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

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

Revised Pages

748 • Chapter 18 / Magnetic Properties

Recording medium

Recording

head

Signal

in Write

Signal

Read out

Width

Gap

Figure 18.23 Schematic

representation showing how

information is stored and

retrieved using a magnetic

storage medium. (From J. U.

Lemke,MRS Bulletin,Vol. XV,

No. 3, p. 31, 1990. Reprinted with

permission.)

Recently, hybrid heads that consist of an inductive-write and a magnetoresistive

read head in a single unit have been introduced. In the magnetoresistive head, the

electrical resistance of the magnetoresistive thin film element is changed as a result

of magnetic field changes when the tape or disk passes by the read head. Higher sen-

sitivities and higher data transfer rates make magnetoresistive heads very attractive.

There are two principal types of magnetic media—particulateandthin film.Par-

ticulate media consist of very small needle-like or acicular particles, normally of

γ-Fe 2 O 3 ferrite or CrO 2 ; these are applied and bonded to a polymeric film (for mag-

netic tapes) or to a metal or polymer disk. During manufacture, these particles are

aligned with their long axes in a direction that parallels the direction of motion past

the head (see Figures 18.23 and 18.24). Each particle is a single domain that may be

magnetized only with its magnetic moment lying along this axis. Two magnetic states

are possible, corresponding to the saturation magnetization in one axial direction,

and its opposite. These two states make possible the storage of information in digital

Figure 18.24 A scanning electron

micrograph showing the microstructure of a

magnetic storage disk. Needle-shaped

particles ofγ-Fe 2 O 3 are oriented and

embedded within an epoxy phenolic resin.

8000 ×. (Photograph courtesy of P. Rayner

and N. L. Head, IBM Corporation.)