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• those used in magnetic heads for recording (writing), playing (reading) and erasing information.
• those used in magnetic storage devices for storage either permanently or temporarily.
  The magnetic media can be flexible (audio and video cassettes and floppy disks) or rigid (hard disk of computer).

RECORDING

The basic principle of recording in all the devices is same. For example, consider magnetic recording on an audio tape which is infact simply a polymer backing tape with magnetic material on it. The information to be stored is converted into a current signal i(t) and passed through the toroid wrapped on a core as shown in fig.1.
FIG.1

This assembly of toroid type electromagnet is known as the recording head. The thin air gap in this electromagnet is of the dimension around 1mm and is responsible for the recording. When current i(t) passes through the toroid, it produces magnetic field in the core. As the core is of high permeability magnetic field is confided in the core. However in the air gap it fringes outwards. The recording in the magnetic tape is due to this fringing magnetic field. As the fringing magnetic field depends upon i(t), hence the magnetic field stored in the region of tape directly under the head depends upon i(t). By moving the tape at speed synchronized with the electric input signal the input is recorded as spatial magnetic pattern in tape. The fringing fields of recording head modulate magnetization in the tape in the direction of motion, put differentially along the length of the tape. This type of magnetic information storage is called longitudinal recording.

PLAYING

For reading the stored data, the tape is passed through a second head called play head. It converts the spatial variations in magnetization of tape into a voltage signal which is amplified and conditioned for playback. When the tape passes under the play head the portion of tape directly under the air gap of play head core produces magnetic field in its vicinity and due to high permeability of core material most of the magnetic field is confided in it. As the tape moves, the magnetic field linked with the core varies changing flux, thus generating voltage v(t) which is a direct consequence of stored magnetic field in tape and hence of current i(t). In most cases the same head is used for recording and reading.

Suppose input signal has frequency f and speed of tape is u. Then the magnetic pattern repeats after 1/f seconds and in this interval tape advances by D x=u/f. Hence D x is a spatial wavelength l that indicates the repetition of spatial magnetic pattern representing information. Hence the smaller l (or greater f) allows greater storage of information. Typical video tapes have l in the submicron range. The actual recording in video tape is far more complex involving helical motion of head across film which increases tape speed and hence induced voltage.

The recording of digital information is very simple as it involves binary system and head reads or writes as presence or absence of magnetization on tape. For analog recording, the signal is imposed on an a.c. bias signal. However analog recording can be saved as digital one using encoding procedure.

STORAGE MATERIALS

RECORDING HEAD MATERIAL

The material of recording head must be magnetically soft so that it can be easily magnetized (by input current i(t) or by magnetic field intensity H). Also it must produce strong fringing field in the air gap so that the tape may be magnetized. For this it must have large saturation magnetization and small coercivity i.e. it should be soft magnetic material having as large relative permeability as possible. The material which satisfy these properties and are generally used are

• Sendust (Fe-Al-Si alloy)
• Permalloys (Ni-Fe alloy)
• Sintered soft ferrites (e.g. MnZn and NiZn ferrites)
• Amorphous metals (such as CoZrNb alloys)

Laminated sheets of the above materials with thin insulation between them form the core. This method of assembly reduces eddy current losses. For high frequency recording ferrites are preferred as they are insulators and suffer no eddy current loss. However ferrites have one great disadvantage that magnetization in air gap is not proportional to input current. To overcome this problem the pole is covered with high magnetization alloy e.g. Sendust or CuZrNb. This type of head is called metal-in-gap (MIG) head. More recently recording head devices have been made using thin film deposition techniques. This reduces eddy current losses at high frequencies

The magnetic core is in the form of a thin film of thickness in microns and width equal to that of tape. The air gap has width equal to tape but spacing is of sub micron range. A spiral type coil is made by depositing nonmagnetic metal thin film which threads the core. If the core is a metallic material then the coil has to be insulated from the core.

MAGNETIC STORAGE MEDIA MATERIALS

The most important property required for magnetic storage is that the material should be able to retain the spatial magnetization patterns stored in it by the recording head. This requires high remanent magnetization which is also essential for the production of magnetic flux that induces voltage in reading head. Also the storage material must have high coercivity so that it should be difficult to erase magnetic information from it under stray fields. On the other hand if coercivity is too high it would be impossible to record magnetic data on it. Therefore a balance has to be achieved.

For typical storage media (e.g. audio and video tapes) a flexible polymeric sheet coated with magnetic storage media is used. Typical materials used for coating are gaama -Fe₂O₃ , Co-modified gamma -Fe₂O₃ or Co(gamma Fe₂0₃), Cr0₂ and metallic particles (Fe). Elongated particles of magnetic materials are magnetically hard due to the fact that they tend to be single domains and due to the shape anisotropy. They prefer to be magnetized along their lengths. The properties of magnetic coating not only depend upon the nature of magnetic material used for coating but also on the concentration as well as distribution of coating. An optimum concentration and distribution are required for a balance in the magnetic properties.

Another form of magnetic storage is in the form of magnetic thin films deposited onto various hard substrates. Deposition of films can be done by vacuum evaporation of magnetic material using electron beam to heat it or by electroplating.

STORAGE DEVICES

FLOPPY DISK

The term "floppy disk" is an exact descriptive term. Inside its protective shell, the floppy disk medium is floppy i.e. it is a wide, flat and flexible disk. The disks are stamped out from wide rolls of magnetic medium. The material of these wide rolls is same as that of a recording tape i.e. a plastic substrate on which a magnetic oxide is deposited. However unlike a tape, floppy disks are coated with magnetic material on both sides. Even in case of single sided disks both sides are coated but only one of the sides is tested and verified. Different magnetic media determine the magnetic volume of the disk. High density disks use magnetic media with finer grain allowing more disk space. Floppy disks have been available in three sizes:

• 8-inch
• 5 1/4-inch
• 3 1/2 inch

8-INCH FLOPPY DISK

8-inch floppy was introduced in 1971 but it has become obsolete now.

5 1/4-INCH FLOPPY DISK

The 5 1/4-inch floppy disk consists of an outer shell of tough plastic welded together. Inside the shell is a layer of non-woven cloth, the liner which reduces friction of disk against shell during spinning. The large hole in the center of the shell allows drive hub of the disk drive to fit through and spin the disk without slippage. The hub is shaped like a cone which forces the disk to clamp itself into proper position. This portion of disk is most liable to wear and tear. To reduce this wear and tear many disks are equipped with protective hub rings which reinforce this vital center area of disk. A smaller hole in the floppy disk not far from hub hole is called index hole which allows mechanical indexing of the disk. A large oval hole or slot in both sides of disk shell is called head access aperture. It allows head of drive to contact disk surface.

3 1/2-INCH FLOPPY DISK

3 1/2-inch floppy disk has greater magnetic space as compared to 5 1/4-inch floppy disk. Also it has tougher shell which gives greater protection. An additional feature is the insertion key which prevents improper insertion. Otherwise it’s basic structure is quite similar to the 5 1/4-inch floppy disk.

HARD DISK

Hard disk differs from floppy disk as it uses hard or rigid substrate for its magnetic medium. The hard disk is actually a combination device, part electronic and part mechanical. Electrically the hard disk performs the function of converting electronic digital data into permanent magnetic fields. Mechanically it arranges magnetic storage and locates information stored on the disk.

The basic elements of a hard disk include a stack of one or more platters, that are covered with magnetic medium on which data can be stored. Together the platters rotate as a unit on a shaft called the spindle. Generally the shaft connects directly to a spindle motor that spins the entire assembly (at about 3600 r.p.m.). Most hard disks use servo-controlled spindle motors which constantly monitor their own speed using optical or magnetic sensors and automatically compensate for any variation. The storage capacity of a hard disk is determined by the number of platters that the drive uses. Greater the number of platters, greater the surface area available and greater will be its storage capability. Typically the platters consist of an Aluminium alloy precisely machined to extreme tolerance. The Al-alloy serves as substrate to which magnetic media is affixed either with a binder or mechanically. Initially oxide media was used in hard disks but due to its uneven surface roughness which reduces maximum storage density, it has become obsolete. Nowadays thin-film media is used which consists of a microscopically skinny layer of pure metal or mixture of metals mechanically bound to platter’s surface. Thin films can be applied either by plating or by sputtering (vapor plating in vacuum). The thinness of film media allows greater capacity of hard disk and it’s hardness reduces head crashes (wear of medium when head passes over it). If head was stationary it would only be able to utilize a narrow section of the disk. Hence a mechanism called head actuator is used that moves the head assembly so that the complete recordable area is utilized. Usually head assembly is pivoted and swung across the disk by a special head actuator solenoid or motor. Precision of modern head actuators helps increase the capacity of hard disks. The head actuators are basically of two types:

• Open-loop actuators (which do not get direct feed back)
• Closed-loop actuators (which get direct feed back)

TAPES

A tape consists of two basic layers, the backing and the coating. The backing provides support strength to hold the tape together. Nowadays polyester of double-knit leisure suit fame is used as backing. The coating is done by doping backing by magnetic oxide, by coating of pure metal particles in binders or by vapor plating metal films.

OPEN-REEL TAPE

In the beginning open-reel tapes were used. It was termed as open-reel tape as it is not kept inside protective shell. Although when first made it was a great step forward but now it is rarely used due to its small storage capacity and large size which leads to greater expenses. It’s one advantage is its greater reliability.

CASSETTE TAPE

The basic cassette mechanism consists of two spools of open-reel tape put inside a plastic shell. It’s main advantage over open-reel tape is its smaller size and easy handling. The shell protects the tape which is attached to both the spools. The sides of cassette shell holds the tape so that it does not pop out. Two Teflon slip sheets (one on each side of the tape) help eliminate the friction of tape against the shell. A clear window on either side of the tape let’s us look at how much tape is on either spool. The reels inside the cassette tapes are just hubs that the tape can wrap around. A small clip that forms part of perimeter of hub holds the end of the tape to the hub. At various points around the inside of the shell guides are provided to assure the tape travels in correct paths.

CARTRIDGE TAPE

In cartridge tapes a thin isoelastic belt stretches throughout the cartridge mechanism looping around and making contact with the supply and spools on their outer perimeter. The belt also passes around a rubber drive wheel which contacts a capstan in the tape drive . The capstan keeps the belt from touching the tape. The friction and hence wear and tear of the tape is reduced. For sturdiness the cartridge is built around an Aluminium baseplate. The cover of cartridge is transparent plastic which allows the mechanism to be readily observed.