8/7/2019 Lec_18_Magnetic_Storage_W03[1]
1/32
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
2/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
3/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
4/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
5/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
6/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
7/32
Oersted
A Danish physicist, Hans Christian Oersted in 1819
discovered that
a current carrying conductor produces magnetic
field around it.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
8/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
9/32
R/W head
The R/W head consists of a coil of wire wrapped
around a soft iron core.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
10/32
How we magnetize the surface of a disk?
The R/W head consists of a coil of wire wrapped
around a soft iron core.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
11/32
How we magnetize the surface of a disk?
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
12/32
How we magnetize the surface of a disk?
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
13/32
Faraday
Michael Faraday in 1831 found that if a conductor is
placed in a moving magnetic field, an electricalcurrent is generated.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
14/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
15/32
How we Read the surface of a disk?
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
16/32
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)
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
17/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
18/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
19/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
20/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
21/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
22/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
23/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
24/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
25/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
26/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
27/32
Various Encoding Schemes
Three basic types have been the most popular:
Frequency Modulation
Modified Frequency Modulation
Run Length Limited
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
28/32
Various Encoding Schemes
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
29/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
30/32
Binary Unit Prefixes
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
31/32
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.
8/7/2019 Lec_18_Magnetic_Storage_W03[1]
32/32
Areal Density