RECORDING HEAD TECHNOLOGY BASIC School of Mechanical Engineering Institute of Engineering Suranaree University of Technology
Outline • Magnetic and Magnetism • History of Magnetic Recording • Digital Data Encoding and Decoding • HDD Write Head Technology • HDD Read Head and MR Technology • HDD Recording Material • Introduction to Head Fabrications • Introduction to HDD Head Test
Magnetism • Magnetism is one of the phenomena by which materials exert an attractive or repulsive forces on other materials. • Some well known materials that exhibit easily detectable magnetic properties are nickel, iron, some steels, and the mineral magnetite.
Magnetism • The ancient Greeks, originally those near the city of Magnesia, and also the early Chinese knew about strange and rare stones with the power to attract iron. • Chinese found that a steel needle stroked with such a "lodestone" became "magnetic" when freely suspended, pointed north-south. • Around 1600 William Gilbert, proposed an explanation: the Earth itself was a giant magnet, with its magnetic poles some distance away from its geographic ones
Magnetism • Until 1821, only one kind of magnetism was known, the one produced by iron magnets. • Hans Christian Oersted noticed that the current caused a nearby compass needle to move. • Andre-Marie Ampere, who concluded that the nature of magnetism was quite different from what everyone had believed. • It was basically a force between electric currents: two parallel currents in the same direction attract, in opposite directions repel.
Magnetic Dipoles • Normally, magnetic fields are seen as dipoles, having a "South pole" and a "North pole"; • A magnetic field contains energy, and physical systems stabilize into the configuration with the lowest energy. • The magnetic energy, so-called “flux” flows from the north pole to the south pole.
Magnetic Dipoles • Magnetic dipoles result on the atomic scale from the two kinds of movement of electrons. • First: the orbital motion of the electron around the nucleus. • Second: source of electronic magnetic moment is due to a quantum mechanical property called the “spin dipole” magnetic moment
Type of Magnet • Permanent Magnets • Electromagnets
Permanent magnets • A few elements -- especially iron, cobalt, and nickel -- are ferromagnetic at room temperature. • Every ferromagnetic has its own individual temperature, called the Curie temperature, or Curie point, • A long bar magnet has a north pole at one end and a south pole at the other. Near either end the magnetic field falls off inversely with the square of the distance from that pole. • For a magnet of any shape, at distances large compared to its size, the strength of the magnetic field falls off inversely with the cube of the distance from the magnet's centre.
Classification of Magnetic Materials • Diamagnetism • Paramagnetism • Ferromagnetism • Antiferromagnetism • Ferrimagnetism
Diamagnetism • In a diamagnetic material the atoms have no net magnetic moment when there is no applied field. • Under the applied field (H) the spinning electrons produces a magnetisation (M) in the opposite direction to that of the applied field
Paramagnetism • In paramagnetism materials each atom has a magnetic moment which is randomly oriented as a result of thermal agitation. • The magnetic field creates a slight alignment of these moments and hence a low magnetisation in the same direction as the applied field.
Ferromagnetism • Ferromagnetism is only possible when atoms are arranged in a lattice and the atomic magnetic moments can interact to align parallel to each other. • Only Fe, Co and Ni are ferromagnetic at and above room temperature
Antiferromagnetism • Antiferromagnetic materials are very similar to ferromagnetic materials but the exchange interaction between neighboring atoms leads to the anti-parallel alignment of the atomic magnetic moments.
Ferrimagnetism • Ferrimagnetism is only observed in compounds, which have more complex crystal structures than pure elements
Electromagnet • An electromagnet is a wire that has been coiled into one or more loops, known as a solenoid. • When electric current flows through the wire, a magnetic field is generated. • The more loops of wire, the greater the cross-section of each loop, and the greater the current passing through the wire, the stronger the field. • Uses for electromagnets include particle accelerators, electric motors, etc
The Orientation of Magnet • The orientation of this effective magnet is determined via the right hand rule.
Magnetic Phenomena • An electric current produces a magnetic field. • Some materials are easily magnetized when placed in a weak magnetic field. When the field is turned off, the material rapidly demagnetizes. These are called "Soft Magnetic Materials."
Magnetic Phenomena • In some magnetically soft materials the electrical resistance changes when the material is magnetized. The resistance goes back to its original value when the magnetizing field is turned off. This is called "Magneto-Resistance" or the MR Effect. • Certain other materials are magnetized with difficulty but once magnetized, they retain their magnetization when the field is turned off. These are called "Hard Magnetic Materials" or "Permanent Magnets."
HISTORY OF MAGNETIC RECORDERS • In 1888, Oberlin Smith originated the idea of using permanent magnetic impressions to record sounds. • In 1900, Vladeniar Poulsen demonstrated a Telegraphone. It was a device that recorded sounds onto a steel wire. • Although everyone thought it was a great idea, they didn't think it would succeed since you had to use an earphone to hear what was recorded.
HISTORY OF MAGNETIC RECORDERS • Until 1935, all magnetic recording was on steel wire. • Then, at the 1935 German Annual Radio Exposition in Berlin, Fritz Pfleumer demonstrated his Magnetophone. It used a cellulose acetate tape coated with soft iron powder. • The Magnetophone and its "paper" tapes were used until 1947 when the 3M Company introduced the first plastic-based magnetic tape.
HISTORY OF MAGNETIC RECORDERS • In 1956, IBM introduced the next major contribution to magnetic recording - the hard disk drive. The disk was a 24-inch solid metal platter and stored 4.4 megabytes of information. • Later, in 1963, IBM reduced the platter size and introduced a 14-inch hard disk drive.
HISTORY OF MAGNETIC RECORDERS • In 1971, 3M Company introduced the first 1/4-inch magnetic tape cartridge and tape drive. • In that same year, IBM invented the 8-inch floppy disk and disk drive. It used a flexible 8-inch platter of the same material as magnetic tape. • In 1980, a little-known company named Seagate Technology invented the 5-1/4-inch floppy disk drive.
PREREQUISITES FOR MAGNETIC RECORDING • Input Signal • Recording Medium • Magnetic Head
Input Signal • An input signal can come from a microphone, a radio receiver, electrical device, or any other source that's capable of producing a recordable signal. • Some input signals can be recorded immediately, but some must be processed first. • This processing is needed when an input signal is weak, or is out of the Frequency response range of the recorder.
Recording Medium • A recording medium is any material that has the ability to become magnetized, in varying amounts, in small sections along its entire length. • Some examples of this are magnetic tape and magnetic disks
Magnetic Heads • Magnetic heads are the transducers that convert the electrical input signal into the magnetic that are stored on a recording medium. • Magnetic heads do 3 different things. • Transfer signal onto the recording medium. • Recover signal from the recording medium. • Remove signal off the recording medium.
HDD Data Encode and Decode • Digital information is a stream of ones and zeros. • Hard disks store information in the form of magnetic pulses. • In order for the PC's data to be stored on the hard disk, therefore, it must be converted to magnetic information. • When it is read from the disk, it must be converted back to digital information.
HDD Data Encode and Decode • Magnetic information on the disk consists of a stream of very small magnetic fields. • Information is stored on the hard disk by encoding information into a series of magnetic fields. • This is done by placing the magnetic fields in one of two polarities: either N-S, or S-N
HDD Data Encode and Decode • Although it is conceptually simple to match "0 and 1" digital information to “N-S” and “S-N” magnetic fields. • The reality is much more complex: a 1-to-1 correspondence is not possible, and special techniques must be employed to ensure that the data is written and read correctly.
Technical Requirements • Fields vs. Reversals • Synchronization • Field Separation
Fields vs. Reversals • Read/write heads are designed not to measure the actual polarity of the magnetic fields, but rather flux reversals. • Flux reversals occur when the head moves from an area that has N-S polarity to S-N, or vice-versa.
Fields vs. Reversals • The reason the heads are designed based on flux reversals instead of absolute magnetic field, is that reversals are easier to measure. • The encoding of data must be done based on flux reversals, and not the contents of the individual fields.
Synchronization: • Another consideration in the encoding of data is the necessity of using some sort of method of indicating where one bit ends and another begins. • Even if we could use one polarity to represent a "one" and another to represent a "zero", what would happen if we needed to encode on the disk a stream of 1,000 consecutive zeros?
Field Separation • Although we can conceptually think of putting 1000 tiny N-S pole magnets in a row one after the other. They are additive. • Aligning 1000 small magnetic fields near each other would create one large magnetic field, 1000 times the size and strength of the individual components.
Data Encoding • We must encode using flux reversals, not absolute fields. • We must keep the number of consecutive fields of same polarity to a minimum. • To keep track of which bit is where, some sort of clock synchronization must be added to the encoding sequence.
Media Limitation • Each linear inch of space on a track can only store so many flux reversals. • We need to use some flux reversals to provide clock synchronization, these are not available for data. • A prime goal of data encoding methods is therefore to decrease the number of flux reversals used for clocking relative to the number used for real data.
Media Limitation • Over time, better methods that used fewer flux reversals to encode the same amount of information. • Hardware technology strives to allow more bits to be stored in the same area by allowing more flux reversals per linear inch of track. • Encoding methods strive to allow more bits to be stored by allowing more bits to be encoded (on average) per flux reversal.