1、8.5.6 POLYCRYSTALLINE MATERIALS AND THE M VERSUS H BEHAVIOR,The majority of the magnetic materials used in engineering is polycrystalline and therefore have a microstructure that consists of many grains of various sizes and orientations depending on the preparation and thermal history of the compone
2、nt. In an unmagnetized polycrystalline sample, each crystal grain will possess domains, as depicted in Figure 8.30.,The domain structure in each grain will depend on the size and shape of the grain and, to some extent, on the magnetizations in neighboring grains. Although very small grains perhaps s
3、maller than 0.1m, may be single domains, in most cases the majority of the grains will have many domains. Overall, the structure will possess no net magnetization, provided that it was not previously subjected to an applied magnetic field. We can assume that the component was heated to a temperature
4、 above the Curie point and then allowed to cool to room temperature without an applied field.,Suppose that we start applying a very small external magnetic field (0H) along some direction, which we can arbitrarily label as +x. The domain walls within various grains begin to move small distances, and
5、 favorably oriented domains (those with a component of M along +x) grow a little larger at the expense of those pointing sway from the field, as indicated by point an in Figure 8.31. The domain walls that are pinned by imperfections tend to bow out. There is a very small but net magnetization along
6、the field, as indicate by the Oa region in the magnetization versus magnetizing field (M versus H) behavior in Figure 8.31.,As we increase the magnetizing field, the domain motion extend larger distance, as shown for point b in Figure 8.31, and walls encounter various obstacles such as crystal imper
7、fections, impurities, second phase, and so on which tend to attract the walls and thereby hinder their motions. A domain wall that is stuck (or pinned) at an imperfection at a given field cannot move until the field increases sufficiently to provide the necessary force to snap the wall snaps free, w
8、hich then suddenly surges forward to the next obstacle.,As a wall suddenly snaps free and shoots forward to the next obstacle, essentially two causes lead to heat generation: Sudden change in the lattice distortion, due to magnetostriation create lattice waves that carry off some of the energy. Sudd
9、en changes in the magnetization induce eddy currents that dissipate energy via Joule heating(domains have a finite electrical resistance). These processes involve energy conversion to heat and are irreversible. Sudden jerks in the wall motions lead to small jumps in the magnetization of the specimen
10、 as the magnetizing field is increased; the phenomenon is known as the Barkhausen effect. If we could examine the magnetization precisely with a highly sensitive instrument, we would see jumps in the M versus H behavior, as show in the inset in Figure 8.31.,8.5.7 DEMAGNETIZATION The B-H hysteresis c
11、urves, as is Figure 8.32b, that are commonly given for magnetic materials represent B versus H behavior observed under repeated cycling. The applied field intensity H is cycled back and forward between the +x and x directions. If we were to try and demagnetize a specimen with a remnant magnetization
12、 at point e in Figure 8.34 by applying reverse field intensity, then the magnetization would move along from point e to point f. If at point f we were to suddenly switch off the applied field, we would find that B does not actually remain zero but recovers along f to point e and attains some value B
13、r.,The simplest method to demagnetize the sample is first to cycle H with ample magnitude to reach saturation and then to continue cycling H but with a gradually decreasing magnitude, as depicted in Figure 8.35.As H is cycled with a decreasing magnitude, the sample traces out smaller and smaller B-H
14、 loops until the B-H loops are so small that they end up at the origin when H reaches zero.,The demagnetization process in Figure 8.35 is commonly known as diapering. Undesirable magnetization of various magnetic devices such as recording heads is typically removed by this diapering process (for exa
15、mple, a demagnetizing gun brought close to a magnetized recording head implements diapering by applying a cycled H with decreasing magnitude).,8.6 SOFT AND HARD MAGNETIC MATERIALS,8.6.1 DEFINITIONS Based on their B-H behavior, engineering materials are typically classified into soft and hard magneti
16、c materials. Their typical B-H hysteresis curves are shown in Figure 8.37. Soft magnetic materials are easy to magnetize and demagnetize and hence require relatively low magnetic field intensities. Put differently, their B-H loops are narrow, as shown in Figure 8.37. The hysteresis loop has a small
17、area, so the hysteresis power loss per cycle is small.,Soft magnetic materials are typically suitable for application where repeated cycle of magnetization and demagnetization are involved, as in electric motors, transformers, and inductors, where the magnetic field varies cyclically. These applicat
18、ions also require low hysteresis losses, or small hysteresis loop area. Electromagnetic relays that have to be turned on and off require the relay iron to be magnetized and demagnetized and therefore need soft magnetic materials.,Hard magnetic materials, on the other hand, are difficult to magnetize
19、 and demagnetize and hence require relatively large magnetic field intensities, as apparent in Figure 8.37. Their B-H curves are broad and almost rectangular. They possess relatively large coercivities, which mean that they need large applied fields to be demagnetized. The coercive field for hard ma
20、terials can be millions of times greater than those for soft magnetic materials. Their characteristics make hard magnetic materials useful as permanent magnet in a variety of applications.,It is also clean that the magnetization can be switched from one very persistent direction to another very pers
21、istent direction, from +Br to Br, by a suitably large magnetizing field intensity. As the coercivity is strong, both the states +Br and Br persist until a suitable (large) magnetic field intensity switches the field from one direction to the other. It is apparent that hard magnetic materials can als
22、o be used in magnetic storage of digital data, where the states +Br and Br can be made to represent 1 and 0 (or vice versa).,8.8 HARD MAGNETIC MATERIALS EXAMPLES AND USES,An ideal hard magnetic material, as summarized in Table 8.6, has very large coercivity and remanent field. Further, since they ar
23、e used as permanent magnets, the energy stored per unit volume in the external magnetic filed should be as large as possible since this is the energy available to do work.,This energy density (J m-3) in the external field depends on the maximum value of the product BH in the second quadrant of the B-H characteristics and is denoted as (BH)max. It corresponds to the largest rectangular area that fits the B-H curve in the second quadrant, as shown in Figure 8.39.,
