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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.
© 1999 by CRC Press LLC
high-frequency response and reduced inductance due to a small volume of magnetic yoke and allows
higher transfer rate. Very high recording density drives require the use of a magnetoresistive (MR) head
which will be described later.
12.2.1.1 Structure and Fabrication Process of Thin-Film Inductive Heads
Figure 12.6a shows the cross section and the planar view of the thin-film inductive head. The magnetic
gap is located on the air-bearing surface (ABS). The track width is defined in the planar view. In order
to achieve high magnetic yoke efficiency, the track width should be narrower compared with the width
of the recessed yoke area. Figure 12.7 shows the SEM image of a thin-film head. Film heads must be
deposited on a substrate for which a hard Al
2
O
3
–TiC ceramic is usually employed. With few exceptions,
permalloy, which is the alloy of approximately 80 wt% Ni with 20 wt% Fe, is used for the magnetic layer
of the film head, because an annealing process is not necessary to obtain the high permeability (1000 to
3000) and the low coercivity (3 to 5 Oe). As indicated in Figure 12.8, the heat-cured photoresist materials
are used for insulation layers. After plating the coil layer, the surface of coil layer is not smooth; therefore,
the photoresist is coated. The photoresist insulation layer also makes the surface of the upper coil layer
smooth. Figure 12.8 shows the fabrication process of a thin-film head element. Several thousands of the
head elements are fabricated on the same substrate at the same time. The thin-film heads are fabricated
by stacking thin-film layers. First, the magnetic layer is deposited; then the coil layer and the upper
magnetic layer are plated subsequently. The passivation layers are also deposited between the coil layer
FIGURE 12.7
SEM image of the thin-film inductive head.
FIGURE 12.8
The schematics of the slider fabrication process.
© 1999 by CRC Press LLC
and both the upper and lower permalloy magnetic layers. Finally, the thick protective Al
2
O
3
layer (30 to
50 µm) is sputtered for protecting the head element. Then the wafer is sliced into the head sliders.
In a thin-film head fabrication process, permalloy and copper can be deposited by evaporation,
sputtering, or plating. In a photolithography process, a deposited film is etched physically or chemically
through a patterned photoresist. In a electroplating process, a material is plated only on the conductive
layer. Materials cannot be plated where a conductive layer is not exposed. Figure 12.9 shows this electro-
plating process. First, a conductive layer is deposited on all areas of a wafer and a photoresist is coated
and patterned. The patterned photoresist covers a part of a conductive layer. An electroplating material
(permalloy or copper) can be plated only on the exposed area. After removing this frame, patterned
permalloy or copper can be obtained in Figure 12.9. Figure 12.10 shows the SEM images of the frame of
an upper permalloy layer for an electroplating. The copper and the upper permalloy layer is also plated
by using a photoresist frame. This frame is patterned on a conductive layer; permalloy is plated only on
the exposed area of the under conductive layer. After removing the resist frame, the patterned upper
permalloy layer is obtained as shown in Figure 12.7 The top pole width is controlled by the photoresist
patterning width and the track width tolerance of the upper permalloy yoke can be reduced.
12.1.1.2 Head Slider Manufacturing Process
After finishing the wafer process, the wafer must be sliced into the head sliders. First, a wafer
(Figure 12.11a) is sliced to a row of bars (Figure 12.11b). The sliced surface (surface A in Figure 12.11b)
is lapped very carefully, because this surface will be an ABS and the head throat height is controlled
through this lapping process. The throat height of thin-film head is about 1 µm, the tolerance of this row
bar lapping process is required less than 1 µm. The row bar is attached to the toolings for lapping an ABS
( Figure 12.11c) of row bars. This tooling can be bent for obtaining a precise throat height for all head
tips in a row bar. After finishing the throat height lapping, many row bars are aligned and the lapped
surfaces are etched to make the ABS at the same time (Figure 12.11d). Recently, in order to obtain a
constant flying height for all disk radii, a negative pressure air bearing has been widely used. The shape
of a negative-pressure air bearing is not simple; an ion-etching process must be used to make a air-bearing
FIGURE 12.9
Schematics of the framed permalloy plating: (a) after plating permalloy; (b) after removing photoresist.
FIGURE 12.10
SEM image of the photoresist frame for plating permalloy upper yoke.
© 1999 by CRC Press LLC
shape. After the ion-etching process, the head slider can be obtained by dividing a row bar into the head
sliders (Figure 12.11e).
12.2.1.3 Domain Structure in a Thin-Film Head
Magnetic materials are composed of individual domains with local magnetization which is equal to the
saturation magnetization of the materials. Magnetic domain structure is defined by minimizing the total
magnetic energy of the domain wall, the magnetic anisotropy, and the magnetostriction energy. In
general, when the size of the magnetic film is reduced to several hundred microns, the magnetic domain
structure becomes clear. Since the size of a magnetic yoke of a film head is almost the same size, domain
structure affects the read-and-write characteristics and the stability of the film head. A typical domain
structure of the upper magnetic yoke is shown in Figure 12.12. The easy axis is indicated by the arrow
direction, and the magnetization direction of most domain patterns is parallel to the easy axis. The
domains are separated by a 180° Bloch wall. When the magnetic easy axis is in the
x
-direction, a large
portion of a domain aligns the
x
-direction. To reduce total magnetic energy, a domain whose magneti-
zation direction aligns in the
y
-direction appears in the edge region, because domains of the
y
-direction
cancel surface magnetic charges. Also magnetostriction effects must be considered for designing a film
FIGURE 12.11
The schematics of the slider fabrication process.
FIGURE 12.12
Typical domain configurations in an inductive film
head yoke.
[...]... Performance of a hard disk media is mainly determined from two points of view which are the recording density and the reliability for head–disk friction and clash The former depends on both of the magnetic characteristics of a recording thin film and the flyability of a disk (substrate) The latter depends on the head–disk interface and the mechanical characteristics of a substrate/disk 12.2.2.1 Construction of. .. influences the angles of incidence and the kinetic energy of the sputtering atoms because of the collisions between atoms of argon and sputtering material The substrate temperature relates to the diffusion energy of the sputtering atoms The sputtering power is generally in proportion to the number of the sputtering atoms For these reasons, the grain size and the magnetic characteristics of the recording... decrease in roughness of the slider and the disk surfaces The atmosphere, such as temperature and relative humidity, has significant influences on the tribological properties of heads and media At high humidity, adsorbed water causes the stiction between a head slider and a disk, as well as an excessive amount of lubricant At low humidity, the wear debris of the head slider and the disk often transfers onto... construction of a typical hard disk with a recording layer of a thin magnetic film A hard disk consists of a flat rigid substrate, a recording layer (thickness of about 20 nm) usually with an underlayer in thickness of about 100 nm, a protective layer of about 20 nm, and a lubricant layer of less than 4 nm © 1999 by CRC Press LLC FIGURE 12.39 A head/disk interface (HDI) There are two kinds of substrate... on the microstructure of the magnetic film Figure 12.40 shows the structure of the magnetic film, which consists of many grains Equation 12.13 shows the relation between a signal-to-noise ratio (S/N) and the number of magnetic particles which are included in a unit of volume (ν), where W is a track width, v is a relative velocity, and f is a frequency of recorded signal An increase of ν has an equal effect... example, diameters of disks are 5.25, 3.5, 2.5, and 1.8 in., and thicknesses are 1.2, 0.89, 0.635, and 0.389 mm, respectively Figure 12.39 shows the head–disk interface (HDI) schematically to explain the flyability and the reliability for the head–disk friction and collision A glide height means the minimum distance without collision between a head and a disk Taking mechanical clearance of a drive into... flying height of 50 nm In addition, dents must be also removed from a disk surface because some defects of a disk causes errors of the electric signals Recording density of the disk is determined by reproducing voltage and media noise The reproducing voltage depends on the magnetic characteristics, the thickness of the magnetic thin film, and the spacing loss which depends on the flying height of a head... head, W is a width of a head, E is an efficiency of a head, ν is a relative velocity between disk and head, µ0 is the permeability of free space, g is a gap length of a head, d is a head/disk spacing, and a is a transition length of a magnetization A transition length a is represented theoretically by Williams and Comstock in 1971 as Equation 12.9 (Williams and Comstock, 1971): a= f + f2+ M r δd ′ H c... resistance change of its MR sensor, because the cross section of the MR element is very small and the sensor current density is very high, about 1 × 1011 A/m2 The temperature of the MR sensor becomes several tens to 100°C When the sensor hits an asperity on the disk, the MR sensor temperature and the resistance of the MR sensor also changes Figure 12.35a and b show the base-level variation of the output... domain structures and the error rate of the inductive thin-film head called “popcorn noise,” which is controlled by the magnetostriction energy By controlling the composition of Ni and Fe, the magnetostriction of the magnetic yoke material can be optimized Figure 12.13b shows the probability of popcorn noise vs the Fe composition The probability of popcorn noise is the probability of popcorn noise divided . Kondo, H.; et al. “Design and Construction of Magnetic Storage Devices”
Handbook of Micro/ Nanotribology.
Ed. Bharat Bhushan
Boca Raton:. 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
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