Methods of driving polarization inversion in ferroelectric materials and devices
Methods and devices are described for driving ferroelectric perovskite oxide crystals to achieve polarization inversion with reduced coercivity. In some embodiments, the anisotropy in the potential energy surface of a ferroelectric material is employed to drive polarization inversion and switching with a reduced coercive field relative to uniaxial excitation. In some embodiments, polarization inversion with reduced coercivity is produced via the application of an electric field that exhibits a time-dependent orientation, in contrast with conventional uniaxial electrical excitation, thereby causing the central ion (and the crystal structure as a whole) to evolve along a lower-energy path, in which the central ion is driven such that it avoids the potential energy maximum. This may be achieved, for example, by applying at least two non-parallel time-dependent voltages (e.g. bias, potential) such that orientation of the electric field changes with time during the switching cycle.
1. A method of electrically driving a ferroelectric material to achieve polarization inversion, the ferroelectric material having an anisotropic potential energy surface with an energy barrier for a curved switching path that is lower than for the barrier for a direct path associated with uniaxial switching, the method comprising:
applying time-dependent voltages to the ferroelectric material in at least two directions; and
controlling the voltages such that the orientation of the electric field within the ferroelectric material varies with time during the switching cycle, such that the coercive field is reduced relative to the coercive field required for uniaxial excitation;
wherein the voltages are applied such that the coercive field is exceeded during the switching cycle, thereby achieving polarization inversion.
2. The method according to claim 1 wherein the voltages are a first voltage and a second voltage, and wherein the first voltage and the second voltage are applied in directions that are approximately orthogonal.
3. The method according to claim 1 wherein the voltages are applied such that the electric field rotates within the ferroelectric material during the switching cycle.
4. The method according to claim 3 wherein the voltages are applied such that the electric field rotates in a circular orientation with an approximately constant magnitude.
5. The method according to claim 4 wherein the voltages are applied such that the ferroelectric material switches to an intermediate orthorhombic phase prior to achieving polarization inversion.
6. The method according to claim 3 wherein the rotation of the electric field is elliptical.
7. The method according to claim 6 wherein the major axis of the elliptical rotation is oriented approximately along the hard axis of the ferroelectric material.
8. The method according to claim 1 wherein the voltages are applied such that the electric field rotates in a polar configuration.
9. The method according to claim 8 wherein the polar configuration is a lemniscate-like parametric curve.
10. The method according to claim 1 wherein the voltages are configured such that both the magnitude and phase of the voltages are varied during the switching cycle in order to predominantly produce excitation that is associated within a selected axis, thereby achieving polarization inversion with reduced coercivity relative to purely uniaxial excitation, while generating a desired mechanical response associated with the selected axis.
11. The method according to claim 1 wherein the voltages controlled to vary one or more of the amplitude of the electric field in a first direction, the amplitude of the electric field in a second direction, the ratio of the electric field amplitudes in orthogonal directions, the phase delay between electric field components, and the ratio of the frequencies of orthogonal electric field components.
12. The method according to claim 1 wherein the ferroelectric material comprises a ferroelectric perovskite oxide crystal.
13. The method according to claim 1 wherein the ferroelectric material is selected from the group consisting of PZT, PbTiO 3 , BaTiO 3 , and LiNbO 3 .
14. The method according to claim 1 wherein the applied surface power density exceeds approximately 0.5 W/cm 2 .
15. The method according to claim 1 wherein the ferroelectric material is polycrystalline.
16. The method according to claim 1 wherein the ferroelectric material is a single crystal.
17. The method according to claim 1 wherein the ferroelectric material is a component of a therapeutic ultrasonic device.
18. A method of electrically driving an ultrasonic device, the ultrasonic device comprising one or more ferroelectric elements having an anisotropic potential energy surface with an energy barrier for a curved switching path that is lower than for the barrier for a direct path associated with uniaxial switching, the method comprising:
applying time-dependent voltages to the one or more ferroelectric elements in at least two directions; and
controlling the voltages such that the orientation of the electric field within the one or more ferroelectric elements varies with time during the switching cycle, such that the coercive field is reduced relative to the coercive field required for uniaxial excitation;
wherein the voltages are applied such that the coercive field is exceeded during the switching cycle, thereby achieving polarization inversion.
19. The method according to claim 18 wherein the one or more ferroelectric elements are provided in an array.
20. An ultrasonic device comprising:
one or more ferroelectric elements, each ferroelectric element having an anisotropic potential energy surface with an energy barrier for a curved switching path that is lower than for the barrier for a direct path associated with uniaxial switching; and
control circuitry connected to the ferroelectric elements for applying time-dependent voltages to the one or more ferroelectric elements in at least two directions;
wherein the voltages are provided by the control circuitry such that the orientation of the electric field within the one or more ferroelectric elements varies with time during the switching cycle, such that the coercive field is reduced relative to the coercive field required for uniaxial excitation; and
wherein the voltages are provided by the control circuitry such that the coercive field is exceeded during the switching cycle, thereby achieving polarization inversion.