Chapter 10 Magnetic Properties 10. 1 Introduction 10 Chapter 10 Magnetic Properties 10.1 Introduction 10.2 Physical Basis of Magnetism 10.3 Classification of Magnetic Materials 10.4 Diamagnetism and Para-magnetism 10.5 Ferromagnetism 10.6 Anti-ferromagnetism and Ferrimagnetism 10.7 Devices and Applications 10.7.1 Permanent magnets 10.7.2 Transformer cores 10.7.3 Magnetic Storage Devices

10.3 Classification of Magnetic Materials Based on the extent and nature of the interaction between electrons in the solid and an external magnetic field it is possible to group materials into five classes. Three of the classes of magnetic materials-paramagnetic, diamagnetic, and anti-ferromagnetic solids---show almost no response to external magnetic fields . In contrast ferromagnetic and ferromagnetic materials interact strongly with external magnetic fields and are used in a variety of magnetic applications, including electrical transformers, information storage devices (magnetic tapes and computer disks), motors and generators and loudspeakers.

10.4 Diamagnetism and Para-magnetism Diamagnetism and Para-magnetism are both weak forms of interaction between solids and external magnetic fields. In diamagnetic solids the internal magnetic field is anti-parallel to the external field, while in paramagnetic solids the internal and external fields point in the same direction. The orbital motion of an electron around its nucleus always results in a contribution to the diamagnetic response of a material. The electron’s spin, however, may lead to a paramagnetic contribution.

What factors determine whether a material will be dia-or paramagnetic What factors determine whether a material will be dia-or paramagnetic? Diamagnetism is usually observed in solids composed of atoms with completely filled electron shells. The reason is that in a filled shell there are equal numbers of electrons with positive and negative spins, so that the total magnetic moment (from spin) is zero. Examples of diamagnetic materials include most ionic and covalent crystals, almost all organic compounds including polymers, and some metals, notably Cu, Ag, and Au. The interaction between a diamagnetic solid and an external field is very weak. In fact, the magnitude of χ for most diamagnetic solids is on the order of 10-4 to 10-5. as such, diamagnetic materials find few magnetic applications.

Para-magnetism is related to the magnetic moment resulting from unpaired electrons in unfilled inner electron shells (unpaired electrons in the valence shell do not contribute to paramagnetic behavior). Thus, most transition metals are paramagnetic. Figure 10-3 shows the distribution of electron spins in the 3d shells of some transition metals. The number of unpaired spins per atom can be found using Hund’s rule, which states that in an unfilled shell the number of unpaired spins will be as large as possible within the constraints of the Pauli exclusion principle. As with diamagnetic solids, the strength of the interaction between a paramagnetic material and an external magnetic field is limited (χ is on the order of 10-2 to 10-3). As a result paramagnetic materials find few magnetic applications.

10.5 Ferromagnetism Ferromagnetic solids display magnetic susceptibility values much greater than 1 .The primary difference between Para-ferromagnetic and ferromagnetic materials is in the strength of the interaction between adjacent atomic magnetic dipoles. While the dipoles are essentially independent in paramagnetic materials, they interact strongly in ferromagnetic materials.

At low temperatures the exchange interaction between adjacent atomic magnetic dipoles in ferromagnetic solids is strong energy to overcome the thermal fluctuations attempting to randomize the orientation of the magnetic dipoles. The result is that even without an external field, neighboring dipoles align with each other. Detailed calculations show that the requirements for ferromagnetism are: ① The inner electron shell is unfilled ②the unfilled shell must gave a small radius③ The electron energy band associated with the unfilled shell must be narrow.

10.6 Anti-ferromagnetism and Ferrimagnetism In the previous section we showed that adjacent atomic dipoles can interact to form an ordered array of magnetic dipoles in some materials. If, as shown schematically in Figure 10-4a, all the dipoles point in the same direction, the material is said to be ferromagnetic. However, other orientational relationships between neighboring dipoles are possible. For example, the neighboring dipoles can align themselves in an antiparallel configuration. If a material is characterized by a dipoles arrangement, as shown Figure 10-4b, in which the alternating dipoles have equal strength, it is antiferromagnetic. If the alternating dipoles have different strengths, as shown in part 10-4c of the figure, the material is ferrimagnetic.

Most common antiferromagnetic materials are compounds with the chemical formula MXn, where M is a transition metal atom, X is an electronegative atom (O, S, Te, or F), and n is either 1 or 2. The magnetic dipoles occur within the unfilled inner shell of the metal atoms. Since only the metal atoms contribute magnetic dipoles, all the dipoles have equal strength.

Ferrimagnetism occurs when more than one type of metal atom is in the structure. If the two metal atoms have unequal dipole strengths, then even if their dipoles are aligned so as to oppose one another, there will be a nonzero net magnetic dipole. Thus, ferrimagnetic materials have magnetic susceptibilities between those of the anti-ferromagnetic and the ferromagnetic materials. Their properties and applications, however, are similar to those of the ferromagnetic materials.

The three most common types of ferrimagnetic materials, all of which are ceramic oxides, are summarized in table 10-5. The spinels, or cubic ferrites, are used in applications requiring soft magnets, including transformer cores, inductors, and memory devices. The garnets, or rare earth ferrites, are favored in high-frequency applications such as in microwave devices. The magnetoplumbites, or hexagonal ferrites, are preferred in hared magnet applications (i.e, they are used as permanent magnets).

thank you!