Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-18 GTBL042-Callister-v2 September 13, 2007 13:46

Revised Pages

736 • Chapter 18 / Magnetic Properties

Temperature (°C)

Temperature (°F)

Saturation magnetization,

M

(10s

6 A/m)

Saturation flux density,

Bs

(gauss)

–200 0 200 400

Fe 3 O 4

Pure Fe

600 800 1000

0

0.5

1.0

1.5

2.0

–400 0 400 800 1200 1600

25,000

20,000

15,000

10,000

5,000

0

Figure 18.10 Plot

of saturation

magnetization as a

function of

temperature for iron

and Fe 3 O 4. [Adapted

from J. Smit and

H. P. J. Wijn,Ferrites.

Copyright©c1959 by

N. V. Philips Gloeil-

ampenfabrieken,

Eindhoven

(Holland). Reprinted

by permission.]

18.7 DOMAINS AND HYSTERESIS

Any ferromagnetic or ferrimagnetic material that is at a temperature belowTcis

composed of small-volume regions in which there is a mutual alignment in the same

direction of all magnetic dipole moments, as illustrated in Figure 18.11. Such a re-

gion is called a domain, and each one is magnetized to its saturation magnetization.

Adjacent domains are separated by domain boundaries or walls, across which the

direction of magnetization gradually changes (Figure 18.12). Normally, domains are

microscopic in size, and for a polycrystalline specimen, each grain may consist of

more than a single domain. Thus, in a macroscopic piece of material, there will be

a large number of domains, and all may have different magnetization orientations.

The magnitude of theMfield for the entire solid is the vector sum of the magneti-

zations of all the domains, each domain contribution being weighted by its volume

One domain

Domain wall

Another domain

Figure 18.11 Schematic depiction of domains in a

ferromagnetic or ferrimagnetic material; arrows represent

atomic magnetic dipoles. Within each domain, all dipoles are

aligned, whereas the direction of alignment varies from one

domain to another.