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Preface to Volume 16
PREFACE TO VOLUME 16
The Handbook series Magnetic Materials is a continuation of the Handbook series Ferromagnetic Materials. When Peter Wohlfarth started the latter series, his original aim was to combine new developments in magnetism with the achievements of earlier compilations of monographs, producing a worthy successor to Bozorth’s classical and monumental book Ferromagnetism. This is the main reason that Ferromagnetic Materials was initially chosen as title for the Handbook series, although the latter aimed at giving a more complete cross-section of magnetism than Bozorth’s book. In the last few decades magnetism has seen an enormous expansion into a variety of different areas of research, comprising the magnetism of several classes of novel materials that share with truly ferromagnetic materials only the presence of magnetic moments. For this reason the Editor and Publisher of this Handbook series have carefully reconsidered the title of the Handbook series and changed it into Magnetic Materials. It is with much pleasure that I can introduce to you now Volume 16 of this Handbook series. Magnetostrictive materials exhibit strains caused by the orientation of the magnetic moments when exposed to a magnetic field. A new class of magnetostrictive materials was discovered in the 90th of last century by Ullakko and co-workers. These new materials are called Magnetic Shape Memory (MSM) alloys or Ferromagnetic Shape Memory Alloys (FSMA). The thermoelastic martensitic phase transformation in MSM materials leads to a low-symmetry phase characterized by a large magnetocrystalline anisotropy and by highly mobile twin boundaries between the variants. These MSM materials can exhibit giant magnetic field-induced strains (MFIS) that find their origin in the rearrangement of the crystallographic domains (twin variants). When applying a magnetic field, the martensite variants that have their easy axis of magnetization along the field, start to grow due to twin boundary motion and become predominant. This process lowers the magnetization energy. The MFIS of MSM materials are unique as they can generate large strains with fairly high frequencies without changes in the external temperature. The obtainable strains are greater than those of magnetostrictive, piezoelectric or electrostrictive materials. Consequently, MSM materials are regarded as potentially important materials for actuator and sensor applications. A comprehensive survey of this interesting novel class of materials is presented in Chapter 1. The development of modern magnetic materials and their application requires a deep understanding of the magnetization processes that determine the magnetic properties. In this respect, micromagnetics is of paramount importance since it relates the microscopic distribution of the magnetization to the physical and chemical structure of a given material. In fact, micromagnetic modelling has become a most important tool in characterizing v
vi
PREFACE TO VOLUME 16
the magnetic behaviour of many different materials as applied in thin film heads, various types of recording media and nanostructured permanent magnets. The rapid progress of nanotechnology has led to many new functional materials and devices. A prerequisite for the development and application of structured materials is the detailed knowledge of the correlation between the physical and magnetic structure of the system. So-called smart magnetic materials require the possibility of predicting the response of the system to external fields, stress and temperature as a function of time. All these aspects are treated in Chapter 2 where the basic theory of micromagnetism is explained for ferromagnetic and antiferromagnetic materials. The authors introduce various methods for the solution of the micromagnetic equation and deal with different numerical discretization schemes such as the finite difference method and the finite element method. At the end of their chapter, the authors present fully integrated recording simulations that allow to simulate a complete recording process, including the relative movement of the externally magnetized head and the recording medium. A ferrofluid, or magnetic fluid as it is also frequently called, consists of a stable colloidal suspension of single-domain nano-sized particles of a ferrimagnet or ferromagnet in a carrier liquid. A wide range of carrier liquids exists. Many ferrofluids are commercially used to meet particular requirements. For instance in applications such as rotary vacuum lead-troughs, it is essential that the carrier liquids have a very low vapor pressure. In other applications, temperature may be a critical quantity. The first product based on ferrofluid, introduced in the 60th of the last century, was a dynamic seal capable of operating under pressure or vacuum. Since then quite a large number of new applications of the technology have been discovered. Although ferrofluids may be present in many consumer products, they are usually hidden from view. It is estimated that over 200 000 rotary vacuum seals, 350 million loudspeakers, 500 million computer disk drives, and 15 million DVD-ROM drives have been built with ferrofluids. Chapter 3 of this Handbook Volume presents a comprehensible overview of the production of ferrofluids and their colloidal and magnetic stability, their basic properties and the possibility to magnetically control the properties and flows of magnetic liquids by moderate magnetic fields. Moreover various examples for the practical use of ferrofluids will be presented, including those in well established fields but dealing also with novel and innovative applications currently in the focus of actual research. Antiferromagnetic exchange-coupled multilayer films are important ingredients for devices based on the giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) effect as employed in many electronic and magnetic devices such as magnetic recording heads and magnetic random access memories (MRAM). Furthermore, antiferromagnetic exchange-coupled multilayer films are also employed in perpendicular magnetic recording media where they serve as means that can reduce domain noise. Excellent high frequency characteristics have been obtained by using antiferromagnetic/ferromagnetic/antiferromagnetic trilayers. In such devices, antiferromagnetic (AFM) systems based on Mn alloys are useful as exchange biasing films for the ferromagnetic (FM) film and magnetic tunnel junctions. There are many systems having a complex (non-collinear) spin structure, depending on the d-electron number and the crystal structure associated with geometrical frustration of the magnetic moments. Experimental evaluation of their magnetic structures is quite difficult, especially in γ -phase disordered alloys, and hence there still remain the
PREFACE TO VOLUME 16
vii
systems whose magnetic structures have not been established yet. Furthermore, the explanations of the mechanism of the exchange bias-field are still diverging, depending on the proposed models. Chapter 4 presents an overview of the physical aspects of antiferromagnetism of Mn alloys both from theoretical and experimental view points, providing a basis for future fundamental and practical developments. The authors discuss the electronic and magnetic structures of Mn alloys, including results of first-principle calculations. Experimental data of antiferromagnetic transitions of various Mn alloys are reviewed in the light of theoretical calculations. A discussion is also presented of the electrical resistivity and its relation with the Néel transition and the concomitant changes in electronic states. Further topics addressed are the relations between lattice distortions, phase diagrams and spin structures. Magnetovolume and magnetoelastic effects are discussed in terms of spin fluctuations. Finally, exchange-bias fields are reviewed and the mechanism of the exchange-bias field between antiferromagnetic Mn alloy films and a ferromagnetic films are discussed within the framework of the classical Heisenberg model. Nanoparticulate materials have attracted increasing attention in the last decade. The reason for this is the unique combination of small size, exotic properties and good processability, opening the possibility of their use in many technological and biomedical applications. For particle sizes falling into the nanometer range, materials can exhibit unusual and interesting physical and mechanical properties. Many of the properties of magnetic nanoparticles are described in Chapter 5. After a detailed description of the various methods used to synthesize magnetic nanoparticles, the authors give a comprehensible account of the colloidal properties of nanoparticles which largely determine the applicability to these materials. Special emphasis is given by them to several specific methodologies used in the literature such as surface modification of nanoparticles and/or encapsulation which render the magnetic nanoparticles useful in many areas of science and technology. The magnetic behavior of magnetic nanoparticles represents a complex and challenging problem. It is clear that the potential applicability of the nanoparticulate systems requires a deep knowledge of their magnetic properties and these properties are extensively discussed by the authors. The last part of this chapter specifically deals with a detailed description of the biomedical applications of magnetic nanoparticles such as NMR imaging, hyperthermia, drug targeting, separation and selection. Volume 16 of the Handbook on the Properties of Magnetic Materials, as the preceding volumes, has a dual purpose. As a textbook it is intended to be of assistance to those who wish to be introduced to a given topic in the field of magnetism without the need to read the vast amount of literature published. As a work of reference it is intended for scientists active in magnetism research. To this dual purpose, Volume 16 of the Handbook is composed of topical review articles written by leading authorities. In each of these articles an extensive description is given in graphical as well as in tabular form, much emphasis being placed on the discussion of the experimental material in the framework of physics, chemistry and material science. The task to provide the readership with novel trends and achievements in magnetism would have been extremely difficult without the professionalism of the North Holland
viii
PREFACE TO VOLUME 16
Physics Division of Elsevier Science B.V., and I wish to thank Wim Spaans for his great help and expertise. K.H.J. B USCHOW VAN DER WAALS -Z EEMAN I NSTITUTE U NIVERSITY OF A MSTERDAM , N ETHERLANDS
The Handbook series Magnetic Materials is a continuation of the Handbook series Ferromagnetic Materials. When Peter Wohlfarth started the latter series, his original aim was to combine new developments in magnetism with the achievements of earlier compilations of monographs, producing a worthy successor to Bozorth’s classical and monumental book Ferromagnetism. This is the main reason that Ferromagnetic Materials was initially chosen as title for the Handbook series, although the latter aimed at giving a more complete cross-section of magnetism than Bozorth’s book. In the last few decades magnetism has seen an enormous expansion into a variety of different areas of research, comprising the magnetism of several classes of novel materials that share with truly ferromagnetic materials only the presence of magnetic moments. For this reason the Editor and Publisher of this Handbook series have carefully reconsidered the title of the Handbook series and changed it into Magnetic Materials. It is with much pleasure that I can introduce to you now Volume 16 of this Handbook series. Magnetostrictive materials exhibit strains caused by the orientation of the magnetic moments when exposed to a magnetic field. A new class of magnetostrictive materials was discovered in the 90th of last century by Ullakko and co-workers. These new materials are called Magnetic Shape Memory (MSM) alloys or Ferromagnetic Shape Memory Alloys (FSMA). The thermoelastic martensitic phase transformation in MSM materials leads to a low-symmetry phase characterized by a large magnetocrystalline anisotropy and by highly mobile twin boundaries between the variants. These MSM materials can exhibit giant magnetic field-induced strains (MFIS) that find their origin in the rearrangement of the crystallographic domains (twin variants). When applying a magnetic field, the martensite variants that have their easy axis of magnetization along the field, start to grow due to twin boundary motion and become predominant. This process lowers the magnetization energy. The MFIS of MSM materials are unique as they can generate large strains with fairly high frequencies without changes in the external temperature. The obtainable strains are greater than those of magnetostrictive, piezoelectric or electrostrictive materials. Consequently, MSM materials are regarded as potentially important materials for actuator and sensor applications. A comprehensive survey of this interesting novel class of materials is presented in Chapter 1. The development of modern magnetic materials and their application requires a deep understanding of the magnetization processes that determine the magnetic properties. In this respect, micromagnetics is of paramount importance since it relates the microscopic distribution of the magnetization to the physical and chemical structure of a given material. In fact, micromagnetic modelling has become a most important tool in characterizing v
vi
PREFACE TO VOLUME 16
the magnetic behaviour of many different materials as applied in thin film heads, various types of recording media and nanostructured permanent magnets. The rapid progress of nanotechnology has led to many new functional materials and devices. A prerequisite for the development and application of structured materials is the detailed knowledge of the correlation between the physical and magnetic structure of the system. So-called smart magnetic materials require the possibility of predicting the response of the system to external fields, stress and temperature as a function of time. All these aspects are treated in Chapter 2 where the basic theory of micromagnetism is explained for ferromagnetic and antiferromagnetic materials. The authors introduce various methods for the solution of the micromagnetic equation and deal with different numerical discretization schemes such as the finite difference method and the finite element method. At the end of their chapter, the authors present fully integrated recording simulations that allow to simulate a complete recording process, including the relative movement of the externally magnetized head and the recording medium. A ferrofluid, or magnetic fluid as it is also frequently called, consists of a stable colloidal suspension of single-domain nano-sized particles of a ferrimagnet or ferromagnet in a carrier liquid. A wide range of carrier liquids exists. Many ferrofluids are commercially used to meet particular requirements. For instance in applications such as rotary vacuum lead-troughs, it is essential that the carrier liquids have a very low vapor pressure. In other applications, temperature may be a critical quantity. The first product based on ferrofluid, introduced in the 60th of the last century, was a dynamic seal capable of operating under pressure or vacuum. Since then quite a large number of new applications of the technology have been discovered. Although ferrofluids may be present in many consumer products, they are usually hidden from view. It is estimated that over 200 000 rotary vacuum seals, 350 million loudspeakers, 500 million computer disk drives, and 15 million DVD-ROM drives have been built with ferrofluids. Chapter 3 of this Handbook Volume presents a comprehensible overview of the production of ferrofluids and their colloidal and magnetic stability, their basic properties and the possibility to magnetically control the properties and flows of magnetic liquids by moderate magnetic fields. Moreover various examples for the practical use of ferrofluids will be presented, including those in well established fields but dealing also with novel and innovative applications currently in the focus of actual research. Antiferromagnetic exchange-coupled multilayer films are important ingredients for devices based on the giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) effect as employed in many electronic and magnetic devices such as magnetic recording heads and magnetic random access memories (MRAM). Furthermore, antiferromagnetic exchange-coupled multilayer films are also employed in perpendicular magnetic recording media where they serve as means that can reduce domain noise. Excellent high frequency characteristics have been obtained by using antiferromagnetic/ferromagnetic/antiferromagnetic trilayers. In such devices, antiferromagnetic (AFM) systems based on Mn alloys are useful as exchange biasing films for the ferromagnetic (FM) film and magnetic tunnel junctions. There are many systems having a complex (non-collinear) spin structure, depending on the d-electron number and the crystal structure associated with geometrical frustration of the magnetic moments. Experimental evaluation of their magnetic structures is quite difficult, especially in γ -phase disordered alloys, and hence there still remain the
PREFACE TO VOLUME 16
vii
systems whose magnetic structures have not been established yet. Furthermore, the explanations of the mechanism of the exchange bias-field are still diverging, depending on the proposed models. Chapter 4 presents an overview of the physical aspects of antiferromagnetism of Mn alloys both from theoretical and experimental view points, providing a basis for future fundamental and practical developments. The authors discuss the electronic and magnetic structures of Mn alloys, including results of first-principle calculations. Experimental data of antiferromagnetic transitions of various Mn alloys are reviewed in the light of theoretical calculations. A discussion is also presented of the electrical resistivity and its relation with the Néel transition and the concomitant changes in electronic states. Further topics addressed are the relations between lattice distortions, phase diagrams and spin structures. Magnetovolume and magnetoelastic effects are discussed in terms of spin fluctuations. Finally, exchange-bias fields are reviewed and the mechanism of the exchange-bias field between antiferromagnetic Mn alloy films and a ferromagnetic films are discussed within the framework of the classical Heisenberg model. Nanoparticulate materials have attracted increasing attention in the last decade. The reason for this is the unique combination of small size, exotic properties and good processability, opening the possibility of their use in many technological and biomedical applications. For particle sizes falling into the nanometer range, materials can exhibit unusual and interesting physical and mechanical properties. Many of the properties of magnetic nanoparticles are described in Chapter 5. After a detailed description of the various methods used to synthesize magnetic nanoparticles, the authors give a comprehensible account of the colloidal properties of nanoparticles which largely determine the applicability to these materials. Special emphasis is given by them to several specific methodologies used in the literature such as surface modification of nanoparticles and/or encapsulation which render the magnetic nanoparticles useful in many areas of science and technology. The magnetic behavior of magnetic nanoparticles represents a complex and challenging problem. It is clear that the potential applicability of the nanoparticulate systems requires a deep knowledge of their magnetic properties and these properties are extensively discussed by the authors. The last part of this chapter specifically deals with a detailed description of the biomedical applications of magnetic nanoparticles such as NMR imaging, hyperthermia, drug targeting, separation and selection. Volume 16 of the Handbook on the Properties of Magnetic Materials, as the preceding volumes, has a dual purpose. As a textbook it is intended to be of assistance to those who wish to be introduced to a given topic in the field of magnetism without the need to read the vast amount of literature published. As a work of reference it is intended for scientists active in magnetism research. To this dual purpose, Volume 16 of the Handbook is composed of topical review articles written by leading authorities. In each of these articles an extensive description is given in graphical as well as in tabular form, much emphasis being placed on the discussion of the experimental material in the framework of physics, chemistry and material science. The task to provide the readership with novel trends and achievements in magnetism would have been extremely difficult without the professionalism of the North Holland
viii
PREFACE TO VOLUME 16
Physics Division of Elsevier Science B.V., and I wish to thank Wim Spaans for his great help and expertise. K.H.J. B USCHOW VAN DER WAALS -Z EEMAN I NSTITUTE U NIVERSITY OF A MSTERDAM , N ETHERLANDS