magnetic recording

1. The recording of a signal, such as sound or computer instructions, in the form of a magnetic pattern on a magnetizable surface for storage and subsequent retrieval.
2. A surface containing a magnetic recording.

The technique of storing information as a magnetic pattern on a moving magnetic medium. The medium may be a disk, either flexible (floppy) or rigid, or a tape.

All materials respond in some way to an applied magnetic field, but the term “magnetic material” generally means one that maintains a magnetic polarization in the absence of an applied field. This remanence depends upon the magnetic field history to which the material was exposed. This history can be plotted on a graph of magnetization versus magnetic field, giving rise to hysteresis loops (Fig. 1). If the material is initially completely demagnetized (A in Fig. 1, sometimes referred to as the ac erase state), and the applied field is increased to some intermediate value and then reversed, a minor loop (BCDE) is obtained. At zero magnetic field there is a remanence. If the field is increased to the point that further increases in the field result in no further increases in magnetization, the material is said to be saturated. The remanence, at zero magnetic field, is a function of the maximum magnetic field to which the material has been exposed. Also, it takes a finite field in the reverse direction to drive the magnetization to zero. This field is called the coercivity of the material, and is an important parameter in magnetic recording. See also Magnetic materials; Magnetization.

Typical magnetization curves. At A, material is completely demagnetized. If the magnetic field is increased to B and then reversed, the minor loop BCDE is obtained. The inset shows remanence as a function of maximum field; points C and G correspond to the hysteresis loops shown.

The hysteretic behavior of magnetic materials, and in particular, the field dependence of the remanence, is the basis for recording sound. The basic idea is to use the electric current from a microphone to generate a magnetic field (Ampère's law) that magnetizes portions of a magnetic medium in proportion to this current. The resulting magnetic pattern along the medium can then be read back as a voltage induced in a pick-up coil (Faraday's induction law) as the fringing fields from the magnetic medium pass` by. See also Faraday's law of induction.

The writing field associated with a coil can be enhanced by filling the coil with a magnetic material. The reason is that the magnetic flux density generated by an electric current is proportional to the current through a constant of proportionality called the permeability. In free space, this quantity is usually denoted by μ₀. The permeability of a magnetic material, μ, however, is much larger. For example, the nickel-iron alloy permalloy has a relative permeability, μ/μ₀, of the order of 10,000. Therefore, the magnetic flux density inside a permalloy core would be 10,000 times that of an air core.

A high-permeability core also serves to confine the flux density. The field in a very narrow gap in the magnetic material can therefore be relatively large. Thus, recording generally employs an electromagnet with a narrow gap (Fig. 2). The writing and reading element is referred to as the head.

Writing and reading process. (a) Motion of the magnetic medium past the electromagnet in the form of a ring head. (b) Variation with time of input current and output voltage.

The relationship between the remanence and the field is very nonlinear (Fig. 1), which led to a good deal of distortion in early recorders. However, if an alternating current (ac) is added to the signal current, the resulting remanence, called the anhysteretic remanence, becomes linear at low fields. The amplitude of this bias must be sufficient to produce a field greater than the coercivity, and the bias frequency must be higher than the highest signal frequency. See also Sound recording.

To extend magnetic recording to video recording with signals as high as 5 MHz requires increasing the speed between the tape and the recording head. Video recording is based on an approach in which a number of heads are mounted on the face of a drum that rotates rapidly in a direction transverse to the direction of the tape motion.

Magnetic recording was applied to the storage of data in the early 1950s. Data generally means information represented in a digital form, that is, a sequence of 0's and 1's. In the first tape system for data storage, the data were recorded longitudinally along seven tracks. In a disk system, data are stored along concentric tracks. The figure of merit is the areal bit density, which is the product of the linear density along a track and the track density. See also Computer storage technology.

Early tapes and disks utilized particulate media in which the magnetic ingredient consisted of microscopic particles of a magnetic oxide. These particles were immersed in a polymeric binder that served to separate the particles from one another and bind them to the substrate. The particles first used were the gamma form of iron oxide. Higher levels of magnetization and coercivity can be obtained in particles made of the ferromagnetic elements, iron and cobalt, and their alloys than is possible with oxide particles. However, metal particles tend to corrode in the atmosphere and to react with binders and so must be passivated at some cost in saturation magnetization. The particles are also difficult to disperse and are much more expensive than particles of iron oxide. Metal particles having coercivities in the range 700–1150 Oe (56–92 kA/m) are used in premium audio tapes, and particles with coercivities of 1350–1550 Oe (107–123 kA/m) are used in 8-mm video tapes.

Metallic films are used on magnetic disks to reduce the thickness of the magnetic medium and retain a large magnetization. Among the magnetic elements (iron, cobalt, and nickel), cobalt has a hexagonal crystalline structure that leads to a large coercivity. Therefore, many metallic media consist of cobalt with additional elements to stabilize the hexagonal phase, and also, for longitudinal recording, to ensure that the hexagonal axis lies in the plane of the film.

Placing information (data, voice, video, etc.) on a ferromagnetic storage medium. See magnetic storage and ferromagnetic.

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