Multiferroic Materials

Properties, Techniques, and Applications

Junling Wang

• Publisher: CRC Press
• Year: 2016
• eBook ISBN: 9781315355269
• Topic: Physical Sciences
• Subtopic: Condensed Matter

"a very detailed book on multiferroics that will be useful for PhD students and researchers interested in this emerging field of materials science"

—Dr. Wilfrid Prellier, Research Director, CNRS, Caen, France

Multiferroics has emerged as one of the hottest topics in solid state physics in this millennium. The coexistence of multiple ferroic/antiferroic properties makes them useful both for fundamental studies and practical applications such as revolutionary new memory technologies and next-generation spintronics devices. This book provides an historical introduction to the field, followed by a summary of recent progress in single-phase multiferroics (type-I and type-II), multiferroic composites (bulk and nano composites), and emerging areas such as domain walls and vortices. Each chapter addresses potential technological implications. There is also a section dedicated to theoretical approaches, both phenomenological and first-principles calculations.

1

Coupled Electricity and Magnetism in Solids

Multiferroics and Beyond

Daniel I. Khomskii

Contents

1.1Introduction

1.2Some Historical Notes

1.3Magnetoelectric Effect; Symmetry Considerations

1.4Multiferroics

1.4.1General Considerations

1.5Different Types of Multiferroics

1.6Type-I Multiferroics

1.7Type-II Multiferroics

1.8Beyond Multiferroics

1.8.1Electric Activity of Magnetic Domain Walls

1.8.2Spiral Magnetic Structures on Metal Surfaces

1.8.3Magnetoelectric Effects in Magnetic Vortices and Skyrmions

1.8.4Electric Activity of Spin Waves

1.9Conclusions

References

1.1Introduction

The intrinsic coupling of electricity and magnetism is one of the cornerstones of modern physics. It goes back to the famous Maxwell equations, or even earlier, to Michael Faraday, and one can even find earlier reports pointing in that direction. This coupling plays crucial roles in all modern physics, and it is one of the foundations of modern technology, for example, in the generation of electricity in electric power stations, and in electric transformers. Recently this field acquired new life in spintronics—the idea of being able to use not only charge but also spin of electrons for electronic applications. In this field, one predominantly deals with the influence of magnetic field and/or magnetic ordering on transport properties of materials; for example, the well-known magnetoresistance or the work of magnetic tunnel junctions. But very interesting such effects can also exist in insulators. These are, for example, the (linear) magnetoelectric effect, or the coexistence and mutual influence of two types of ordering, magnetic and ferroelectric ordering in multiferroics. Such phenomena are very interesting in terms of its physical features, and are very promising for practical applications, such as for addressing magnetic memory electrically without the use of currents, or as very efficient magnetic sensors. These factors probably are reasons for creating such a significant interest in this field. It is now one of the hottest topics in condensed matter physics, and in addition to magnetoelectrics and multiferroics per se, the study of these has many spin-offs in the related fields of physics, such as the study of magnetoelectric effects in different magnetic textures (domain walls, magnetic vortices, skyrmions, etc.).

There are already several good reviews of this field [1–7], and there exists a very complete and useful collection of short reviews on multiferroics in the special issue of the Journal of Physics of Condensed Matter [8]. There is also a chapter on multiferroics in my recent book [9]. In this chapter, I will more or less follow the general outline of my short review on “Multiferroics for pedestrians,” published in Physics: Trends [7]—of course with significant additions.

This book is mainly devoted to real multiferroics—materials with coexisting magnetic and ferroelectric orderings. These materials are extremely interesting as to their physics, and they promise many important practical applications. However, one has to realize that for many practical applications, such as attempts to write and read magnetic memory in hard discs electrically, using electric fields rather than currents (e.g., with gate voltage devices), one needs not so much multiferroics but rather materials with good magnetoelectric properties: one must be able to modify the magnetic state by a changing electric field. But the idea is that it is precisely multiferroic materials in which the change of magnetic state by electric field, or vice versa, may be especially strong. From this point of view, various textures in magnetic materials, which can have magnetoelectric response, such as certain domain walls of skyrmions, also attract now considerable attention. These topics are mentioned later in this chapter and extensively discussed in several chapters of this book.

1.2Some Historical Notes

When describing the field of magnetoelectrics and multiferroics, the first reference one usually gives is the paper of Curie [10], who shortly noticed the possibility of having both magnetic and electric orderings in one material. But the real story began with a short remark in one of the famous books on theoretical physics by Landau and Lifshitz [11], who wrote in 1959:

Let us point out two more phenomena, which, in principle, could exist. One is piezomagnetism, which consists of linear coupling between a magnetic field in a solid and a deformation (analogous to piezoelectricity). The other is a linear coupling between magnetic and electric fields in a media, which would cause, for example, a magnetization proportional to an electric field. Both these phenomena could exist for certain classes of magnetocrystalline symmetry. We will not however discuss these phenomena in more detail because it seems that till present, presumably, they have not been observed in any substance.

Indeed, at the moment of publication of that volume there were no known real examples of magnetoelectric or multiferroic systems. But already less than a year after its publication, the seminal paper by Dzyaloshinskii [12] appeared, who on symmetry grounds predicted that the well-known antiferromagnet Cr₂O₃ should exhibit the linear magnetoelectric effect. And the following year, this effect was indeed observed in Cr₂O₃ by Astrov [13]. After that a rapid development of this field followed, initially in the study of magnetoelectrics, see Reference [14]. But very soon the ideas of not only the magnetoelectric effect, but also those of real multiferroics were put forth. Soon the first multiferroic—a material in which (antiferro) magnetic and ferroelectric orderings are present simultaneously—was discovered by Ascher et al. [15]—the Ni–I boracite. (It was in fact Schmid [16] who later coined the very term “multiferroics” in connection with such materials.) An active program to synthesize such materials artificially was initiated, predominantly by two groups in the former Soviet Union: in the group of Smolenskii in Leningrad (present-day St. Petersburg) and in the group of Venevtsev in Moscow.

However, after considerable activity in the 1960s and 1970s, the interest in this field faded somewhat. A new surge of activity appeared at around 2000, and there were three factors that stimulated it.

The first was the realization of an interesting and challenging problem in the physics of magnetic and ferroelectric materials, mostly on the example of perovskites. There are quite a lot of magnetic perovskites, including the famous colossal magnetoresistance manganites, or the “two-dimensional perovskite” La₂CuO₄—the parent material of high-temperature ...

Table of contents

• Cover
• Half title
• Title Page
• Copyright Page
• Contents
• Preface
• Editor
• Contributors
• CHAPTER 1 Coupled Electricity and Magnetism in Solids: Multiferroics and Beyond
• SECTION I Single-Phase Multiferroics
• SECTION II Multiferroic Composites
• SECTION III Theoretical Approaches in Multiferroic Study
• SECTION IV Emerging Topics in the Field
• Index