Magnetic Materials, Basics, Classification, Ferrites, Ferro/Para-magnetic Materials and Components.

INTRODUCTION TO MAGNETIC MATERIALS

Magnetic materials are used in electric motors, transformers, loudspeakers, cranes, data processing, and households. Hard magnets must retain their magnetization even in stray magnetic fields, and soft magnets must change their magnetization with the lowest possible resistance. In this chapter, we explore the different magnetic materials and the processing that endows them with the desired properties. In order to do that, we first review the concepts of magnetism and examine how material becomes magnetic.

We are all familiar with magnets: they are used in the home to attach notes to the refrigerator door, as magnetic catches holding the refrigerator door closed, or to pick up small metallic objects. With these magnets, we require that the magnetization be strong enough and that it be permanent; we do not wish the magnetization to be weakened or modified by contact with other magnets or by stray magnetic fields. Such devices use hard ferromagnetic materials. Advances in the magnetic strength of these materials allow for greater efficiency and miniaturization in motors.

Magnetic fields, induction and Magnetization:

The ability to attract steel and redistribute iron filings in a characteristic pattern is familiar evidence that a magnetic field (H) is established in space by a bar magnet (figure 1(A)). A magnetic field is distributed geometrically the same way by a solenoid as shown in figure 1(A) and 1(B)

1(A)

1(B)

1(C)

Figure 1. (A) Magnetic field surrounding a bar magnet. (B) Magnetic field surrounding an air-filled solenoid. (C) Magnetic field surrounding a solenoid with an iron core.

Magnetic Susceptibility:

Magnetic field produces lines of force that penetrate the medium to which the field is applied.
The density of the lines of force is known as the magnetic flux density.
In a vacuum, the magnetic field and the magnetic flux density are related by the permeability of free space, μ₀. B = μ₀ H
If a magnetic material is placed in the field, it can increase or decrease the flux density.

Magnetic Descriptions of Atoms & Ions

Diamagnetic - Atoms or ions with a closed shell of electrons, all of the electrons are paired.

Paramagnetic - Atoms or ions with unpaired electrons, where the moment of an atom with unpaired electrons is given by the spin, S, and orbital angular, L and total momentum, J, quantum numbers.

The field of the sample in the applied field is known as its magnetization, M, where H is the applied field.

The magnetic flux density, B, is given by:

B = μ₀(H+M)

μ₀ is the permeability of free space,

where H is the symbol for henry

μ₀H is the induction generated by the field alone

μ₀M is the additional induction contributed by the sample

Typically, the magnetization is discussed in terms of the magnetic susceptibility,

Ferromagnetism – Magnetic moments of atoms align to produce a strong magnetic effect. For ferromagnetism, the Curie Law becomes , where T_C is the Curie Temperature.

Anti ferromagnetism – magnetic moments of atoms align anti-parallel to produce a strong magnetic effect. For anti ferro magnetism, the Curie Law becomes , where T_N is the Neel temperature.

Magnetic Susceptibility vs. Temperature

Interplay of applied field and thermal randomization leads to temperature dependence described by the Curie Law, = C/T (where C is a constant known as the Curie constant, and T is in Kelvin)

Paramagnetic substances with localized, weakly interacting electrons obey the Curie-Weiss law.

where is the molar magnetic susceptibility, C = Curie constant, and θ = Weiss constant.

Hysteresis Curves

Magnetic behavior of different ferromagnetic substances is demonstrated by hysteresis curves, a plot of magnetic flux density (B) against applied magnetic field (H).

Starting with a nonmagnetic sample (domains randomly aligned) B and H are zero, but as the field is increased the flux density also increases.
Upon reaching the maximum value of magnetization all the spins are aligned in the sample, but when the applied field is reduced the flux density does not follow the initial curve, because of the difficulty of reversing processes where domains have grown through crystal imperfections.
A sufficiently large magnetic field in the reverse direction must be applied before the magnetization process can be reversed.

The magnetization where H is zero, but B is not zero is known as the remnant magnetization.

The field that needs to be applied in the reverse direction to reduce magnetization to zero is the coercive force.

Materials that are magnetically soft are those of low coercivity, H_c

Soft materials have low permeability and a hysteresis loop that is 'narrow at the waist' and of small area. Materials that are magnetically hard are those of high coercivity, H_c, and a high M_r (B_r)
Hard materials are not easily demagnetized, find use a permanent magnets.

Energy Losses in an Alternating Magnetic Field

When the ferromagnet is subjected to an alternating magnetic field, such as in a transformer, energy is lost at each cycle by two mechanisms: hysteresis losses and eddy currents.

1. Hysteresis Losses

At each cycle of the alternating field, an energy corresponding to the area inside the hysteresis curve is lost and transformed into heat. If, for instance, the coercive field is H^c=500 A/m and the saturation induction is B^s=2 T, the energy loss is 4,000 J/m³ at each cycle. With a frequency of 60 Hz, this represents 240 kW/m³.

2. Eddy Currents

The magnetic core can be considered to be made of many loops in which the time variation dB/dt induces a voltage. If the magnet is electrically conductive, this causes an electric current to flow. This current is proportional to the frequency. This current heats the material and constitutes an important loss of energy, especially at high frequencies. (Such energy absorption is used in heating by inductive coils and in microwave ovens.)

Soft Magnets

When the magnetization is produced by an ac current, as for instance in electric transformers, one uses magnetic materials with a small coercive field to reduce hysteresis losses. These are the soft magnets.

Hard Magnets

When it is desired that a magnet conserves its magnetization in the presence of external magnetic fields, one selects a material with a high coercive field H^c.

Note that the coercive field in the hard magnets is indicated in kA/m and in A/m for soft

Ferrimagnetism

Ferrimagnets are ceramics, generally oxides. They have a smaller saturation magnetization than the metallic ferromagnets; these materials are electric insulators and thereby avoid the losses by eddy currents.

Ferrites

The development of soft ferrites dates to the mid-1930s when important advances in magnetic materials began to unfold. Over the years important compositions were developed; they included (MnZn)Fe²O⁴, (MnCu)Fe²O⁴, and (NiZn)Fe²O⁴.

Ferrite products are produced by powder pressing and sintering, and one of the common shapes produced is the toroid. When wound with wire, toroid make efficient transformer cores, and because they are electrically insulating, there is very little eddy-current loss even when they are operated at microwave frequencies. This, coupled with high saturation magnetization, has enabled ferrites to find applications in television and radio components such as line transformers, deflection coils, tuners, rod antennas, as well as in small to medium power supplies.

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