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We present investigations on the resistive switching effect in SrRuO3∕PbZr0.2Ti0.8O3∕Pt ferroelectric capacitors. Using a conductive atomic force microscope, the out-of-plane piezoelectric response and the capacitive and resistive current were simultaneously measured as a function of applied bias voltage. We observed two independent switching phenomena, one attributed to the ferroelectric switching process and the other to resistive switching.We show that I-V curves alone are not sufficient in ferroelectric materials to clarify the underlying switching mechanism and must be used with sufficient caution.

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The in-plane image of piezoresponse force microscopy ͑PFM͒ generally exhibits a higher resolution and less noise than the out-of-plane image. Geometrical considerations indicate that the optical in-plane amplification is Ϸ40 times larger than the out-of-plane amplification. We experimentally confirm this explanation in a dedicated setup. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1878153͔ Ferroelectric materials have been successfully characterized and manipulated on the micro-and nanometer scale by piezoresponse force microscopy ͑PFM͒ in recent years. [1][2][3][4][5][6][7] In this method a conducting tip is brought into contact with the sample and an ac voltage is applied to the tip. The in-plane and out-of-plane response of the piezoelectric material is optically detected as a deflection of the cantilever. In many cases the in-plane signal, as first conducted on thin films by Roelofs et al.,8 is substantially larger than the out-of-plane signal ͑usually more than one order of magnitude͒ and therefore shows more details and less noise. Taking the field distribution, the morphology and the piezoelectric coefficients ͑given for PbTiO 3 and BaTiO 3 in Table I͒ into account the out-of-plane piezoresponse should be larger than the in-plane response. 9 An example of the difference of in-plane and outof-plane PFM is given in Fig. 1 where the piezoresponse of nanograins prepared by chemical solution deposition 2 is shown.The most common detection method for the deflection of the cantilever is by measuring the position of a reflected laser beam on a photosensitive detector. The out-of-plane position is given by ͓͑a + b͒ − ͑c + d͔͒ / ͑a + b + c + d͒. This so-called optical lever arm method is presented in Fig. 2. The "lever amplification:"L is a factor of about one thousand. 10 Using the same method for detecting the in-plane deflection of the cantilever ͑tor-sion͒ the two left and two right quadrants of the photodiode have to be regarded as one, i.e. ͓͑a + c͒ − ͑b + d͔͒ / ͑a + b + c + d͒. The simplified movement of the tip is shown in Fig. 3.We assume that the apex of the tip moves a lateral distance ␦ whereas the middle of the base of the tip remains stationary. In the out-of-plane case the laser is deflected vertically, in the in-plane case horizontally. This is accounted for by the fact that the left and right quadrants of the photodiode are regarded as one unit instead of the bottom and top quadrants. From Fig. 3 it follows for small ␣:where h is the height of the tip plus the thickness of the cantilever. The change of the irradiated area of the left and right parts of the photodiode is then a linear function of the displacement:With this equation the amplification factor can be determined:The ratio R between these in-plane and

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