[ Team LiB ] Previous Section Next Section

Magnetic Storage

The most popular mass storage systems in modern computers are based on magnetism. Hard disks use magnetic storage, as do floppy disks and tape drives. Magnetic storage is one of the oldest storage mediums used by computers (only punch cards predate its application), and it ranks as one of the most reliable. It also yields systems with the lowest cost per byte of storage.

Magnetic systems are popular both because of the unique properties of magnetism and because of its heritage. It has been used so long that its operating principles are well understood. It gives engineers a head start when they look for ways to improve the technology—and improve it they have. In 20 years, the typical capacity of a hard disk system has grown by a factor of 10,000, while the disk mechanism itself has actually shrunk. Every time some new breakthrough technology threatens to beat the storage capabilities of magnetic media, engineers simply roll out a refinement or two that lets the old medium easily eclipse the new.

The original electronic mass storage system was magnetic tape—that thin strip of paper (in the United States) upon which a thin layer of refined rust had been glued. Later, the paper gave way to plastic, and the iron oxide coating gave way to a number of improved magnetic particle based on iron, chrome dioxide, and various mixtures of similar compounds.

The machine that recorded upon these rust-covered ribbons was the Magnetophon, the first practical tape recorder, created by the German division of the General Electric Company, Allgemeine Elektricitaets Gesellschaft (AEG) in 1934. Continually improved but essentially secret through the years of World War II, despite its use at German radio stations, the Magnetophon was the first device to record and play back sound indistinguishable from live performances. After its introduction to the United States (in a demonstration by John T. Mullin to the Institute of Radio Engineers in San Francisco on May 16, 1946), tape recording quickly became the premiere recording medium and within a decade gained the ability to record video and digital data. Today, both data cassettes and streaming tape systems are based on the direct offspring of the first Magnetophon.

The principle is simple. Some materials become magnetized under the influence of a magnetic field. Once the material becomes magnetized, it retains its magnetic field. The magnetic field turns a suitable mixture or compound based on one of the magnetic materials into a permanent magnet with its own magnetic field. A galvanometer or similar device can later detect the resulting magnetic field and determine that the material has been magnetized. The magnet material remembers.


Key to the memory of magnetism is permanence. Magnetic fields have the wonderful property of being static and semi-permanent. On their own, they don't move or change. The electricity used by electronic circuits is just the opposite. It is constantly on the go and seeks to dissipate itself as quickly as possible. The difference is fundamental. Magnetic fields are set up by the spins of atoms physically locked in place. Electric charges are carried by mobile particles—mostly electrons—that not only refuse to stay in place but also are individually resistant to predictions of where they are or where they are going.

Given the right force in the right amount, however, magnetic spins can be upset, twisted from one orientation to another. Because magnetic fields are amenable to change rather than being entirely permanent, magnetism is useful for data storage. After all, if a magnetic field were permanent and unchangeable, it would present no means of recording information. If it couldn't be changed, nothing about it could be altered to reflect the addition of information.

At the elemental particle level, magnetic spins are eternal, but taken collectively, they can be made to come and go. A single spin can be oriented in only one direction, but in virtually any direction. If two adjacent particles spin in opposite directions, they cancel one another out when viewed from a larger, macroscopic perspective.

Altering those spin orientations takes a force of some kind, and that's the key to making magnetic storage work. That force can make an alteration to a magnetic field, and after the field has changed, it will keep its new state until some other force acts upon it.

The force that most readily changes one magnetic field is another magnetic field. (Yes, some permanent magnets can be demagnetized just by heating them sufficiently, but the demagnetization is actually an effect of the interaction of the many minute magnetic fields of the magnetic material.)

Despite their different behavior in electronics and storage systems, magnetism and electricity are manifestations of the same underlying elemental force. Both are electromagnetic phenomena. One result of that commonalty makes magnetic storage particularly desirable to electronics designers—magnetic fields can be created by the flow of electrical energy. Consequently, evanescent electricity can be used to create and alter semi-permanent magnetic fields.

When set up, magnetic fields are essentially self-sustaining. They require no energy to maintain, because they are fundamentally a characteristic displayed by the minute particles that make up the entire universe (at least according to current physical theories). On the submicroscopic scale of elemental particles, the spins that form magnetic fields are, for the most part, unchangeable and unchanging. Nothing is normally subtracted from them—they don't give up energy, even when they are put to work. They can affect other electromagnetic phenomena (for example, they can be used in mass to divert the flow of electricity). In such a case, however, all the energy in the system comes from the electrical flow—the magnetism is a gate, but the cattle that escape from the corral are solely electrons.

The magnetic fields that are useful in storage systems are those large enough to measure and effect changes on things that we can see. This magnetism is the macroscopic result of the sum of many microscopic magnetic fields, many elemental spins. Magnetism is a characteristic of submicroscopic particles. (Strictly speaking, in modern science magnetism is made from particles itself, but we don't have to be quite so particular for the purpose of understanding magnetic computer storage.)

Magnetic Materials

Three chemical elements are magnetic—iron, nickel, and cobalt. The macroscopic strength as well as other properties of these magnetic materials can be improved by alloying them, together and with nonmagnetic materials, particularly rare earths such as samarium.

Many particles at the molecular level have their own intrinsic magnetic fields. At the observable (macroscopic) level, they do not behave like magnets because their constituent particles are organized—or disorganized—randomly so that in bulk, the cumulative effects of all their magnetic fields tend to cancel out. In contrast, the majority of the minute magnetic particles of a permanent magnet are oriented in the same direction. The majority prevails, and the material has a net magnetic field.

Some materials can be magnetized. That is, their constituent microscopic magnetic fields can be realigned so that they reveal a net macroscopic magnetic field. For instance, by subjecting a piece of soft iron to a strong magnetic field, the iron will become magnetized.

Magnetic Storage

If that strong magnetic field is produced by an electromagnet, all the constituents of a magnetic storage system become available. Electrical energy can be use to alter a magnetic field, which can be later detected. Put a lump of soft iron within the confines of an electromagnet that has not been energized. Any time you return, you can determine whether the electromagnet has been energized in your absence by checking for the presence of a magnetic field in the iron. In effect, you have stored exactly one bit of information.

To store more, you need to be able to organize the information. You need to know the order of the bits. In magnetic storage systems, information is arranged physically by the way data travels serially in time. Instead of being electronic blips that flicker on and off as the milliseconds tick off, magnetic pulses are stored like a row of dots on a piece of paper—a long chain with a beginning and end. This physical arrangement can be directly translated to the temporal arrangement of data used in a serial transmission system, just by scanning the dots across the paper. The first dot becomes the first pulse in the serial stream, and each subsequent dot follows neatly in the data stream as the paper is scanned.

Instead of paper, magnetic storage systems use one or another form of media—generally a disk or long ribbon of plastic tape—covered with a magnetically reactive mixture. The form of medium directly influences the speed at which information can be retrieved from the system.

In operation, the tape moves from left to right past a stationary read/write head. When a current is passed through an electromagnetic coil in this head, it creates the magnetic field needed to write data onto the tape. When the tape is later passed in front of this head, the moving magnetic field generated by the magnetized particles on the tape induces a minuscule current in the head. This current is then amplified and converted into digital data. The write current used in putting data on the tape overpowers whatever fields already exist on the tape, both erasing them and imposing a new magnetic orientation to the particles representing the information to be recorded.

No matter whether it's tape or disk, when a magnetic storage medium is blank from the factory, it contains no information. The various magnetic domains on it are randomly oriented. Recording on the medium reorients the magnetic domains into a pattern that represents the stored information, as shown in Figure 15.1.

Figure 15.1. Orientation of magnetic domains in blank and recorded media.


After you record on a magnetic medium, you can erase it by overwriting it with a strong magnetic field. In practice, you cannot reproduce the true random orientation of magnetic domains of the unused medium. However, by recording a patter with a frequency out of the range of the reading or playback system—a very high or low frequency—you can obscure previously recorded data and make the medium act as if it were blank.

Digital Magnetic Systems

Computer mass storage systems differ in principle and operation from tape systems used for audio and video recording. Whereas audio and video cassettes record analog signals on tape, computers use digital signals.

In the next few years, this situation will likely change as digital audio and video tape recorders become increasingly available. Eventually, the analog audio and video tape will become historical footnotes, much as the analog vinyl phonograph record was replaced by the all-digital compact disc.

In analog systems, the strength of the magnetic field written on a tape varies in correspondence with the signal being recorded. The intensity of the recorded field can span a range of more than six orders of magnitude. Digital systems generally use a code that relies on patterns of pulses, and all the pulses have exactly the same intensity.

The technological shift from analog to digital is rooted in some of the characteristics of digital storage that make it the top choice where accuracy is concerned. Digital storage resists the intrusion of noise that inevitably pollutes and degrades analog storage. Every time a copy is made of an analog recording, the noise that accompanies the desired signal essentially doubles because the background noise of the original source is added to the background noise of the new recording medium; however, the desired signal does not change. This addition of noise is necessary to preserve the nuances of the analog recording—every twitch in the analog signal adds information to the whole. The analog system cannot distinguish between noise and nuance.

In digital recording, however, there's a sharp line between noise and signal. Noise below the digital threshold can be ignored without losing the nuances of the signal. Consequently, a digital recording system can eliminate the noise built up in making copies. Moreover, noise can creep into analog recordings as the storage medium deteriorates, whereas the digital system can ignore most of the noise added by age. In fact, properly designed digital systems can even correct minor errors that get added to their signals.


Digital recordings avoid noise because they ignore all strength variations of the magnetic field except the most dramatic. They just look for the unambiguous "it's either there or not" style of digital pulses of information. Analog systems achieve their varying strengths of field by aligning the tiny molecular magnets in the medium. A stronger electromagnetic field causes a greater percentage of the fields of these molecules to line up with the field, almost in direct proportion to the field strength, to produce an analog recording. Because digital systems need not worry about intermediate levels of signal, they can lay down the strongest possible field that the tape can hold. This level of signal is called saturation because much as a saturated sponge can suck up no more water, the particles on the tape cannot produce a stronger magnetic field.

Although going from no magnetic field to a saturated field would seem to be the widest discrepancy possible in magnetic recording—and therefore the least ambiguous and most suitable for digital information—this contrast is not the greatest possible or easy to achieve. Magnetic systems attempt to store information as densely as possible, trying to cram the information in so that every magnetic particle holds one data bit. Magnetic particles are extremely difficult to demagnetize, but the polarity of their magnetic orientation is relatively easy to change. Digital magnetic systems exploit this capability to change polarity and record data as shifts between the orientations of the magnetic fields of the particles on the tape. The difference between the tape being saturated with a field in one direction and the tape being saturated with a field in the opposite direction is the greatest contrast possible in a magnetic system and is exploited by nearly all of today's digital magnetic storage systems.


One word that you may encounter in the description of a magnetic medium is coercivity, a term that describes how strongly a magnetic field resists change, which translates into how strong of a magnetic field a particular medium can store. Stronger stored fields are better because the more intense field stands out better against the random background noise that is present in any storage medium. Because a higher coercivity medium resists change better than a low coercivity material, it also is less likely to change or degrade because of the effects of external influences. Of course, a higher coercivity and its greater resistance to change means that a recording system requires a more powerful magnetic field to maximally magnetize the medium. Equipment must be particularly designed to take advantage of high-coercivity materials.

With hard disks, which characteristically mate the medium with the mechanisms for life, matching the coercivity of a medium with the recording equipment is permanently handled by the manufacturer. The two are matched permanently when a drive is made. Removable media devices—floppy disks, tape cartridges, cassettes, and so on—pose more of a problem. If media are interchangeable and have different coercivities, you face the possibility of using the wrong media in a particular drive. Such problems often occur with floppy disks, particularly when you want to skimp and use cheaper double-density media in high-density or extra-density drives.

Moreover, the need for matching drive and medium makes upgrading a less-than-simple matter. Obtaining optimum performance requires that changes in media be matched by hardware upgrades. Even when better media are developed, they may not deliver better results with existing equipment.

The unit of measurement for coercivity is the Oersted. As storage media have been miniaturized, the coercivity of the magnetic materials as measured in Oersteds has generally increased. The greater intrinsic field strength makes up for the smaller area upon which data is recorded. With higher coercivities, more information can be squeezed into the tighter confines of the newer storage formats. For example, old 5.25-inch floppy disks had a coercivity of 300 Oersteds. Today's high-density 3.5-inch floppies have coercivities of 750 Oersteds. Similarly, the coercivities of the tapes used in today's high-capacity quarter-inch cartridges are greater than those of the last generation. Older standards used 550 Oersted media; data cartridges with capacities in excess of 1.5GB and minicartridges with capacities beyond 128MB require 900-Oersted tape. Although invisible to you, the coercivities of tiny modern hard disk drives are much higher than big old drives.

Coercivity is a temperature-dependent property. As the temperature of a medium increases, its resistance to magnetic change declines. That's one reason you can demagnetize an otherwise permanent magnet by heating it red hot. Magnetic media dramatically shift from being unchangeable to changeable—meaning a drop in coercivity—at a material-dependent temperature called the Curie temperature. Magneto-optical recording systems take advantage of this coercivity shift by using a laser beam to heat a small area of magnetic medium that is under the influence of a magnetic field otherwise not strong enough to affect the medium.

At room temperature, the media used by magneto-optical systems have coercivities on the order of 6000 Oersteds; when heated by a laser, that coercivity falls to a few hundred Oersteds. Because of this dramatic change in coercivity, the magnetic field applied to the magneto-optical medium changes only the area heated by the laser above its Curie temperature (rather than the whole area under the magnetic influence). Because a laser can be tightly focused to a much smaller spot than is possible with traditional disk read/write heads, using such a laser-boosted system allows data to be defined by tinier areas of a recording medium. A disk of a given size thus can store more data when its magnetic storage is optically assisted. Such media are resistant to the effects of stray magnetic fields (which may change low coercivity fields) as long as they are kept at room temperature.


Another term that appears in the descriptions of magnetic media is retentivity, which measures how well a particular medium retains or remembers the field that it is subjected to. Although magnetic media are sometimes depended upon to last forever—think of the master tapes of phonograph records—the stored magnetic fields begin to degrade as soon as they have been recorded. A higher retentivity ensures a longer life for the signals recorded on the medium.

No practical magnetic material has perfect retentivity, however; the random element of modern physical theories ensures that. Even the best hard disks slowly deteriorate with age, showing an increasing number of errors as time passes after data has been written. To avoid such deterioration of so-called permanent records, many computer professionals believe that magnetically stored recordings should be periodically refreshed. For example, they exercise tapes stored in mainframe computer libraries periodically (in intervals from several months to several years, depending on the personal philosophy and paranoia of the person managing the storage). Although noticeable degradation may require several years (perhaps a decade or more), these tape caretakers do not want to stake their data—and their jobs—on media written long ago.

Disk-makers view such precautions as verging on paranoia. The magnetic medium used in modern disk drives is nothing like that in the old, self-erasing tapes. Their plated media (see Chapter 17, "Magnetic Storage") have much higher retentivities. Moreover, new technologies such as S.M.A.R.T. let drives detect any degradation in the magnetic medium and warn you before you stand a chance of losing your data.

Old floppy disks and backup tapes are another matter. They are likely to deteriorate with time, enough so that the data on decade-old disks and tapes may be at risk. If you have old records on old media, for safety's sake you should consider copying the records to fresher media or a newer technology, such as an optical disc.

    [ Team LiB ] Previous Section Next Section