Lec_18_Magnetic_Storage_W03[1]

8/7/2019 Lec_18_Magnetic_Storage_W03[1]

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History of Magnetic Storage

The history of magnetic storage dates back to June

1949, when a group of IBM engineers and scientists

began working on a new storage device.

On May 21, 1952, IBM announced the IBM 726

Tape Unit with the IBM 701 Defense Calculator,marking the transition from punched-card

calculators to electronic computers.


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First Disk Drive

On September 13, 1956, a small team of IBM

engineers in San Jose, California, introduced the

first computer disk storage system as part of the 305

RAMAC (Random Access Method of Accounting

and Control) computer. The 305 RAMAC drive could store five million

characters (thats right, only 5MB!) of data on 50

disks, each a whopping 24 inches in diameter.


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First Floppy Drive

Not only did IBM invent computer magnetic tape

storage as well as the hard disk drive, but it also

invented the floppy drive. The same San Jose

facility where the hard drive was created introduced

the first floppy drive, then using eight-inch diameterfloppy disks, in 1971.


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IBM

Since then, IBM has pioneered advanced magnetic data

encoding schemes, such as Modified Frequency Modulation(MFM) and Run Length Limited (RLL);

drive head designs, such as Thin Film, magneto-resistive

(MR), and Giant magneto-resistive (GMR) heads; and

drive technologies, such as Partial Response Maximum

Likelihood (PRML), No-ID recording, and Self-Monitoring

Analysis and Reporting Technology (S.M.A.R.T.).

Today, IBM is arguably the leader in developing andimplementing new drive technology.


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How Magnetic Fields are used to

Store and Read Data

All magnetic storage devices, such as floppy disk

drives and hard disk drives, read and write data by

using electromagnetism.

Two basic principles of Physics (electromagnetism)discovered by Oersted (1819) and Faraday (1831)

are used to record data on a disk and reading that

data back.


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Oersted

A Danish physicist, Hans Christian Oersted in 1819

discovered that

a current carrying conductor produces magnetic

field around it.


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How we magnetize the surface of a disk?

Our data are 0s and 1s.

In digital electronic circuits we represent them with

two discreet voltages (e.g., 0v and 5v).

On magnetic surface we represent them with

polarized magnets (orientation of tiny magnets).

Recording information on a magnetic disk involves

converting electrical signal to magnetic signal.


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R/W head

The R/W head consists of a coil of wire wrapped

around a soft iron core.


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The R/W head consists of a coil of wire wrapped

around a soft iron core.


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Data is written on the disk by pulsing the coil with a

surge of current, which produces magnetic lines of

flux in the soft iron core.

At the air gap, the lines of flux dip down into the

disk's magnetic coating because of its low reluctance(compared to iron).

This, in turn, causes the magnetic domains in the

recording surface to align themselves in a directiondictated by the direction of current flow in the coil.


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The magnetic domains of the surface can assume

one of three possible states, depending on thedirection of current flow through the R/W head:

No current - Unmagnetized (randomly arranged

domains) Current in positive direction - Magnetized in a

positive direction

Current in negative direction - Magnetized in a

negative direction.


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Faraday

Michael Faraday in 1831 found that if a conductor is

placed in a moving magnetic field, an electricalcurrent is generated.


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How we Read the surface of a disk? We make use ofFaraday's law of induced voltage.

Faraday said "If there is a relative change between a wire

and the magnetic field, there will be an induced voltage

across the wire."

Lenz's law ofdirection of the induced voltage, simplystates that the direction of the induced current must be such

that its own magnetic field will oppose the action that

produced the induced current.

As the magnetized spots on the surface pass by the head,changes in magnetic polarity causes lines of flux into the

iron core to change. This, in turn, induces a small voltage in

the coil.


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How we Read the surface of a disk?


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Read/Write Head Designs

Five main types of heads have been used in hard disk

drives over the years:

Ferrite

Thin-Film (TF)

Metal-In-Gap (MIG)

Magneto-resistive (MR)

Giant Magneto-resistive (GMR)


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Ferrite

These heads have an iron-oxide core wrapped with

electromagnetic coils.

Ferrite heads are largerand heavierthan thin-film

heads and therefore require a larger floating height

to prevent contact with the disk while it is spinning. monolithic ferrite head and

a composite ferrite head.


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Metal-In-Gap Metal-In-Gap heads are a specially enhanced version of the composite

ferrite design. In MIG heads, a metal substance is applied to the heads recording

gap.

Two versions of MIG heads are available:

single-sided and double-sided.

MIG heads are designed with a layer of magnetic alloy placed along

the trailing edge of the gap.

This magnetic alloy has twice the magnetization capability of raw

ferrite and enables the head to write to the higher coercivity thin-film

media needed at the higher densities.

MIG heads also produce a sharper gradient in the magnetic field for a

better-defined magnetic pulse.

They are still used today in LS-120 (SuperDisk) drives.


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Thin Film Thin-film heads are manufactured much the same way as a

semiconductor chipthrough a photo-lithographic process. This process creates many thousands of heads on a single

circular wafer and produces a very small, high-quality

product.

TF heads have an extremely narrow and controlled head

gap.

The core is a combination ofiron and nickel alloy that has

two to four times more magnetic power than a ferrite headcore.

Many of the drives in the 100MB to 2GB range used TF

heads, especially in the smaller form factors.


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Magneto-Resistive Heads Another magnetic effect that is well known today is being used in

modern drives. When a wire is passed through a magnetic field, notonly does the wire generate a small current, but the resistance of the

wire also changes. Standard read heads use the head as a tiny

generator, relying on the fact that the heads will generate a pulsed

current when passed over magnetic flux transitions. A newer type of

head design pioneered by IBM instead relies on the fact that theresistance in the head wires will also change.

Rather than use the head to generate tiny currents, which must then be

filtered, amplified, and decoded, a magneto-resistive head uses the

head as a resistor. A circuit passes a voltage through the head andwatches for the voltage to change, which will occur when the

resistance of the head changes as it passes through the flux reversals

on the media. This mechanism for using the head results in a much

strongerand clearer signal of what was on the media and enables the

density to be increased.


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Magneto-Resistive Heads

MR heads rely on the fact that the resistance of a

conductor changes slightly when an externalmagnetic field is present.

The output of MR head is three or more times morepowerful than a TF head during a read.

In effect, MR heads are power-read heads, acting

more like sensors than generators.


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Two Heads

Because the MR principle can only read data and is

not used for writing, MR heads are really two headsin one.

The assembly includes a standard inductive TF head

for writing data and an MR head for reading.


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Giant Magneto-Resistive Heads In the quest for even more density, IBM introduced a new

type of MR head in 1997. Called giant magneto-resistive(GMR) heads, they are physically smaller than standard MR

heads but are so named for the GMR effect on which they

are based.

The design is very similar; however, additional layers

replace the single NiFe layer in a conventional MR design.

In MR heads, a single NiFe film changes resistance in

response to a flux reversal on the disk.

In GMR heads, two films (separated by a very thin copper

conducting layer) perform this function.


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Giant Magneto-Resistive Heads

IBM announced the first commercially available

drive using GMR heads (a 16.8GB 3.5-inch drive) inDecember 1997.

Since then, GMR heads have become the standard in

most drives of 20GB and beyond. The latest GMR drives have data densities

exceeding 20GB per platter, enabling drives as large

as 80GB to be produced in the standard 3.5-inchwide, 1-inch high form factor.


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Data Encoding Schemes Magnetic storage is essentially an analog medium. The data a PC

stores on it, however, is digital information

that is, 1s and 0s. When the drive sends digital information to a magnetic recording

head, the head creates magnetic domains on the storage medium with

specific polarities corresponding to the positive and negative voltages

the drive applies to the head.

It is the flux reversals that form the boundaries between the areas of

positive and negative polarity that the drive controller uses to encode

the digital data onto the analog medium.

During a read operation, each flux reversal the drive detects generates

a positive or negative pulse that the device uses to reconstruct theoriginal binary data.


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Data Encoding Schemes To optimize the placement of flux transitions during

magnetic storage, the drive passes the raw digital input datathrough a device called an encoder/decoder(endec), which

converts the raw binary information to a waveform designed

to optimally place the flux transitions (pulses) on the media.

During a read operation, the endec reverses the process and

decodes the pulse train back into the original binary data.

Over the years, several schemes for encoding data in this

manner have been developed; some are better or more

efficient than others.


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Various Encoding Schemes

Three basic types have been the most popular:

Frequency Modulation

Modified Frequency Modulation

Run Length Limited


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Various Encoding Schemes


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Capacity Measurements

In December 1998, the International

Electrotechnical Commission (IEC) - the leadinginternational organization for worldwide

standardization in electrotechnologyapproved as

an IEC International Standard names and symbolsfor prefixes for binary multiples for use in the fields

of data processing and data transmission.

Prior to this there had been a lot of confusion as to

whether a megabyte stood for1 million bytes (10 6 )

or1,048,576 bytes (2 20 ). Even so, these new

prefixes have yet to be widely adopted and

confusion still reigns.


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Binary Unit Prefixes


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Areal Density Areal density is often used as a technology growth-rate indicator for

the hard disk drive industry. Areal density is defined as the product of the linear bits per inch

(BPI), measured along the length of the tracks around the disk,

multiplied by the number of tracks per inch (TPI), measured radially

on the disk (see Figure 9.9).

The results are expressed in units of megabits or gigabits per square

inch (Mbit/sq.-inch or Gbit/sq.-inch) and are used as a measure of

efficiency in drive recording technology.

Current high-end 3.5-inch drives record at a real densities of

10Gbit/sq.-inch20Gbit/sq.-inch. Prototype drives with densities ashigh as 40Gbit/sq.-inch now exist, which will allow 3.5-inch drives

with capacities of 400GB or more in the next few years.


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Areal Density

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Lec_18_Magnetic_Storage_W03[1]