4. Periodically-poled lithium niobate (PPLN) is a standard sample used for PFM b

Question

4. Periodically-poled lithium niobate (PPLN) is a standard sample used for PFM because t is engineered to have stable ferroelectric stripe domains. This means there are neighboring areas of opposite ferroelectric polarizations. Assume that the polarizations are either in the positive or negative z directions, which is the direction the electric field is applied here in the PFM set up. The figure below shows the PFM signal amplitude (left), PFM signal phase (right), and a line profile across each graph (below) (Figure from S. Kim, et al, Scientific Reports, 7, 41657 (2017)) 4 (a.u.) 180° 180 180 90 0 90 180 10 Distance (um) 5 15 20 a. Using what you have learned about ferroelectrics, piezoelectrics, and PFM, explain why you see what you do for the amplitude and phase signals. b. Can you definitively tell which for which stripes the polarization points in the +2 and 2 directions? Why or why not?

Explanation / Answer

(a) PFM stands for piezoresonsive force microscopy, and is used especially for ferroelectrics. In this form of microscopy, an AC field is passed through a conductive tip. The AC field causes periodic deformations of the piezoresponsive sample (here it is Lithium Niobate). These periodic deformations are very small, but can be picked up by a cantilever, and the signal can be used to reconstruct the topography as well as domains of the sample.

The amplitude of the PFM signal shows periodic dips followed by plateau regions. Precisely where the dip in amplitude occurs, the phase experiences a jump from -90 to +90. These may be explained as follows: For the actual scan shown in the top left panel (d), the dark lines represent domain walls between +90o phase domain and -90o domain. The phase angle color code shown below panel (e), which is just a schematic for the actual measurement shown in panel (d). The amplitude of the PFM signal is high in the bulk of a domain, and suggests that the domain is responsive to the AC input. At the domain wall, the piezoresponse amplitude dips sharly, since at the domain wall, the polarization is almost zero. Concomitantly, the phase switches from -90 to +90 or vice-versa.

(b) It is not possible to tell from the data alone, that the polarization points in a specific direction. Since the conductive tip is just a metal, which is not sensitive to the direction of the polarization. The two polarization directions are energetically equally favourable, i.e degenerate in energy. Thus, the conductive tip will induce exactly identical deformations in domains pointing in either +z or -z directions.

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