- Email: [email protected]
Theory and modelling of ferroelectric materials
Current Opinion in Solid State and Materials Science 9 (2005) 99
Editorial overview
Theory and modelling of ferroelectric materials P.D. Bristowe
*
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke St., Cambridge CB2 3QZ, United Kingdom
Understanding the properties of ferroelectric materials is important for future device applications but presents a challenge to theory and modelling because of the range of length and time scales involved in ferroelectric processes. This is particularly true for memory devices such as FRAMs where the ferroelectric material is in the form of a thin-film bonded to electrodes and subject to polarisation switching. An atomistic model is essential for understanding the physics of ferroelectric distortions at the unit cell level but it is domain wall motion which controls the ultimate switching time of a memory device and hence at this level a macroscopic model is desirable which incorporates the effects of microstructure and domain dynamics as well as the application of an external electric field, mechanical stress and temperature. This section reviews some of the contributions made by theory and modelling to our understanding of ferroelectric phenomena. It focuses on electronic, atomistic and micromechanical models and illustrates the advances made in these areas in recent years. The first article by Huber describes the development of micromechanical models and emphasises the inclusion of domain wall dynamics and grain-to-grain interactions. It also identifies the need to connect modelling approaches across different length scales, i.e., multi-scale modelling, whereby atomistic detail can be incorporated into macroscopic models where this is useful and appropriate. The second article by Sepliarsky and co-workers reviews progress in modelling ferroelectric oxides using classical interatomic potentials. It addresses the advantages of this approach, the need to validate the potentials (mainly shell models) and describes various applications. Phase transitions and finite temperature behaviour in perovskites have been the main areas of study where notable success has been achieved. However, improved potentials are required if more complex ferroelec-
*
Tel.: +44 1223 334305; fax: +44 1223 334567. E-mail address: [email protected]
1359-0286/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2006.06.004
trics are to be modelled and in this regard charge transfer potentials look promising. Ponomareva and co-workers follow with a paper that describes atomistic modelling of low-dimensional perovskite ferroelectrics using a first-principles-derived effective Hamiltonian approach. This approach is a useful alternative to shell model potentials and enables the application of various electrical and mechanical boundary conditions. The authors describe applications of the method to various nano-structures of lead zirconium titanate and the results in some cases have revealed unusual dipole patterns of potential technological importance. The next article by Rabe highlights recent results in the theory of epitaxially strained perovskite ferroelectrics. This is of importance in thin-film systems often found in devices. The author places particular emphasis on first-principles methods, phenomenological Landau–Devonshire functionals and the phase-field approach. Results are presented for individual materials and include calculations of total energy, dielectric constant, polarisation and structure as a function of misfit strain. Like other authors Rabe discusses the structural and electrical complexity of real systems (point defects, extended defects, domains, interfaces and depolarisation fields) and the need to integrate modelling approaches on different scales. The section concludes with an article by Ederer and Spaldin on the first principles modelling of multiferroic materials. These are relatively new materials that display both ferroelectric and magnetic order and could be used to design novel electronic devices. Examples that are discussed by the authors include BiFeO3 and YMnO3. The paper reviews recent studies of these materials using density functional theory aimed at explaining their unusual magnetisation and polarisation properties and also predicting new multiferroics whose characteristics are further improved.
Editorial overview
Theory and modelling of ferroelectric materials P.D. Bristowe
*
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke St., Cambridge CB2 3QZ, United Kingdom
Understanding the properties of ferroelectric materials is important for future device applications but presents a challenge to theory and modelling because of the range of length and time scales involved in ferroelectric processes. This is particularly true for memory devices such as FRAMs where the ferroelectric material is in the form of a thin-film bonded to electrodes and subject to polarisation switching. An atomistic model is essential for understanding the physics of ferroelectric distortions at the unit cell level but it is domain wall motion which controls the ultimate switching time of a memory device and hence at this level a macroscopic model is desirable which incorporates the effects of microstructure and domain dynamics as well as the application of an external electric field, mechanical stress and temperature. This section reviews some of the contributions made by theory and modelling to our understanding of ferroelectric phenomena. It focuses on electronic, atomistic and micromechanical models and illustrates the advances made in these areas in recent years. The first article by Huber describes the development of micromechanical models and emphasises the inclusion of domain wall dynamics and grain-to-grain interactions. It also identifies the need to connect modelling approaches across different length scales, i.e., multi-scale modelling, whereby atomistic detail can be incorporated into macroscopic models where this is useful and appropriate. The second article by Sepliarsky and co-workers reviews progress in modelling ferroelectric oxides using classical interatomic potentials. It addresses the advantages of this approach, the need to validate the potentials (mainly shell models) and describes various applications. Phase transitions and finite temperature behaviour in perovskites have been the main areas of study where notable success has been achieved. However, improved potentials are required if more complex ferroelec-
*
Tel.: +44 1223 334305; fax: +44 1223 334567. E-mail address: [email protected]
1359-0286/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2006.06.004
trics are to be modelled and in this regard charge transfer potentials look promising. Ponomareva and co-workers follow with a paper that describes atomistic modelling of low-dimensional perovskite ferroelectrics using a first-principles-derived effective Hamiltonian approach. This approach is a useful alternative to shell model potentials and enables the application of various electrical and mechanical boundary conditions. The authors describe applications of the method to various nano-structures of lead zirconium titanate and the results in some cases have revealed unusual dipole patterns of potential technological importance. The next article by Rabe highlights recent results in the theory of epitaxially strained perovskite ferroelectrics. This is of importance in thin-film systems often found in devices. The author places particular emphasis on first-principles methods, phenomenological Landau–Devonshire functionals and the phase-field approach. Results are presented for individual materials and include calculations of total energy, dielectric constant, polarisation and structure as a function of misfit strain. Like other authors Rabe discusses the structural and electrical complexity of real systems (point defects, extended defects, domains, interfaces and depolarisation fields) and the need to integrate modelling approaches on different scales. The section concludes with an article by Ederer and Spaldin on the first principles modelling of multiferroic materials. These are relatively new materials that display both ferroelectric and magnetic order and could be used to design novel electronic devices. Examples that are discussed by the authors include BiFeO3 and YMnO3. The paper reviews recent studies of these materials using density functional theory aimed at explaining their unusual magnetisation and polarisation properties and also predicting new multiferroics whose characteristics are further improved.