Tài liệu Handbook of Micro and Nano Tribology P12 doc

Kondo, H.; et al. “Design and Construction of Magnetic Storage Devices”

Handbook of Micro/Nanotribology.

Ed. Bharat Bhushan

Boca Raton: CRC Press LLC, 1999

© 1999 by CRC Press LLC

© 1999 by CRC Press LLC

Part II

Applications

© 1999 by CRC Press LLC

12

Design and

Construction

of Magnetic

Storage Devices

Hirofumi Kondo, Hiroshi Takino,

Hiroyuki Osaki, Norio Saito,

and Hiroshi Kano

12.1 Introduction

12.2 Hard Disk Files Heads • Construction of the Magnetoresistive Head • The

Disk • The Head-Disk Interface

12.3 Tape Systems

The Recording Head • Magnetic Tapes • The Head–Tape

Interface

12.4 Floppy Disk Files

Floppy Disk Heads • Floppy Disks • High-Storage-Capacity

Floppy Disks • Head–Floppy Disk Interface

References

12.1 Introduction

Magnetic recording is the most common technology used to store many different types of signals. Analog

recording of sound was the first and is still a major application. Digital recording of encoded computer

data on disk and tape recorders has evolved as another major use. Hard disk drives use high signal

frequencies coupled with high medium speeds, and emphasize small access times together with high

reliability. A third large application area is video recording for professional or consumer use. The high

video frequencies are normally recorded using rotatory-head drums. Despite the availability of other

methods of storing data, such as optical recording and semiconductor devices, magnetic recording media

has the following advantages: (1) inexpensive media, (2) stable storage, (3) relatively high data rate,

(4) high volumetric density.

In principle, a magnetic recording medium consists of a permanent magnet and a pattern of remanent

magnetization can be formed along the length of a single track, or a number of parallel tracks on its

surface. Magnetic recording is accomplished by relative motion between a magnetic medium (tape or

© 1999 by CRC Press LLC

disk) against a stationary or rotatory read/write head. The one track example is given in Figure 12.1a.

The medium is in the form of a magnetic layer supported on a nonmagnetic substrate. The recording

or the reproducing head is a ring-shaped electromagnet with a gap at the surface facing the medium.

When the head is fed with a current representing the signal to be recorded, the fringing field from the

gap magnetizes the medium as shown in Figure 12.1b. For a constant medium velocity, the spatial

variations in remanent magnetization along the length of the medium reflect the temporal variations in

the head current, and constitute a recording of the signal.

The recording magnetization creates a pattern of external and internal fields, in the simplest case, to

a series of contiguous bar magnets. When the recorded medium is passed over the same head, or a

reproducing head of similar construction, the flux emanating from the medium surface is intercepted

by the head core, and a voltage is induced in the coil proportional to the rate of change of this flux. The

voltage is not an exact replica of the recording signal, but it constitutes a reproduction of it in that

information describing the recording signal can be obtained from this voltage by appropriate electrical

processing. The combination of a ring head and a medium having longitudinal anisotropy tends to

produce a recorded magnetization. This combination has been the one used traditionally, and it still

FIGURE 12.1 (a) Illustration of the recording and reproducing process. (b) Schematic of cross-sectional view

showing the magnetic field at the gap.

© 1999 by CRC Press LLC

dominates all major analog and digital applications. Ideally, the pattern of magnetization created by a

square-wave recording signal would be like that shown in Figure 12.1a.

In between recording and reproduction, the recorded signal can be stored indefinitely even if the

medium is not exposed to magnetic fields comparable in strength to those used in recording. Whenever

recording is no longer required, it can be erased by means of a strong field applied by the same head as

that used for recording or by a separate erase head. After erasure, the medium is ready for a new recording.

Overwriting an old signal with a new one, without a separate erase step, is available for writing.

Figure 12.2 shows a road map of magnetic storage devices including hard disks (fixed and removable),

magnetic tapes, and floppy disks and of optical storage device. The recording density has been increasing

continuously over the years and a plot of logarithm of the areal density vs. year almost gives a straight

line. The areal density of the hard disk is almost the same as the optical medium. For high areal recording

density, the linear flux density and the track density should be as high as possible. Reproduced signal

amplitude decreases rapidly with a decrease in the recording wavelength and track width. The signal loss

is a function of magnetic properties and thickness of the magnetic coating, read gap length, and

head–medium spacing. For high recording densities, high magnetic flux density and coercivity of a

medium are needed. Regarding the materials, metal magnetic powder (MP) and a monolithic cobalt

alloy thin film of higher magnetic saturation and coercivity have been launched in recent media. So as

to a magnetic head, higher frequency response and sensitivity are required.

It is known that the signal loss as a result of spacing can be reduced exponentially by reducing the

separation between the head and medium. A physical contact between the medium and the head occurs

during starting and stopping operation and a load-carrying air film is developed at the interface in the

relative motion. Closer flying heights lead to undesirable collision of asperities and increased wear so

that this air film should be thick enough to mitigate any asperity contacts; on the contrary it must be

thin enough to attain a large reproduced signal. Thus, the head–medium interface should be designed

with optimum conditions.

The achievement of higher recording densities requires smoother surfaces. The ultimate objective is

to use two smooth surfaces in contact for recording provided the tribological issues can be resolved.

Smooth surfaces lead to an increase in adhesion, friction, and interface temperatures. Friction and wear

issues are resolved by appropriate selection of interface materials and lubricants, by controlling the

FIGURE 12.2 Areal density migration of magnetic recording media. Optical media shown for comparison.

© 1999 by CRC Press LLC

dynamics of the head and medium, and the environment. A fundamental understanding of the tribology

of the magnetic head–medium interface becomes crucial for the continuous growth of the magnetic

storage industry.

In this chapter materials and construction used in the modern media and heads are reviewed. Selected

interesting fabrication processes of these devices are also described.

12.2 Hard Disk Files

Magnetic heads for rigid disk drives are discussed in this section. Figure 12.3 shows the schematic of the

rigid disk drive. A 3.5-in.-diameter disk is widely used and two to three disks are typically stacked in one

hard disk drive. For very high storage density drives, up to about ten disks are stacked. Writing and

reading are done with magnetic heads attached to a spring suspension. The slider surface (air-bearing

surface) is designed to develop a hydrodynamic force to maintain an adequate spacing (~50 nm) between

a head slider and a disk surface. The magnetic head assembly is actuated by a stepper motor or voice

coil motor to access the data on the disk. The magnetic head-suspension assembly is high, and the fast

access speed can be achieved. From these characteristics, hard disk drives have an advantage of fast access

speed and high storage density.

12.2.1 Heads

The areal density of the rigid disk drives have been increasing 60% per year; the magnetic recording head

performance must be improved continuously to maintain this high growth rate of the areal recording

density. The track width of the recording head must be narrower and narrower and the transfer rate

FIGURE 12.3 Schematic diagram of hard disk drive.

© 1999 by CRC Press LLC

becomes higher and higher. The ferrite bulk head (monolithic head, Figure 12.4) and the composite MIG

head (metal-in-gap head Figure 12.5) were widely used for the rigid disk drives. Since these two types

of bulk recording heads are fabricated mainly by conventional machining processes, it is difficult to

control a narrow track width down to 10 µm. On the other hand, thin-film inductive heads are fabricated

by using the same photolithography processes that are used for semiconductor devices, which allows

control of a narrow track width. The coil inductance must be reduced for the high transfer rate appli￾cation. The yoke size of the monolithic head is almost the same as that of the MIG head shown in

Figures 12.4 and 12.5 (Jones, 1980). Figure 12.6a shows the eight-turn thin-film inductive head and

Figure 12.6b shows the slider with a thin-film head. Minimizing the total magnetic ring yoke size of the

film head, the coil inductance of the thin-film head can be reduced. Film heads have an advantage of the

FIGURE 12.4 The schematic diagram of the ferrite monolithic head.

FIGURE 12.5 The schematic diagram of the composite head.

FIGURE 12.6 The schematic diagram of the thin-film head.