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Objectives
- The student will identify the typical physical components
of magnetic media.
- The student will describe the size and composition of magnetic
coatings.
- Given a track format and medium, the student will estimate
the data capacity of the medium.
- The student will view electron microscopic images and identify
the magnetic storage material depicted.
History
Magnetic media date from the 1940s when thin wire was used
as a medium for audio recording. The wire was brittle and hard
to handle, and was soon replaced by a flexible backing of acetate-coated
with magnetic particles. The backing material evolved to MylarÒ and other modern plastics, but the basic construction
of the tape is still very similar today.
Magnetic Tape
A cross-section of magnetic tape would show a "sandwich"
composed from top to bottom:
Magnetic coating
Adhesive
Backing materials
Back coating and lubricant
The coarseness of the magnetic coating determines the density
of data and amount of wear that the medium will support. Some
tape formats, such as audio and video, use the abrasive nature
of the media intentionally to clean the heads as the tape passes
by. As flexible magnetic media are used, the magnetic coatings
frequently flake away from the backing material, creating gaps
in the recorded data.
Information
In Ferromagnetic materials, groups of atoms tend to line up
in magnetic domains to produce overall magnetism in the material.
A magnetic domain is an area where magnetic moments all align
in the same direction. Magnetic media are composed of many magnetic
domains. Once created, each of the magnetic domains present
in the material are roughly equal in energy. Boundary effects,
and the need for additional energy to cause a change in state,
tend to keep domains stable and prevent "flipping"
of magnetic moments.
When a magnetic domain moves past a coil, such as those used
on the pick up head, an electrical charge is created which varies
proportionally with changes in the magnetic domain. Information
can be stored by changing the magnetic moments, and retrieved
by reading the alignment patterns in the magnetic domains.
Tracks
Data is encoded in specific areas of the media called tracks.
The alignment between the head and track make reading recorded
data possible. When the reading head does not correctly align
with the track the data cannot be read. For a given medium,
increasing the number of tracks increases the amount of data
that can be stored. The space between tracks as measured from
center to center is called the track pitch.
Correct and incorrect alignment
Data is recorded on the surface of the tape in a series of
tracks. Typically, audio tape uses parallel tracks running the
length of the tape. The tracks are described in terms of the
amount of the tape that they cover, such as full-, half -, quarter
-, or eight-track tape.
Video
Helical data tracks, used for digital audio tape (DAT) and
video recording, use rotating heads to stack diagonal tracks
along the length of the tape. Using helical tracks permits the
use of greater surface area on the recording media and provides
greater data density for a given tape size.
Digital Audio Tape (DAT) uses cassettes similar to but smaller
than small video tape. The audio data encoded is a digital signal.
The ease of recording/erasure/re-recording and digital quality
make DAT a popular medium for recording studios and other high
quality audio production.
Video signals are compiled from several data sources, including
audio, control track, and video information about brightness
and color. Due to this complexity, video recording use a combination
of track formats to encode the information needed to recreate
the video signals. On a given tape, formats may include a helical
video track, and linear tracks for audio, and the control track
to provide timing information needed to reconstruct the video
image.
Disk Drives
Disk drives were developed as extensions of tape and drum recorders.
The first disk drive was the IBM 350, introduced in 1957, which
used 50, 24-inch diameter disks. The storage capacity was 5
MB. The disk rotated at 1200 rpm and provided a data transfer
rate of 12.5 kilobytes per second. Today, disks can rotate at
up to 7,000 rpm and in a fraction of the space required even
a few years ago, the storage capacity can exceed 18 gigabytes.
Disk drives use a small record/read head that moves rapidly
just above the surface of the disk. To record data, a small
charge is created to alter the magnetic domains of the media.
To read data, the magnetic domains create small electrical fields
in the coil, which are amplified and processed.
Diagram to photograph
The ability to increase storage relied on progress in engineering
to scale down three design elements:
- Flying height, or distance from the head to the magnetic
medium – 125 – 250 nm
- Gap size of the head- 0.2 - 1 mm
- Thickness of the magnetic medium
- Size of surface anomalies that can cause problems –
10 nm
Recording Heads
Early magnetic recording heads were made of Mu-metal, an alloy
composed of nickel, iron, molybdenum, and copper (NiFeMoCu).
Ferrite heads replaced the Mu-metal heads in the mid-1960s and
permitted smaller head gaps and smaller bit sizes, resulting
in significantly increased storage capacity. Ferrite head development
continued to reduce head size and increase data capacity. Thin
film heads, introduced in 1979, used photographic-lithographic
etching techniques taken from the semiconductor industry. Magnetoresistive
heads, introduced in 1991, offered increased sensitivity and
permitted a further increase in recording and playback density.
Disk Drive Heads
Disk drives use a small electromagnetic head composed of two
coils separated by a small gap, approximately 0.4 m
m wide. To record information, the magnetic field created between
the coils extends across the gap between the head and the medium
changing the direction of the magnetic alignment. To read information,
the coils in the head pass over the magnetic medium and small
changes in the electrical charge, or flux, are created by the
magnetic alignment of the medium.
Recording requires conversion of the data to electrical signals.
Data is stored when the magnetic field generated by the current
in the coils of the head alters the magnetic domain in the coating
of the tape or disk. Effective recording requires a balance
of the strength of the signal, design of the head, distance
from head to medium, and properties of the magnetic coating
material. The magnetic surface is divided into tracks when the
medium is formatted or information recorded. The size and length
of the tracks are two factors that determine the amount of data
that can be stored.
Analog/Digital
Changes in magnitude of the patterns of North and South alignment
can convey analog information. Digital data can be encoded by
using a change in state to represent a 1, or no change for 0.
Reading Data
Reading information from the medium requires three components;
a transport mechanism to move the medium; a head to detect changes
in the magnetic information; and electronics capable of detecting
and amplifying very small changes in the voltage generated in
the coils in the head by the magnetic domain.
The record/read heads are composed of a coil attached to magnetic
material, such as ferrite. When an electrical signal passes
through the coil, a magnetic field is created in the head. The
head is produced with a small gap across which the magnetic
field will "jump," producing a directed extension
of the field that can reach from the surface of the head to
the recording medium.
Heads
The design of the coil, size of the head and gap determine
the size and strength of the field, and the capacity of a drive
to read and write data. Initially heads were manufactured by
machining and layering the materials. Today thin film heads
are produced like semiconductors with material built up in layers
then etched to create a tiny head gap. Head gap lengths are
in the order of 100s of nanometers.
Zip Disk Tracks
For example, this is an image of the surface of a Zip disk
shown at X magnification. As you can see, the
data tracks are approximately X m m wide, and the spacing in between, the track pitch,
is approximately X m
m.
Zip Disk
The sizes of the head and track are directly proportional.
The smaller the head, the smaller the track size, and greater
the amount of data that can be stored in a given space.
Disk Drive Recording
The recording surface of a disk drive is typically divided
like a pie into sectors composed of linear areas called servo
fields that mark the beginning of the field and the data storage
area in between these fields. Each sector is further divided
into tracks with a servo indicator at the beginning of each
track.
Disk Drive Recording Error
Factors causing error in disk drive recording include:
- noise from electronics used to ampliy the signal
- interference accross tracks due to high data densities
- defects in the media
- track/head misalignment of tracks and head.
- inter-symbol interference, such as misreading transitions
of magnetic domains
Oscillators are used to generate a clock against which to compare
data. Timing of tracks and spacing can be corrected by comparing
data tracks against the clock track to minimize interference
and improve accuracy of reading data.
Current spacing between the head and disk is in the 125-250nm
range. At this degree of separation, surface anomalies of 10nm
in height become important. Atomic Force Microscopes (AFM) are
used to examine heads and disk surfaces to study head/disk interference.
Atomic Force Microscopes use less contact force than other techniques,
provide better vertical height accuracy, and resolve atomic
dimensions in three axes, permitting three-dimensional measurements.
Electron microscopic mages of magnetic storage media show characteristics
that can be used to identify the format. For example:
Video tape data is stored in relatively roughly defined tracks
of stacked, parallel segments.
Video
Floppy disk data is stored in stacked, parallel segments with
relatively large blank areas between the data tracks.
Floppy Disk
Hard disks have more precise control of head and track alignment,
which permits greater data storage density. Images of hard disks
show tightly stacked accurately aligned data elements with relatively
narrow blank areas between the tracks.
Calculations
The maximum capacity of a given disk can be calculated by:
Capacity Maximum = surface area (unit2)(Maximum
bit density per track)(Maximum track density for the unit)
or
Cmax = (_ri2)(r02)(Maximum
bit density per track)(Maximum track density for the unit)
The density of information on a hard disk can be calculated
by multiplying the density of the data (in bits per unit of
area) by the area of the medium. An indication of the change
in storage on a disk can be seen in the changes in the density
and bit area over time:
Date Density (Mb/in2)
Bit Area (m m2)
1980 1.25 52
1987 36 18
1990 100 7
1994 500 1.29
1997 1000 0.65
2000 (6000) 0.110
To give an idea of the scale involved in data storage, densities
of 1gb/in2 involve magnetic features smaller that the wavelength
of light.
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