict the dielectric permittivity of the films over a range of three orders of magnitude for frequency and one order of magnitude of field amplitude. Biaxial strain fields were applied and the nonlinear behavior was measured. It was found that for both orientations of the films, 180° domain wall motion was the major source of dielectric nonlinearity. The temperature dependence of the nonlinear behavior showed a reduction of both Rayleigh parameters at decreasing temperatures, indicating a reduction of the mobility of the interfaces. As the temperature increased towards the Curie temperature, the Rayleigh parameters showed a net increase in the reversible Rayleigh parameters and a much smaller increase in the irreversible Rayleigh parameter, indicating higher reversible mobility of the interfaces. The piezoelectric Rayleigh parameters also showed logarithmic dependence on frequency and a higher concentration of mobile interfaces in {100} oriented films. Electric field dependent dielectric and piezoelectric nonlinearities showed comparable amounts of extrinsic contributions to the nonlinear response, under the same applied bias field levels. A normalized frequency dependence of the Rayleigh parameters showed comparable trends for initå and initd, and åá and dá, indicating similar phenomena are responsible for the dielectric and piezoelectric nonlinearities. In the literature, only ferroelastic non-180° domain walls are considered to contribute to the piezoelectric effect. However in many ferroelectric films, their mobility is considerably reduced. A dynamic poling model is proposed, allowing a largely reversible 180° domain wall motion contribution to the piezoelectric nonlinearity. This model predicts a Rayleigh-like behavior of the piezoelectric coefficient with electric field, accompanied by the creation of a second order harmonic of strain. Experimental measurements of the higher order harmonics of polarization and strain confirmed the model’s predictions. Measurement of the fe,31 piezoelectric coefficients as a function of increasing ac strain levels didn’t show piezoelectric nonlinearities. Ferroelectric 180° domain wall motion, in fact, can’t be activated by application of homogeneous strain fields. The second order harmonic of the piezoelectric response proved to be at least one order of magnitude smaller than the first and third order harmonics. The observation of a Rayleigh behavior in fd,33along with a strong second harmonic in the strain response for electric field drive, coupled with the lack of the amplitude dependence for a strain excitation is consistent with the dynamic poling model. It was therefore demonstrated that piezoelectric nonlinearity can appear in thin films even in cases where only 180° domain wall motion is operative. This mechanism will be important in any ferroelectric with large-scale, nearly reversible motion of domain walls (tilted hysteresis loops).