Multiferroic materials exhibit significant potential applications in the fields of novel multifunctional magnetic‐electric devices, spintronics devices, and high performance information storage and processing, etc. Besides, multiferroic has become a hot topic due to its rich connotation in condensed matter physics concerning charge, spin, orbital, and lattice.

The possibility of an intrinsic magnetoelectric (ME) effect in some crystals had been predicted by Pierre Curie in 1894. The research on magnetoelectric physics and materials was quite slow in the whole twentieth century due to the rare of magnetoelectric materials and the poor magnetoelectric performance. Schmid coined a new terminology of multiferroics in 1994, which denotes the coexistence of multiple ferroic (ferroelectric, ferromagnetic and ferroelastic) orders in a single‐phase material. The research on multiferroic materials resurged because of the two unexpected breakthroughs (epitaxy BiFeO₃ thin films and TbMnO₃) in 2003. It stimulated numerous subsequent investigations on single‐phase multiferroic, multiferroic composites, and multiferroic heterostructures (oxides and metallic/ferroelectric).

What is this Book?

The book presents a unified summary of multiferroic materials, multiferroic simulations and multiferroic prototype devices. Specifically, it covers a broad variety of multiferroic materials, including single phase multiferroic, oxides and metallic/ferroelectric multiferroic heterostructures, bulk, thin film and nanostructure multiferroic materials. And for each family of materials, their magnetoelectric coupling mechanisms and multiferroic simulations (first‐principle calculation, phase simulation and theoretical modes of ME coupling in multiferroic heterostructures) are also extensively discussed. Some prototype devices, including tunable RF/microwave devices (antenna, inductor, bandpass/stop filters and phase shifter), multiferroic memories, multiferroic sensors and integration of multiferroics on chip were presented. Novel multiferroic composites and devices were also prospected. Given these rich contents, it provides readers an introductory overview of multiferroic materials and devices, both beneficial for beginner and experienced researchers. I believe that such a book will invaluable reference for the multiferroic community.

Meanwhile, there are numerous reviews on single‐phase multiferroic, multiferroic composites, or multiferroic heterostructures, respectively. Theoretical modes and prototype devices were briefly mentioned in these reviews. Books introducing widespread multiferroic materials and prototype devices together with the required basics and theory are rare. With this book, we fill this gap.

Why this Book?

The book is aimed at advanced undergraduate and graduate students of the materials science, electronic devices design and physics. Since these are usually recruited from most natural sciences, i.e. physics, materials, electronic devices, we addressed the book to this readership. Readers would definitely profit from a sound knowledge of materials and physics. However, all authors are engaged in materials science, physics and electronic devices for many years and achieved outstanding achievements in these field. Hopefully, you will find that they came upon good solutions. In case you see room for improvement, please let me know.

Is this Book for You?

Students, who require an in‐depth knowledge, should begin at their level of knowledge, either in Chapters 1 (Introduction to multiferroics and its application) or 2 (Multiferroics materials). To deeply understand the physical mechanism of magnetoelectric coupling effect and simulations of multiferroic materials. Then, they should proceed through Chapters 3 (Mechanisms of multiferroic material) and 4 (Multiferroic simulations). Chapters introduce the application and prototype devices of multiferroic materials and Chapter 9 prospects the novel multiferroic composites and devices. They should be studied according to interest and requirement.

Acknowledgments

Finally, I would like to thank some people that contributed directly and indirectly to this book. First of all, I would like to name Prof. Dr. Nian X. Sun, Prof. Dr. Gopalan Srinivasan, Prof. Dr. Gail Brandon, Prof. Dr. Cewen Nan, and Prof. Dr. Shuxiang Dong. As mentioned, they encouraged me to write this book and given many valuable opinions during this project. Furthermore, I would like to thank all authors, including Dr. Bing Peng, Dr. Jing Ma, Prof. Dr. Chungang Duan, Dr. Xi Yang, Dr. Brandon Howe, Prof. Dr. Zhongqiang Hu, Prof. Dr. Zhiguang Wang, Dr. Menghui, Dr. Tianxiang Nan, Dr. Yuan Gao, and Dr. Qu Yang, who invested their expertise, time and energy in writing, correcting and finalizing their respective chapters. All are very respected colleagues, and some of them became friends during this project. Also, I acknowledge the project‐editors responsible at Wiley‐VCH for this project, Dr. Andreas Sendtko and Dr. Zai Yu, who sincerely supported this project and showed a very professional patience, when yet another delay occurred, but also pushed, when required.

This chapter gives an introduction to multiferroics including the concept, characteristics, advantages, and existing researches toward potential applications. Voltage‐controlled ferromagnetism based on multiferroic heterostructures is focused here because of the capacity for low energy dissipation, high signal‐to‐noise ratio, etc. We discuss the basic understanding and potential applications.

1.1 Concept of Multiferroics and the Existing Magnetization Manipulation Methods for Practical Applications

Of late, multiferroic materials have been very popular in spintronics [1]. They simultaneously occupy ferromagnetic (FM) and ferroelectric (FE) orders, enabling magnetism to be manipulated by an electric field (E‐field) or vice versa [2–19]. Therefore, multiferroic materials are very promising in producing multifunctional, miniature, high‐speed devices [1] . So far, several methods (e.g. electric currents, voltages, thickness, or temperature) based on multiferroic materials have been well established to manipulate magnetization to realize applications like sensors, magnetic random access memories (MRAMs), radiofrequency (RF)/microwave systems, and so on [20–22]. Methods like electric currents manage to control high‐anisotropy magnetic cells through the current‐induced spin/strain‐transfer torque (STT), thus holding out prospects for magnetic devices like information storage devices [23]. Multiferroic devices with voltage controlling techniques have low energy dissipation and high signal‐to‐noise ratio due to the absence of electromagnets [18, 22 ]. These methods can largely reduce the accumulation of heat as well as increase the integrated quality by substitutional magnetoelectric (ME) coupling [ 18 , 20 ]. Meanwhile, accompanied by increasing memory density and decreasing mass, the voltage modulation is preferred for satellite, radar, and portable electronic devices where volume, mass, and energy consumption are precious [22] .

1.2 Typical Multiferroic Heterostructures and Their Characteristics

Although extensive work has been carried out in single‐phase multiferroic compounds like BiFeO₃, they are still limited in achieving controllable modulation with ME coupling while at room temperature [24]. On the contrary, multiferroic heterostructures that integrate individual magnetic and FE materials have strong room‐temperature ME effects, and are more likely to be utilized in ME devices in the near future [24] . Besides, they are also favored for the flexibility of material choices and device designs [24] . Multiferroic heterostructures, like Fe₃O₄/PMN–PT (lead magnesium niobate–lead titanate), FeGaB/Si/PMN–PT, and YIG (yttrium iron garnet)/PMN–PT, have been explored on the basis of particular FE crystal material (PMN–PT) with a large piezoelectric coefficient [ 1 ,5]. With the external electric field (E‐field) applied along the PMN–PT substrates, these heterostructures should obtain strains and charge accumulations [ 1 , 20 ]. It provides a great opportunity for the adjacent magnetic layers to achieve magnetic anisotropy and, eventually, to obtain a large change of ferromagnetic resonance (FMR) through the inverse magnetoelastic coupling [ 1 , 20 ]. What is more, it is also demonstrated that FM/FE heterostructures are exceptionally useful in the applications of STT random access memory due to the strain‐induced magnetostatic surface spin waves as well as the strain‐controlled repeatable and nonvolatile magnetic anisotropy reorientation [20] . Here, we mainly focus on the voltage‐controlled ferromagnetism based on multiferroic heterostructures and discuss recent progress in the fundamental understanding and the potential applications.