Modelling and Simulations of Structures and Structural Transitions

Abstract

The main themes of the work presented in this thesis were, broadly, motivated by ideas of designing newer and more compact nano-structured materials for information storage. Both ferroelectric and ferromagnetic materials form potentially strong candidates to be used as components in non-volatile memory devices, since in these materials the direction of electric or magnetic polarization can be used to signify binary states, 0 and 1. The properties of compact arrays of such materials cannot necessarily be simply derived from a knowledge of the corresponding bulk materials. Thus, an additional theme that emerges is the desire to see how finite size affects properties. Properties of materials can change, both qualitatively and quantitatively, when their sizes are reduced to nanoscales from three-dimensional bulk structures. For example, bulk BaTiO3 is seen to undergo a sequence of phase transitions from cubic paraelectric to tetragonal, orthorhombic and rhombohedral ferroelectric phases as temperature is decreased; however, epitaxial thin films of the same material displays only a single transition from paraelectric to tetragonal ferroelectric phase. It was only until recently that Fong et al., showed that 1.2 nm thin films of PbTiO3 displayed ferroelectric properties [1]. From first-principles Density Functional Theory calculations Junquera and Ghosez showed that ferroelectricity survived in 24 ˚A BaTiO3 film sandwiched between electrodes [2].