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A ferrite is a
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
material made by mixing and firing large proportions of iron(III) oxide (Fe2O3,
rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
) blended with small proportions of one or more additional
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
lic elements, such as strontium, barium,
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
,
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
, and
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
. They are
ferrimagnetic A ferrimagnetic material is a material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism, but these moments are unequal in magnitude so a spontaneous magnetization remains. This can for example occur when ...
, meaning they can be
magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nicke ...
ized or attracted to a magnet. Unlike other ferromagnetic materials, most ferrites are not electrically
conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gene ...
, making them useful in applications like magnetic cores for
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s to suppress
eddy current Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnet ...
s. Ferrites can be divided into two families based on their resistance to being demagnetized (magnetic
coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
). Hard ferrites have high
coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
, so are difficult to demagnetize. They are used to make permanent
magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nicke ...
s for applications such as
refrigerator magnet A refrigerator magnet or fridge magnet is a small magnet, often attached to an artistic or whimsical ornament, which may be used to post items such as shopping lists, Christmas cards, child art or reminders on a refrigerator door, or which sim ...
s,
loudspeaker A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or ...
s, and small
electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate for ...
s. Soft ferrites have low
coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
, so they easily change their magnetization and act as conductors of magnetic fields. They are used in the electronics industry to make efficient magnetic cores called
ferrite core In electronics, a ferrite core is a type of magnetic core made of ferrite on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability couple ...
s for high-frequency
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s,
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s and antennas, and in various
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
components. Ferrite compounds are extremely low cost, being made of mostly iron oxide, and have excellent corrosion resistance. Yogoro Kato and Takeshi Takei of the
Tokyo Institute of Technology is a national research university located in Greater Tokyo Area, Japan. Tokyo Tech is the largest institution for higher education in Japan dedicated to science and technology, one of first five Designated National University and selected as ...
synthesized the first ferrite compounds in 1930.


Composition, structure, and properties

Ferrites are usually
ferrimagnetic A ferrimagnetic material is a material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism, but these moments are unequal in magnitude so a spontaneous magnetization remains. This can for example occur when ...
ceramic compounds derived from iron oxides.
Magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
(Fe3O4) is a famous example. Like most of the other ceramics, ferrites are hard,
brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Br ...
, and poor conductors of electricity. Many ferrites adopt the spinel structure with the formula AB2O4, where A and B represent various metal cations, usually including iron (Fe). Spinel ferrites usually adopt a crystal motif consisting of cubic close-packed (fcc) oxides (O2−) with A cations occupying one eighth of the tetrahedral holes and B cations occupying half of the octahedral holes, i.e., . Ferrite crystals do not adopt the ordinary spinel structure, but rather the inverse spinel structure: One eighth of the tetrahedral holes are occupied by B cations, one fourth of the octahedral sites are occupied by A cations. and the other one fourth by B cation. It is also possible to have mixed structure spinel ferrites with formula 2+1−δFe3+δM2+δFe3+2−δ]O4 where δ is the degree of inversion. The magnetic material known as "ZnFe" has the formula ZnFe2O4, with Fe3+ occupying the octahedral sites and Zn2+ occupy the tetrahedral sites, it is an example of normal structure spinel ferrite. Some ferrites adopt hexagonal crystal structure, like barium and strontium ferrites BaFe12O19 (BaO:6Fe2O3) and SrFe12O19 (SrO:6Fe2O3). In terms of their magnetic properties, the different ferrites are often classified as "soft", "semi-hard" or "hard", which refers to their low or high magnetic
coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
, as follows.


Soft ferrites

Ferrites that are used in
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
or Electromagnetism, electromagnetic cores contain
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
,
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
, and/or
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
compounds. Soft ferrites are not permanent magnets. They have magnetism (much like mild steel), but when the magnetic field is removed, the magnetism decreases. Soft ferrites are commonly used as transformers (to change the voltage from primary to secondary windings). As a result, soft ferrites are also called transformer ferrites.They have a low
coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
. The low coercivity means the material's magnetization can easily reverse direction without dissipating much energy ( hysteresis losses), while the material's high
resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
prevents
eddy current Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnet ...
s in the core, another source of energy loss. Because of their comparatively low losses at high frequencies, they are extensively used in the cores of RF transformers and
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s in applications such as
switched-mode power supplies A switched-mode power supply (switching-mode power supply, switch-mode power supply, switched power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like ...
and loopstick antennas used in AM radios. The most common soft ferrites are: * Manganese-zinc ferrite (MnZn, with the formula ). MnZn have higher permeability and saturation induction than NiZn. * Nickel-zinc ferrite (NiZn, with the formula ). NiZn ferrites exhibit higher resistivity than MnZn, and are therefore more suitable for frequencies above 1 MHz. For applications below 5 MHz, MnZn ferrites are used; above that, NiZn is the usual choice. The exception is with common mode inductors, where the threshold of choice is at 70 MHz.


Semi-hard ferrites

* Cobalt ferrite, CoFe2O4 (CoO·Fe2O3), is in between soft and hard magnetic material and is usually classified as a semi-hard material. It is mainly used for its magnetostrictive applications like sensors and actuators thanks to its high saturation
magnetostriction Magnetostriction (cf. electrostriction) is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field chan ...
(~200 ppm). CoFe2O4 has also the benefits to be
rare-earth The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
free, which makes it a good substitute for
Terfenol-D Terfenol-D, an alloy of the formula (''x'' ≈ 0.3), is a magnetostrictive material. It was initially developed in the 1970s by the Naval Ordnance Laboratory in the United States. The technology for manufacturing the material efficiently was d ...
. Moreover, its magnetostrictive properties can be tuned by inducing a magnetic uniaxial anisotropy. This can be done by magnetic annealing, magnetic field assisted compaction, or reaction under uniaxial pressure. This last solution has the advantage to be ultra fast (20 min) thanks to the use of
spark plasma sintering Spark plasma sintering (SPS), also known as field assisted sintering technique (FAST) or pulsed electric current sintering (PECS), or plasma pressure compaction (P2C) is a sintering technique. The main characteristic of SPS is that the pulsed or u ...
. The induced magnetic anisotropy in cobalt ferrite is also beneficial to enhance the
magnetoelectric effect In its most general form, the magnetoelectric effect (ME) denotes any coupling between the magnetic and the electric properties of a material. The first example of such an effect was described by Wilhelm Röntgen in 1888, who found that a dielectric ...
in composite.


Hard ferrites

In contrast, permanent ferrite
magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nicke ...
s are made of hard ferrites, which have a high
coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
and high remanence after magnetization. Iron oxide and barium or
strontium carbonate Strontium carbonate (SrCO3) is the carbonate salt of strontium that has the appearance of a white or grey powder. It occurs in nature as the mineral strontianite. Chemical properties Strontium carbonate is a white, odorless, tasteless powder. B ...
are used in manufacturing of hard ferrite magnets. The high coercivity means the materials are very resistant to becoming demagnetized, an essential characteristic for a permanent magnet. They also have high magnetic permeability. These so-called ''ceramic magnets'' are cheap, and are widely used in household products such as
refrigerator magnet A refrigerator magnet or fridge magnet is a small magnet, often attached to an artistic or whimsical ornament, which may be used to post items such as shopping lists, Christmas cards, child art or reminders on a refrigerator door, or which sim ...
s. The maximum magnetic field ''B'' is about 0.35 tesla and the magnetic field strength ''H'' is about 30 to 160 kiloampere turns per meter (400 to 2000
oersted The oersted (symbol Oe) is the coherent derived unit of the auxiliary magnetic field H in the centimetre–gram–second system of units (CGS). It is equivalent to 1 dyne per maxwell. Difference between CGS and SI systems In the CGS system, ...
s). The density of ferrite magnets is about 5 g/cm3. The most common hard ferrites are: * Strontium ferrite, SrFe12O19 (SrO·6Fe2O3), used in small electric motors, micro-wave devices, recording media, magneto-optic media, telecommunication and electronic industry. Strontium hexaferrite (SrFe12O19) is well known for its high
coercivity Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
due to its magnetocrystalline anisotropy. It has been widely used in industrial applications as permanent magnets and, because they can be powdered and formed easily, they are finding their applications into micro and nano-types systems such as biomarkers, bio diagnostics and biosensors. * Barium ferrite, BaFe12O19 (BaO·6Fe2O3), a common material for permanent magnet applications. Barium ferrites are robust ceramics that are generally stable to moisture and corrosion-resistant. They are used in e.g.
loudspeaker A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or ...
magnets and as a medium for
magnetic recording Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetizable material to store data and is a form of non-volatile memory. The information is ac ...
, e.g. on
magnetic stripe card The term digital card can refer to a physical item, such as a memory card on a camera, or, increasingly since 2017, to the digital content hosted as a virtual card or cloud card, as a digital virtual representation of a physical card. They share ...
s.


Production

Ferrites are produced by heating a mixture of the oxides of the constituent metals at high temperatures, as shown in this idealized equation: :Fe2O3 + ZnO → ZnFe2O4 In some cases, the mixture of finely-powdered precursors is pressed into a mold. For barium and strontium ferrites, these metals are typically supplied as their carbonates, BaCO3 or SrCO3. During the heating process, these carbonates undergo
calcination Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), gener ...
: :MCO3 → MO + CO2 After this step, the two oxides combine to give the ferrite. The resulting mixture of oxides undergoes
sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...
.


Processing

Having obtained the ferrite, the cooled product is milled to particles smaller than 2 µm, sufficiently small that each particle consists of a single magnetic domain. 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 Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
). 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 Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
shortage. The chemical composition and especially the structure vary strongly between the precursor and the sintered product. To allow efficient stacking of product in the furnace during sintering and prevent parts sticking together, many manufacturers separate ware using ceramic powder separator sheets. These sheets are available in various materials such as alumina, zirconia and magnesia. They are also available in fine, medium and coarse particle sizes. By matching the material and particle size to the ware being sintered, surface damage and contamination can be reduced while maximizing furnace loading.


Uses

Ferrite core In electronics, a ferrite core is a type of magnetic core made of ferrite on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability couple ...
s are used in electronic
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s,
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s, and
electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in ...
s where the high
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual paralle ...
of the ferrite leads to very low
eddy current Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnet ...
losses. Ferrites are also found as a lump in a computer cable, called a ferrite bead, which helps to prevent high frequency electrical noise (
radio frequency interference Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrost ...
) from exiting or entering the equipment; these types of ferrites are made with lossy materials to not just block (reflect), but also absorb and dissipate as heat, the unwanted higher-frequency energy. Early computer memories stored data in the residual magnetic fields of hard ferrite cores, which were assembled into arrays of ''
core memory Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...
''. Ferrite powders are used in the coatings of magnetic recording tapes. Ferrite particles are also used as a component of radar-absorbing materials or coatings used in stealth aircraft and in the absorption tiles lining the rooms used for electromagnetic compatibility measurements. Most common audio magnets, including those used in loudspeakers and electromagnetic instrument pickups, are ferrite magnets. Except for certain "vintage" products, ferrite magnets have largely displaced the more expensive
Alnico Alnico is a family of iron alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni), and cobalt (Co), hence the acronym ''al-ni-co''. They also include copper, and sometimes titanium. Alnico alloys are ferromagnetic, ...
magnets in these applications. In particular, for hard hexaferrites today the most common uses are still as permanent magnets in refrigerator seal gaskets, microphones and loud speakers, small motors for cordless appliances and in automobile applications. Ferrite nanoparticles exhibit superparamagnetic properties.


History

Yogoro Kato and Takeshi Takei of the
Tokyo Institute of Technology is a national research university located in Greater Tokyo Area, Japan. Tokyo Tech is the largest institution for higher education in Japan dedicated to science and technology, one of first five Designated National University and selected as ...
synthesized the first ferrite compounds in 1930. This led to the founding of
TDK is a Japanese multinational electronics corporation that manufactures electronic components and recording and data-storage media. Its motto is "Contribute to culture and industry through creativity". "TDK" is an initialism of the original Ja ...
Corporation in 1935, to manufacture the material. Barium hexaferrite (BaO•6Fe2O3) was discovered in 1950 at the
Philips Natuurkundig Laboratorium The Philips Natuurkundig Laboratorium (English translation: ''Philips Physics Laboratory'') or NatLab was the Dutch section of the Philips research department, which did research for the product divisions of that company. Originally located in the ...
(''Philips Physics Laboratory''). The discovery was somewhat accidental—due to a mistake by an assistant who was supposed to be preparing a sample of hexagonal
lanthanum Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lant ...
ferrite for a team investigating its use as a semiconductor material. On discovering that it was actually a magnetic material, and confirming its structure by
X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
, they passed it on to the magnetic research group. Barium hexaferrite has both high coercivity (170 kA/m) and low raw material costs. It was developed as a product by
Philips Koninklijke Philips N.V. (), commonly shortened to Philips, is a Dutch multinational conglomerate corporation that was founded in Eindhoven in 1891. Since 1997, it has been mostly headquartered in Amsterdam, though the Benelux headquarters i ...
Industries (Netherlands) and from 1952 was marketed under the trade name ''Ferroxdure''. The low price and good performance led to a rapid increase in the use of permanent magnets. In the 1960s Philips developed strontium hexaferrite (SrO•6Fe2O3), with better properties than barium hexaferrite. Barium and strontium hexaferrite dominate the market due to their low costs. Other materials have been found with improved properties. BaO•2(FeO)•8(Fe2O3) came in 1980. and Ba2ZnFe18O23 came in 1991.Raul Valenzuela, ''Magnetic Ceramics'', p. 76-77, Cambridge University Press, 2005 .


See also

* Ferromagnetic material properties * Cobalt Ferrite


References


External links


International Magnetics Association


Sources

*MMPA 0100-00
''Standard Specifications for Permanent Magnet Materials''
*Meeldijk, Victor ''Electronic Components: Selection and Application Guidelines'', 1997 Wiley *Ott, Henry ''Noise Reduction Techniques in Electronic Systems'' 1988 Wiley *Luecke, Gerald and others ''General Radiotelephone Operator License Plus Radar Endorsement'' 2004, Master Pub. *Bartlett, Bruce and other
''Practical Recording Techniques''
2005 Focal Press *Schaller, George E
''Ferrite Processing & Effects on Material Performance''
{{DEFAULTSORT:Ferrite (Magnet) Ceramic materials Ferromagnetic materials Types of magnets Loudspeakers Ferrites