Nanoscale Ferroelectrics and Multiferroics: Key Processing and Characterization Issues, and Nanoscale Effects, 2 Volumes

This two volume set reviews the key issues in processing and characterization of nanoscale ferroelectrics and multiferroics, and provides a comprehensive description of their properties, with an emphasis in differentiating size effects of extrinsic ones like boundary or interface effects. Recently described nanoscale novel phenomena are also addressed. Organized into three parts it addresses key issues in processing (nanostructuring), characterization (of the nanostructured materials) and nanoscale effects.

Taking full advantage of the synergies between nanoscale ferroelectrics and multiferroics, the text covers materials nanostructured at all levels, from ceramic technologies like ferroelectric nanopowders, bulk nanostructured ceramics and thick films, and magnetoelectric nanocomposites, to thin films, either polycrystalline layer heterostructures or epitaxial  systems, and to nanoscale free standing objects with specific geometries, such as nanowires and tubes at different levels of development.

This set is developed from the high level European scientific knowledge platform built within the COST (European Cooperation in Science and Technology) Action on Single and multiphase ferroics and multiferroics with restricted geometries (SIMUFER, ref. MP0904). Chapter contributors have been carefully selected, and have all made major contributions to knowledge of the respective topics, and overall, they are among most respected scientists in the field.

• Language: English
• Publisher: Wiley
• Release date: Mar 24, 2016
• ISBN: 9781118935675

Book preview

List of Contributors

Andrew R. Akbashev Drexel University, USA

Miguel Algueró Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Spain

Andreas Amann Tyndall National Institute, University College Cork, Ireland; and School of Mathematical Sciences, University College Cork, Ireland

Harvey Amorín Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Spain

Nicoleta Apostol National Institute of Materials Physics, Romania

Juras Banys Faculty of Physics, Vilnius University, Lithuania

Laurent Bellaiche Physics Department, University of Arkansas, USA

Daniela C. Berger Departement of Inorganic Chemistry, Physical-Chemistry and Electrochemistry, Politehnica University of Bucharest, Romania

J.D. Bobic Institute for Multidisciplinary Research, University of Belgrade, Serbia

Alexei A. Bokov Department of Chemistry and 4D LABS, Simon Fraser University, Canada

Andra G. Boni National Institute of Materials Physics, Romania

Mihaela Botea National Institute of Materials Physics, Romania

Iñnigo Bretos Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Spain

Maria Teresa Buscaglia Institute of Energetics and Interphases, National Research Council, Italy

Vincenzo Buscaglia Institute of Energetics and Interphases, National Research Council, Italy

Michael P.D. Campbell School of Maths and Physics, Queen's University Belfast, Northern Ireland, United Kingdom

Arianna Casiraghi NanoSpin, Department of Applied Physics, Aalto University School of Science, Finland

Alichandra Castro CICECO – Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, Portugal

Alicia Castro Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain

Li-Wu Chang School of Maths and Physics, Queen's University Belfast, Northern Ireland, United Kingdom

Cristina Chirila National Institute of Materials Physics, Romania

Frank Clemens Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Switzerland

Covadonga Correas Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Spain; and College of Engineering, Swansea University, United Kingdom

Lavinia P. Curecheriu Department of Physics, Alexandru Ioan Cuza University, Romania

Nitin Deepak Tyndall National Institute, University College Cork, Ireland

Marco Deluca Materials Center Leoben Forschung GmbH, and Institut für Struktur- und Funktionskeramik, Montanuniversitaet Leoben, Austria

Xiangdong Ding State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, China

Christopher De Dobbelaere Inorganic and Physical Chemistry Group, Institute for Materials Research, Universiteit Hasselt and imec vzw, Division imomec, Belgium

C. Doubrovsky Laboratoire de Physique des Solides, Université de Paris Sud, Campus d'Orsay, France

Ahmad Faraz Tyndall National Institute, University College Cork, Ireland

Liliana P. Ferreira Biosystems and Integrative Sciences Institute, Department of Physics, University of Coimbra, Portugal

Paula Ferreira CICECO – Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, Portugal

Pascale Foury-Leylekian Laboratoire de Physique des Solides, Université de Paris Sud, Campus d'Orsay, France

Kévin J. A. Franke NanoSpin, Department of Applied Physics, Aalto University School of Science, Finland

Vladimir M. Fridkin Drexel University, USA; and Russian Academy of Sciences, Moscow, Russian Federation

Huaxiang Fu Physics Department, University of Arkansas, USA

Andreja Gajović Molecular Physics Laboratory, Institute Rudjer Boskovic, Croatia

Corneliu Ghica National Institute of Materials Physics, Romania

Margarida Godinho Biosystems and Integrative Sciences Institute, Department of Physics, Faculty of Sciences, University of Lisbon, Portugal

Martha Greenblatt Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, USA

J. Marty Gregg School of Maths and Physics, Queen's University Belfast, Northern Ireland, United Kingdom

R. Grigalaitis Faculty of Physics, Vilnius University, Lithuania

Sampo J. Hämäläinen NanoSpin, Department of Applied Physics, Aalto University School of Science, Finland

An Hardy Inorganic and Physical Chemistry Group, Institute for Materials Research, Universiteit Hasselt and imec vzw, Division imomec, Belgium

Luminita M. Hrib National Institute of Materials Physics, Romania

Teresa Hungria Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Spain; and Centre de Microcaractérisation Raimond Castaing, UMS 3623, France

Alan J. Hurd Los Alamos National Laboratory, USA

Adelina-Carmen Ianculescu Department of Oxide Materials Science and Engineering, Politehnica University of Bucharest, Romania

Alin Iuga National Institute of Materials Physics, Romania

Maksim Ivanov Faculty of Physics, Vilnius University, Lithuania

Ricardo Jiménez Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Spain

Lynette Keeney Tyndall National Institute, University College Cork, Ireland

Michael R. Koblischka Institute of Experimental Physics, Saarland University, Germany

Anjela Koblischka-Veneva Institute of Experimental Physics, Saarland University, Germany

Amit Kumar School of Maths and Physics, Queen's University Belfast, Northern Ireland,United Kingdom

Tuomas H.E. Lahtinen NanoSpin, Department of Applied Physics, Aalto University School of Science, Finland

Diego López González NanoSpin, Department of Applied Physics, Aalto University School of Science, Finland

M. Lourdes Calzada Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Spain

Axel Lubk Triebenberg Laboratory, Technische Universität Dresden, German

Tony Lusiola Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Switzerland

Leo J.McGilly Ceramics Laboratory, École Polytechnique Féedérale de Lausanne (EPFL), Switzerland

Raymond G.P. McQuaid School of Maths and Physics, Queen's University Belfast, Northern Ireland, United Kingdom

J. Macutkevic Faculty of Physics, Vilnius University, Lithuania

César Magén Instituto de Nanociencia de Aragón-ARAID, Universidad de Zaragoza, Spain

Tuhin Maity Tyndall National Institute, University College Cork, Ireland

Barbara Mali.c Electronic Ceramics Department, Jo.zef Stefan Institute, Slovenia

Liliana Mitoseriu Faculty of Physics, University Alexandru Ioan Cuza, Romania

Evagelia G. Moshopoulou Institute of Nanoscience and Nanotechnology, National Center for Scientific Research Demokritos, Greece; and Laboratoire de Physique des Solides, Universitéde Paris Sud, Campus d'Orsay, France

Ivan I. Naumov Carnegie Institution of Washington, USA

Raluca Negrea National Institute of Materials Physics, Romania

Marius Olariu Department of Electrical Measurements & Materials, Technical University Gh. Asachi Iasi, Romania

Leontin Padurariu University Alexandru Ioan Cuza, Romania

Katharine Page Spallation Neutron Source, Oak Ridge National Laboratory, USA

Iuliana Pasuk National Institute of Materials Physics, Romania

Martyn E. Pemble Tyndall National Institute, University College Cork, Ireland; and Department of Chemistry, University College Cork, Ireland

Nikolay Petkov Tyndall National Institute, University College Cork, Ireland

M.M. Vijatovic Petrovic Institute for Multidisciplinary Research, University of Belgrade, Serbia

Ioana Pintilie National Institute of Materials Physics, Romania

Lucian Pintilie National Institute of Materials Physics, Romania

Sergei Prosandeev Physics Department, University of Arkansas, USA and Physics Department and Research Institute of Physics, Southern Federal University, Russia

Jesús Ricot Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Spain

Brian J. Rodriguez Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Ireland

Saibal Roy Tyndall National Institute, University College Cork, Ireland

Ekhard. K.H. Salje Department of Earth Sciences, University of Cambridge, United Kingdom

Alina Schilling School of Maths and Physics, Queen'sUniversity Belfast, Northern Ireland, United Kingdom

Michael Schmidt Tyndall National Institute, University College Cork, Ireland

Theodor Schneller Institut für Werkstoffe der Elektrotechnik II, RWTH Aachen University, Germany

Stella Skiadopoulou CICECO – Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, Portugal

Etienne Snoeck Centre d'Elaboration de Matériaux et d'Etudes Structurales, CNRS, France

Jonathan E. Spanier Drexel University, USA

Biljana D. Stojanovic Institute for Multidisciplinary Research, University of Belgrade, Serbia

Bosiljka Tadić Department of Theoretical Physics, Jožef Stefan Institute, Slovenia

Cristian M. Teodorescu National Institute of Materials Physics, Romania

Gregor Trefalt Electronic Ceramics Department, Jožef Stefan Institute and Center of Excellence NAMASTE, Slovenia; and University of Geneva, Switzerland

Lucian Trupina National Institute of Materials Physics, Romania

Roxana Trusşca S.C. METAV – Research and Development, Romania

Marina Tyunina Microelectronics and Materials Physics Laboratories, University of Oulu, Finland

Marlies K. Van Bael Inorganic and Physical Chemistry Group, Institute for Materials Research, Universiteit Hasselt and imec vzw, Division imomec, Belgium

Sebastiaan van Dijken NanoSpin, Department of Applied Physics, Aalto University School of Science, Finland

Bogdan S. Vasile Department of Oxide Materials Science and Engineering, Politehnica University of Bucharest, Romania

Catalina A. Vasilescu Department of Oxide Materials Science and Engineering, Politehnica University of Bucharest, Romania

PaulaM. Vilarinho CICECO – Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, Portugal

Roger W. Whatmore Tyndall National Institute, University College Cork, Ireland; Department of Chemistry, University College Cork, Ireland and Department of Materials, Royal School of Mines, Imperial College London, United Kingdom

Zuo-Guang Ye Department of Chemistry and 4D LABS, Simon Fraser University, Canada

Preface

Multiferroics have been at the cutting edge of research and development in materials for ICTs for over a decade. During this period steady improvements in fundamental knowledge have been made. At the same time, nanoscale phenomena have assumed an increasing importance. Progress has benefited from the strong synergies with activities in nanoscale ferroelectrics, which are at a more mature stage. Multifunctionality and nanoscaling are widely acknowledged at present as the keys to the miniaturization of solid-state electronics, and specifically nanoferronics, which is emerging as a new area with large technological potential. The topic has now reached a maturity level that allows, and actually requires, books that provide a comprehensive revision of the topic, and an in-depth analysis of future trends. These are the objectives of Nanoscale Ferroelectrics and Multiferroics: Key Processing and Characterization Issues, and Nanoscale Effects.

It is intended to provide the increasing number of scientists and engineers, who are approaching the topic from a range of backgrounds, with a reference/guide text that should help them to roadmap their R&Dactivities. The volume reviews the key issues in processing and characterization of nanoscale ferroelectrics and multiferroics, and provides a comprehensive description of their properties. An emphasis is put on differentiating size effects of extrinsic ones like boundary or interface effects. Nanoscale novel, recently described, phenomena that are bound to be behind major advancement in the field during the coming years are also addressed.

The book is devised to stress, and take full advantage of, the synergies between nanoscale ferroelectrics and multiferroics. It covers materials nanostructured at all levels, from ceramic technologies like ferroelectric nanopowders, bulk nanostructured ceramics and thick films, and magnetoelectric nanocomposites, to thin films, either polycrystalline layer heterostructures or epitaxial systems, and to nanoscale free-standing objects with specific geometries, such as nanowires and tubes at different levels of development, but all technologically relevant. Nanostructuring is a requirement of the current tendency to miniaturization of ceramic technologies for microelectronics that imposes stringent conditions on processing, and has a deep impact on functional properties. Also, nanostructuring ultimately results in the ever-decreasing processing temperatures of thin films, a key issue to the integration of these multifunctional oxides with silicon devices and flexoelectronics. Last but not least, a range of novel physical phenomena has been described in nanoscale ferroelectrics and multiferroics that have the potential to enable a range of disruptive technologies, like magnetoelectric memory. Overall, the book reviews the current state of the art of these materials, stressing a range of specific topics at the cutting edge of research.

This project springs from the high-level European scientific knowledge platform built within the COST Action Single and Multiphase Ferroics and Multiferroics with Restricted Geometries (SIMUFER, ref. MP0904), active between March 2010 and May 2014. COST (European Cooperation in Science and Technology) is a pan-European networking instrument that allows researchers from COST member countries and cooperating states to jointly develop their ideas and initiatives in a field or topic of common interest. SIMUFER established a multidisciplinary scientific network of groups from 24 European countries and 7 non-COST countries, experienced in synthesis, advanced characterization, and modeling of all nanoscale ferroics, single-phase multiferroics, and ferroic-based combinations of dissimilar materials. This book project arises primarily from their expertise, though it has been open to world renowned experts when necessary. Chapter contributors have been carefully selected and have all made major contributions to knowledge of the respective topics; overall, they are among the most respected scientists in the field.

Introduction

Why Nanoscale Ferroelectrics and Multiferroics?

Miguel Algueró¹, J. Marty Gregg², and Liliana Mitoseriu³

¹Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Cientίficas (CSIC), Spain

²School of Maths and Physics, Queen’s University Belfast, Northern Ireland, United Kingdom

³Faculty of Physics, University Alexandru Ioan Cuza, Romania

I.1 Ferroics and Multiferroics

Single-phase ferroics are compounds that present one of the three (currently expanded to four) ferroic properties: ferroelectricity, ferromagnetism, or ferroelasticity, to which ferrotoroidicity has recently been added. The common feature of the four types of ferroics is the appearance of the ferroic order; either it is a spontaneous electrical polarization, magnetization, strain, or toroidal moment, in a phase transition from a high-temperature prototype phase to the low-temperature ferroic phase, related by a group/subgroup relationship. This transition is always accompanied by a decrease in symmetry and the splitting of the ferroic phase into domains (regions with a different orientation of the order parameter). A second feature, the direct consequence of the switchable nature of the order parameter and of the domain dynamics, is the characteristic ferroic hysteresis loop; that is, a distinctive hysteretic dependence of the order parameter on its conjugated field (electric or magnetic field, mechanical stress or toroidal source vector, respectively), with two remnant states of opposite sign [1]. The four phenomena are schematically shown in Figure I.1. Ferroics are highly topical, advanced functional materials that have not only enabled a range of mature and ubiquitous related technologies (like magnetic or ferroelectric information recording, ceramic ultrasound transducers, or shape memory alloys, to name only a few examples) but are also under extensive research for a number of novel, potentially disruptive, applications [2].

Figure I.1 Schematics of domains and hysteretic switching for the four ferroic phenomena. Adapted by permission of IOP Publishing from [1]. © IOP Publishing. All rights reserved.

There are also compounds that simultaneously present two (or more) ferroic phenomena, known as multiferroics, among which those showing coexistence of ferroelectricity and ferromagnetism (initially termed ferroelectromagnets) are receiving increasing attention [4]. This is not only because of their inherent multifunctionality but also for the fact that they are liable to show magnetoelectric coupling, and have thus the potential to enable the electrical control of magnetism (and the magnetic control of polarization) [5].

This book specifically deals with ferroelectrics and ferroelectromagnets (either robust ferromagnetic materials or canted antiferromagnets showing weak ferromagnetism), though the general term multiferroics will be used following the current tendency to name ferroelectric–ferromagnetic materials, and in general any type of magnetic ordering compounds, in this way.

The choice of addressing them together only acknowledges the deep-rooted relationship between the two sets of materials; multiferroism requires ferroelectricity and thus multiferroics have to be electrically insulating to be functional (an issue not always acknowledged). This feature is not easily found in magnetic materials, most of them being metallic or narrow-band gap semiconductors. Indeed, chemical bonding requirements suggest the two ferroic phenomena to be incompatible [6]; transition metal or rare earth atomic species with partially filled outer d or f electronic shells (and unpaired electrons) are necessary for magnetism, while model ferroelectric perovskite oxides are characterized by covalently bonded transition metals (to oxygen) with empty d orbitals [7]. Nevertheless, an ever-increasing number of multiferroic single-phase materials have been reported over the last decade, exploring alternative mechanisms of ferroelectricity.

I.2 Ferroelectric Materials and Related Technologies

Ferroelectrics are thus materials that present a spontaneous electrical polarization, whose direction can be reversed with an electric field (by nucleation and growth of inversion domains, resulting in the distinctive ferroelectric hysteresis). The ferroic phase appears at a ferroelectric transition, driven by electrical polarization [8], which can be either of a displacive type, the most common one, associated with a crystal structure instability induced by condensation of a transverse optical phonon (the soft mode) [9], or of an order–disorder type. Its macroscopic phenomenological description according to Landau's theory of phase transitions can be found in Chapter 19.

All ferroelectrics are also pyroelectric and piezoelectric, as well as electrooptic, which turns them into a prototype of multifunctionality (even before magnetic order is added). Moreover, they are the only materials that can present these properties, intrinsically linked to the crystal structure, in polycrystalline form (thanks to the ability to reorient the polarization under an electric field).

Though ferroelectricity was first described for hydrogen-bonded compounds (Rochelle salt being the first one in 1921), and there are also examples among tellurides, fluorides, and iodides [10], a number of electroactive polymers like poly(vinylidene fluoride) [11], and recent reports of ferroelectric metal organic frameworks [12], clearly oxides stand out as the ones that have enabled a range of successful ferroelectric technologies.

I.2.1 Ferroelectric Bulk Technologies

Perhaps the best-known ferroelectric, and also the first oxide shown to be so in 1944, is BaTiO3 with a perovskite structure. This model compound presents the ferroelectric transition at ∼393 K and a succession of low-temperature polymorphic phase transitions between ferroelectric phases with decreasing symmetry, from tetragonal to orthorhombic and to rhombohedral, for which the polar axis (and thus the direction of the spontaneous polarization defined by the displacement of the Ti⁴+ cation from the centre of the oxygen octahedra) changes. Polymorphism is a quite common phenomenon in ferroelectric perovskite oxides and plays a very important role in their functionality. BaTiO3 is also the base composition of multilayer ceramic capacitors (after the chemical tailoring of the ferroelectric transition down to room temperature), one of the two large-scale, mature bulk ceramic ferroelectric technologies. This material and its modifications are extensively addressed in this book (see Chapters 1, 11, 12, 15, and 18), for the miniaturization of these capacitors is a case study of the current trends in microelectronics that require the nanostructuring of the ceramic layers. In the last few years, nanostructured BaTiO3 and its solid solutions have become the main candidates for active materials used in capacitive building blocks for energy storage applications.

The second successful technology is piezoelectric ceramics for electromechanical transduction. The state of the art material for these applications, which range from sensors and actuators (like accelerometers or positioning systems for scanning probe microscopy, respectively) and their combination in smart systems (to implement active vibration damping), to ultrasound generation and sensing (for medical imaging or non-destructive testing), and to submarine acoustics, is Pb(Zr,Ti)O3, which also has a perovskite structure. This is an oxide solid solution, for which the best properties are found at a morphotropic phase boundary (MPB) between rhombohedral and tetragonal ferroelectric polymorphs, for which a monoclinic phase has been recently described [13]. This material can be regarded as a modification of PbTiO3, a second model ferroelectric oxide that also shows a succession of polymorphic phase transitions, yet induced by hydrostatic pressure instead of temperature [14], which has been placed at one of these ferroelectric instabilities (the MPB) by building up chemical pressure (achieved by substitution of Zr⁴+ for Ti⁴+). As a matter of fact, the very good electromechanical response of this material is a combination of two effects, a crystal contribution, associated with the existence of a transverse lattice instability at the monoclinic tetragonal boundary [15], and an extrinsic contribution, associated with the fact that ferroelectric perovskite oxides are also ferroelastic (and therefore multiferroic, but not ferroelectromagnets). The spontaneous strain develops at the ferroelectric transition along with the polarization (the two parameters are intrinsically coupled), and as a consequence ferrroelectric–ferroelastic domains appear (in addition to polarization inversion domains).The non-180° (90° in the tetragonal case) domain walls are mobile under stress and electric field (unlike 180° walls that only move under an electric field), giving way to a wall contribution to the piezoelectric effect [16]. Moreover, the domain dynamics is enhanced at the morphotropic phase boundary. In addition, chemical (or doping) engineering of Pb(Zr,Ti)O3 has been developed that enables a range of soft and hard piezoelectric ceramics with tailored properties for specific applications. Piezoelectric ceramics are also being considered for novel applications, such as energy harvesting [17] and magnetoelectric composites (see later). Further explanations of the mechanisms, along with a review of alternative materials, can be found in Chapter 16. This technology is not oblivious to the general miniaturization trend and nanostructuring can also be anticipated.

Other examples of ferroelectric ceramic bulk technologies are infrared (IR) cameras for night vision (and, in general, IR detectors for a range of applications exploiting the pyroelectric effect) and electrooptic devices. Modifications of PbTiO3 like (Pb,La)TiO3 and transparent (Pb,La)(Zr,Ti)O3 are usually the material choices, respectively. An excellent review of ferroelectric ceramics and related technologies can be found in [18].

Also successful ferroelectric single-crystal technologies are presently available. The best examples are surface acoustic wave (SAW) devices for radio frequency and microwave signal conditioning, based on ferroelectric LiNbO3 substrates. At the very end of the last century, ultrahigh piezoelectricity and strain under an electric field was reported for single crystals of the relaxor–ferroelectric solid solutions Pb(Zn1/3Nb2/3)O3–PbTiO3 and Pb(Mg1/3Nb2/3)O3–PbTiO3 [19]. Their very large electromechanical response is obtained for plates cut along off-polar directions by a mechanism of polarization rotation that takes full advantage of the already mentioned lattice transverse instability [21]. This crystal technology is now reaching reliability standards for commercialization and is especially suitable for ultrasonic and acoustic transducers [22].

I.2.2 Ferroelectric Thin-Film Technologies

Ferroelectric thin-film technologies can be seen as a first step in miniaturization, and comprise a range of microelectronic devices where the oxide layer is usually the integrated, active component with CMOS control electronics. They can also be reckoned as a first example of a nanostructured ferroelectric, for films on silicon-based substrates are usually polycrystalline or columnar with a grain size (or column diameter) below 100 nm, even though thickness is in the micrometer or submicrometer range.

Three main technologies have been proposed, although only one has succeeded in full commercialization: the ferroelectric random access memory (FeRAM) [24]. This is a non-volatile memory, where the two binary states are defined as the two opposite directions of (reversible) spontaneous polarization. It is a low-power, fast-access device, whose capacity has steadily increased from 4 kbytes in 1992, when mass production started, to 64 Mbytes [25]. Commercial products are based on either columnar Pb(Zr,Ti)O3 films with conducting oxide electrodes or SrBi2Ta2O7 with Pt electrodes [26], and are used in applications such as IC cards and radio frequency tags [27]. SrBi2Ta2O7 is an example of a wide family of oxides with a layered perovskite, Aurivillius-type crystal structure, which are also used as high-temperature piezoelectrics (see Chapter 17); multiferrroic examples have also been reported (see Chapter 25).

The second ferroelectric thin-film technology is the piezoelectric microelectromechanical system (pMEMS). These are bulk or surface micromachined structures, commonly in silicon, on to which a piezoelectric oxide layer has been integrated to implement sensing and/or actuation capabilities, along with the control electronics. Most commonly used microstructures are cantilevers and membranes. One device based on such a microcantilever is the piezoelectric force sensor for scanning probe microscopy, while cantilever arrays have been considered for applications like adaptative optics, micromirror image projectors, or radio frequency switches. Ultrasonic micromotors based on piezoelectrically actuated membranes have been fabricated, and micromembrane arrays are the basis of piezoelectric micromachined ultrasonic transducers (pMUTs) [25]. They are also being considered for ink-jet printing heads (an example out of a range of applications in microfluidics) [28].

Microwave microelectronics making use of the material high-dielectric permittivity and its tunability under the electric field can be considered the third ferroelectric thin-film technology. (Ba,Sr)TiO3 films are being considered for varactors, even if non-negligible dielectric losses have hindered its development. A successful product, though based on non-ferroelectric piezoelectric AlN, is the bulk acoustic wave (BAW) thin-film resonator, which has been key to the miniaturization of mobile communication devices like cellular telephones [25, 28].

Integrated infrared sensors [29] and thin-film capacitors [30] are other examples of ferroelectric thin-film technologies. Electrothermal energy interconversion by means of the electrocaloric effect is also under extensive research [31]. A thorough description of applications and related material issues can be found in recent reviews [10, 25, 28].

I.3 Multiferroic Materials for Enabling Magnetoelectric Technologies

Multiferroics are liable to present different magnetoelectric coupling phenomena. They all show linear dependences of magnetic susceptibility and dielectric permittivity on the electric and magnetic fields (originating from linear–quadratic terms in the free energy: ∝ ExH² and HxE², respectively ), and can also show a large linear magnetoelectric effect as compared with non-multiferroic magnetoelectrics like Cr2O3 (this time from a bilinear ∝ HxE term). This latter effect is the development of an electrical polarization proportional to an applied magnetic field (direct effect) and of magnetization in response to an electric field (converse effect). Magnetoelectric materials are being investigated as an alternative to superconducting quantum interference devices (SQUIDs) for weak magnetic field detection and for electrically tunable microwave magnetic devices such as filters, resonators, and phase shifters [33].

They are also liable to show magnetoelectric switching, which would be the reversal of the electric polarization with a magnetic field or of the magnetization with an electric field, phenomenon that could enable novel potentially disruptive information technologies like the magnetoelectric random access memory (MeRAM). This device would combine the best characteristics of ferroelectric memories (FeRAMs): an electrical low-power and ultra-fast WRITE operation, with a magnetic non-destructive (no reset) READ operation [10, 34]. Destructive reading is a main shortcoming of FeRAMs, which determine architecture and ultimately limit downscaling and, thus, storage capacity.

Research and development are being intensified along two lines that progress in parallel: single-phase multiferroics and magnetoelectric composite materials.

I.3.1 Single-Phase Multiferroics

A lot of research has been, and still is, concentrated on magnetic or spin-induced ferroelectrics. These are improper ferroelectrics, in which a small spontaneous polarization (≪1 μC cm−2) develops as a byproduct of a magnetic transition, often involving complex incommensurate antiferromagnetic states like sinusoidal or spiral spin configurations. Such complex magnetic orders result from competing exchange interactions in systems with several different magnetic cations and from consequent magnetic frustration. First examples were orthorhombic Terbium manganites like TbMnO3 and TbMn2O5 with ordering temperatures around 40 K. These oxides developed a ferroelectric state at low-temperature successive magnetic transitions involving spin reorientation (one system of this type is chosen in Chapter 12 to show the application of high-resolution synchrotron X-ray diffraction in the study of the crystal mechanisms underlying multiferroism). Large magnetoelectric effects, basically changes of permittivity and reorientation of polarization, even direction reversal, have been reported under high magnetic fields associated with phase-change responses at the instabilities of the antiferromagnetic state [36]. Following works concentrated on finding systems that showed similar phenomenology but higher magnetic ordering temperatures, among which CuO at 230 K [37] and Sr3Co4Fe24O41 at 670 K stand out [38].

Similar effects have been described for different improper ferroelectrics like geometric or electronic ones, for which a comparatively large spontaneous polarization (>1 μC cm−2) develops independently of the magnetic order, usually at a high-temperature transition, either involving structural rearrangements or charge ordering. The best known examples are YMnO3 and LuFe2O4, respectively [40]. Nonetheless, magnetic ordering temperatures are also below room temperature and magnetoelectric effects again are phase-change responses associated with additional polarization components that appear at low-temperature magnetic transitions. Furthermore, the ferroelectricity of LuFe2O4 has been recently questioned [41]. Besides, reports on the electrical control of magnetization, let alone on its reversal (the key enabling property for the magnetoelectric memory), are scarce. An example is the development of a ferromagnetic Ho³+ state in HoMnO3 under the electric field, switchable but not remnant below the ordering temperature of 80 K [42]. Therefore, and though research goes on covering an ever wider range of manganites, ferrites, chromites, and cobaltites [43], it is not clear that they can provide room temperature, electrically driven magnetization switching. Alternatives thus need to be (and are being) investigated.

An alternative approach to obtain oxides that do show independent ferroic orders is to chemically design ABO3 perovskite systems where ferroelectricity and magnetism originate at different crystal lattice sites. BiMnO3 stands out as an insulating ferromagnet, which has also been claimed to be ferroelectric, and is considered a model system of this strategy. Ferromagnetic order of the Mn³+ at the B-site develops at 100 K, while ferroelectric off-center distortion would be driven by the stereochemically active Bi+3 lone pair at the A-site [44]. However, later detailed structural characterizations, electrical measurements down to 77 K, and studies of the lattice dynamics have shown this oxide not to be ferroelectric [47]. This is consistent with studies of the xBiMnO3 – (1 – x)PbTiO3 solid solution that clearly indicated the disappearance of ferroelectricity for x ≥ 0.5 [48].

BiFeO3 is undoubtedly a multiferroic material, and perhaps the only compound that shows magnetic ordering and proper ferroelectricity at room temperature. This perovskite oxide has a ferroelectric transition temperature of 1100 K and G-type antiferromagnetism with an ordering temperature of 643 K. Crystal magnetoelectric coupling does exist and results in spin canting as well as a long-range incommensurate cycloid superstructure that cancels out the ferromagnetic component [49]. Weak ferromagnetism and a linear magnetoelectric effect are only obtained when the latter spin cycloid is destroyed by chemical modification [50], epitaxial strain [51], or due to a size effect [52]. These two latter phenomena are examples of multiferroism tailored by means of nanostructuring (see Figure I.2).

Figure I.2 Uncovering the latent magnetization in BiFeO3 (a) thin films (reprinted with permission from [51]. Copyright [2005], AIP Publishing LLC) and (b) nanoparticles (reprinted with permission from [52]. Copyright (2007) American Chemical Society).

Recently, a strong phase-change magnetoelectric response has been anticipated by a first-principles investigation of phases in the BiFeO3–BiCoO3 perovskite binary system, associated with the existence of a morphotropic phase boundary (MPB) between multiferroic polymorphs of rhombohedral and tetragonal symmetries [53]. This might be a general property of multiferroic MPBs, like the enhancements of polarizability and of the electromechanical response are of analogous ferroelectric MPBs, and is a novel promising approach for room temperature magnetoelectricity. The mechanism would be the rotation of the magnetization easy axis following that of spontaneous polarization under the electric field, facilitated by the lattice transverse instability at the MPB, and the final electric field induced transformation from one ferroelectric (multiferroic) polymorphic phase to the other. This approach requires the identification and study of suitable material systems, like the BiFeO3–PbTiO3 solid solution, recently shown to present such a multiferroic MPB, for which the enhancement of properties was described [55].

Nonetheless, the presence of coexisting, independently developed ferroic orders and of linear or/and phase-change magnetoelectric effects in a single-phase material does not guarantee the occurrence of magnetoelectric switching. This phenomenon requires direct coupling of the magnetic and ferroelectric domain configurations. This coupling has been demonstrated for antiferromagnetic multiferroics like YMnO3 and BiFeO3 and enables the reorientation of the antiferromagnetic domains following electrical switching of ferroelectric/ferroelastic ones [58]. However, an analogous phenomenon has not been found for ferromagnetic multiferroics.

A milestone in research of single-phase multiferroics, partially reversible ferroelectric domain reorientations under magnetic fields have been very recently described for (1 – x) Pb(Zr0.53Ti0.47)O3–x Pb(Fe1/2Ta1/2)O3, with x = 0.4 at room temperature [59]. This is an example out of a number of perovskite solid solutions between ferroelectric phases and relaxor compounds (it is the second time that these polar materials are mentioned without being introduced; they are examples of intrinsically nanostructured compounds that will be addressed later on) containing magnetically active cations like Pb(Fe1/2Nb1/2)O3, Pb(Fe2/3W1/3)O3, and Pb(Fe1/2Ta1/2)O3 itself, which show weak ferromagnetism in coexistence with a range of polar states and different magnetoelectric coupling phenomena [61]. Though the underlying mechanisms have not been elucidated, spin clustering is thought to be responsible for the ferromagnetic component in the otherwise expected paramagnetic state, while elastic interactions between the proposed magnetic nanoregions and the ferroelectric matrix would cause the magnetoelectric effects.

Ferroelectric domain reorientations under magnetic fields have also been recently reported for Aurivillius structure multiferroic single-phase materials [62]. These oxides are the topic of Chapter 25.

I.3.2 Magnetoelectric Composites

In spite of the extensive research briefly reviewed in the previous section, a single-phase material multiferroic at room temperature and with strong magnetoelectric response, either a linear response or fully controllable magnetoelectric switching, has not yet been reported.

An alternative to single-phase materials for room temperature magnetoelectricity are two-phase ferromagnetic–ferroelectric composites [64]. In these materials, the magnetoelectric effect is obtained as a product property of the magnetostriction and piezoelectricity of the phases (it is thus strain mediated). Best results have been obtained with laminate composites of a high-permeability magnetostrictive FeBSiC alloy and ultra-high piezoelectricity Pb(Mg1/3Nb2/3)O3–PbTiO3 fibers with a 2–1 connectivity, which showed a magnetoelectric coefficient αE > 50 V cm−1 Oe−1 (to be compared with 0.017 V cm-1 Oe-1 for the Sr3Co4Fe24O41 hexaferrite). Magnetic field sensors with extremely low equivalent magnetic noise and sensitivity as high as 10 pT have been demonstrated with these laminates [65]. Ferromagnetic–ferroelectric composites are also promising bitunable materials (field-dependent permittivity and permeability) with a high potential for miniaturized low-weight wearable antennas (mostly in the radiofrequency range) with enhanced bandwidth, improved directivity, high efficiency, and tunable resonant frequency.

This is the last example of a technology that seems to be ready for commercialization: bulk magnetoelectric composites of ultrahigh magnetostriction alloys, such as terfenol or metglass, and high-sensitivity piezoelectrics. Research is also very active in all-oxide composites with 2–2 and 0–3 connectivities, for which spinel structure compounds based on NiFe2O4 are commonly selected as magnetostrictive components (Chapter 3 introduces one of the most topical nanotechnologies for the processing of these composites) and in polymeric-based ones.

The challenge is now to develop comparable materials in thin-film form for integrated technologies. It is clear that films on silicon-based substrates are required for the magnetoelectric memory, but integrated sensors and microwave components are also targeted. There is also a parallel activity on films of single phases, some of which have already been mentioned [62].

However, the current, most promising approaches for obtaining magnetoelectric switching are based on multiferroic epitaxial heterostructures. This is already within the realm of nanoscale ferroelectrics and multiferroics.

I.4 Nanoscale Ferroelectrics and Multiferroics

We have already presented several examples of how the miniaturization of bulk ferroelectric technologies requires the nanostructuring of the ceramic layers. We have also noted that thin films used for FeRAMs are nanostructured, and further miniaturization must lead to even smaller characteristic sizes. The successive FeRAM generations have complied with their own Moore law up to now and must continue doing it in the future for increasing capacity [25].

However, before introducing nanoscale ferroelectrics and multiferroics, we consider it necessary to make some remarks about the concept of critical size for ferroelectricity, highly topical at the turn of the last century. Ferroic phenomena, and thus ferroelectricity, are solid-state cooperative phenomena that result from the interaction of individual entities (electrical dipoles originating from the ion arrangement in the non-centrosymmetric crystal structure in this latter case). Therefore, one can anticipate a critical size below which the interacting units are not enough for the ferroic transition to take place. Even within these limits, ferroelectrics have been shown to withstand successive levels of nanostructuring down to very low sizes while preserving polar order. For instance, PbTiO3 epitaxial films as thin as 3 monolayers (1D nanostructuring down to 1.2 nm) are ferroelectric and show inversion domains. These are formed to reduce the electrostatic energy associated with the increasing depolarization field. Ferroelectricity thus is only limited by an antiferrodistortive surface reconstruction [66]. Moreover, BaTiO3 nanowires with 3 nm diameter (2D nanostructuring) are still ferroelectric, and extrapolation of the transition temperature trend allows their existence to be predicted down to 0.8 nm. In this case the depolarization field is thought to be screened by surface adsorbed chemical species [67]. Finally, linearly ordered and monodomain polarization states have been described in BaTiO3 individual isolated cubic nanocrystals with characteristic dimensions below 10 nm (3D nanostructuring) [68]. This latter example is illustrated in Figure I.3.

Figure I.3 (a) Atomic resolution reconstructed phase image and (b) titanium displacement map of a single BaTiO3 nanocrystal and (c) ferroelectric switching (followed with piezoresponse force microscopy) in a 10 nm crystal. Reprinted by permission from Macmillan Publishers Ltd (Nature Materials) [68], copyright (2012).

Therefore, there remains plenty of room at the bottom for the nanostructuring of ferroelectrics before the ferroic order vanishes. This has shifted attention from determining the critical size to the description and the understanding of the effects of nanostructuring on the properties of ferroelectric materials. These do not only result from the size decrease, but also from the specific mechanical and electrical boundary conditions. The three levels of nanostructuring are being investigated extensively.

I.4.1 1D Nanostructured Ferroelectrics and Multiferroics

The ideal 1D nanostructured system would be a free-standing single-crystal plate. BaTiO3 nanoscale thickness plates have been machined out of bulk single-crystal samples by focused ion beam milling and their domain configuration and dynamics, as well as electrical properties successfully characterized. This research has been the key to building up an understanding of the effect of the confined geometry in ferroelectrics [69]. However, the most topical and technologically relevant 1D nanostructured systems are epitaxial films (and specifically those very recently grown on buffered silicon; see Chapter 20).

I.4.1.1 Nanoscale Epitaxial Films for Novel Ferroelectric Memories

Ultra-thin epitaxial ferroelectric oxide films do not only maintain ferroelectricity but their properties can be tailored to the epitaxial strain, an approach referred to as strain engineering, which allows, for instance, enhanced spontaneous electrical polarization to be obtained [70]. In addition, these films are enabling materials for novel memory designs proposed to overcome current FeRAMs limitations, basically the capacitive and destructive reading, which hinders miniaturization. Three devices are under extensive investigation:

The first one to be raised was the ferroelectric field effect transistor (FeFET) memory. This is a one-transistor (1T) device as compared with one-transistor one-capacitor (1T–1C) current architectures. They are basically a metal oxide-semiconductor FET (MOSFET), for which the gate dielectric has been replaced by a ferroelectric layer. In MOSFETs, the charge carrier density within the channel and the current between the source and drain are controlled by polarization within the gate oxide. This layer being ferroelectric, a hysteretic resistance loop with two well-differentiated remnant states appears to be directly associated with the ferroelectric hysteresis and the two directions of the remnant polarization. The memory is a ferroelectric one, for information is still stored in the polarization state, but the reading is resistive and non-destructive, and the architecture is simplified. In spite of these advantages, reliability issues originating from the quality of the ferroelectric oxide–semiconductor interface have prevented full development (epitaxial growth is used, and still surface electronic states often compensate the electrical polarization and screen the electric field within the channel) [27]. This topic is addressed in Chapter 20.

A second promising, related, device is the ferroelectric tunnel junction (FeTJ) memory. It is based on the dependence of the tunnel electroresistance through an ultra-thin ferroelectric film capacitor structure on the polarization within the dielectric. This shows two well-differentiated resistance states, again directly associated with the two polarization directions. It is therefore also a ferroelectric memory with a resistive, non-destructive reading, but with a vertical instead of a planar configuration, and a 1C architecture that is also advantageous for miniaturization. All-oxide epitaxial capacitors are investigated, which provide very high quality interfaces. More information can be found in recent reviews [72].

The last device is the ferroelectric scanning probe memory. This is an example of scanning probe technologies currently under extensive research, specifically based on the ability of writing arrays of very small ferroelectric domains in ultra-thin ferroelectric films with SPM electrically biased microcantilevers, and of probing their orientation with the piezoresponse force mode. Indeed, dense patterns up to 28 Gbit cm−2 have been written and imaged in epitaxial Pb(Zr,Ti)O3 films with 37 nm thickness [73]. This technology requires parallel operation of large microcantilever arrays for fast access. SPM technologies for ferroelectrics are the topic of Chapters 13 and 14.

I.4.1.2 Nanoscale Film Epitaxial Heterostructures for Magnetoelectricity

Epitaxial multiferroic films are also under focus, and multiferroism has been described in films of BiMnO3, not ferroelectric in bulk, and of (Bi,La)MnO3, with which multiferroic tunnel junctions have been built and four-state memory effect demonstrated [74]. Multiferroism has also been strain engineered in EuTiO3, taking advantage of spin-lattice coupling [75].

Moreover, magnetoelectricity has been shown to appear in a number of oxide epitaxial heterostructures consisting of ferroelectric and magnetic layers, and thus multiferroic by design. Different mechanisms of interaction between the ferroic layers, capable of generating the magnetoelectric response, have been described.

The first one to be reported was elastic interaction of the layers (or strain-mediated magnetoelectricity). This is the same mechanism responsible for the high magnetoelectric response of bulk composites. The simplest configuration is a magnetic layer deposited on to a ferroelectric single crystal, and indeed strong magnetoelectric effects can be obtained with such systems. An example is the electrical tuning of ferromagnetic resonance. This effect has been illustrated for a range of oxide heterostructures, like Fe3O4 films on Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals, throughout the variation of magnetic susceptibility due to in-plane piezoelectric strains [76]. Direct electric field control of magnetism has been obtained on two-component systems with elastically coupled ferromagnetic and ferroelectric/ferroelastic domains (the topic of Chapter 21).

However, this approach is not directly transferable to epitaxial heterostructures on non-active substrates, such as SrTiO3, or even better SrTiO3/Si for their integration with electronics, for the piezoelectric or ferroelastic strains are strongly limited by the substrate in this configuration. This is why attention has mainly concentrated on epitaxial columnar nanostructures of magnetic spinel and ferroelectric perovskite structure oxides such as CoFe2O4 and BaTiO3 or BiFeO3 [77]. A 1–3 connectivity is engineered in these multiferroic epitaxial thin-film composites to avoid substrate clamping effects. Though magnetoelectric switching has been locally demonstrated and controlled [78], an analogous macroscopic effect has not been achieved, and only very recently an unambiguous magnetoelectric response was reported, though with low coefficients of 0.06 V cm−1 Oe−1 [79]. The substrate clamping issue can also be minimized by selecting 0–3 connectivity, and the most promising results have been found for thin-film particulate nanocomposites of Co and BaTiO3 with magnetoelectric coefficients of 0.16 V cm−1 Oe−1 [80]. Chapter 6 presents a novel process for such thin-film composites.

Magnetoelectricity has also been obtained from magnetic interaction of the layers (exchange-mediated). This requires a heterostructure formed by an antiferromagnetic multiferroic with coupled domains, such as YMnO3 or BiFeO3, and a ferromagnet. Electrical control of the magnetism in the ferromagnetic component is obtained by means of exchange interaction with the multiferroic one, specifically by electrical modulation of the exchange bias [82].

Electrostatic interaction can also result in magnetoelectricity. This has been described for heterostructures of ferroelectrics and strongly correlated magnetic systems like PbZr0.2Ti0.8O3/La0.8Sr0.2MnO3/SrTiO3 film epitaxial ones. The mechanism in this case is modification of the density of the charge carriers in the manganite compound and formation of depleted/accumulated layers, with the electrostatic field associated with the switchable electrical polarization in the ferroelectric oxide. This has a direct effect on magnetism (charge-mediated magnetoelectricity). Note the analogy with the ferroelectric field effect transistor [83].

Both effects can be operative and add up, such as in BiFeO3/(La,Sr)MnO3 heterostructures, for which reversible electric control of the exchange bias has been demonstrated using a field effect device [84]. Two examples of magnetoelectric response in epitaxial heterostructures are given in Figure I.4.

Figure I.4 (a) Magnetoelectric response for a BiFeO3/CoFe2O4 thin-film nanocomposite (reprinted with permission from [79]. Copyright [2010], AIP Publishing LLC) and (b) magnetoelectric hysteresis curve for a Pb(Zr,Ti)O3/(La,Sr)MnO3 heterostructure (reproduced with permission from [83]. Copyright © [2009] John Wiley & Sons).

A phenomenon that is attracting increasing attention is interface room temperature multiferroism [85], such as that recently reported for BaTiO3/Fe heterostructures [86]; see Chapter 10 for an atomic resolution analysis of the chemical nature of the interface. Ferroelectric control of the current spin polarization through magnetic tunnel junctions based on BaTiO3/Fe has been demonstrated [87]. Multiferroic tunnel junctions consisting of a ferroelectric barrier between dissimilar ferromagnetic electrodes are an example out of a range of multiferroic devices under extensive investigation for spintronics [88]. Interface multiferroism has also been described for ferroelectric domain walls, the topic of Chapter 24.

A transversal issue for all type of memories, whether they are based on either ferroelectric or multiferroic film structures, is their patterning to obtain nanoscale cell arrays for very high densities. Different approaches used to prepare ordered arrays of 3D nanostructured ferroic elements on substrates are reviewed in Chapter 5.

I.4.2 2D Nanostructured Ferroelectrics and Multiferroics

Single-crystal nanowires do not only preserve ferroelectricity down to very small diameters but they can also show enhancement of axial polarization with the decrease in size [89] and a significant piezoelectric response [90]. They are being considered for a range of applications [91], among which energy harvesting stands out [92].

Single-crystal nanotubes have not been reported, though polycrystalline analogs are under extensive study. These are not actual 2D nanostructured systems, but ensembles of 3D nanostructured crystals under distinctive boundary conditions imposed by shape. Finite curvature effects leading to better downscaling behavior than films of a comparable thickness have been reported [93]. They are also under consideration for different applications [94], such as microfluidics, making use of piezoelectricity and random access memories. Coaxial capacitors can be formed with a large effective surface, providing increased specific charge.

These systems are extensively addressed in Chapters 8 and 9.

I.4.3 3D Nanostructured Ferroelectrics and Multiferroics

Though isolated ferroelectric and multiferroic single-crystal nanoparticles have been used for piezo- or photocatalysis [91], they are basically precursors, as ceramic nanopowders, for the processing of high-performance electroceramics and also an enabling technology for the nanostructuring of ceramic layers (see Chapters 1, 2, and 4).

In fact, most topical and technologically relevant 3D nanostructured systems are nanostructured ceramics, films, and nanotubes, which are actually ensembles of densely packed nanoparticles. Nanostructuring is a requirement of the current tendency to miniaturization of ceramic and film technologies. This has a deep impact on functional properties, not only as a consequence of actual size effects but also of the boundary conditions. This subject is dealt with in Chapters 15, 16, 17, and 18. Also, nanostructuring ultimately results from the ever-decreasing processing temperatures, a key issue to the integration of these multifunctional oxides into silicon devices and flexoelectronics (see Chapter 7).

I.4.4 Intrinsically Nanostructured Ferroelectrics and Multiferroics

There are single-crystal compounds that show nanoscale features, which highly determine their properties. This is the case of the hierarchical domain configuration characteristic of Pb(Zr,Ti)O3 at the MPB, consisting of a first level of submicrometer size non-180° lamellar domains, within which a substructure, or second level, of nanoscale regular twinned domains exist. It is the dynamics of these nanotwins that is responsible for the high domain wall contribution to the piezoelectric coefficients [95].

However, the best known example is relaxor ferroelectrics like the model Pb(Mg1/3 Nb2/3)O3, for which nanoscale chemical ordered regions (CORs) self-assemble within a disorder matrix, driven by electrostatic and elastic forces, and seed the nucleation of polar nanoregions (PNRs) [96]. The dynamics of these PNRs embedded within a paraelectric matrix is responsible for the distinctive electrical behavior of these short-range ordered polar and extensively studied materials [97]. Magnetoelectric relaxors, also named electric/magnetic birelaxors, have been described. Relaxors are extensively addressed in Chapters 11 and 17, and also in Chapter 23, where the intriguing reentrant phenomena are addressed.

I.5 Book Plan

Therefore, nanoscale ferroelectrics are a highly topical research field that is driven by technological interests. They are also being considered for novel applications like magnetoelectric and photovoltaic ones (here chosen as examples; see Chapters 25 and 26). Additionally, a range of novel physical phenomena have been described in nanoscale ferroelectrics and multiferroics, mainly relating to domain configurations in confined systems. The best known example is the finding of polarization closure domain configurations in nanoscale objects, within a general search of the predicted dipolar vortexes (electrical ferrotoroic order) [98].This is the topic of Chapter 22.

This book aims at reviewing the current state of the art of these materials, stressing a range of specific topics at the cutting edge of research. Key issues in processing (nanostructuring), characterization (of the nanostructured materials), and nanoscale effects are successively addressed in three specific sections.

References

(1) Schmid, H. (2008) Some symmetry aspects of ferroics and single phase multiferroics. Journal of Physics: Condensed Matter, 20, art. no. 434201.

(2) Bibes, M. (2012) Nanoferronics is a winning combination. Nature Materials, 11, 354–357.

(3) Spaldin, N.A. and Fiebig, M. (2005) The renaissance of magnetoelectric multiferroics. Science, 309, 391–392.

(4) Eerenstein, W., Mathur, N.D., and Scott, J.F. (2006) Multiferroic and magnetoelectric materials. Nature, 442, 759–765.

(5) Fiebig, M. (2005) Revival of the magnetoelectric effect. Journal of Physics D: Applied Physics, 38, R123–R152.

(6) Hill, N.A. (2000) Why are there so few magnetic ferroelectrics? Journal of Physical Chemistry B, 104, 6694–6709.

(7) Cohen, R.E. (1992) Origin of ferroelectricity in perovskite oxides. Nature, 358, 136–138.

(8) Devonshire, A.F. (1954) Theory of ferroelectrics. Advances in Physics, 3, 85–130.

(9) Cochran, W. (1960) Crystal stability and the theory of ferroelectricity. Advances in Physics, 9, 387–423.

(10) Scott, J.F. (2007) Applications of modern ferroelectrics. Science, 315, 954–959.

(11) Martins, P., Lopes, A.C., and Lanceros-Mendez, S. (2014) Electroactive phases of poly(vinylidene fluoride): determination, processing and applications. Progress in Polymer Science, 39, 683–706.

(12) Guo, M., Cai, H.L., and Xiong, R.G. (2010) Ferroelectric metal organic framework (MOF). Inorganic Chemistry Communications, 13, 1590–1598.

(13) Noheda, B., Gonzalo, J.A., Cross, L.E., et al. (2000) Tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: the structure of PbZr0.52Ti0.48O3. Physical Review B, 61, 8687–8695.

(14) Ahart, M., Somayazulu, M., Cohen, R.E., et al. (2008) Origin of morphotropic phase boundaries in ferroelectrics. Nature, 451, 545–549.

(15) Damjanovic, D. (2009) Comments on origins of enhanced piezoelectric properties in ferroelectrics. IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 56, 1574–1585.

(16) Damjanovic, D. (1998) Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics. Reports on Progress in Physics, 61, 1267–1324.

(17) Anton, S.R. and Sodano, H.A. (2007) A review of power harvesting using piezoelectric materials (2003–2006). Smart Materials and Structuctures, 16, R1–R21.

(18) Haertling, G.H. (1999) Ferroelectric ceramics: history and technology. Journal of the American Ceramic Society, 82, 797–818.

(19) Park, S.E. and Shrout, T.R. (1997) Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals. Journal of Applied Physics, 82, 1804–1811.

(20) Fu, H. and Cohen, R.E. (2000) Polarization rotation mechanism for ultrahigh electromechanical response in single crystal piezoelectrics. Nature, 403, 281–283.

(21) Kutnjak, Z., Petzelt, J., and Blinc, R. (2006) The giant electromechanical response in ferroelectric relaxors as a critical phenomenon. Nature, 441, 956–959.

(22) Zhang, S., Li, F., Jiang, X., et al. (2015) Advantages and challenges of relaxor-PbTiO3 ferroelectric crystals for electroacoustic transducers – a review. Progress in Materials Science, 68, 1–66.

(23) Scott, J.F. and Paz de Araujo, C.A. (1989) Ferroelectric memories. Science, 246, 1400–1405.

(24) Scott, J.F. (2000) Ferroelectric Memories, in Advanced Microelectronics Series, edited by K. Itoh and T. Sakurai, Springer-Verlag, Berlin, Heidelberg, Germany.

(25) Setter, N., Damjanovic, D., Eng, L., et al. (2006) Ferroelectric thin films: review of materials, properties and applications. Journal of Applied Physics, 100, art. no. 051606.

(26) Paz de Araujo, C.A., Cuchiaro, J.D., McMillan, L.D., et al. (1995) Fatigue-free ferroelectric capacitors with platinum electodes. Nature, 374, 627–629.

(27) Ishiwara, H. (2012) Ferroelectric random access memories. Journal of Nanoscience and Nanotechnology, 12, 7619–7627.

(28) Eom, C.B. and Trolier-McKinstry, S. (2012) Thin-film piezoelectric MEMS. MRS Bulletin, 37, 1007–1017.

(29) Whatmore, R.W., Zhang, Q., Shaw, C.P., et al. (2007) Pyroelectric ceramics and thin films for applications in uncooled infra-red sensor arrays. Physica Scripta, T129, 6–11.

(30) Brennecka, G.L., Ihlefeld, J.F., Maria, J.P., et al. (2010) Processing technologies for high permittivity thin films in capacitor applications. Journal of the American Ceramic Society, 93, 3935–3954.

(31) Alpay, S.P., Mantese, J., Trolier-McKinstry, S., et al. (2014) Next-generation electrocaloric and pyroelectric materials for solid-state electrothermal energy interconversion. MRS Bulletin, 39, 1099–1109.

(32) Sott, J.F. (2012) Applications of magnetoelectrics. Journal of Materials Chemistry, 22, 4567–4574.

(33) Ma, J., Hu, J.M., Li, Z., and Nan, C.W. (2011) Recent progress in multiferroic magnetoelectric composites: from bulk to thin films. Advanced Materials, 23, 1062–1087.

(34) Bibes, M. and Barthelémy, A. (2008) Towards a magnetoelectric memory. Nature Materials, 7, 425–426.

(35) Kimura, T., Goto, T., Shintani, H., et al. (2003) Magnetic control of ferroelectric polarization. Nature, 426, 55–58.

(36) Hur, N., Park, S., Sharma, P.A., et al. (2004) Electric polarization reversal and memory in a multiferroic material induced by magnetic fields. Nature, 429, 392–395.

(37) Kimura, T., Sekio, Y., Nakamura, H., et al. (2008) Cupric oxide as an induced multiferroic with high TC. Nature Materials, 7, 291–294.

(38) Kitagawa, Y., Hiraoka, Y., Honda, T., et al. (2010) Low field magnetoelectric effect at room temperature. Nature Materials, 9, 797–802.

(39) Van Aken, B.B., Palstra, T.T.M., Filippetti, A., and Spaldin, N.A. (2004) The origin of ferroelectricity in magnetoelectric YMnO3. Nature Materials, 3, 164–170.

(40) Ikeda, N., Ohsumi, H., Ohwada, K., et al. (2005) Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe2O4. Nature, 436, 1136–1138.

(41) Lafuerza, S., Garcia, J., Subias, G., et al. (2013) Intrinsic electrical properties of LuFe2O4. Physical Review B, 88, art. no. 085130.

(42) Lottermoser, T., Lonkai, T., Amann, U., et al. (2004) Magnetic phase control by an electric field. Nature, 430, 541–544.

(43) Preethi-Meher, K.R.S., Martin, C., Caignaert, V., et al. (2014) Multiferroics and magnetoelectrics: a comparison between some chromites and cobaltites. Chemistry of Materials, 26, 830–836.

(44) Seshadri, R. and Hill, N.A. (2001) Visualizing the role of Bi 6s lone pairs in the off-center distortion in ferromagnetic BiMnO3. Chemistry of Materials, 13, 2892–2899.

(45) Belik, A.A., Kodama, K., Igaea, N., et al. (2010) Crystal and magnetic structures and properties of BiMnO3+δ. Journal of the American Chemical Society, 132, 8137–8144.

(46) Castro, A., Correas, C., Peña, O., et al. (2012) Nanostructured BiMnO3+δ obtained at ambient pressure: analysis of its multiferroicity. Journal of Materials Chemistry, 22, 9928–9938.

(47) Golan, V., Kamba, S., Savinov, M., et al. (2012) Absence of ferroelectricity in BiMnO3 ceramics. Journal of Applied Physics, 112, art. no. 074112.

(48) Castro, A., Vila, E., Jiménez, R., et al. (2010) Synthesis, structural characterization, and properties of perovskites belonging to the x BiMnO3–(1 – x) PbTiO3 system. Chemistry of Materials, 22, 541–550.

(49) Catalan, G. and Scott, J.F. (2009) Physics and applications of bismuth ferrite. Advanced Materials, 21, 2463–2485.

(50) Khomchenko, V.A., Kiselev, D.A., Vieira, J.M., et al. (2008) Effect of diamagnetic Ca, Sr, Pb, and Ba substitution on the crystal structure and multiferroic properties of the BiFeO3 perovskite. Journal of Applied Physics, 103, art. no. 024105.

(51) Bai, F.M., Wang, J.L., Wuttig, M., et al. (2005) Destruction of spin cycloid in (111)c-oriented BiFeO3 thin films by epitiaxial constraint: enhanced polarization and release of latent magnetization. Applied Physics Letters, 86, art. no. 032511.

(52) Park, T.J., Papaefthymiou, G.C., Viescas A.J., et al. (2007) Size-dependent magnetic properties of single-crystalline multiferroic BiFeO3 nanoparticles. Nano Letters, 7, 766–772.

(53) Dieguez, O. and Iñiquez, J. (2011) First-principles investigation of morphotropic transitions and phase-change functional responses in BiFeO3–BiCoO3 multiferroic solid solutions. Physical Review Letters, 107, art. no. 057601.

(54) Amorín, H., Correas, C., Ramos, P., et al. (2012) Very high remnant polarization and phase-change electromechanical response of BiFeO3–PbTiO3. Applied Physics Letters, 101, art. no. 172908.

(55) Amorin, H., Correas, C., Fernandez-Posada, C.M., et al. (2014) Multiferroism and enhancement of material properties across the morphotropic phase boundary of BiFeO3–PbTiO3. Journal of Applied Physics, 115, art. no. 104104.

(56) Fiebig, M., Lottermoser, Th., Fröhlich, D., et al. (2002) Observation of coupled magnetic and electric domains. Nature, 419, 818–820.

(57) Zhao, T., Scholl, A., Zavaliche, F., et al. (2006) Electrical control of antiferromagnetic domains in multiferroic BiFeO3 films at room temperature. Nature Materials, 5, 823–829.

(58) Lebeugle, D., Colson, D., Forget, A., et al. (2008) Electric field induced spin flop in BiFeO3 single crystals at room temperature. Physical Review Letters, 100, art. no. 227602.

(59) Evans, D.M., Schilling, A., Kumar, A., et al. (2013) Magnetic switching of ferroelectric domains at room temperature in multiferroic PZTFT. Nature Communications, 4, art. no. 1534.

(60) Sánchez, D.A., Ortega, N., Kumar, A., et al. (2011) Symmetries and multiferroic properties of novel room-temperature magnetoelectrics: lead ion tantalate–lead zirconate titanate (PFT/PZT). AIP Advances, 1, art. no. 042169.

(61) Sánchez, D., Ortega, N., Kumar, A., et al. (2013) Room temperature single-phase multiferroic magnetoelectrics: Pb(Fe,M)x(Zr,Ti)1-xO3 [M = Ta,Nb]. Journal of Applied Physics, 113, art. no. 074105.

(62) Keeney, L., Maity, T., Scmidt, M., et al. (2013) Magnetic field induced ferroelectric switching in multiferroic Aurivillius phase thin films at room temperature. Journal of the American Ceramic Society, 96, 2339–2357.

(63) Nan, C.W., Bichurin, M.I., Dong, S., et al. (2008) Multiferroic magnetoelectric composites: historical perspective, status, and future directions. Journal of Applied Physics, 103, art. no. 031101.

(64) Vaz, C.A.F., Hoffman, J., Ahn, C.H., and Ramesh, R. (2010) Magnetoelectric coupling effects in multiferroic complex oxide composite structures. Advanced Materials, 22, 2900–2818.

(65) Wang, Y.J., Gray, D., Berry, D., et al. (2011) An extremely low equivalent magnetic noise magnetoelectric sensor. Advanced Materials, 23, 4111–4114.

(66) Fong, D.D., Stephenson, G.B., Streiffer, S.K., et al. (2004) Ferroelectricity in ultrathin perovskite films. Science, 304, 1650–1653.

(67) Spanier, J.E., Kolpak, A.M., Urban, J.J., et al. (2006) Ferroelectric phase transition in individual single-crystalline BaTiO3 nanowires. Nano Letters, 6, 735–739.

(68) Polking, M.J., Han, M.G., Yourdkhani, A., et al. (2012) Ferroelectric order in individual nanometre scale crystals. Nature Materials, 11, 700–709.

(69) Sharma, P., McQuaid, R.G.P., McGilly, L.J., et al. (2013) Nanoscale dynamics of superdomain boundaries in single crystal BaTiO3 lamellae. Advanced Materials, 25, 1323–1330.

(70) N. Schlom, D.G., Chen, L.Q., Eom, C.B., et al. (2007) Strain tuning of ferroelectric thin films. Annual Review of Materials Research, 37, 589–626.

(71) Chanthbouala, A. Crassous, A., Garcia, V., et al. (2012) Solid-state memories based on ferroelectric tunnel junctions. Nature Nanotechnology, 7 (2), 101–104.

(72) Garcia, V. and Bibes, M. (2014) Ferroelectric tunnel junctions for information storage and processing. Nature Communications, 5, art. no. 4289.

(73) Tybell, T., Paruch, P., Giamarchi, T., and Triscone, J.M. (2002) Domain wall creep in epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 thin films. Physical Review Letters, 89, art. no. 097601.

(74) Gajek, M., Bibes, M., Fusil, S., et al. (2007) Tunnel junctions with multiferroic barriers. Nature Materials, 6, 296–302.

(75) Lee, J.H., Fang, L., Vlahos, E., et al. (2010) A strong ferroelectric ferromagnet created by means of spin-lattice coupling. Nature, 466, 954–959.

(76) Liu, M., Obi, O., Lou, J., et al. (2009) Giant electric field tuning of magnetic properties in multiferroic ferrite/ferroelectric heterostructures. Advanced Functional Materials, 19, 1826–1831.

(77) Zheng, H., Wang, J., Lofland, S.E., et al. (2004) Multiferroic BaTiO3–CoFe2O4 nanostructures. Science, 303, 661–663.

(78) Zavaliche, F., Zheng, H., Mohaddes-Ardabili, L., et al. (2005) Electric field induced magnetization switching in epitaxial columnar nanostructures. Nano Letters, 5, 1793–1796.

(79) Oh, Y.S., Crane, S., Zheng, H., et al. (2010) Quantitative determination of anistropic magnetoelectric coupling in BiFeO3–CoFe2O4 nanostructures. Applied Physics Letters, 97, art. no. 052902.

(80) Park, J.H., Jang, H.M., Kim, H.S., et al. (2008) Strain-mediated magnetoelectric coupling in BaTiO3–Co nanocomposite thin films. Applied Physics Letters, 92, art. no. 062908.

(81) Laukhin, V., Skumryev, V., Marti, X., et al. (2006) Electric-field control of exchange bias in multiferroic epitaxial heterostructures. Physical Review Letters, 97, art. no. 227201.

(82) Chu, Y.H., Martin, L.W. Holcomb, M.B., et al. (2008) Electric field control of local ferromagnetism using a magnetoelectric multiferroic. Nature Materials, 7, 476–482.

(83) Molegraaf, H.J.A., Hoffman, J., Vaz, C.A.F., et al. (2009) Magnetoelectric effects in complex oxides with competing ground states. Advanced Materials, 21, 3470–3474.

(84) Wu, S.M., Cybart, S.A., Yu, P., et al. (2010) Reversible electric control of exchange bias in a multiferroic filed effect device. Nature Materials, 9, 756–761.

(85) Rondinelli, J.M., Stengel, M., and Spaldin, N.A. (2008) Carrier-mediated magetoelectricity in complex oxide heterostructures. Nature Nanotechnology, 3, 46–50.

(86) Valencia, S., Crassous, A., Bocher, L., et al. (2011) Interface-induced room temperature multiferroicity in BaTiO3. Nature Materials, 10, 753–758.

(87) García, V., Bibes, M., Bocher, L., et al. (2010) Ferroelectric control of spin polarization. Science, 327, 1106–1110.

(88) Tsymbal, E.Y., Gruverman, A., García, V., et al. (2012) Ferroelectric and multiferroic tunnel junctions. MRS Bulletin, 37, 138–143.

(89) Cai, M.Q., Zheng, Y., Wang, B., and Yang, G.W. (2009) Nanosize confinement induced enhancement of spontaneous polarization in a ferroelectric nanowire. Applied Physics Letters, 95, art. no. 232901.

(90) Wang, Z., Hu, J., Suryavanshi, A.P., et al. (2007) Voltage generation from individual BaTiO3 nanowires