Overview No. 151Multiferroic and magnetoelectric heterostructures
Introduction
Complex oxides represent a broad class of materials that have a wide range of crystal structures and functionalities. Among them, the study of magnetic, ferroelectric, and more recently, multiferroic properties has stimulated considerable interest. The study of multiferroic and magnetoelectric materials, in particular, has experienced a dramatic increase in research effort over the past decade and today we see the emergence of real-life applications based on these efforts. This work has been driven, in part, by the development of new thin film growth techniques and the resulting access to high-quality materials for further study. In general, the field of functional oxide materials has experienced unprecedented growth during the past decades in terms of the discovery of new materials systems and physical phenomena, advances in characterization, and development of deeper understanding of the fundamental properties and how to control these properties through systematic changes in crystal chemistry (i.e. doping or alloying), strain, and other variables. Perhaps the most interesting recent manifestation of the complex and rich diversity of physical phenomena is the emergence of coupled behavior, in which the lattice, orbital, spin, and charge degrees of freedom are coupled through either quantum mechanics or through engineering design of artificial heterostructures. Of course, such coupling between phenomena is not necessarily new. Piezoelectrics exhibit coupling between mechanics and electrical degrees of freedom and ferromagnetic shape memory alloys exhibit coupling between magnetism and mechanics. But in the last decade, renewed interest in the potential of coupling between the spin and charge degrees of freedom has fascinated researchers and engineers not only from a fundamental perspective, but from an applied direction as well. Imagine a world in which one can control and manipulate magnetism with electric fields (which are intrinsically much easier to use in an actual device, especially in small dimensions, and can potentially provide routes to lower power/energy consumption in systems), thus eliminating currents and magnetic fields.
Creating novel materials and combinations of materials is thus a critical component that enables the exploration of such fascinating phenomena. The power of advanced materials synthesis has been demonstrated in a large number of instances; for example, semiconductor epitaxy has led not only to a large number of technologies, also to several Nobel prizes. Researchers in oxide science have taken a page out of the semiconductor lexicon and consequently, materials synthesis plays a critical role in enabling the study of such novel materials. In this paper, the recent advances in the growth and characterization of multiferroic and magnetoelectric oxide materials (in particular the model multiferroic BiFeO3) and the interplay between synthesis, theory, and experimental probes will be reviewed. We will summarize with a look to the future of complex oxide materials with special attention given to possible areas of impact for future technologies.
Section snippets
The crystal chemistry of complex oxides
The general field of metal oxide materials has been the focus of much study because of the broad range of structures, properties, and exciting phenomena present in these materials [1], [2]. The perovskite structure, which has the chemical formula ABO3 (i.e. CaTiO3, SrRuO3, BiFeO3), is made up of corner-sharing octahedra with the A-cation coordinated with twelve oxygen ions and the B-cation with six. The structure can easily accommodate a wide range of valence states on both the A- and B-sites
Multiferroism and magnetoelectricity
A hallmark of perovskites is the large variety of functional responses such as ferroelectricity, piezoelectricity, pyroelectricity, ferromagnetism, antiferromagnetism, etc., that form the underpinnings for both advanced technologies (such as ferroelectric nonvolatile memories, SONAR, transformers, etc.) and a wealth of basic science (for example in the colossal magnetoresistant (CMR) manganites). Such a broad range of responses can be enabled within a framework of the oxygen coordination
Engineering new functionalities with multiferroics
One of the major questions in the study of multiferroics today is how and when will multiferroics make their way into a room-temperature device and what will these devices look like? In early 2005, a number of so-called magnetoelectronic devices based on magnetoelectric materials were proposed [111]. The idea was a simple one, namely to use the net magnetic moment created by an electric field in a magnetoelectric thin film to change the magnetization of a neighboring ferromagnetic layer through
Future directions and conclusions
We hope that this review has captured some of the exciting new developments in the field of complex oxides, multiferroics and magnetoelectrics, especially from a thin film perspective. New developments are occurring at a rapid pace, throwing further light onto the complexities inherent to these materials. The dramatic progress in thin film heterostructure and nanostructure growth has been a key enabler fueling these discoveries. Since the advent of the superconducting cuprates, complex oxides
Acknowledgements
L.W.M. acknowledges the support of the Army Research Office under Grant W911NF-10-1-0482 and the Samsung Electronics Co. Ltd under Grant 919 Samsung 2010-06795. R.R. acknowledges the support of the Director, Office of Basic Energy Sciences, Materials Science Division of the US Department of Energy under Contract No. DE-AC02-05CH11231, support from several ARO and ONR MURI contracts, and the Western Institute of Nanoelectronics program, as well as significant intellectual and financial support
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