Basically, a floppy disk drive reads
and writes data to a small, circular piece of metal-coated plastic similar to audio
cassette tape.
- Both use a thin plastic base material coated with iron
oxide. This oxide is a ferromagnetic material, meaning that if you expose
it to a magnetic field it is permanently magnetized by the field.
- Both can record information instantly.
- Both can be erased and reused many times.
- Both are very inexpensive and easy to use.
A floppy disk, like a cassette tape,
is made from a thin piece of plastic coated with a magnetic material on both
sides. However, it is shaped like a disk rather than a long thin ribbon. The
tracks are arranged in concentric rings so that the software can jump from each
file easily. The diskette spins like a record and the heads move to the correct
track. The write head puts data on the diskette by magnetizing small, iron,
bar-magnet particles embedded in the plastic surface. The magnetized particles
have their north and south poles oriented in such a way that their pattern may
be detected and read on a subsequent read operation. This type of storage is a
common type in all computers. However due to the small amout
information able to be held on a
magnetic disk, technology has advanced and brought us optical disk
drives. Plastics are used because they are inexpensive to make and you
can get similar conductive properties by adding the oxide layer.
Magnetic storage
Longitudinal recording and perpendicular recording, two types of writing heads on a hard disk.
Magnetic storage (or magnetic recording) is the storage of data on a magnetized medium.
Magnetic storage uses different patterns of magnetization
in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using one or more read/write
heads.
As of 2013, magnetic storage media, primarily hard disks,
are widely used to store computer data
as well as audio
and video
signals. In the field of computing, the term magnetic storage is
preferred and in the field of audio and video production, the term magnetic
recording is more commonly used. The distinction is less technical and more
a matter of preference. Other examples of magnetic storage media include floppy disks,
magnetic recording tape, and magnetic stripes on credit cards.
History
Magnetic storage in the form of wire recording—audio
recording on a wire—was publicized by Oberlin Smith
in 1888. He filed a patent in September, 1878 but did not pursue the idea as
his business was machine tools. The first publicly demonstrated (Paris
Exposition of 1900) magnetic recorder was invented by Valdemar Poulsen in 1898. Poulsen's device recorded a signal on a wire wrapped around a drum. In
1928, Fritz Pfleumer developed the first magnetic tape recorder.
Early magnetic storage devices were designed to record analog
audio signals. Computer and now most audio and video magnetic storage devices
record digital data.
In old computers, magnetic storage
was also used for primary storage in a form of magnetic drum,
or core memory,
core rope memory, thin film memory, twistor memory
or bubble memory. Unlike modern computers, magnetic
tape was also often used for secondary storage.
Design
Hard drives use magnetic memory to
store giga- and terabytes of data in computers.
Information is written to and read
from the storage medium as it moves past devices called read-and-write
heads that operate very close (often tens
of nanometers) over the magnetic surface. The read-and-write head is used to
detect and modify the magnetization of the material immediately under it. There
are two magnetic polarities, each of which is used to represent either 0 or 1.
The magnetic surface is conceptually
divided into many small sub-micrometer-sized
magnetic regions, referred to as magnetic domains, (although these are not magnetic domains in a rigorous physical sense), each of which has a mostly
uniform magnetization. Due to the polycrystalline
nature of the magnetic material each of these magnetic regions is composed of a
few hundred magnetic grains. Magnetic grains are typically 10 nm in size and each
form a single true magnetic domain.
Each magnetic region in total forms a magnetic dipole
which generates a magnetic field. In older hard disk drive
(HDD) designs the regions were oriented horizontally and parallel to the disk
surface, but beginning about 2005, the orientation was changed to perpendicular to allow for closer magnetic domain spacing.
For reliable storage of data, the
recording material needs to resist self-demagnetization, which occurs when the
magnetic domains repel each other. Magnetic domains written too densely
together to a weakly magnetizable material will degrade over time due to
rotation of the magnetic moment one or more domains to cancel out these forces. The domains
rotate sideways to a halfway position that weakens the readability of the
domain and relieves the magnetic stresses. Older hard disk drives used iron(III) oxide
as the magnetic material, but current disks use a cobalt-based alloy.
A write head magnetizes a region by
generating a strong local magnetic field, and a read head detects the
magnetization of the regions. Early HDDs used an electromagnet
both to magnetize the region and to then read its magnetic field by using electromagnetic
induction. Later versions of inductive heads
included metal in Gap (MIG) heads and thin film
heads. As data density increased, read heads using magnetoresistance (MR) came into use; the electrical resistance of the head
changed according to the strength of the magnetism from the platter. Later
development made use of spintronics;
in read heads, the magnetoresistive effect was much greater than in earlier
types, and was dubbed "giant"
magnetoresistance (GMR). In today's heads, the read
and write elements are separate, but in close proximity, on the head portion of
an actuator arm. The read element is typically magneto-resistive while the write element is typically thin-film inductive.
The heads are kept from contacting
the platter surface by the air that is extremely close to the platter; that air
moves at or near the platter speed. The record and playback head are mounted on
a block called a slider, and the surface next to the platter is shaped to keep
it just barely out of contact. This forms a type of air bearing.
Magnetic
recording classes
Analog
recording
Analog recording is based on the fact that remnant magnetization of a given
material depends on the magnitude of the applied field. The magnetic material
is normally in the form of tape, with the tape in its blank form being
initially demagnetized. When recording, the tape runs at a constant speed. The
writing head magnetizes the tape with current proportional to the signal. A
magnetization distribution is achieved along the magnetic tape. Finally, the
distribution of the magnetization can be read out, reproducing the original
signal. The magnetic tape is typically made by embedding magnetic particles in
a plastic binder on polyester film tape. The commonly used magnetic particles
are Iron oxide particles or Chromium oxide and metal particles with size of 0.5
micrometers.Analog recording was very popular in audio and video recording. In
the past 20 years, however, tape recording has been gradually replaced by
digital recording.
Digital
recording
Instead of creating a magnetization
distribution in analog recording, digital recording only needs two stable magnetic states, which are the +Ms
and -Ms on the hysteresis loop. Examples of digital recording are floppy disks and HDDs.
Magneto-optical
recording
Magneto-optical recording
writes/reads optically. When writing, the magnetic medium is heated locally by
a laser,
which induces a rapid decrease of coercive field. Then, a small magnetic field
can be used to switch the magnetization. The reading process is based on
magneto-optical Kerr effect. The magnetic medium are typically amorphous R-FeCo thin
film (R being a rare earth element). Magneto-optical recording is not very
popular. One famous example is Minidisc
developed by Sony.
Domain
propagation memory
Domain propagation memory is also
called bubble memory. The basic idea is to control domain wall motion in a
magnetic medium that is free of microstructure. Bubble refers to a stable
cylindrical domain. Data is then recorded by the presence/absence of a bubble
domain.
Technical
details
Access
method
Magnetic storage media can be
classified as either sequential
access memory or random access memory although in some cases the distinction is not perfectly
clear. The access time can be defined as the average time needed to gain access
to stored records. In the case of magnetic wire, the read/write head only
covers a very small part of the recording surface at any given time. Accessing
different parts of the wire involves winding the wire forward or backward until
the point of interest is found. The time to access this point depends on how
far away it is from the starting point. The case of ferrite-core memory is the
opposite. Every core location is immediately accessible at any given time.
Hard disks and modern linear
serpentine tape drives do not precisely fit into either category. Both have
many parallel tracks across the width of the media and the read/write heads
take time to switch between tracks and to scan within tracks. Different spots
on the storage media take different amounts of time to access. For a hard disk
this time is typically less than 10 ms, but tapes might take as much as 100 s.
Current
usage
As of 2011, common uses of magnetic storage media are for computer
data mass storage on hard disks and the recording of analog audio and video
works on analog tape. Since much of audio and video production is moving to
digital systems, the usage of hard disks is expected to increase at the expense
of analog tape. Digital
tape and tape libraries
are popular for the high capacity data storage of archives and backups. Floppy disks
see some marginal usage, particularly in dealing with older computer systems
and software. Magnetic storage is also widely used in some specific
applications, such as bank cheques (MICR) and credit/debit cards (mag stripes).
Future
A new type of magnetic storage,
called Magnetoresistive Random Access Memory or MRAM, is being produced that stores data in magnetic bits based on the tunnel
magnetoresistance (TMR) effect. Its advantage is
non-volatility, low power usage, and good shock robustness. The 1st generation
that was developed was produced by Everspin Technologies, and utilized field induced writing. The 2nd generation is
being developed through two approaches: Thermal
Assisted Switching (TAS)which is currently being
developed by Crocus Technology, and Spin Torque Transfer (STT) on which Crocus,
Hynix.
Submitted by- Gaurav kocher
Class-Bsc1st cs
Roll no. 4263
Assigment of computer science
No comments:
Post a Comment