In this letter a method to simultaneously measure the physical and the polarization thickness of a 90°d omain wall in a ferroelectric perovskite is presented. This method combines accurate atomic force microscopy and piezoresponse force microscopy scans of the same area with little drift and an analysis of the entire scanned area. It is found that the physical thickness is significantly narrower ͑about seven and a half times͒ than the polarization thickness in a 90°domain wall in BaTiO 3 . Evidence of the trapping of defects at such domain walls is also found. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2185640͔ Ferroelectric materials offer great potential for next generation high density storage devices, such as nonvolatile random memory access and high strain actuators in microelectromechanical systems applications due to their inherent high dielectric properties and high strain response. 1-3 Their macroscopic response to mechanical, electrical and optical loads is strongly related to their microstructural domain patterns. 4,5 The formations and kinetics of these domains are governed by the underlying atomistic structure, and their interaction with domain walls and material imperfections. As the size of the devices reduces to micro-and nano-scale, the motion and interaction of twin ͑domain͒ walls significantly influences the overall piezoelectric, optical, and mechanical response of the device. 6 In order to predict and control the phenomenological behavior of twin wall kinetics, their physical and electrical properties need to be accurately determined.Many of the current experimental techniques, including x-ray diffraction and high-resolution transmission electron microscopy ͑TEM͒ are capable of imaging domain walls, but are often unable to provide information about their polarization. 7,8 Also, since these techniques are based on diffraction principles, they only provide averaged information about the physical twin wall thickness. 9 Current far field techniques are limited in their resolution by the wavelength of the light and, although near-field techniques such as NSOM provide useful information, they are still difficult to use.An experimental approach for determining the physical and electrical twin wall thickness in a ferroelectric perovskite material BaTiO 3 by simultaneously using a combination of atomic force microscopy ͑AFM͒ and piezoresponse force microscopy ͑PFM͒ is described here. The recorded AFM and PFM images are quantitatively compared to their respective displacement and polarization fields using the widely used and accepted Devonshire-Ginzburg-Landau ͑DGL͒ phenomenological model. [10][11][12] As has been shown by Shilo, Ravichandran, and Bhattacharya 9 for the case of PbTiO 3 , fitting the measured AFM displacement field to the DGL model by means of a least-squares method produces excellent results with nanometer resolution. Here, this approach is extended to include PFM measurements to determine the polarization domain wall thickness simultaneously along with the physical domain ...