Ferrite (magnet)
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[[Image:Ceramic magnets.jpg|thumb|A stack of ferrite magnets]]
'''Ferrites''' are a class of [[chemical compound]]s with the [[Chemical formula|formula]] AB<sub>2</sub>O<sub>4</sub>, where A and B represent various metal [[cation]]s, usually including iron. These [[ceramic]] materials are used in applications ranging from magnetic components in microelectronics.
Ferrites are a class of [[spinel]]s, materials that adopt a crystal motif consisting of cubic close-packed (FCC) oxides (O<sup>2-</sup>) with A cations occupying one eighth of the octahedral holes and B cations occupying half of the octahedral holes. The magnetic material known as "ZnFe" has the deceptively simple formula ZnFe<sub>2</sub>O<sub>4</sub>, with Fe<sup>3+</sup> occupying the octahedral sites and half of the tetrahedral sites. The remaining tetrahedral sites in this spinel are occupied by Zn<sup>2+</sup>.<ref>Shriver, D. F.; Atkins, P. W.; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A. “Inorganic Chemistry” W. H. Freeman, New York, 2006. ISBN: 0-7167-4878-9.</ref>
== Properties ==
Ferrites are usually non-conductive [[ferrimagnetism|ferrimagnetic]] ceramic compounds derived from [[iron oxide]]s such as [[Hematite|hematite]] (Fe<sub>2</sub>O<sub>3</sub>) or [[Magnetite|magnetite]] (Fe<sub>3</sub>O<sub>4</sub>) as well as [[oxides]] of other metals. Ferrites are, like most other ceramics, hard and [[brittle]]. In terms of the magnetic properties, ferrites are often classified as "soft" and "hard" which refers to their low or high [[coercivity]] of their magnetism, respectively.
===Soft ferrites===
Ferrites that are used in [[transformer]] or [[electromagnetic]] [[Magnetic core|cores]] contain [[nickel]], [[zinc]], or [[manganese]] compounds. They have a low [[coercivity]] and are called '''soft ferrites'''. Because of their comparatively low losses at high frequencies, they are extensively used in the cores of [[Switched-mode power supply|Switched-Mode Power Supply (SMPS)]] and [[Radio Frequency|RF]] transformers and [[inductor]]s. A common ferrite, abbreviated "MnZn," is composed of the oxides of manganese and zinc.
===Hard ferrites===
In contrast, permanent ferrite magnets (or "hard ferrites"), which have a high [[remanence]] after magnetization, are composed of iron and [[barium]] or [[strontium]] oxides. In a magnetically [[Saturation (magnetic)|saturated]] state they conduct [[magnetic flux]] well and have a high magnetic [[permeability (electromagnetism)|permeability]]. This enables these so-called ''ceramic magnets'' to store stronger [[magnetic field]]s than iron itself. They are the most commonly used magnets in radios. The maximum magnetic field ''B'' is about 0.35 [[tesla (unit)|tesla]] and the magnetic field strength ''H'' is about 30 to 160 kiloampere turns per meter (400 to 2000 [[oersted]]s). (Hill 2006)
== Production ==
Ferrites are produced by heating an intimate mixture of powdered precursors are heated and pressed in a mold. During the heating process, calcination of [[carbonate]]s occurs:
:MCO<sub>3</sub> → MO + CO<sub>2</sub>
The oxides of barium and strontium are typically supplied as their carbonates, [[Barium carbonate|BaCO<sub>3</sub>]] or [[Strontium carbonate|SrCO<sub>3</sub>]].
The resulting mixture of oxides undergoes [[Sintering|sinter]]ing. Afterwards the cooled product is milled to particles smaller than 2 μm in order to produce [[Weiss domains]] in the size of one particle. Next the powder is pressed into a shape, dried, and re-sintered. The shaping may be performed in an external magnetic field, in order to achieve a preferred orientation of the particles ([[anisotropy]]).
Small and geometrically easy shapes may be produced with dry pressing. However, in such a process small particles may agglomerate and lead to poorer magnetic properties compared to the wet pressing process. Direct calcination and sintering without re-milling is possible as well but leads to poor magnetic properties.
Electromagnets are pre-sintered as well (pre-reaction), milled and pressed. However, the sintering takes place in a specific atmosphere, for instance one with an [[oxygen]] shortage). The chemical composition and especially the structure vary strongly between the precursor and the sintered product.
==Uses==
[[ferrite bead|Ferrite cores]] are used in electronic [[inductor]]s, [[transformer]]s, and [[electromagnet]]s where the high [[electrical resistance]] of the ferrite leads to very low [[eddy current]] losses. They are commonly seen as a lump in a computer cable, called a [[ferrite bead]], which helps to prevent high frequency electrical noise ([[radio frequency interference]]) from exiting or entering the equipment.
Early [[computer memory|computer memories]] stored data in the residual magnetic fields of hard ferrite cores, which were assembled into arrays of ''[[core memory]]''. Ferrite powders are used in the coatings of [[magnetic tape|magnetic recording tapes]]. One such type of material is [[iron (III) oxide]].
Ferrite particles are also used as a component of radar-absorbing materials or coatings used in [[Stealth technology#Stealth principles|stealth]] aircraft and in the expensive absorption tiles lining the rooms used for [[electromagnetic compatibility]] measurements.
Most common radio magnets, including those used in loudspeakers, are ferrite magnets. Ferrite magnets have largely displaced [[Alnico]] magnets in these applications.
It is a common magnetic material for [[pickup (music)#Magnetic pickups|electromagnetic instrument pickups]], because of price and relatively high output. However, such pickups lack certain sonic qualities found in other pickups, such as those that use Alnico alloys or more sophisticated magnets.
==References==
<references/>
==External links==
* [http://computer.howstuffworks.com/question352.htm What are the bumps at the end of computer cables?]
==Manufacturers==
* [http://www.ti-electronic.com/ferrite/en TI-Electronic]
* [http://www.acme.com.my/ Acme Ferrite Products Malaysia]
* [http://www.acme-ferrite.com.tw/ Acme-Ferrite]
* [http://www.tdk.co.jp/ TDK]
* [http://sfil.exportersindia.com/ Strontium Ferriten India Limited]
==Sources==
* Meeldijk, Victor ''Electronic Components: Selection and Application Guidelines'', 1997 Wiley [ISBN 0-471-18972-3]
* Ott, Henry ''Noise Reduction Techniques in Electronic Systems'' 1988 Wiley [ISBN 0-471-85068-3]
* Luecke, Gerald and others ''General Radiotelephone Operator License Plus Radar Endorsement'' 2004, Master Pub. [ISBN 0-945053-14-2]
* Bartlett, Bruce and others ''Practical Recording Techniques'' 2005 Focal Press [ISBN 0-240-80685-9]
* Hill Technical Sales [http://www.hilltech.com/products/emc_components/Amorphous_Shielding.html]
* Schaller, George E. ''Ferrite Processing & Effects on Material Performance'' [http://www.cmi-ferrite.com/News/Papers/ferpro.pdf]
[[Category:Ceramic materials]]
[[Category:Types of magnets]]
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