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Magnetic storage

Magnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using one or more read/write heads. As of 2009, magnetic storage media, primarily hard disks, are widely used to store computer data as well as audio and video signals. In the field of computing, the term magnetic storage is preferred and in the field of audio and video production, the term magnetic recording is more commonly used. The distinction is less technical and more a matter of preference.

Contents

History

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. Poulsen'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 the a material depends on applied fields. The magnetic materials are normally in the form a tape and the tape is initially demagnetized. When recording, the tape runs at a constant speed. The writing head magnetizes the tape with current that proportional to the signal. Then a magnetization distribution is achieved along the magnetic tape. Finally, the distribution of the magnetization can be read out and change back to original signal. The magnetic tape is typically made by embedding magnetic particles in a plastic binder on a polyester film tape. The commonly used magnetic particles are Iron oxide particles or Chromium oxide and metal particles with size of 0.5 micrometers[1]. Analog recording was very popular in audio and video recording. In the past 20 years,however, they were gradually replaced by digital recording[2].

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 by used to switch the magnetization. The reading process is based on magneto-optical Kerr effect. The magnetic medium 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 are usually in space and aeronautics.

Technical details

Access method

Magnetic storage media can be classified as either sequential access memory or random access memory although in some cases the distinction is not perfectly clear. In the case of magnetic wire, the read/write head only covers a very small part of the recording surface at any given time. Accessing different parts of the wire involves winding the wire forward or backward until the point of interest is found. The time to access this point depends on how far away it is from the starting point. The case of ferrite-core memory is the opposite. Every core location is immediately accessible at any given time.

Hard disks and modern linear serpentine tape drives do not precisely fit into either category. Both have many parallel tracks across the width of the media and the read/write heads take time to switch between tracks and to scan within tracks. Different spots on the storage media take different amounts of time to access. For a hard disk this time is typically less than 10 ms, but tapes might take as much as 100 s.

Current usage

As of 2008, common uses of magnetic storage media are for computer data mass storage on hard disks and the recording of analog audio and video works on analog tape. Since much of audio and video production is moving to digital systems, the usage of hard disks is expected to increase at the expense of analog tape. Digital tape and tape libraries are popular for the high capacity data storage of archives and backups. Floppy disks see some marginal usage, particularly in dealing with older computer systems and software. Magnetic storage is also widely used in some specific applications, such as bank checks (MICR) and credit/debit cards (mag stripes).

Future

A new type of magnetic storage, called MRAM, is being produced that stores data in magnetic bits based on the tunnel magnetoresistance (TMR) effect. Its advantage is non-volatility, low power usage, and good shock robustness. The 1st generation that was developed was produced by Everspin Technologies, and utilized field induced writing[3]. The 2nd generation is being develped through two approaches: Thermal Assisted Switching (TAS)[4] which is currently being developed by Crocus Technology, and Spin Torque Transfer (STT) on which Crocus, Hynix, IBM, and several other companies are working[5]. However, with storage density and capacity orders of magnitude smaller than an HDD, MRAM is useful in applications where moderate amounts of storage with a need for very frequent updates are required, which flash memory cannot support due to its limited write endurance.

Related

  • Data storage, a broader perspective
  • Magnetism, the phenomenon
  • Magnetization, the property of an object that is affected by magnetism
  • Magnetic tape sound recording
  • Disk storage, which can store data magnetically and by other means
  • Marvin Camras, another innovator in the field
  • MRAM, one of the newest types of magnetic storage

References

  1. ^ http://hyperphysics.phy-astr.gsu.edu/hbase/audio/tape2.html
  2. ^ E. du Trémolete de Lacheisserie, D. Gignoux, and M. Schlenker (editors), Magnetism: Fundamentals, Springer, 2005
  3. ^ http://www.everspin.com/technology.html
  4. ^ The Emergence of Practical MRAM http://www.crocus-technology.com/pdf/BH%20GSA%20Article.pdf
  5. ^ http://www.eetimes.com/news/latest/showArticle.jhtml?articleID=218000269

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