Callister - An introduction - 8th edition

826 • Chapter 20 / Magnetic Properties
Read head
V read
Write head
Main pole
Figure 20.23 Schematic
diagram of a hard disk
drive that employs the
perpendicular magnetic
recording medium; also
shown are inductive write
and magnetoresistive-read
heads.
Recording
medium
Soft
underlayer
Return pole
Flux line
above and in close proximity to the magnetic medium by a self-generating air bearing
as the medium passes beneath at relatively high rotational speeds. 6 On the other
hand, tapes make physical contact with the heads during read and write operations.
Tape velocities run as high as 10 m/s.
As noted previously, there are two principal types of magnetic media—hard
disk drives (HDDs) and magnetic tapes—both of which we now briefly discuss.
Hard Disk Drives
Hard disk magnetic storage hard drives consist of rigid circular disks [having diameters
that range between about 65 mm (2.5 in.) and 95 mm (3.75 in)]. During read
and write processes, disks rotate at relatively high velocities—5400 and 7200 revolutions
per minute are common. Rapid rates of data storage and retrieval are possible
using HDDs, as are also high storage densities.
For the current HDD technology, “magnetic bits” point up or down perpendicular
to the plane of the disk surface; this scheme is appropriately called perpendicular
magnetic recording (abbreviated PMR), and is represented schematically in
Figure 20.23.
Data (or bits) are introduced (written) into the storage medium using an inductive
write head. For one head design, shown in Figure 20.23, a time-varying write magnetic
flux is generated at the tip of the main pole—a ferromagnetic/ferrimagnetic core
material around which a wire coil is wound—by an electric current (also timevariable)
that passes through the coil. This flux penetrates through the magnetic storage
layer into a magnetically soft underlayer and then reenters the head assembly
through a return pole (Figure 20.23). A very intense magnetic field is concentrated
in the storage layer beneath the tip of the main pole. At this point, data are written
as a very small region of the storage layer becomes magnetized. Upon removal of the
field (i.e., as the disk continues its rotation), the magnetization remains; that is, the
signal (i.e., data) has been stored. Digital data storage (i.e., as ones and zeros) is in
the form of minute magnetization patterns; the ones and zeros correspond to the presence
or absence of magnetic reversal directions between adjacent regions.
6 It is sometimes stated that the head “flies” over the disk.