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Download book Magnetic Materials : Fundamentals and Applications. This book covers the fundamentals of magnetism and the basic theories and applications of conventional magnetic materials.
MRL Wednesday May 13th: Special Class Session. Magnetism in Atoms; Diamagnetism: pdf file. Paramagnetism: pdf file. Ferromagnetism: pdf file.
Particularly renowned for her research in multiferroics and magnetoelectrics, her current research focuses on using electronic structure methods to design and understand materials that combine magnetism with additional functionalities.
John Simonds, Magnetoelectronics today and tomorrow, Physics Today, April Before we can begin our discussion of magnetic materials we need to understand some of the basic concepts of magnetism, such as what causes magnetic fields, and what effects magnetic fields have on their surroundings.
These fundamental issues are the subject of this first chapter. Unfortunately, we are going to immediately run into a complication. There are two complementary ways of developing the theory and definitions of magnetism.
The "physicist's way" is in terms of circulating currents, and the "engineer's way" is in terms of magnetic poles such as we find at the ends of a bar magnet. The two developments lead to different views of which interactions are more fundamental, to slightly different-looking equations, and to really confuse things to two different sets of units. Most books that you'll read choose one convention or the other and stick with it. Instead, throughout this book we are going to follow what happens in "real life" or at least at scientific conferences on magnetism and use whichever convention is most appropriate to the particular problem.
To avoid total confusion later, we will give our definitions in this chapter and the next from both viewpoints, and provide a conversion chart for units and equations at the end of Chapter 2. Reference  provides an excellent light-hearted discussion of the unit systems used in describing magnetism.
Magnetic field Magnetic polesSo let's begin by defining the magnetic field, H, in terms of magnetic poles. This is the order in which things happened historically -the law of interaction between magnetic poles was discovered by Michell in England in , and by Coulomb in France in , a few decades before magnetism was linked to the flow of electric current.
They can, however, be approximated by one end of a very long bar magnet, which is how the experiments were carried out. By convention, the end of a freely suspended bar magnet which points towards magnetic north is called the north pole, and the opposite end is called the south pole. Turning Eq. The unit of pole strength does not have a name in the cgs system. In SI units, the constant of proportionality in Eq. To understand what causes the force, we can think of the first pole generating a magnetic field, H, which in turn exerts a force on the second pole.
By convention, the north pole is the source of the magnetic field, and the south pole is the sink, so we can sketch the magnetic field lines around a bar magnet as shown in Fig. The units of magnetic field are oersteds Oe in cgs units, so a field of unit strength has an intensity of 1 oersted. In the SI system, the analogous equation for the force one pole exerts on another is The earth's magnetic field has an intensity of around one-tenth of an oersted, and the field at the end of a typical kindergarten toy bar magnet is around Oe.
Magnetic fluxIt's appropriate next to introduce another rather abstract concept, that of magnetic flux,. The idea behind the term "flux" is that the field of a magnetic pole is conveyed to a distant place by something which we call a flux. Rigorously the flux is defined as the surface integral of the normal component of the magnetic field.
This means that the amount of flux passing through unit area perpendicular to the field is equal to the field strength. Magnetic flux is important because a changing flux generates an electric current in any circuit which it intersects. The electromotive force provides the potential difference which drives electric current around the circuit.
The minus sign in Eq. This is known as Lenz's law. Circulating currentsThe next development in the history of magnetism took place in Denmark in when Oersted discovered that a magnetic compass needle is deflected in the neighborhood of an electric current. This was really a huge breakthrough because it unified two sciences. The new science of electromagnetism, which dealt with forces between moving charges and magnets, encompassed both electricity, which described the forces between charges, and magnetism, which described the forces between magnets.
By small we mean small with respect to the distance at which the magnetic field is observed. The north pole of a bar magnet corresponds to current circulating in a counter-clockwise direction, whereas clockwise current is equivalent to the south pole, as shown in Fig. Today it's believed that magnetic effects are caused by the orbital and spin angular momenta of electrons. Of course the next obvious question to ask is what happens if the wire is not straight.
What magnetic field does a general circuit produce? In fact the total current, I, is equal to the line integral of the magnetic field around a closed path containing the current. We will look at some examples later.
Field from a straight wireTo show that these laws are equivalent, let's use them both to calculate the magnetic field generated by a current flowing in a straight wire. The geometry of the problem is shown in Fig. If we assume that the field lines go around the wire in closed circles by symmetry this is a fairly safe assumption then the field, H, has the same value at all points on a circle concentric with the wire.
This makes the line integral of Eq. The geometry for calculating the field at a point P at a distance a from the wire is shown in Fig. Unfortunately, analytic expressions for the field produced by a current can only be obtained for conductors with rather simple geometries. For more complicated shapes the field must be calculated numerically. Numerical calculation of magnetic fields is an active research area, and is tremendously important in the design of electromagnetic devices.
A review is given in . Magnetic momentNext we need to introduce the concept of magnetic moment, which is the moment of the couple exerted on either a bar magnet or a current loop when it is in an applied field.
Again we can define the magnetic moment either in terms of poles or in terms of currents. We showed in Section 1. Our notation here is to represent vector quantities by bold italic type, and their magnitudes by regular italic type. This gives a definition:The magnetic moment is the moment of the couple exerted on a magnet when it is perpendicular to a uniform field of 1 oersted.
Alternatively, if a current loop has area A and carries a current I , then its magnetic moment is defined as The cgs unit of magnetic moment is the emu. In SI units, magnetic moment is measured in A m 2.
Magnetic dipoleA magnetic dipole is defined as either the magnetic moment, m, of a bar magnet in the limit of small length but finite moment, or the magnetic moment, m, of a current loop in the limit of small area but finite moment. The field lines around a magnetic dipole are shown in Fig. The energy of a magnetic dipole is defined as zero when the dipole is perpendicular to a magnetic field. We will be using the concept of magnetic dipole, and this expression for its energy in a magnetic field, extensively throughout this book.
DefinitionsFinally for this chapter, let's review the definitions which we've introduced so far. Here we give all the definitions in cgs units. Magnetic pole, p. A pole of unit strength is one which exerts a force of 1 dyne on another unit pole located at a distance of 1 centimeter. Magnetic field, H. A field of unit strength is one which exerts a force of 1 dyne on a unit pole.
Magnetic flux,. Magnetic moment, m. The magnetic moment of a magnet is the moment of the couple exerted on the magnet when it is perpendicular to a uniform field of 1 oersted.
Magnetic dipole. HomeworkExercises 1. If not, how might we go about calculating magnetic fields for generalized geometries? Assume that the magnetic moment of the first electron is aligned parallel to the field from the second electron.
What qualitative feature of the field is significant in each case? Suggest a use for each pair of Helmholtz coils. In fact this is not the case, and in fact the unit of magnetic induction is called the gauss. Indeed, mixing up gauss and oersteds is a sure way to upset magnetism scientists at parties. If you have trouble remembering which is which, it can be safer to work in the SI units which we discuss next. Flux densityThe magnetic induction, B, is the same thing as the density of flux, , inside the medium.
In general the flux density inside a material is different from that outside. In fact magnetic materials can be classified according to the difference between their internal and external flux. If inside is less than outside then the material is known as diamagnetic. Examples of diamagnetic materials include Bi and He.
These materials tend to exclude the magnetic field from their interior. We'll see later that the atoms or ions which make up diamagnetic materials have zero magnetic dipole moment. If inside is slightly more than outside then the material is either paramagnetic e. Na or Al or antiferromagnetic e. MnO or FeO. In many paramagnetic and antiferromagnetic materials, the constituent atoms or ions have a magnetic dipole moment.
Magnetism is a class of physical phenomena that are mediated by magnetic fields. Electric currents and the magnetic moments of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. Magnetism is one aspect of the combined phenomenon of electromagnetism. The most familiar effects occur in ferromagnetic materials, which are strongly attracted by magnetic fields and can be magnetized to become permanent magnets , producing magnetic fields themselves. Demagnetizing a magnet is also possible. Only a few substances are ferromagnetic; the most common ones are iron , cobalt and nickel and their alloys.
Machine generated contents note: Part I. Basics: 1. Review of basic magnetostatics; 2. Magnetization and magnetic materials; 3. Atomic origins of magnetism; 4.
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This book provides a comprehensive discussion of magnetism, magnetic materials, and related applications. Beginning with a description of magnetic phenomena and measurements on a macroscopic scale, it is followed by discussions of intrinsic and phenomenological concepts of magnetism, such as electronic magnetic moments and classical, quantum, and Beginning with a description of magnetic phenomena and measurements on a macroscopic scale, it is followed by discussions of intrinsic and phenomenological concepts of magnetism, such as electronic magnetic moments and classical, quantum, and band theories of magnetic behavior.
Информация уходит. - Вторжение по всем секторам. Сьюзан двигалась как во сне. Подойдя к компьютеру Джаббы, она подняла глаза и увидела своего любимого человека.
Нареченный Детским манежем, Третий узел ничем не напоминал стерильную атмосферу остальной части шифровалки. Его обстановка напоминала домашнюю - мягкий ковер, высокотехнологичная звуковая система, холодильник, полный напитков и всяческой еды, маленькая кухня и даже баскетбольное кольцо.
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