21.9

Magnetic Materials

| Ferromagnetism

The similarity between the magnetic field lines in the neighborhood of a bar magnet and those around a current loop suggests that the magnetism in each case arises from a common cause. The field that surrounds the loop is created by the charges moving in the wire. The magnetic field around a bar magnet is also due to the motion of charges, but the motion is not that of a bulk current through the magnetic material. Instead, the motion responsible for the magnetism is that of the electrons within the atoms of the material.
The magnetism produced by electrons within an atom can arise from two motions. First, each electron orbiting the nucleus behaves like an atomic-sized loop of current that generates a small magnetic field, similar to the field created by the current loop in Figure 21-30a. Second, each electron possesses a spin that also gives rise to a magnetic field. The net magnetic field created by the electrons within an atom is due to the combined fields created by their orbital and spin motions.
In most substances the magnetism produced at the atomic level tends to cancel out, with the result that the substance is nonmagnetic overall. However, there are some materials, known as ferromagnetic materials, in which the cancellation does not occur for groups of approximately 1016 − 1019 neighboring atoms, because they have electron spins that are naturally aligned parallel to each other. This alignment results from a special type of quantum mechanical* interaction between the spins. The result of the interaction is a small but highly magnetized region of about 0.01 to 0.1 mm in size, depending on the nature of the material; this region is called a magnetic domain. Each domain behaves as a small magnet with its own north and south poles. Common ferromagnetic materials are iron, nickel, cobalt, chromium dioxide, and alnico (an aluminum–nickel–cobalt alloy).

| Induced Magnetism

Often the magnetic domains in a ferromagnetic material are arranged randomly, as Figure 21-41a illustrates for a piece of iron. In such a situation, the magnetic fields of the domains cancel each other, so the iron displays little, if any, overall magnetism. However, an unmagnetized piece of iron can be magnetized by placing it in an external magnetic field provided by a permanent magnet or an electromagnet. The external magnetic field penetrates the unmagnetized iron and induces (or brings about) a state of magnetism in the iron by causing two effects. Those domains whose magnetism is parallel or nearly parallel to the external magnetic field grow in size at the expense of other domains that are not so oriented. In Figure 21-41b the growing domains are colored gold. In addition, the magnetic alignment of some domains may rotate and become more oriented in the direction of the external field. The resulting preferred alignment of the domains gives the iron an overall magnetism, so the iron behaves like a magnet with associated north and south poles. In some types of ferromagnetic materials, such as the chromium dioxide used in cassette tapes, the domains remain aligned for the most part when the external magnetic field is removed, and the material thus becomes permanently magnetized.

Figure 21-41 (a) Each magnetic domain is a highly magnetized region that behaves like a small magnet (represented by an arrow whose head indicates a north pole). An unmagnetized piece of iron consists of many domains that are randomly aligned. The size of each domain is exaggerated for clarity. (b) The external magnetic field of the permanent magnet causes those domains that are parallel or nearly parallel to the field to grow in size (shown in gold color).

The magnetism induced in a ferromagnetic material can be surprisingly large, even in the presence of a weak external field. For instance, it is not unusual for the induced magnetic field to be a hundred to a thousand times stronger than the external field that causes the alignment. For this reason, high-field electromagnets are constructed by wrapping the current-carrying wire around a solid core made from iron or other ferromagnetic material.
Induced magnetism explains why a permanent magnet sticks to a refrigerator door and why an electromagnet can pick up scrap iron at a junkyard. Notice in Figure 21-41b that there is a north pole at the end of the iron that is closest to the south pole of the permanent magnet. The net result is that the two opposite poles give rise to an attraction between the iron and the permanent magnet. Conversely, the north pole of the permanent magnet would also attract the piece of iron by inducing a south pole in the nearest side of the iron. In nonferromagnetic materials, such as aluminum and copper, the formation of magnetic domains does not occur, so magnetism cannot be induced into these substances. Consequently, magnets do not stick to aluminum cans or to copper pennies.
The physics of detecting fingerprints. The attraction that induced magnetism creates between a permanent magnet and a ferromagnetic material is used in crime-scene investigations, where powder is dusted onto surfaces to make fingerprints visible. Magnetic fingerprint powder allows investigators to recover evidence from surfaces like the neoprene glove in Figure 21-42, which are very difficult to examine without damaging the prints in the process. Brushing excess conventional powder away ruins the delicate ridges of the pattern that allow the print to be identified reliably. Magnetic fingerprint powder, however, consists of tiny iron flakes coated with an organic material that allows them to stick to the greasy residue in the print. A permanent magnet eliminates the need for brushing away excess powder by creating induced magnetism in the iron and pulling away the powder not stuck directly to the greasy residue.

Figure 21-42 Magnetic fingerprint powder has been used to reveal fingerprints on this neoprene glove. The powder consists of tiny iron flakes coated with an organic material that enables them to stick to the greasy residue in the print. Because of induced magnetism, a permanent magnet can be used to remove (without brushing and possible damage to the print) any excess powder that would obscure the evidence. (James King-Holmes/SPL/Photo Researchers, Inc.)

| Magnetic Tape Recording

The process of magnetic tape recording uses induced magnetism, as Figure 21-43 illustrates. The weak electrical signal from a microphone is routed to an amplifier where it is amplified. The current from the output of the amplifier is then sent to the recording head, which is a coil of wire wrapped around an iron core. The iron core has the approximate shape of a horseshoe with a small gap between the two ends. The ferromagnetic iron substantially enhances the magnetic field produced by the current in the wire.

Figure 21-43 The magnetic fringe field of the recording head penetrates the magnetic coating on the tape and magnetizes it.

The physics of magnetic tape recording.

When there is a current in the coil, the recording head becomes an electromagnet with a north pole at one end and a south pole at the other end. The magnetic field lines pass through the iron core and cross the gap. Within the gap, the lines are directed from the north pole to the south pole. Some of the field lines in the gap “bow outward,” as Figure 21-43 indicates, the bowed region of magnetic field being called the fringe field. The fringe field penetrates the magnetic coating on the tape and induces magnetism in the coating. This induced magnetism is retained when the tape leaves the vicinity of the recording head and, thus, provides a means for storing audio information. Audio information is stored, because at any instant in time the way in which the tape is magnetized depends on the amount and direction of current in the recording head. The current, in turn, depends on the sound picked up by the microphone, so that changes in the sound that occur from moment to moment are preserved as changes in the tape’s induced magnetism.

| Maglev Trains

A magnetically levitated train—maglev, for short—uses forces that arise from induced magnetism to levitate or float above a guideway. Since it rides a few centimeters above the guideway, a maglev does not need wheels. Freed from friction with the guide-way, the train can achieve significantly greater speeds than do conventional trains. For example, the Transrapid maglev in Figure 21-44 has achieved speeds of 110 m/s (250 mph).

Figure 21-44 (a) The Transrapid maglev (a German train) has achieved speeds of 110 m/s (250 mph). The levitation electromagnets are drawn up toward the rail in the guideway, levitating the train. (Courtesy Deutsche Bundesbahn/Transrapid) (b) The magnetic propulsion system.

Figure 21-44a shows that the Transrapid maglev achieves levitation with electromagnets mounted on arms that extend around and under the guideway. When a current is sent to an electromagnet, the resulting magnetic field creates induced magnetism in a rail mounted in the guideway. The upward attractive force from the induced magnetism is balanced by the weight of the train, so the train moves without touching the rail or the guideway.

The physics of a magnetically levitated train.

Magnetic levitation only lifts the train and does not move it forward. Figure 21-44b illustrates how magnetic propulsion is achieved. In addition to the levitation electromagnets, propulsion electromagnets are also placed along the guideway. By controlling the direction of the currents in the train and guideway electromagnets, it is possible to create an unlike pole in the guideway just ahead of each electromagnet on the train and a like pole just behind. Each electromagnet on the train is thus both pulled and pushed forward by electromagnets in the guideway. By adjusting the timing of the like and unlike poles in the guideway, the speed of the train can be adjusted. Reversing the poles in the guideway electromagnets serves to brake the train.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.