Search

Both variable density and variable area optical soundtracks were improved over time spurred by the competition between ERPI and RCA, both of whom strove to improve the components of their systems. By 1933 the 100–5000 Hz frequency response limitation had expanded by an octave and a half to 50–8000 Hz, and other attributes were similarly improved with Western Electric calling its enhancements Wide Range and RCA calling theirs High Fidelity. Improvements were made to the entire chain of hardware beginning with the microphone. Western Electric’s changes included replacing their condenser mike in 1931, nicknamed the cannon, with the moving coil 618A mike. The new microphone used the electrodynamic microphone principle, which was apparently first described by Bell in 1877 who suggested a coil attached to a diaphragm made of a non-magnetic material arranged to vibrate within a magnetic field to generate a voltage proportional to the sound pressure on the diaphragm. The concept is also described in USP 242,816, Telephone, filed November 28, 1877 by Charles Cuttriss and Jerome Redding, but the design principle could not be realized without electronic amplification. RCA’s W. C. Jones (1931) and I. W. Giles figured out how to do just that by using a diaphragm made of duralumin with a dome-shaped center. A coil made of aluminum ribbon wound on an insulator was attached to the diaphragm to move within a magnetic field that was produced by a cobalt steel permanent magnet. Damping of the diaphragm’s movement to prevent distortion was achieved by restricting the flow of air of the cavities on either side of it. The new moving coil microphone achieved a response of about 40–10,000 cycles (Hz).

The current fusion energy development path, based on large volume moderate magnetic B field devices is proving to be slow and expensive. A modest development effort in exploiting new superconductor magnet technology development, and accompanying plasma physics research at high-B, could open up a viable and attractive path for fusion energy development. This path would feature smaller volume, fusion capable devices that could be built more quickly than low-to-moderate field designs based on conventional superconductors. Fusion’s worldwide development could be accelerated by using several small, flexible devices rather than relying solely on a single, very large device. These would be used to obtain the acknowledged science and technology knowledge necessary for fusion energy beyond achievement of high gain. Such a scenario would also permit the testing of multiple confinement configurations while distributing technical and scientific risk among smaller devices. Higher field and small size also allows operation away from well-known operational limits for plasma pressure, density and current. The advantages of this path have been long recognized—earlier US plans for burning plasma experiments (compact ignition tokamak, burning plasma experiment, fusion ignition research experiment) featured compact high-field designs, but these were necessarily pulsed due to the use of copper coils. Underpinning this new approach is the recent industrial maturity of high-temperature, high-field superconductor tapes that would offer a truly “game changing” opportunity for magnetic fusion when developed into large-scale coils. The superconductor tape form and higher operating temperatures also open up the possibility of demountable superconducting magnets in a fusion system, providing a modularity that vastly improves simplicity in the construction, maintenance, and upgrade of the coils and the internal nuclear engineering components required for fusion’s development. Our conclusion is that while tradeoffs exist in design choices, for example coil, cost and stress limits versus size, the potential physics and technology advantages of high-field superconductors are attractive and they should be vigorously pursued for magnetic fusion’s development.

Static magnetic devices used in electromechanical energy conversion processes (e.g., rotating machines, rectifiers, inverters) are inductors and (permanent) magnets: their operation is based on Maxwell’s equations [1] and in particular for low-frequency (f < 50 kHz) applications on Ampere’s and Faraday’s laws neglecting displacement currents. Although the nonlinear characteristics of iron cores will be discussed in one of the following subsections, mostly ideal characteristics, that is, no iron-core losses (PFe = 0) and an infinitely large relative permeability (μr = ∞) will be assumed in this chapter. Computer-aided measurement techniques are introduced for measuring, as a function of frequency, small loss levels (few watts), impedances, resistances and inductances of inductors as used in solid-state energy conversion devices.

Neutron scattering is a comprehensive tool for condensed matter research. After a brief description of the interaction of neutrons with matter, the usefulness of neutrons to probe the physical properties of magnetic materials is illustrated using examples taken from different research areas. Then a description of the crystal structure investigation, including in situ and time-resolved studies is given. The use of polarized or unpolarized neutrons to study magnetic structures or magnetic phase transition is also illustrated. The potential of techniques such as small-angle neutron scattering or neutron scattering on magnetic surfaces is presented showing that neutron scattering now offers a wide range of useful techniques to probe the structural and magnetic properties of magnetic materials whatever their state: polycrystalline, single crystal, amorphous, bulk, or thin films. Examples are taken from a wide range of research fields: hard magnetic materials, nanocomposite soft magnets, multilayers, superlattices, geometrically frustrated magnetic materials, etc. The experimental aspects are not covered in detail but relevant references are given throughout the chapter.

This chapter reviews the development of SmCo-type magnets over the last 40 years. First, the physical metallurgy and crystal structures are considered; then the focus is on the recent developments in high-temperature Sm(CobalFe w Cu x Zr y ) z magnets suitable for operation temperatures up to 500°C. It is elucidated that the evolution of coercivity and microchemistry in the respective phases of the heterogeneous nanostructure as well as magnetic domain structure is very sensitive to details of the processing procedure, especially to the slow cooling ramp as the last step where the hard magnetic properties evolve. These changes give rise to rather complex pinning mechanisms in a three-phase precipitation structure, which again depend in a subtle manner on the microchemistry of the 1:5-type cell boundary phase in the 2:17-type magnets. It is the amount and distribution of Cu in and at the cell boundary phase which is the prevalent factor determining the pinning strength and which can yield a non-monotonic temperature dependence of coercivity. The chapter concludes with an overview of novel non-equilibrium processing routes used to obtain SmCo-type nanocomposites.

The brushless DC motor used in ISAD of Electric and Hybrid Electric Vehicles is introduced in this paper. Magnet field distribution and static characteristic of permanent magnet brushless DC motor are calculated by finite element methods.

In this chapter, microfabrication techniques are discussed to enable miniaturized magnetic machines for use with silicon microturbomachinery. These rotational machines could be used as either motors (for motoring or fluidic pumping) or as generators for electrical power generation. Challenges and methods for microfabricating microscale magnetic machines are described, and examples of previously implemented devices are discussed.

Spin Dynamics: Fast Switching of Macro-spins.- Core-Shell Magnetic Nanoclusters.- Designed Magnetic Nanostructures.- Superconductivity and Magnetism in Silicon and Germanium Clathrates.- Neutron Scattering of Magnetic Materials.- Tunable Exchange Bias Effects.- Dynamics of Domain Wall Motion in Wires with Perpendicular Anisotropy.- Magnetic Nanowires for Domain Wall Logic and Ultrahigh Density Data Storage.- Bit-Patterned Magnetic Recording: Nanoscale Magnetic Islands for Data Storage.- The Magnetic Microstructure of Nanostructured Materials.- Exchange-Coupled Nanocomposite Permanent Magnets.- High-Temperature Samarium Cobalt Permanent Magnets.- Nanostructured Soft Magnetic Materials.- Magnetic Shape Memory Phenomena.- Magnetocaloric Effect and Materials.- Spintronics and Novel Magnetic Materials for Advanced Spintronics.- Growth and Properties of Epitaxial Chromium Dioxide (CrO2) Thin Films and Heterostructures.- FePt and Related Nanoparticles.- Magnetic Manipulation of Colloidal Particles.- Applications of Magnetic Nanoparticles in Biomedicine.- Nano-Magnetophotonics.- Hard Magnetic Materials for MEMS Applications.- Solid-State Magnetic Sensors for Bioapplications.

Exchange-coupled nanocomposite magnets are a new type of permanent magnetic materials. Large amounts of theoretical and experimental research have been carried out in the past two decades in understanding the inter-phase exchange interactions and in processing bulk nanocomposite magnets with enhanced energy products. This chapter reviews recent advancements in both the fundamental research and the materials processing technologies. Details in the new findings about the effects of soft phase properties and interface conditions on the hard/soft phase exchange interactions are presented. Various methods for characterizing the inter-phase exchange coupling are reviewed. In materials processing aspects, the development of the bottom-up approaches is discussed. Novel methodology for nanoparticle synthesis including the salt-matrix annealing and surfactant-assisted ball milling is described. Unconventional compaction techniques including warm compaction and dynamic compaction are recommended because they can be used to retain the desired nanoscale morphology for effective exchange coupling in the nanocomposites. At the end of this chapter, perspectives on fabrication of anisotropic nanocomposite magnets are given.

This chapter discusses applications of magnetic materials in bioengineering and medicine [1–8]. Magnetism and magnetic materials have been used for many decades in many modern medical applications, and several new applications are being developed in part because of the availability of superior electromagnets, superconducting magnets and permanent magnets [9–12]. Advances in the synthesis and characterization of magnetic particles, especially nanomagnetic particles, have also aided in the use of magnetic biomaterials [6–12]. We begin with an introduction to magnetism and magnetic materials, followed by a discussion of the characterization, synthesis techniques and applications of magnetic biomaterials [8, 9]. Magnetic materials can be applied to cell separation, immunoassay, magnetic resonance imaging (MRI), drug and gene delivery, minimally invasive surgery, radionuclide therapy, hyperthermia and artificial muscle applications [1–5, 7]. Physical properties which make magnetic materials attractive for biomedical applications are, first, that they can be manipulated by an external magnetic field – this feature is useful for separation, immunoassay and drug targeting, and second, hysteresis and other losses occur in alternating magnetic fields – this is useful in hyperthermia applications. In biology, there has been much interest in the possible use by bees and pigeons of magnetic materials as biological compasses for navigation. Some magnetotactic bacteria are known to respond to a magnetic field, they contain chains of small magnetite particles and they can navigate to the surface or bottom of the pools that they live in using these particles. These particles can be obtained by disruption of the cell wall followed by magnetic separation; the presence of the lipid layer makes these particles biocompatible and they can be readily functionalized for a variety of biomedical applications. The earliest known biomedical use of naturally occurring magnetic materials involves magnetite (Fe3O4) or lodestone which was used by the Indian surgeon Sucruta around 2,600 years ago. He wrote in the book Ayurveda that magnetite can be used to extract an iron arrow tip. Current areas in medicine to which magnetic biomaterials can be applied include molecular and cell biology, cardiology, neurosurgery, oncology and radiology.

The thrust to develop magnetic particles, both fundamentally and technologically, arose out of the necessity for application of this in many spheres of the human race. Physical and chemical properties of lower dimension (nanoscale) particles are very much different from their bulk counterpart. Magnetic nanocomposites depicted quite a few unique properties like very low as well as very high coercivity, superparamagnetism, blocking temperature etc., which opened up new vistas in this area. It was quite evident that iron oxide, in different phases and alloys formed the basic macroscopic magnetic material. In the past few decades it was clear that besides developing magnetic particles as a pure material, there arose a tremendous compulsion to probe into diversions for an inter disciplinary area engulfing chemical and biological sciences for the benefit of mankind. One such effort resulted in determining the compatibility of nano-scale magnetic particles (metal oxides, particularly iron oxide) with polymeric materials to form organic/inorganic composites. Amongst the many questions, which generated with time, a few have been typically selected and addressed in this chapter. This chapter primarily concentrates on ways to prepare (both chemical and physical processes), understand structural aspects, magnetic behavior, electrical and optical properties of polymer–iron oxide based magnetic nanocomposites. Some interesting applications are discussed at the end of the chapter.

This chapter focuses on the use of electromagnetic transducers for the harvesting of kinetic (vibration) energy. The chapter introduces the fundamental principals of electromagnetism and describes how the voltage is linked to the product of the flux linkage gradient and the velocity. The flux linkage gradient is largely dependent on the magnets used to produce the field, the arrangement of these magnets, and the area and number of turns for the coil. The characteristics of wire-wound and micro-fabricated coils, and the properties of typical magnetic materials, are reviewed. The scaling of electromagnetic energy harvesters and the design limitations imposed by micro-fabrication processes are discussed in detail. Electromagnetic damping is shown to be proportional to the square of the dimension and analysis shows that the decrease in electromagnetic damping with scale cannot be compensated by increasing the number of turns. For a wire wound coil, the effect of increasing coil turns on EM damping is directly cancelled by an increase in coil resistance. For a planar micro-coil increasing the number of turns results in a greater increase in the coil resistance, resulting in an overall decrease in damping. Increasing coil turns will, however, increase the induced voltage which may be desirable for practical reasons. An analysis is also presented that identifies the optimum conditions that maximise the power in the load. Finally, the chapter concludes with a comprehensive review of electromagnetic harvesters presented to date. This analysis includes a comparison of devices that confirms the theoretical comparison between conventional wound and micro-fabricated coils and the influence of device size on performance.

Microwave properties of magnetic materials have been under intensive study over 50 years. The results obtained have been summarized in a number of monographs (see, e. g., Soohoo, 1985; Vittoria, 1993). Recently, the interest to these properties renewed. This is mainly due to the necessity of developing novel high-speed magnetic recording devices (Saito, 1990; Stinnett et al., 1998). To be used as magnetic recording head sensors, it is necessary for a magnetic material to have the largest possible permeability values at frequencies of hundreds of MHz and above. Such materials can be also useful in a number of other applications, for a brief review see the end of this chapter.

Bonded magnets can be manufactured by compression molding, extrusion, injection molding or calendering processes. For compression molding, the magnet powders coated with epoxies are compression molded into required geometries and then cured at an appropriate temperature. Bonded magnets manufactured by this process can be held to tight tolerances, which eliminates the need for secondary or finish machining. For injection molding, extrusion and calendering processes, the magnet powders are first blended with polymers and polymer additives. The mixture is then intensively mixed together at a certain temperature by a compounder to form a “compound”. The compound can then be extruded, injection-molded or calendered. The binder that holds the magnet particles together may produce either a flexible or a rigid bonded magnet. Typical binders for flexible bonded magnets are elastomers, such as nitrile rubber or vinyl. Typical binders for rigid bonded magnets include thermoplastics, thermosetting polymers, and in some cases, metallic binders.

Permanent magnet materials capable of reliably operating at high temperatures (≧400 °C) are required for future advanced power Systems in a proposed more electric aircraft (MEA) initiative. A major objective of the MEA initiative is to increase aircraft reliability, maintainability, and supportability, including drastically reducing the need for ground support equipment (Fingers and Rubertus, 2000). This advancement will be accomplished in part through the development of advanced power components such as magnetic bearings, integrated power units, and internal starter/generators for main propulsion engines. New high-temperature magnets are enabling technologies for the development of these new power components. Power system designers frequently find that magnetic materials impose technological limitations on their designs. Compromises are generally required between the desired Performance and the magnetic, mechanical, and electrical properties of available materials. If new materials can operate at ≧ 400 °C, then new advanced designs will be possible. Air-cooling, rather than complicated liquid cooling and its necessary logistics support, will become an operational capability. Likewise, oil-less/lubeless gas turbine engines and space power Systems will be possible.

With the continuing demand for increasingly efficient magnetic materials for technological applications, the theoretical and experimental effort aimed at the development of novel magnetic materials has increased dramatically in recent years. Of particular interest are artificial materials whose properties are governed by reduced dimensionality in the nanometer length scale, e.g., soft magnetic nanocrystalline alloys, metallic multilayers exhibiting the giant magnetoresistance effect, nanopowders, and rapidly quenched permanent magnets.

Permanent magnets are now widely used in motor, generator, telecommunication, and control devices, and are considered as indispensable materials in everyday life. Until now, the intermetallics composed of the rare earth and 3d elements have been mainly developed as high-performance permanent magnets, such as SmCo5, Sm2 (Co,Fe)17 and Nd2Fei4B (Stmat et al., 1967; Ojima et al., 1977; Buschow, 1977; Mishra et al., 1981; Sagawa et al., 1984; Croat et al., 1984; Hadjipanayis et al., 1984; Sellmyer et al., 1984). As the main, components Sm and Co are particularly expensive, and therefore it is desirable to use the less costly iron-based Compounds in place of the cobalt-based Compounds. Despite the initial promise of the Nd2Fe14B magnet, Problems associated with its poor temperature stability and corrosion resistance have ensured that the search for even better permanent magnetic materials continues unabatedly. Since 1987, worldwide efforts have been made to investigate the magnetic properties of R(Fe1−x M x )12 (M = Ti,V,Mo, Cr,Mn,W, AI or Si)(Yang et al., 1981; Buschow, 1988, 1991; Li and Coey, 1991; Suski, 1996, and references therein). Among them, Sm(Fe,M)12 seems to be the best potential candidate for permanent magnet applications (Schultz and Katter, 1991a). Unfortunately, Sm(Fe,M)12 appears to offer less practical use than Nd2Fe14B. The Curie temperature is almost the same as that of Nd2Fe14B, but its Saturation magnetization is lower than the Sm(Fe,M)12, which leads to the conclusion that the theoretical maximum energy product (BH)max of Sm(Fe,M)12 is only half that of Nd2Fe14B.

After three generations of the discovery of rare-earth permanent magnets, based on SmCo5, Sm2Co17 and Nd2Fe14B Compounds, a great effort has been made on researching new rare-earth transition-metal (R-T) intermetallic Compounds in permanent magnetic materials. The aim is to have a better understanding of structural and magnetic properties of the intermetallics and to find new R-T Compounds possessing good intrinsic magnetic properties, namely, a sufficiently high Curie temperature, high magnetization and high uniaxial magnetic anisotropy.

In recent years, exchange-coupled magnetic nanostructures have attracted much attention in the areas of permanent magnetism, magnetic recording, sensors, soft magnetism and spin-electronics. The reason is that suitable nanostructuring may improve the performance of an artificial material beyond that of naturally occurring substances, realizing what is known as the materials-by-design concept. This chapter focuses on permanent magnets, whose ability to store magnetostatic energy is described by the energy product. In the 20th century, energy product doubled every twelve years, and the present-day record-holder Nd2 Fe14 B has energy products in excess of 451 kJ/m3. However, the outlook for discovering new ternary phases with magnetizations significantly higher than Nd2 Fe14 B has been poor, and new approaches are necessary if the energy product is ever to double again. For example, as predicted in Skomski and Coey(1993), adding a soft material with a high polarization, such as Fe65 Co35 (µ0 M s = 2.43 T), to an oriented hard magnet improves the energy product if the grain size of the soft regions is sufficiently small. Compared to the present-day theoretical limit of 516 kJ/m3 for single-phase Nd2Fe14B, the energy product in suitably nanostructured Sm2 Fe17 N3/Fe65 Co35 composites was predicted to be as high as 1090 kJ/m3 (Skomski and Coey, 1993). Related structures have been investigated by a number of authors (Al-Omari and Sellmyer, 1995; Liu et al., 1998a; Sawitchi et al., 2000; Bowden et al., 2000, Skomski et al., 1999; Hadjipanayis, 1999; Fullerton et al. 1999).

Magnetic materials have inspired human imagination for millennia, and for many centuries they have stimulated progress in science and technology. For a long time, focus has been on naturally occurring magnetic materials, such as iron and magnetite (see Fig. 1.1). In the last few decades, there has been a revolution in the development of magnetic materials. On one hand, atomic-scale quantum-mechanical and relativistic effects have been exploited to create high-performance magnetic materials, such as the alloys SmCo5 and Nd2Fe14B, which are used to produce permanent magnets. On the other hand, geometrically well-defined nanostructures such as multilayers, particle arrays and bulk composites, are now actively explored and used to fabricate magnetic materials for a wide range of applications (Coehoorn et al., 1988; Baibich et al., 1988; Skomski and Coey, 1993; McCurrie, 1994; Himpsel et al., 1998; Comstock, 1999; Wood, 2000; Weller et al., 2000; Ziese and Thornton, 2001; Sellmyer et al., 2002).

The nanocomposite Nd2Fe14 B-based permanent magnets was produced by researchers at Philips Research Laboratories for the first time (Buschow et al., 1988; Coehoorn et al., 1988). The materials comprised of Fe3B, Nd2Fe14B, and a small amount of α-Fe. The content of Nd in these magnets is about one third of conventional Nd2Fe14 B-based magnets, and hence a lower material cost was anticipated. Later, a semi-reversible demagnetization behavior originating from nearly reversible rotation of magnetic moments in the magnetically softer components was noted, and the term “exchange-spring magnet” was coined by Kneller and Hawig (Kneller and Hawig, 1991). More recently, Skomski and Coey, pointed out the possibility of realization of magnetic energy products exceeding 1 MJ/m3 (Skomski and Coey, 1993), which is about 2.5 times greater than the value (444 kJ/m3) achieved in the best Nd2Fe14 B-based magnet reported up to this date (Kaneko, 2000). These experimental and theoretical findings have kindled intense research on nanocomposite permanent magnets.

In this chapter the term “magnetic polarization” J is used, which is related to “magnetization” M and “magnetic flux density” B, by J = μ 0 M = B − μ 0 H, where magnetic constant (permeability of a vacuum) μ 0 =4π × 10−7(H/m). In general, SI units were used instead of cgs units. However, the symbols and units in the figures were not changed from those used in the original papers, and so cgs units sometimes remain. In addition, for the units of magnetic field, both amperes/metre [A/m] (related to H) and Tesla [T] (related to μ 0 H) were used.

The performance of permanent magnets has greatly been improved by introducing rare-earth elements (R) to their constituents (Herbst, 1991). It is doubtless that the high coercivity of these magnets comes from the large magnetic anisotropy originating from the crystalline electric field (CEF) acting on R ions with large orbital angular momentum. Magnetization measurements up to the high-field region where the hard-axis magnetization saturates are indispensable in order to obtain the basic insight of the magnetic anisotropy. Since the discovery of sintered Nd-Fe-B magnets (Sagawa et al., 1984), we have been investigating systematically high-field magnetization processes in a series of Nd2Fe14B-type compounds using mainly single crystal samples (Nakagawa et al., 1990). On the other hand, we developed a method of analyzing these magnetization curves, which consists of a simplified Hamiltonian taking exchange and crystal field at R ions into account, with Fe sublattice being treated phenomenologically (Yamada et al., 1988). Similar analyses were made by Givord et al. (1988), Cadogan et al. (1988) and Radwanski et al. (1990b). A comparison of crystalline magnetization process (CEF) and molecular field parameters determined by these groups has been made by Nakagawa et al. (1990). The essential feature in these analysis models is the coupling of the two different types of sublattices. One is the R sublattice, which gives large magnetic anisotropy owing to the CEF interaction.

The world of sensors is characterized by a large variety of devices, appropriately selected to answer the needs to detect different external conditions. For instance, multitudes of thermal sensors are present as contact and non-contact sensors, devices based on the measure of heat flux or temperature changes, and basic sensing technologies for temperature probes include thermocouple [1], thermistor [2], solid state [3] and fibre optic [4]. Magnetic sensors are equally variegated in their types and shapes according to the different physical principles they make use of: from those that are actuated by the presence of a permanent magnet, where their operating principle is based on the use of reed contacts, to fluxometric sensors which use the voltage induced by the changes in permeability of a ferromagnetic core.

Magnetic field strength has always been an important parameter to consider for Magnetic Resonance Imaging (MRI). It is generally agreed that the Signal to Noise Ratio (SNR) is approximately proportional to magnetic field strength [1,2], although other more subtle effects, such as chemical shift dispersion and susceptibility, also scale with field strength and can cause problems for good anatomical imaging. However, it is no surprise to learn that the engineering challenges presented by the commercial construction of higher field MRI systems are formidable. This chapter is an opportunity to introduce these to a wider audience.

The advancement of MRI as a radiological instrument has been associated with a constant drive toward higher magnetic field strengths, resulting in higher operational frequencies. More powerful magnets bring the promise of enhanced signal to noise ratio resulting in exquisite resolution, and reduced scan times. At the same time, however, operating MRI at higher frequencies adds significant physical and engineering complexities to the MRI experiment, most notably in designing safe, versatile, and high-performance radiofrequency (RF) coils. This chapter provides RF coil studies that span frequencies ranging form 1.5 to approx. 12 T. The results and conclusions are based on experimental findings using 8 and 1.5T whole-body MRI systems, computational electromagnetics using the finite-difference time-domain method, and analytical derivations using electromagnetic theory. The outcome of these studies is then utilized to provide new avenues and techniques to improve the performance of RF head coils for human MRI at very high fields.