Onset of ferrielectricity and the hidden nature of nanoscale polarization in ferroelectric thin films

MATIAS NUNEZ AND MARCO BUONGIORNO NARDELLI

PHYSICAL REVIEW LETTERS

Año: 2008

0031-9007

Using calculations from first principles, we have elucidated the nanoscale organization and local polarization in ferroelectric thin films. The  spontaneous electric polarization exhibited by particular classes of materials, where individual crystal dipoles orient in a fixed direction, is a property known as ferroelectricity and it  is at the  basis of a broad range of modern technological applications. Although ferroelectricity has been understood for a long time in bulk crystals, the recent drive towards miniaturization of electronic devices has unveiled novel characteristics when the size of the material is scaled to the microscopic dimensions of thin films. Our results unveil a peculiar spatial pattern of the local microscopic dipoles where individual atomic layers acquire uncompensated opposing polarizations in what was originally thought to be a simple domain of homogeneous orientation. This behavior arises as consequence of the complex energetic competition between the interface effects, the internal electric fields due to uncompensated interface charges, and the orientation and mutual interaction of the layer dipoles. Moreover, as the thickness of the film is varied, we discovered the appearance of a dielectric phase transition that we believe is a universal feature of ferroelectric materials at the nanoscale.