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

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18.10 Hard Magnetic Materials • 745

B

H

Hd

Bd  Hd < (BH)max

Bd

(BH)max

Figure 18.22 Schematic

magnetization curve that

displays hysteresis. Within the

second quadrant are drawn

twoB–Henergy product

rectangles; the area of that

rectangle labeled (BH)maxis

the largest possible, which is

greater than the area defined

byBd–Hd.

Concept Check 18.6

It is possible, by various means (i.e., alteration of microstructure and impurity ad-

ditions), to control the ease with which domain walls move as the magnetic field is

changed for ferromagnetic and ferrimagnetic materials. Sketch a schematicB-versus-

Hhysteresis loop for a ferromagnetic material, and superimpose on this plot the loop

alterations that would occur if domain boundary movement were hindered.

[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]

Conventional Hard Magnetic Materials

Hard magnetic materials fall within two main categories—conventional and high

energy. The conventional materials have (BH)maxvalues that range between about 2

and 80 kJ/m^3 (0.25 and 10 MGOe). These include ferromagnetic materials—magnet

steels, cunife (Cu–Ni–Fe) alloys, alnico (Al–Ni–Co) alloys—as well as the hexagonal

ferrites (BaO–6Fe 2 O 3 ). Table 18.6 presents some of the critical properties of several

of these hard magnetic materials.

The hard magnet steels are normally alloyed with tungsten and/or chromium.

Under the proper heat-treating conditions these two elements readily combine with

carbon in the steel to form tungsten and chromium carbide precipitate particles, which

are especially effective in obstructing domain wall motion. For the other metal alloys,

an appropriate heat treatment forms extremely small single-domain and strongly

magnetic iron-cobalt particles within a nonmagnetic matrix phase.

High-Energy Hard Magnetic Materials

Permanent magnetic materials having energy products in excess of about 80 kJ/m^3 (10

MGOe) are considered to be of the high-energy type. These are recently developed

intermetallic compounds that have a variety of compositions; the two that have