Magnetic Nanomaterials E-Book

• Herausgeber: Wiley-VCH Verlag GmbH & Co. KGaA
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 1165

Veröffentlichungsjahr: 2017

Beschreibung

Timely and comprehensive, this book presents recent advances in magnetic nanomaterials research, covering the latest developments, including the design and preparation of magnetic nanoparticles, their physical and chemical properties as well as their applications in different fields, including biomedicine, magnetic energy storage, wave-absorbing and water remediation.
By allowing researchers to get to the forefront developments related to magnetic nanomaterials in various disciplines, this is invaluable reading for the nano, magnetic, energy, medical, and environmental communities.

Leseprobe

CONTENTS

Cover

Title Page

Copyright

List of Contributors

Preface

Part One: Fundamentals

Chapter 1: Overview of Magnetic Nanomaterials

1.1 Introduction

1.2 Typical Characterization of Magnetic Nanomaterials

1.3 Conclusions

References

Chapter 2: Magnetism of Nanomaterials

2.1 Introduction

2.2 Nanomagnetic Phenomena of Atomic Origin

2.3 Micromagnetics

2.4 Spin-Dependent Transport

Appendices

Appendix 2.A: Functional Derivatives and Materials Equations

Appendix 2.B: Relativistic Physics

Appendix 2.C: Unit Conversion in Magnetism

Acknowledgments

References

Part Two: Synthesis

Chapter 3: Overview of Synthesis of Magnetic Nanomaterials

3.1 Introduction

3.2 General Synthesis Mechanism of Magnetic Nanoparticles

3.3 Typical Methods and Equipment of Magnetic Nanomaterials Synthetic Techniques: Chemical Approaches

3.4 Typical Methods and Equipment of Magnetic Nanomaterials Synthetic Techniques: Physical Approaches

3.5 Conclusions and Perspectives

References

Chapter 4: Synthesis of Soft Magnetic Nanomaterials and Alloys

4.1 Introduction

4.2 Nanoparticles

4.3 Nanorods

4.4 Thin Films

4.5 Ribbons

4.6 Conclusions

References

Chapter 5: Synthesis of Nanostructured Rare-Earth Permanent Magnets

5.1 Introduction

5.2 RCox-Based (R = Sm, Pr, Y, La) Nanostructured Magnets

5.3 R2Fe14B-Based (R = Pr, Nd, Tb, Dy) Magnets

5.4 Conclusions and Perspectives

References

Chapter 6: Synthesis of Rare Earth Free Permanent Magnets

6.1 Introduction

6.2 Tetragonal L10 FeCo

6.3 MnBi Low-Temperature Phase

6.4 Conclusions and Perspective

Acknowledgment

References

Chapter 7: Synthesis and Properties of Magnetic Chalcogenide Nanostructures

7.1 Introduction

7.2 Synthesis Methods of Binary Magnetic Chalcogenide Nanostructures

7.3 Synthesis Methods of Ternary and Higher Order Magnetic Chalcogenides Nanostructures

7.4 Structural and Magnetic Characterizations of Magnetic Chalcogenide Nanostructures

7.5 Potential Applications of Magnetic Chalcogenide Nanostructures

7.6 Conclusions and Perspectives

Acknowledgments

References

Chapter 8: Magnetic Multicomponent Heterostructured Nanocrystals

8.1 Introduction

8.2 Synthesis of Heterostructured Nanocrystals: Basic Concepts and Guiding Criteria

8.3 Heterostructures with Core/Shell Geometries

8.4 Nanohetero-Oligomer Architectures

8.5 Conclusions

Acknowledgment

References

Chapter 9: Wet-Phase Synthesis of Typical Magnetic Nanoparticles with Controlled Morphologies

9.1 Introduction

9.2 Synthesis of Hollow/Porous Magnetic Nanoparticles

9.3 Conclusions and Perspectives

Acknowledgment

References

Chapter 10: Self-Assembly of Co Nanocrystals Self-Assembled in 2D and 3D Superlattices: Chemical and Physical Specific Properties

10.1 Introduction

10.2 Control of Crystalline Structure of Nanoparticles (Nanocrystallinity) and the Nanocrystal Size

10.3 Nano-Kirkendall Effect on Co Nanocrystals: Influence of Size and Nanocrystallinity [51–54]

10.4 3D Self-Assemblies of Magnetic Supracrystals: Various Structures and Specific Behaviors

10.5 3D Self-Assemblies of Magnetic Supracrystals: Physical Properties

10.6 Conclusions

Acknowledgment

References

Part Three: Applications

Chapter 11: Magnetic Nanoparticles for Bioseparation, Biosensing, and Regenerative Medicine

11.1 Introduction

11.2 Synthesis and Modification of High-Moment Magnetic Nanoparticles

11.3 Magnetic Nanomaterials for Bioseparation

11.4 Magnetic Nanoparticles for Magnetic Biosensing

11.5 Magnetic Nanoparticles for Regenerative Medicine

11.6 Challenges and Perspectives

References

Chapter 12: Magnetic Nanomaterials for Diagnostics

12.1 Introduction

12.2 Biocompatibility of Magnetic Nanoparticles

12.3 Surface Functionalization of Magnetic Nanomaterials

12.4 Magnetic Resonance Imaging (MRI)

12.5 Magnetoacoustic Tomography (MAT)

12.6 Magnetic Particle Imaging (MPI)

12.7 Multimodality Imaging

12.8 Conclusions and Perspectives

References

Chapter 13: Magnetic Nanomaterials for Therapy

13.1 Introduction

13.2 Imaging-Guided Therapy Using Magnetic Nanomaterials

13.3 Magnetic Hyperthermia

13.4 Targeted Drug Delivery

13.5 Targeted Gene Delivery

13.6 Manipulation of Cellular Functions

13.7 Conclusions and Perspectives

Acknowledgments

References

Chapter 14: Magnetic Nanomaterials for Data Storage

14.1 Introduction: Magnetic Data Storage and its Requirements on Magnetic Nanomaterials

14.2 Nanostructured Magnetic Thin Films for Data Storage: Overview of Perpendicular Recording (PMR) Media

14.3 Nanostructured Magnetic Thin Films for Data Storage: Overview of FePt Media for Heat-Assisted Magnetic Recording (HAMR)

14.4 Monodisperse Magnetic Nanoparticles: Synthesis, Phase Transition, Orientation Control, and Nanocomposites

14.5 Patterned Magnetic Nanostructures for Bit Patterned Media Through Bottom-Up Approach: Self-Assembly and Guided Assembly of Block Copolymer

14.6 Patterned Magnetic Nanostructures for Bit Patterned Media Through Top-Down Approach: Lithograph

14.7 Conclusions and Perspectives

References

Chapter 15: Magnetic Nanomaterials for Electromagnetic Wave Absorption

15.1 Introduction

15.2 Magnetic Nanosized Powders and Composites

15.3 Nanosized Carbon Materials with Magnetic Components

15.4 Concluding Remarks

References

Chapter 16: Magnetic Nanomaterials for Water Remediation

16.1 Introduction

16.2 Magnetic Nanomaterials for Adsorption and Removal of Pollutants in Water

16.3 Magnetic Nanomaterials for Catalytic Degradation of Wastewater

16.4 Magnetic Nanomaterials for Wastewater Resources Recovery

16.5 Magnetic Nanomaterials for Monitoring and Analysis Technologies

16.6 Conclusion and Perspectives

Acknowledgment

References

Index

End User License Agreement

List of Tables

Table 1.1

Table 1.2

Table 3.1

Table 4.1

Table 4.2

Table 13.1

Table 13.2

Table 13.3

Table 14.1

Table 16.1

Table 16.2

Table 16.3

Table 16.4

Table 16.5

Table 16.6

Table 16.7

List of Illustrations

Figure 1.1

Figure 1.2

Figure 1.3

Figure 1.4

Figure 1.5

Figure 1.6

Figure 1.7

Figure 1.8

Figure 1.9

Figure 1.10

Figure 1.11

Figure 1.12

Figure 1.13

Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 2.7

Figure 2.8

Figure 2.9

Figure 2.10

Figure 2.11

Figure 2.12

Figure 2.13

Figure 2.14

Figure 2.15

Figure 2.16

Figure 2.17

Figure 2.18

Figure 2.19

Figure 2.20

Figure 2.21

Figure 2.22

Figure 2.23

Figure 2.24

Figure 2.25

Figure 2.26

Scheme 3.1

Figure 3.1

Figure 3.2

Figure 3.3

Scheme 3.2

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Figure 3.9

Figure 3.10

Figure 3.11

Figure 3.12

Figure 3.13

Figure 3.14

Figure 3.15

Figure 3.16

Figure 3.17

Figure 3.18

Figure 4.1

Figure 4.2

Figure 4.3

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7

Figure 4.8

Figure 4.9

Figure 5.1

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

Figure 5.6

Figure 5.7

Figure 5.8

Figure 5.9

Figure 5.10

Figure 5.11

Figure 5.12

Figure 5.13

Figure 5.14

Figure 6.1

Figure 6.2

Figure 6.3

Figure 6.4

Figure 6.5

Figure 6.6

Figure 6.7

Figure 6.8

Figure 6.9

Figure 6.10

Figure 6.11

Figure 7.1

Figure 7.2

Figure 7.3

Figure 7.4

Figure 7.5

Figure 7.6

Figure 7.7

Figure 7.8

Scheme 8.1

Scheme 8.2

Figure 8.1

Figure 8.2

Figure 8.3

Scheme 8.3

Figure 8.4

Figure 8.5

Figure 8.6

Figure 8.7

Figure 8.8

Figure 8.9

Figure 8.10

Figure 8.11

Scheme 8.4

Figure 8.12

Figure 8.13

Figure 8.14

Figure 9.1

Figure 9.2

Figure 9.3

Figure 9.4

Figure 9.5

Figure 9.6

Figure 9.7

Figure 9.8

Figure 9.9

Figure 9.10

Figure 9.11

Figure 9.12

Figure 9.13

Figure 9.14

Figure 9.15

Figure 9.16

Figure 9.17

Figure 9.18

Figure 9.19

Figure 10.1

Figure 10.2

Figure 10.3

Figure 10.4

Figure 10.5

Figure 10.6

Figure 11.1

Figure 11.2

Figure 11.3

Figure 11.4

Figure 11.5

Figure 11.6

Figure 12.1

Figure 12.2

Figure 12.3

Figure 12.4

Figure 12.5

Figure 12.6

Figure 12.7

Figure 12.8

Figure 12.9

Figure 12.10

Figure 12.11

Figure 12.12

Figure 13.1

Figure 13.2

Figure 13.3

Figure 13.4

Figure 13.5

Figure 13.6

Figure 13.7

Figure 13.8

Figure 13.9

Figure 13.10

Figure 13.11

Figure 13.12

Figure 13.13

Figure 13.14

Figure 13.15

Figure 13.16

Figure 13.17

Figure 13.18

Figure 13.19

Figure 14.1

Figure 14.2

Figure 14.3

Figure 14.4

Figure 14.5

Figure 14.6

Figure 14.7

Figure 14.8

Figure 14.9

Figure 14.10

Figure 14.11

Figure 14.12

Figure 14.13

Figure 14.14

Figure 14.15

Figure 14.16

Figure 14.17

Figure 14.18

Figure 15.1

Figure 15.2

Figure 15.3

Figure 15.4

Figure 15.5

Figure 15.6

Figure 15.7

Figure 15.8

Figure 15.9

Figure 15.10

Figure 15.11

Figure 15.12

Figure 15.13

Figure 15.14

Figure 15.15

Figure 15.16

Figure 15.17

Figure 15.18

Figure 15.19

Figure 15.20

Figure 15.21

Figure 15.22

Figure 15.23

Figure 15.24

Figure 15.25

Figure 15.26

Figure 15.27

Figure 15.28

Figure 15.29

Figure 15.30

Figure 16.1

Figure 16.2

Figure 16.3

Figure 16.4

Figure 16.5

Figure 16.6

Figure 16.7

Figure 16.8

Figure 16.9

Figure 16.10

Figure 16.11

Figure 16.12

Guide

Cover

Table of Contents

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Part 1

Chapter 1

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Magnetic Nanomaterials

Fundamentals, Synthesis and Applications

Edited by Yanglong Hou and David J. Sellmyer

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

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© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

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Print ISBN: 978-3-527-34134-4

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List of Contributors

Balamurugan Balasubramanian

University of Nebraska

Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience

855 N 16th Street

Lincoln, NE 68588

USA

Luigi Carbone

CNR NANOTEC

Institute of Nanotechnology

sede di Lecce, c/o Campus Ecotekne, via Monteroni

73100 Lecce

Italy

Xiaoyuan Chen

National Institute of Health

National Institute of Biomedical Imaging and Bioengineering

Laboratory of Molecular Imaging and Nanomedicine

35A Convent Dr

Bethesda, MD 20892

USA

Xin Chu

Peking University

College of Engineering

Department of Materials Science and Engineering

5 Yi He Yuan Road

Beijing 100871

China

Pantaleo Davide Cozzoli

CNR NANOTEC

Institute of Nanotechnology

sede di Lecce, c/o Campus Ecotekne, via Monteroni

73100 Lecce

Italy

and

University of Salento

Department of Mathematics and Physics “E. De Giorgi”

c/o Campus Ecotekne, via Monteroni

73100 Lecce

Italy

Angela Fiore

CNR NANOTEC

Institute of Nanotechnology

sede di Lecce, c/o Campus Ecotekne, via Monteroni

73100 Lecce

Italy

and

University of Salento

Department of Mathematics and Physics “E. De Giorgi”

c/o Campus Ecotekne, via Monteroni

73100 Lecce

Italy

Arunava Gupta

University of Alabama

Center for Materials for Information Technology

Tom Bevill Building, 201 7th Avenue

Tuscaloosa, AL 35487

USA

George C. Hadjipanayis

University of Delaware

Department of Physics and Astronomy

104 The Green

Newark, DE 19716

USA

Yiyuan Han

Nanyang Technological University

School of Chemical and Biomedical Engineering

70 Nanyang Drive

637457 Singapore

Singapore

Yu Hong

Beijing Forestry University

Beijing Key Lab for Source Control Technology of Water Pollution

No. 35 East Qinghua Road, Haidian District

100083 Beijing

China

Yanglong Hou

Peking University

College of Engineering

Department of Materials Science and Engineering

5 Yi He Yuan Road

Beijing 100871

China

Taeghwan Hyeon

Institute for Basic Science (IBS)

Center for Nanoparticle Research

1 Gwanak-ro, Gwanak-gu

151-742 Seoul

Korea

and

Seoul National University

School of Chemical and Biological Engineering

1 Gwanak-ro, Gwanak-gu

151-742 Seoul

Korea

Ling Bing Kong

Nanyang Technological University

School of Materials Science and Engineering

50 Nanyang Avenue

639798 Singapore

Singapore

Chih-Huang Lai

National Tsing Hua University

College of Engineering

Department of Materials Science and Engineering

101, Sec. 2, KungFu Rd.

300 Hsinchu

Taiwan

Song Lan

Case Western Reserve University

Department of Materials Science and Engineering

10900 Euclid Avenue

Cleveland, OH44106

USA

Sean Li

The University of New South Wales

School of Materials Science and Engineering

Sydney

NSW 2502

Australia

Jung-Wei Liao

University of Cambridge

Cavendish Laboratory

J. J. Thomson Avenue

CB3 0HE Cambridge

United Kingdom

and

National Tsing Hua University

College of Engineering

Department of Materials Science and Engineering

101, Sec. 2, KungFu Rd.

300 Hsinchu

Taiwan

Daishun Ling

Zhejiang University

College of Pharmaceutical Sciences

866 Yuhangtang Road

310058 Hangzhou

China

and

Zhejiang University

College of Biomedical Engineering & Instrument Science

38 Zhejiang University Road

310058 Hangzhou

China

Jia Liu

Beijing Institute of Technology

School of Materials Science & Engineering

Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications

5 South Zhongguancun Street, Haidian District

Beijing 100081

China

Jiajia Liu

Beijing Institute of Technology

School of Materials Science & Engineering

Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications

5 South Zhongguancun Street, Haidian District

Beijing 100081

China

Lie Liu

General Test Systems Inc.

Taohuayuan Science and Technology Innovation Park

Shenzhen

518102 Guangdong

China

Peirui Liu

Beijing Forestry University

Beijing Key Lab for Source Control Technology of Water Pollution

No. 35 East Qinghua Road, Haidian District

100083 Beijing

China

Concetta Nobile

CNR NANOTEC

Institute of Nanotechnology

sede di Lecce, c/o Campus Ecotekne, via Monteroni

73100 Lecce

Italy

Soubantika Palchoudhury

University of Tennessee at Chattanooga

Civil and Chemical Engineering

615 McCallie Ave

Chattanooga, TN 37403

USA

Marie-Paule Pileni

CEA Saclay

IRAMIS

Gif-sur-Yvette

91191 Paris

France

Shuang Qiao

Peking University

College of Engineering

Department of Materials Science and Engineering

5 Yi He Yuan Road

Beijing 100871

China

Karthik Ramasamy

Los Alamos National Laboratory

Center for Integrated Nanotechnologies

PO Box 5800, MS 1315

Albuquerque, NM 87185

USA

Shenqiang Ren

Temple University

Temple Material Institute and Mechanical Engineering

1947 North 12th Street

Philadelphia, PA 19122

USA

Riccardo Scarfiello

CNR NANOTEC

Institute of Nanotechnology

sede di Lecce, c/o Campus Ecotekne, via Monteroni

73100 Lecce

Italy

and

University of Salento

Department of Mathematics and Physics “E. De Giorgi”

c/o Campus Ecotekne, via Monteroni

73100 Lecce

Italy

David J. Sellmyer

University of Nebraska

Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience

855 N 16th Street

Lincoln, NE 68588

USA

Ralph Skomski

University of Nebraska

Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience

855 N 16th Street

Lincoln, NE 68588

USA

Shouheng Sun

Brown University

Department of Chemistry

324 Brook Street

Providence, RI 02912

USA

Min Wang

Nanyang Technological University

School of Chemical and Biomedical Engineering

70 Nanyang Drive

637457 Singapore

Singapore

Matthew A. Willard

Case Western Reserve University

Department of Materials Science and Engineering

10900 Euclid Avenue

Cleveland, OH44106

USA

Chenjie Xu

Nanyang Technological University

School of Chemical and Biomedical Engineering

70 Nanyang Drive

637457 Singapore

Singapore

and

Nanyang Technological University

NTU-Northwestern Institute of Nanomedicine

50 Nanyang Avenue

639798 Singapore

Singapore

Meng Xu

Beijing Institute of Technology

School of Materials Science & Engineering

Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications

5 South Zhongguancun Street, Haidian District

Beijing 100081

China

Jinbo Yang

Peking University

School of Physics

Collaborative Innovation Center of Quantum Matter

5 Yi He Yuan Road

Haidian District

Beijing 100871

China

Zhihong Yang

Nanjing University of Aeronautics and Astronautics

College of Materials Science and Technology

Changkong Road, Baixia District

211100 Nanjing

China

Ziyu Yang

Peking University

College of Engineering

Department of Materials Science and Engineering

5 Yi He Yuan Road

Beijing 100871

China

Ming Yue

Beijing University of Technology

College of Materials Science and Engineering

100 Pingleyuan

100124 Beijing

China

Hong-Wei Zhang

Nanyang Institute of Technology

School of Electronic and Electrical Engineering

Nanyang

473004 Henan

China

Jiatao Zhang

Beijing Institute of Technology

School of Materials Science & Engineering

Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications

5 South Zhongguancun Street, Haidian District

Beijing 100081

China

Tianshu Zhang

Anhui Target Advanced Ceramics Technology Co. Ltd.

Hightech Zone, Yulan Road No. 767

Hefei

Anhui

China

Zijian Zhou

National Institute of Health

National Institute of Biomedical Imaging and Bioengineering

Laboratory of Molecular Imaging and Nanomedicine

35A Convent Dr

Bethesda, MD 20892

USA

Preface

The basic study and applications of magnetic materials have a long history. About a thousand years ago, the “South pointer,” a Chinese spoon made from a lodestone on a smooth plate, was an early magnetic device used for geolocation. However, there was little development of magnetic materials until the discovery of Faraday's law of electromagnetic induction. New hard and soft magnetic are emerging for electronic devices, communications, and information storage, and these are influencing our daily lives and modern industries. Especially with the development of quantum mechanics and electronics, theoretical study of magnetism has expanded in recent years and the research emphasis gradually has shifted from macroscale to nanoscale.

Nanotechnology is one of the most important developments in science and technology in recent years. Because matter behaves differently in the nanoworld, investigations of magnetic nanomaterials have brought revolutionary progress in materials processing and characterization. In contrast to bulk magnetic materials, magnetic nanomaterials have stimulated a series of novel theories, synthesis methods, and characterization techniques. In fact, although early researchers did not focus on the nanoscale, theoretical work revealed that nanoscale correlations exist in magnetic materials and control their properties. By virtue of their very nature, magnetic materials are a class of nanoscale materials. Some important concepts in magnetism, such as exchange coupling that controls magnetic domain thickness and the exchange length, have great influence on magnetic properties of nanomaterials. We have organized this book to systematically summarize our present knowledge of magnetic nanomaterials.

The book is structured into three parts – Fundamentals, Synthesis, and Applications – offering a broad introduction to magnetic nanomaterials. These three parts are divided into 16 chapters. Part One includes two chapters: Chapter 1 is an overview of nanomaterials, giving basic concepts and the time development of magnetic nanomaterials. Chapter 2 focuses on the magnetism of nanomaterials and demonstrates the differences between theories for bulk magnets and nanomagnets. Part Two includes eight chapters. Chapter 3 provides an overview of the synthesis of magnetic nanomaterials, including chemical and physical methods. Following this, Chapters 4–6 introduce the synthesis of soft magnetic nanomaterials, hard magnetic nanomaterials that consist of rare earth permanent nanocomposite magnets, and rare earth-free permanent nanocomposite magnets, respectively. The synthesis of magnetic chalcogenides is separately dealt with in Chapter 7. Because of their importance, Chapters 8–10 are devoted to wet-phase synthesis of typical magnetic nanoparticles with controlled morphologies and organized ensembles of magnetic. Part Three includes six chapters. At present, magnetic nanomaterials have notable advantages and potential in applications due to their increasing popularity and relevance for current applications and future prospects. Especially in biotechnology and biomedicine, magnetic nanomaterials can target a given cell or virus, provide accurate information on biological processes, and act as a multifunctional tool in cancer therapy. Chapters 11–13 introduce applications in biotechnology, diagnostics, and therapy mainly to present their superiority and distinguishing features. Applications of magnetic nanomaterials in data storage and electromagnetic wave absorption are covered in Chapters 14 and 15. Finally, Chapter 16 is about water remediation, which is a novel application. Magnetic nanomaterials show excellent performance in this field based on their magnetic recycling property.

In summary, this book attempts to exemplify emerging new materials, novel phenomena, and the most exciting developments in materials research and device applications. Discussions about reaction mechanisms and future prospects are also presented. We hope that this book will be a resource for future research in the related fields of magnetism and nanoscience.

We thank Dr. Peter Gregory, Dr. Gudrun Walter, and Dr. Lifen Yang from Wiley who initiated this book and did a great deal of work to bring it to completion. Ms. Nina Stadthaus worked diligently in collecting all the manuscripts, figures, and related paperwork. One of us (DJS) is grateful to the US Department of Energy, Office of Basic Energy Sciences, and the National Science Foundation for sustained research support. YH also thanks for the financial support of both National Natural Science Foundation of China and National Basic Research Program of China. Finally, we thank all of our authors who contributed their very informative and in-depth chapters that made this book a reality.

Peking University, Beijing, China Yanglong Hou

University of Nebraska, Lincoln, NE, USA David J. Sellmyer

Part OneFundamentals

1Overview of Magnetic Nanomaterials

Ziyu Yang,1 Shuang Qiao,1 Shouheng Sun,2 and Yanglong Hou1

1Peking University, College of Engineering, Department of Materials Science and Engineering, 5 Yi He Yuan Road, Beijing 100871, China

2Brown University, Department of Chemistry, 324 Brook Street, Providence, RI 02912, USA

1.1 Introduction

Magnetic nanomaterials have long been investigated due to their scientific and technological importance in many areas, such as magnetic data storage, magnetic fluids, catalysis, biomedicine, magnetic resonance imaging, hyperthermia, magnetic refrigeration, and environmental remediation [1–11]. The unique effects induced by the nanoscale distinguish the magnetism of the nanomaterials from their bulk counterparts. When a material is cut into smaller dimensions, the number of magnetic domains included in the material is decreased and magnetic coercivity is increased. When the size reaches a critical value, the material can only support a single magnetic domain and magnetic behavior of this single-domain material depends mostly on magnetization rotation. Depending on the magnetic characteristics, the single domain size of a material can be in tens, hundreds, or even thousands of nanometers. When the material dimension continues to shrink below the single domain size, surface atoms and temperature start to affect magnetic behaviors drastically and the material can become superparamagnetic at room temperature in which state it can be magnetized as a ferromagnetic material, but its magnetization direction is randomized by thermal agitation, showing zero remanent magnetization and no coercivity.

The key issues related to magnetic nanomaterials are in the synthesis with the desired size, shape, and structure controls. Due to the large surface area, surface energy, and magnetic dipolar interactions, magnetic nanomaterials should also be stabilized by a layer of organic or inorganic matrix. The coating chemistry developed further allows proper functionalization of these nanomaterials for varied applications. The controlled synthesis and coating for stabilization can now be realized from organic-phase synthesis protocols [12,13].

In this chapter, we summarize some widely explored magnetic nanomaterials of (i) metals and alloys, especially single metal M (M = Fe/Co/Ni) and their related alloys, such as MN (N = noble metal) and alloys of M1M2 (M1, M2 = Fe, Co, Ni); (ii) Fe/Co/Ni/Mn oxides; (iii) metal carbides and nitrides; (iv) rare earth (RE)-based permanent magnets, specially RE-Fe (Co), RE-Fe (Co)-B, and RE-Fe (Co)-C/N magnets. In addition to the synthesis, we also review some of the common methods used to characterize these magnetic nanomaterials to better understand their phases, morphologies, micromagnetic structures, and bonding structures. We focus on the tools of X-ray magnetic circular dichroism (XMCD) spectroscopy, Lorentz transmission electron microscope and Mössbauer spectroscopy, magnetic extended X-ray absorption fine structure (MEXAFS), magnetic force microscopy (MFM), and superconducting quantum interference device (SQUID).

1.1.1 Typical Magnetic Nanomaterials

1.1.1.1 Magnetic Nanomaterials of Metal (Fe, Co, and Ni)

Fe, Co, and Ni are three common ferromagnetic elements that serve as structural backbones for tons of magnetic materials in our modern life. The common magnetic properties of Fe, Co, and Ni are given in Table 1.1. Study on the nanoscale Fe, Co, and Ni is both fundamentally and technologically important. With controlled stabilization and surface treatment techniques, these nanoscale materials can find many important applications, ranging from ferrofluids, biomedicine, to catalysis [14–16].

Table 1.1 Properties of ferromagnetic Fe/Co/Ni elements with SI units [17].

Elements

σ

s

/0 K, A·m

2

·kg

−1

σ

s

/293 K, A·m

2

·kg

−1

T

c

/K

Fe

222

218

1043

Co

162

161

1388

Ni

57

54

627

σ

s

: saturation magnetization;

T

c

Curie temperature.

Synthesizing these nanomaterials with the desired dimension and property controls is critical for their practical applications. In the early stage (1950s), Fe was prepared in the nanoparticle (NP) form in mercury [18]. The improved methods involve the use of organic solvents for better Fe(0) protection and stabilization. The common precursor used to obtain metallic Fe is Fe(CO)5. This is a metastable organometallic compound with the standard enthalpy of formation – 185 kcal mol−1 [19]. Despite its easy decomposition potential, it does have a complicated decomposition kinetics due to the formation of various clusters [20,21].

In general, utilizing the decomposition of Fe(CO)5 to generate Fe nanomaterials can be accomplished by heat or sonication. Once produced, these nanomaterials need some proper protections to stay both chemically and colloidally stable for further applications, especially for biomedical applications [22]. Recent synthetic advances have allowed the preparation of monodisperse Fe NPs from the solution-phase reaction. For example, Sun and coworkers developed a simple one-pot reaction to produce monodispersed Fe NPs in the presence of oleylamine, as shown in Figure 1.1a and b [23]. Further studies indicate that the Fe growth process can be controlled by adding Cl ions, as these ions have a strong binding to the present NP surface and slow the Fe growth kinetics, and facilitate the addition of Fe atoms in a thermodynamically more stable manner into better crystalline body-centered cubic (bcc) Fe, as shown in Figure 1.1c–e [24,25].

Figure 1.1 (a) TEM images of 4 nm/2.5 nm Fe@Fe3O4 NPs. Inset is HRTEM image of the Fe@Fe3O4 NPs. (b) Self-assembled Fe@Fe3O4 NPs superlattice. (c) SEM image of the platelike Fe NP assembly obtained directly from the synthesis solution. (d) TEM image of the 15 nm NPs obtained from the redispersion of the plate assembly in hexanes. (e) HRTEM image of a single NP revealing the metallic bcc-Fe core and Fe3O4 shell with Fe (110) and Fe3O4 (222) planes indicated [23,24]. Reproduced with permission of American Chemical Society.

In addition to the decomposition of Fe(CO)5, the metal–organic precursor Fe[N(SiMe3)2]2 (Me = —CH3) is also used for the generation of Fe NPs [26,27]. As reported by Chaudret et al., the metal–organic precursor is decomposed in the presence of hexadecylamine (HDA) and a long-chain acid (oleic acid (OA) or hexadecylammonium chloride (HDAC)) under a H2 atmosphere. The reaction gives bcc-Fe cubes with their Ms at 223 A·m2 kg−1, which is close to the bulk value [26]. Fe[N(SiMe3)2]2 can also be replaced by its dimers {Fe[N(SiMe3)2]22 and the decomposition in the presence of palmitic acid/HDA mixture produces ultrasmall (∼1.5 nm) Fe(0) clusters, or polycrystalline spherical NPs with a mean diameter between 5 and 10 nm, or single nanocubes over 13 nm [28].

The reduction of iron salts can also generate Fe NPs. The reverse micelle method is commonly used for this reduction approach as the particle size can be better controlled by the micelle dimension [29,30]. For example, 3 nm Fe NPs were synthesized by using trioctylphosphine oxide (TOPO) as stabilizing agent, Fe(AOT)2 (AOT = bis(2-ethylhexyl) sulfosuccinate) as iron precursor, and NaBH4 as reducing media in an aqueous solution [31].

The methods developed for the synthesis of Fe NPs can be extended to the preparation of Co NPs as well. The metallic Co NPs in the size range of 1.8–4.4 nm show size-dependent blocking temperature, magnetic moments, and hysteresis behaviors [32]. In addition, the morphology of the final products could also be tuned by varying the reaction parameters such as the reaction temperature [33]. Here, the Co particles nucleate and grow in the nanosized cages of inverse micelles formed by dissolving the surfactant molecules in an apolar solvent. Unfortunately, the size of the final products could not be precisely tuned by changing the water/surfactant ratio. To solve this problem, a germ-growth method is developed to verify that Co particles could be germinated in the first stage of the reaction as seeds, and then as nucleation sites to grow into larger particles [34]. The method leads to the formation of face center cubic (fcc)-Co NPs and their spontaneous 2D monolayer assemblies [35].

Benefiting from the nonthermodynamically controlled property of solution synthesis, monodisperse 9 nm Co NPs with the cubic symmetry of the β-phase of manganese (denoted as ϵ-Co) are synthesized by the reduction of CoCl2 with lithium triethyl borohydride, as shown in Figure 1.2a and b. The as-synthesized ϵ-Co NPs are magnetically soft with reduced dipole interaction between the NPs, which is beneficial to stabilize the individual NPs during the size-selective process and facilitate the formation of ordered NP arrays [36]. In addition, the shapes of the Co NPs can be readily controlled by the NP structure. For example, hcp-Co nanodisks were obtained by the thermal decomposition of di-cobalt-carbonyl Co2(CO)8 in anhydrous o-dichlorobenzene (DCB) assisted with oleic acid [37]. A versatile pathway was proposed to tune the shape and the aspect ratio of Co nanocrystals to get Co nanorods through the decomposition of organometallic precursors [Co(η3-C8H13)(η4-C8H12)] in anisole and via a self-organization process, as shown in Figure 1.2c–e [38,39].

Figure 1.2 (a) TEM image of a 2D assembly of 9 nm cobalt NPs. Inset: High-resolution TEM image of a single particle. The scale bar is 48 nm. (b) TEM image of a 3D superlattice of 9 nm cobalt NPs assembled on amorphous carbon film at 70 °C. TEM micrographs of nanorods synthesized using hexadecylamine and (c) stearic acid, sample was prepared by ultramicrotomy, (d) lauric acid, and (e) stearic acid. The scale bar is 30 nm [36,38]. Reproduced with permission of AIP Publishing LLC.

Recent focus on Ni NPs is on their shape controls. It is widely accepted that the selective coordination of a special ligand on the initial nuclei is essential for the anisotropic growth [40–44]. However, the common ligands used for NP stabilization contain strong π-accepting characters, which degrades the magnetic performance of the NPs [45]. Recently, Pick and Dreysse suggested that the amine ligands might be a favorable alternative [46]. They studied the effects of hexadecylamine on the shape and magnetic properties of Ni NPs prepared by the reduction of Ni(COD)2 (COD, cycloocta-1,5-diene). Results show that increasing the concentration of amines favors the growth of Ni nanorods and stronger ferromagnetism [47].

1.1.1.2 Magnetic Nanomaterials of MN Alloys (M = Fe, Co, and Ni, N = Noble Metal)

Nanomaterials of FePt alloys have long been a hot topic [48]. These FePt alloys can have a chemically disordered face-centered cubic (fcc) structure (commonly referred to as A1 structure) and face-centered tetragonal (fct) structure (commonly referred to as L10 structure) [49]. The fct-FePt has a magnetocrystalline anisotropy constant K (measuring the ease of magnetization reversal) of 107 J m−3, one of the largest among all known hard magnetic materials. This high K value arises from the unique layered Fe–Pt arrangement and strong spin–orbit coupling between Fe 3d and Pt 5d states along the Fe–Pt layer direction [50–52]. Also, the high K value endows Fe–Pt the thermal stable size down to around 2 nm, making them a promising candidate as high-density magnetic recording media [53]. FePt NPs are commonly prepared by thermal decomposition of Fe(CO)5 and reduction of Pt(acac)2, or by the coreduction of metal salts. Reduction of Pt(acac)2 and decomposition of Fe(CO)5 in the presence of OA and OAm around 300 °C can lead to the formation of monodisperse NPs with their size controlled from 3 to 10 nm and compositions controlled by the metal precursor ratios. As prepared, the FePt NPs often adopt the fcc structure. However, via thermal annealing at temperature higher than 500 °C, the fcc-FePt NPs with near 1/1 Fe/Pt atomic ratios can be converted to fct-FePt, as shown in Figure 1.3. The annealed FePt assemblies are ferromagnetic, supporting high-density magnetization reversal transitions [7]. To obtain the fct-structure at lower temperatures, the third element such as Ag, Cu, Sn, Pb, Sb, and Bi can be introduced [54–57].

Figure 1.3 (a) TEM micrograph of a 3D assembly of 6 nm as-synthesized Fe50Pt50 particles deposited from a hexane/octane (v/v 1/1) dispersion onto a SiO-coated copper grid. (b) TEM micrograph of a 3D assembly of 6 nm Fe50Pt50 sample after replacing oleic acid/oleyl amine with hexanoic acid/hexylamine. (c) HRSEM image of a ∼180 nm thick, 4 nm Fe52Pt48 NP assembly annealed at 560 °C for 30 min under 1 atm of N2 gas. (d) High-resolution TEM image of 4 nm Fe52Pt48 NPs annealed at 560 °C for 30 min on a SiO-coated copper grid [7]. Reproduced with permission of the American Association for the Advancement of Science.

FeAu alloys are also widely studied. Due to their limited solubility, the chemically ordered compounds in the FeAu bulk is not permitted when in a small size. However, the chemically ordered L12-type Fe3Au and FeAu3 compounds can be made to sub-10 nm NPs [58]. The ab initio first principle calculations suggest that Fe3Au has a high saturation magnetization of about 143.6 emu g−1, while FeAu3 is antiferromagnetic with the net magnetization arising only from uncompensated surface spins [58]. In addition to FeAu alloys, the Au pyramids can also grow on highly faceted Fe NPs in an epitaxial growth manner [59].

In the case of CoPt, which is another magnetic system that is similar to FePt, the CoPt alloys have a high magnetocrystalline anisotropy of 4 × 107 ergs cm−3, and a saturation magnetization of 800 emu cm−3 [60,61]. Modifying the syntheses of FePt NPs, various kinds of CoPt alloy nanomaterials can be obtained [62–64]. Understanding size, shape, and composition effects on the formation of the fct phase is necessary to develop hard magnetic FePt and CoPt. Takahashi et al. reported the size effect of fcc–fct ordering of FePt NPs by transmission electron microscopy (TEM). The study show that the ordering does not progress when the particle size has a diameter of less than 4 nm [65]. Concerning the morphologies of CoPt and FePt below 3 nm, ab initio calculations show that the ordered multiply twinned morphologies will not show hard magnetic behavior, which is in consistent with the experimental difficulties to stabilize the L10 phase in uncoated FePt NPs by thermal treatment [66–69].

When the alloys of CoPt, FePt, FePd, CoRn, and so on are in their state of chemically ordered structure, the highest value of Ku and the largest coercivity from each structure can be obtained [70–72]. Factors that influence the transition temperature TC (order–disorder transition) have been further studied and size effects are found to dominate the structure ordering in FePt, FePd, and CoPt [73–81]. For example, 2.4∼3 nm CoPt NPs can have a TC in the range of 500–650 °C [82].

1.1.1.3 Magnetic Nanomaterials of M1M2 Alloys (M1, M2 = Fe, Co, and Ni)

When in an alloy state, the magnetic and mechanic properties are affected by the varied heteroelements, thus influencing their thermodynamic and elastic properties and phase stabilities. For example, when the Ni concentration is at about 35%, the thermal expansion coefficient of the fcc-FeNi steel vanishes, which is known as the Invar effect [83,84]. Exploring the magnetic and chemical order–disorder phenomena is necessary for the study of their intrinsic magnetism and certain important realistic effects such as chemical disorder-induced local lattice deformation [85–87]. Table 1.2 illustrates the magnetic ordering temperatures of various FeNi geometries.

Table 1.2 Monte Carlo and measured magnetic ordering temperatures in Kelvin [85].

Alloy

Monte Carlo simulation

Measured

T

C

random

T

C

ordered

T

C

equil

T

C

random

T

C

ordered

FeNi

3

870

1180

970

850

940

FeNi

820

1020

910

785

840

Fe

3

Ni

520

580

580

\

\

The clusters are of special interests due to their unique role in the understanding of both magnetism and catalysis. For the FeNi clusters containing up to four Fe and four Ni atoms, magnetic moments per atom of these clusters are almost insensitive to the specific geometry, but in Fen−1Ni (n = 9, 15, and 27) geometry similar to the bulk bcc structure, the clusters are ferromagnetic with magnetic moments higher than the bulk value [88]. Regarding the synthesis of FeNi alloy NPs, various methods have been introduced such as coprecipitation methods, reduction methods, direct current electrodecomposition, and sol–gel fabrication methods [89–93].

FeCo alloys, with high Curie temperature and high magnetization, are excellent soft magnetic materials for applications in magnetic sensors, magnetic recording head, motors, and generators in electric vehicles [94,95]. With the development of new synthetic techniques, the FeCo NP sizes, shapes, and compositions can now be well controlled, and shape anisotropy can be used to tune the FeCo into “hard” magnets (shape anisotropy can be estimated by coherent rotation mechanism Ks = (µ0/4)·Ms2, where Ks is the calculated room temperature anisotropy constant) [96]. As a result, the oriented FeCo wire arrays show a large coercivity (approximately 2800 Oe) and high squareness (approximately 0.9) [97]. Based on first-principles theory, it is found that the structurally distorted FeCo shows a uniaxial magnetic anisotropy energy (MAE) 50% larger than that of FePt [98]. This MAE increases enormously and reaches to a value on the order of 700–800 µeV/atom while in the state of c/a ratio of 1.20–1.25 and Co concentration of 60%. In addition, in the concentrations where the uniaxial MAE is very large, the magnetic moments is on the order of 2.1 µB, still a high value, as shown in Figure 1.4a. FeCo is also an efficient probe for magnetic heating, which can be used to improve the efficiency of chemotherapy and radiotherapy [99–102]. The specific absorption rate (SAR) of magnetic materials in hyperthermia is given by SAR = A·f, where A is the specific area of the hysteresis loop (specific losses) at the frequency f and magnetic field H at which the experiment is conducted. Under H = 300 Oe and f = 310 kHz, the FeCo nanowires have a SAR value of around 1500 W g−1 [103].

Figure 1.4 (a) Calculated uniaxial Ku (upper panel) and saturation magnetic moments (lower panel) of tetragonal Fe1−xCox as a function of the c/a ratio and the Co concentration x. (b and c) T1-weighted MR images of a rabbit before (b) and 30 min after (c) initial injection of a solution of ∼4 nm FeCo/single-graphitic-shell NPs (metal dose of about 9.6 µmol kg−1 for about 5 kg rabbit) [98,104]. Reproduced with permission of American Physical Society.

FeCo is also a sensitive probe for magnetic resonance imaging agent. Dai et al. prepared high-moment FeCo/single-graphitic-shell NPs that have r1 and r2 relativities of 70 and 644 mM−1 s−1, respectively. The FeCo/single-graphitic-shell NPs also possess high optical absorbance in the near-infrared region, and show long-circulating positive-contrast enhancement at low metal dosages, as shown in Figure 1.4b and c [104].

Nanowires of CoNi alloys can also be prepared by separating nucleation and growth steps in the synthesis to show large coercivity [105]. Tuning the Co composition in CoNi nanowires enables the control of the effective anisotropy determined by the balance between the hexagonal close-packed (hcp) and fcc magnetocrystalline and shape anisotropy [106,107].

1.1.1.4 Magnetic Nanomaterials of Carbides and Nitrides M–C/N (M = Fe, Co, and Ni)

The electron-rich M–C/N (M = Fe, Co, and Ni) bonding endows carbides and nitrides with peculiar magnetic, catalytic, and electronic properties [108–113]. The origin use of carbide nanostructures initiate from the usage of Damascus steel [114]. Regarding the iron carbides compounds, the complex phase diagrams attract wide theoretical and experimental interests. From the perspective of Fe-C based molecules, the geometry and electronic structures of various neutral and ionic Fe-C clusters were reported, including Fe2C [115,116], FeCn (n = 2–5) [117], FenC/FenC+/FenC− (n = 1–6) [118,119], and clusters with greater carbon entities content FeCn (n = 1–4), Fe2Cn (n = 1–3) [120,121]. From the perspective of theoretical simulation results, the strong Fe—C bonding reduces the local magnetic moment of Fe atoms [122]. In a recent study by Hou et al., varied phases of iron carbide NPs of hexagonal (hexa) and monoclinic (mono) Fe2C, monoclinic Fe5C2, and orthorhombic (ortho) Fe3C NPs were presented utilizing a “seed conversion” manner. All the NPs are in a near monodisperse state, with the highest saturation magnetization Ms value of 101.2 emu g−1 and the highest Curie temperature TC of 497.8 K. All the obtained NPs exhibit lower Ms than that of the bcc-Fe NPs, which is in accordance with the theoretical simulations, as shown in Figure 1.5 [123].

Figure 1.5 TEM images of hexa-Fe2C (a), mono-Fe2C (b), mono-Fe5C2 (c), and ortho-Fe3C (d). M versus H curves at 300 K (e) and 2 K (f) of the initial bcc-Fe and the preformed iron carbide NPs [123].

Recently, the Co2C and Co3C nanomaterials are synthesized possessing unusually large coercivities and energy products. Through first-principles electronic structures study using GGA + U functional, it is suggested that the C atom intercalation is responsible for the magnetic anisotropy enhancement [124,125].

The nitrides magnetic nanomaterials are also fascinating areas with their unique catalytic and magnetic properties. Regarding the magnetic properties of iron nitrides, various phases such as Fe16N2, Fe4N, Fe3N, Fe2N, and mononitride FeN possess complicated magnetic properties from ferromagnetic to antiferromagnetic and nonmagnetic, and are considered to be potential in nanomagnetism and spintronic areas [126]. Perhaps the most amazing magnetism is in Fe16N2, in which the most distant Fe atoms from the N atoms possess the large 3d hole in both the down-spin and up-spin states [127]. It is suggested that the penetration of N atoms expanded the Fe lattice with enhanced magnetic moments in terms of hopping electrons and preventing the exchanged splitting. Regarding nickel carbides and nitrides, the distinctions between Ni3C and hcp-Ni are the most debating topic. Taking into consideration the early reported hexagonal lattice (a = 2.628 Å, c = 4.308 Å), the results that Ni3C is of rhombohedral lattice (3R−, a = 4.553 Å, c = 12.92 Å) are very different, while other reports show that the Ni3C possesses the orthorhombic cementite-type structure [128–130]. The intrinsic magnetism are also debated with reported results from ferromagnetic to nonmagnetic [131–134]. It has been reported that the electron transformations from the Ni 3d states to the C 2p band promoted the forced exchange splitting in the case of ferromagnetic solution with the Fermi level at the upper part of the Ni 3d states [135]. With the increased C concentrations in Ni—C bonding, the calculated magnetic moment decreases and quenches at a critical value. In the case of Ni—N bonding, the fcc-NiN0.125 and fcc-NiN0.25 with the Ni sublattice exhibiting little distortion from the fcc lattice possess significant local magnetic moments [135].

1.1.1.5 Magnetic Nanomaterials of Metal Oxides

Iron oxides are widespread in nature, due to their special properties such as low toxicity, robust chemical stability, high corrosion resistance and excellent magnetic properties [12]. Fe3O4 has a cubic inverse spinel structure. The lattice constant of Fe3O4 crystal is a = 0.839. The unit cell of Fe3O4 has 32 O2− ions that are regular cubic close-packed along the [111] direction. Also, the Fe3O4 crystal structure includes two different iron sites: tetrahedral sites made up of Fe3+ and octahedral sites made up of both Fe2+ and Fe3+ [136]. Based on its unique structure, recent research on Fe3O4 has attracted much attention for its fundamental nanomagnetism and its potential applications [137,138]. Researchers have developed various methods to synthesize iron oxides. The most widely used methods are based on organic-phase synthesis [13,138]. Park et al. have synthesized monodisperse Fe3O4 NPs by a thermal decomposed method [139]. In their synthesis, metal chloride first reacts with Na–oleate and metal–oleate complex with NaCl is obtained. Thereafter, metal–oleate complex is mixed with octadecene and heated to more than 300 °C and the metal–oleate decomposes into metal oxide with monodisperse morphology. Sun et al. have reported a simple process for preparing monodisperse FeO NPs through a high-temperature reaction of Fe(acac)3 with oleic acid and oleylamine. The sizes of the NPs are tunable from 10 to 100 nm and the shapes can be controlled to be either spherical or truncated octahedral [140]. Under controlled annealing conditions, the as-synthesized FeO NPs are converted into Fe3O4, γ-Fe2O3, α-Fe2O3, or they undergo disproportionation to form Fe–Fe3O4 composite NPs. These chemical conversions of the paramagnetic FeO NPs facilitate the one-step production of various iron-based NPs with controlled sizes and tunable magnetic properties for various nanoscale magnetic and catalytic applications.

Cobalt oxide has three types of common polymorphs: monoxide or cobaltous oxide (CoO), cobaltic oxide (Co2O3), and cobaltosic oxide or cobalt cobaltite (Co3O4). Co3O4 has a spinel-type structure consisting of both Co2+ and Co3+, which is the most stable cobalt oxide. It is a simple as well as an efficient way of preparing cobalt oxide (Co3O4) NPs by precipitation. In a report by Agilandeswari and Rubankumar, Co3O4 NPs with agglomerated assembled spheres are synthesized, the magnetic characterizations exhibit weak ferromagnetic behavior [141].

Commonly, nickel oxides have three types: NiO, NiO2, and Ni2O3. By changing the synthesis condition and methods, one can obtain nickel oxides in different crystalline phases, such as monoclinic, cubic, hexagonal, and rhombohedral crystals [142]. Song et al. used nickel ethylene glycol as the precursor, successfully synthesized flower-like NiO. The NiO nanoflowers show a very small hysteresis loop with a Hc of 100 Oe and a Mr of 0.015 emu·g−1, indicating a typical ferromagnetic behavior [143].

Manganese oxides have several different forms of MnO, MnO2, Mn2O3, and Mn3O4. MnO2 has several crystalline structures, including α-MnO2, β-MnO2, γ-MnO2, and δ-MnO2. Previous methods to prepare MnO2 include thermal decomposition, electrodeposition, physical mixing, sol–gel, and microwave-assisted synthesis [144]. Moreover, by controlling the reactant concentration, reaction temperature, and reaction time, MnO2 with different sizes and structures were obtained, and nanobelts, nanowires, flowerlike, and tubular architectures were also reported [145–147]. Magnetic susceptibility of MnO between room temperature and about 130 K follows a Curie−Weiss type of behavior, while that of Mn3O4 is strongly enhanced over MnO due to the larger effective moment (theoretical µeff = 9.15 µB) [148].

1.1.1.6 Rare Earth-Based Permanent Magnets

The focus for the development of permanent magnets lies in the increase of reliability, strength, and ability to store energy, which are characterized by the energy product – (BH)max (the key figure of merit of permanent magnets) [149]. During the past decades, the (BH)max has been enhanced, starting from about 1 MGOe for steels to about 3 MGOe for hexagonal ferrites, and finally reaching to about 56 MGOe for neodymium–iron–boron magnets [150].

SmCo5, Sm2Co17, and Nd2Nd14B are well-known rare earth (RE) permanent magnets. SmCo alloy magnets have a high (BH)max ranging from 14 to 28 MGOe and better temperature characteristics than the NdFeB magnets at higher temperature working conditions (Curie temperature of 750 °C for SmCo5, more than 800 °C for Sm2Co17, compared with about 400 °C for Nd2Fe14B and 465 °C for ferrite) [149]. SmCo5 is one of the most important hard magnets among the SmCo-type permanent magnets, the crystalline structure is shown in Figure 1.6. Regarding the preparation of Sm-Co magnets, Hou et al. have prepared SmCo5 by a facile chemical method, just annealing the core/shell-structured Co/Sm2O3 NPs at high temperatures [151]. The results show that the product has a Hc as high as 24 kOe (1 kOe = 0.08 A m−1) at 100 K and 8 kOe at room temperature, while the remnant magnetic moment keeps 40–50 emu g−1. Recently, the 2 : 17 type SmCo permanent magnets (namely, Sm2Co17) have also attracted much attention due to their superior magnetic properties and excellent thermal stability [149,152].

Figure 1.6 The crystal structure of SmCo5.

Chaban et al. investigated the phase diagram of Nd–Fe–B near the composition 2 : 14 : 1 in 1979; afterward, the composition and crystal structure of Nd2Fe14B was confirmed. The structure of the typical compound Nd2Fe14B is shown in Figure 1.7, in which Fe nets providing high magnetization are layered with RE elements (Nd) [153,154]. Today Nd–Fe–B magnets are the leading RE permanent magnets, due to their high coercivity and large (BH)max. There are several methods to prepare Nd–Fe–B magnets, for example, sintering, melt spinning, hydrogen treating Nd–Fe–B alloys, mechanical alloying, or hot working [149].

Figure 1.7 The crystal structure of Nd2Fe14B.

Taking into consideration that the Sm-Co and Nd-Fe-B magnets contain costly Co and Nd elements, the research of low-cost permanent magnets is necessary. Sm-Fe-N(C) magnets have attracted much attention owing to their excellent magnetic properties such as high-saturation magnetization. In the 1990s, Coey et al. found that R2Fe17 could change to R2Fe17Nx by absorbing N2 above 300 °C in a solid–gas reaction. N atoms enter the unit cell of R2Fe17 as interstitial atoms, and the unit cell can sustain Th2Zn17 or Th2Ni17 structure [155]. The anisotropy field at room temperature is 14 T, almost twice of that of Nd2Fe14B.

1.2 Typical Characterization of Magnetic Nanomaterials

1.2.1 X-Ray Magnetic Circular Dichroism Spectroscopy

The X-ray magnetic circular dichroism, which consists of the difference in absorption of left- and right-circularly polarized X-rays, has evolved over 25 years, with the physics similar to the UV-vis MCD known from 1897 [156]. It possesses great advantage in the detection of elemental specificity coming with the core electron spectroscopies, and in proving quantitative information about the distribution of spin and orbital angular momenta. Moreover, the XMCD also finds its strength in the capacity of determining spin orientations and inferring spin states from magnetization curves, and in the ability of separating magnetic and nonmagnetic entities [157,158]. The key ingredients of XMCD consist of a source of circularly polarized X-rays, a monochromator and optics, and the X-ray absorption detection system. To extend the strength of XMCD, the spatially resolving instruments are often introduced to obtain element-specific images of magnetic properties [159,160]. Generally, the combined microscopes are divided into three categories: (i) recording the transmission or secondary yield of fluorescence or photoelectrons by a scanning apparatus, in which a focused X-ray spot is raster-scanned across the examined sample; (ii) operating in a mode similar to transmission light microscopes, in which the condenser and analyzer lenses are replaced by zone plates for the X-ray region; (iii) imaging the photoelectron yield from different regions of the sample utilizing the electron microscope optics, as shown in Figure 1.8 [157].

Figure 1.8 Schematic illustration of (a) a scanning X-ray microscope, (b) a photoelectron microscope, and (c) an imaging transmission microscope [157]. Reproduced with permission of Elsevier.

Utilizing the XMCD, the valence, and spin states in spinel MnFe2O4 was reported [161]. It is declared that there exist FeA3+ (Td) ions in MnFe2O4 by comparing the XMCD spectrum with that of GaFeO3 and γ-Fe2O3, and an inversion of about 10% of Fe ions from B (Oh) sites to A (Td) sites, as shown in Figure 1.9.

Figure 1.9 Comparison of the Fe 2p XMCD spectrum of MnFe2O4 with those of γ-Fe2O3 and GaFeO3 (left) and their weighted sum (left) and also those of the calculated Fe 2p XMCD spectra for FeB3+ (Oh) and FeA3+ (Td) and their weighted sum (right) [161]. Reproduced with permission of American Physical Society.

1.2.2 Lorentz Transmission Electron Microscope

Lorentz transmission electron microscope (LTEM) has been intensively investigated to explore the magnetic domain structure and magnetization reversal mechanisms for more than 40 years. When utilizing a TEM to analyze the magnetic domain structures, it is suggested that many applicable magnetic properties are extrinsic rather than intrinsic [162,163]. The difficulty encountered when utilizing a TEM to study the magnetic materials is that the specimen is usually immersed in the high magnetic field of the objective lens [164]. In order to solve this problem, numerous ways are proposed: (i) switching off the standard objective lens; (ii) changing the position of the specimen; (iii) changing the pole pieces to retain the specimen in its standard position, and once again to provide a nonimmersion environment; (iv) adding super minilenses in addition to the standard objective lens and once again switching off [163,165–168].

The LTEM is a sufficient way of observing domain-wall pinning and motion [169]. Besides, the LTEM also finds its role in the detection of topological spin textures. For example, the skyrmion is a vortex spin structure in which the spins pointing in all directions are wrapped to a sphere and is characterized by a topological number of -1 that was first found in B20-type MnSi [170–176]. Generally, the skyrmions are stabilized in the chiral crystal structure and tend to crystallize basically in a hexagonal lattice or tetragonal/cubic lattice form [177]. Except for small-angle neutron diffraction study, skyrmions could be directly observed by LTEM in B20-type alloys [178]. In a study by Seki et al., magnetoelectric skyrmion in an insulating chiral–lattice magnet Cu2OSeO3 was observed by LTEM, and the skyrmion could magnetically induce electric polarization, as shown in Figure 1.10 [170].

Figure 1.10