Magnetic storage devices

        "Magnetic storage" refers to systems that keep information or data on a magnetized material. The concept of using a magnetic medium to save and retrieve information dates back to 1888, when Oberlin Smith invented a way to store sound on a wire. In 1900, Valdemar Poulsen developed a wire recorder that used a drum around which a magnetized wire was wrapped. This led to the invention of the first magnetic tape recorder by Fritz Pfleumer in 1928.


        Magnetic storage in the form of audio recording on a wire was publicized by Oberlin Smith in 1888. He filed a patent in September, 1878 but did not pursue the idea as his business was machine tools. The first publicly demonstrated (Paris Exposition of 1900) magnetic recorder was invented by Valdemar Poulsen in 1898. Paulsen's device recorded a signal on a wire wrapped around a drum. In 1928, Fritz Pfleumer developed the first magnetic tape recorder. Early magnetic storage devices were designed to record analog audio signals. Computer and now most audio and video magnetic storage devices record digital data.

        In old computers, magnetic storage was also used for primary storage in a form of magnetic drum, or core memory, core rope memory, thin film memory, twistor memory or bubble memory. Unlike modern computers, magnetic tape was also often used for secondary storage.

Magnetic recording classes:

Analog recording:

        Analog recording is based on the fact that remnant magnetization of a given material depends on the magnitude of the applied field. The magnetic material is normally in the form of tape, with the tape in its blank form being initially demagnetized. When recording, the tape runs at a constant speed. The writing head magnetizes the tape with current proportional to the signal. A magnetization distribution is achieved along the magnetic tape. Finally, the distribution of the magnetization can be read out, reproducing the original signal. The magnetic tape is typically made by embedding magnetic particles in a plastic binder on polyester film tape. The commonly used magnetic particles are Iron oxide particles or Chromium oxide and metal particles with size of 0.5 micrometer. Analog recording was very popular in audio and video recording. In the past 20 years, however, tape recording has been gradually replaced by digital recording.

Analog recording

Digital recording:

        Instead of creating a magnetization distribution in analog recording, digital recording only need two stable magnetic states, which are the +Ms and -Ms on the hysteresis loop. Examples of digital recording are floppy disks and HDDs. Since digital recording is the main process nowadays and probably in the coming future, the details of magnetic recording will be discussed in the rest of the project using the HDD as an example.

Magneto-optical recording

        Magneto-optical recording writes/reads optically. When writing, the magnetic medium is heated locally by a laser, which induces a rapid decrease of coercive field. Then, a small magnetic field can be used to switch the magnetization. The reading process is based on magneto-optical Kerr effect. The magnetic mediums are typically amorphous R-FeCo thin film (R being a rare earth element). Magneto-optical recording is not very popular. One famous example is Minidisc developed by Sony.

Domain propagation memory

Domain propagation memory is also called bubble

memory. The basic idea is to control domain wall motion in a magnetic medium that free of microstructure. Bubble refers to stable cylindrical domain. The information is then recorded by the presence/absence of bubble domain. Domain propagation memory has high insensitivity to shock and vibration, so its application is usually in space and aeronautics.

Magnetic disk:

Magnetic disks or cartridges have a few things in common:

  • They use a thin plastic or metal base material coated with iron oxide.
  • They can record information instantly.
  • They can be erased and reused many times.
  • They are reasonably inexpensive and easy to use.

If you have ever used an audio cassette, you know that it has one big disadvantage -- it is a sequential device. The tape has a beginning and an end, and to move the tape to later song you have to use the fast forward and rewind buttons to find the start of the song. This is because the tape heads are stationary.

A disk or cartridge, like a cassette tape, is made from a thin piece of plastic coated with magnetic material on both sides. However, it is shaped like a disk rather than a long, thin ribbon. The tracks are arranged in concentric rings so the software can jump from "file 1" to "file 19" without having to fast forward through files 2 through 18. The disk or cartridge spins like a record and the heads move to the correct track, providing what is known as direct-access storage. Some removable devices actually have a platter of magnetic disks, similar to the set-up in a hard drive. Tape is still used for some long-term storage, such as backing up a server's hard drive, in which quick access to the data is not essential.

The read/write heads ("writing" is saving new information to the storage media) do not touch the media when the heads are traveling between tracks. There is normally some type of mechanism that you can set to protect a disk or cartridge from being written to. For example, electronic optics check for the presence of an opening in the lower corner of a 3.5-inch diskette (or a notch in the side of a 5.25-inch diskette) to see if the user wants to prevent data from being written to it.

Zip drive:

The Zip drive is a medium-capacity removable disk storage system, introduced by Iomega in late 1994.

The format became the most popular of the super-floppy type products but never reached the status of a quasi-standard to replace the 3.5-inch floppy disk. Later, rewritable CDs largely replaced Zip drives, and the internal and external CD writers known as Zip-650 or Zip-CD were sold under the Zip brand.A Zip drive is a small, portable disk drive used primarily for backing up and archiving personal computer files. The trademarked Zip drive was developed and is sold by Iomega Corporation. Zip drives and disks come in two sizes. The 100 megabyte size actually holds 100,431,872 bytes of data or the equivalent of 70 floppy diskettes. There is also a 250 megabyte drive and disk. The Iomega Zip drive comes with a software utility that lets you copy the entire contents of your hard drive to one or more Zip disks.

Magnetic stripe card:

        In most magnetic stripe cards, the magnetic stripe is contained in a plastic-like film. The magnetic stripe is located 0.223 inches (5.56 mm) from the edge of the card, and is 0.375 inches (9.52 mm) wide. The magnetic stripe contains three tracks, each 0.110 inches (2.79 mm) wide. Tracks one and three are typically recorded at 210 bits per inch (8.27 bits per mm), while track two typically has a recording density of 75 bits per inch (2.95 bits per mm). Each track can either contain 7-bit alphanumeric characters, or 5-bit numeric characters. Track 1 standards were created by the airlines industry (IATA). Track 2 standards were created by the banking industry (ABA). Track 3 standards were created by the Thrift-Savings industry.

Magnetic strip card

History of magnetic strip card:

        The process of attaching a magnetic stripe to a plastic card was invented by IBM under a contract with the US government for a security system. Forrest Parry, an IBM Engineer, had the idea of securing a piece of magnetic tape, the predominant storage medium at the time, to a plastic card base. He became frustrated because every adhesive he tried produced unacceptable results. The tape strip either warped or its characteristics were affected by the adhesive, rendering the tape strip unusable. After a frustrating day in the laboratory, trying to get the right adhesive, he came home with several pieces of magnetic tape and several plastic cards. As he walked in the door at home, his wife was ironing and watching TV. She immediately saw the frustration on his face and asked what was wrong. He explained the source of his frustration: inability to get the tape to "stick" to the plastic in a way that would work. She said, "Here, let me try the iron." She did and the problem was solved. The heat of the iron was just high enough to bond the tape to the card.


  1. Article on Forrest Parry, pages 3-4.
  4. E. du Trmolete de Lacheisserie, D. Gignoux, and M. Schlenker (editors), Magnetism: Fundamentals, Springer, 2005

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