
A ferrite is one of a family of
iron oxide
An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust.
Iron ...
-containing magnetic
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, porcela ...
materials. 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 wh ...
, meaning they are attracted by magnetic fields and can be
magnetized to become
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, nickel, c ...
s. Unlike many
ferromagnetic
Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagne ...
materials, most ferrites are not electrically
conductive, making them useful in applications like
magnetic cores for
transformers to suppress
eddy currents.
Ferrites can be divided into two groups based on their 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 Magnetization, demagnetized. Coercivity is usual ...
, their resistance to being demagnetized:
"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 Magnetization, demagnetized. Coercivity is usual ...
, so are difficult to demagnetize. They are used to make permanent magnets for applications such as
refrigerator magnets,
loudspeakers, and small
electric motor
An electric motor is a 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 electromagnetic coil, wire winding to gene ...
s.
"Soft" ferrites have low coercivity, 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 cores for high-frequency
inductors,
transformers and
antennas, and in various
microwave
Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
components.
Ferrite compounds are extremely low cost, being made mostly of iron oxide, and have excellent corrosion resistance. Yogoro Kato and Takeshi Takei of the
Tokyo Institute of Technology 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 wh ...
ceramic compounds derived from
iron oxide
An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust.
Iron ...
s, with either a body-centered cubic or hexagonal
crystal structure. Like most of the other
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, porcela ...
s, 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. ...
, and poor
conductors of electricity.
They are typically composed of α-
iron(III) oxide (e.g.
hematite ) with one, or more additional,
metallic element oxides, usually with an approximately stoichiometric formula of ''M''O·Fe
2O
3 such as
Fe(II) such as in the common mineral
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...
composed of
Fe(II)-Fe(III)2O4. Above 585 °C
Fe(II)-Fe(III)2O4 transforms into the non-magnetic gamma phase. Fe(II)-Fe(III)
2O
4 is commonly seen as the black
iron(II) oxide coating the surface of cast-iron cookware). The other pattern is ''M''·Fe(III)
2O
3, where ''M'' is another metallic element. Common, naturally occurring ferrites (typically members of the
spinel group) include those with
nickel (NiFe
2O
4) which occurs as the mineral
trevorite,
magnesium
Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...
containing
magnesioferrite (MgFe
2O
4),
cobalt (
cobalt ferrite), or
manganese
Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...
(MnFe
2O
4) which occurs naturally as the mineral
jacobsite. Less often
bismuth,
strontium,
zinc
Zinc is a chemical element; it has symbol Zn and atomic number 30. It 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 periodic tabl ...
as found in
franklinite,
aluminum
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
,
yittrium, or
barium ferrites are used
In addition, more complex synthetic alloys are often used for specific applications.
Many ferrites adopt the
spinel chemical structure with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...
, where A and B represent various metal
cations, one of which is usually iron (Fe). Spinel ferrites usually adopt a crystal motif consisting of cubic close-packed (fcc) oxides (
O) with A cations occupying one eighth of the tetrahedral holes, and B cations occupying half of the octahedral holes, i.e., . An exception exists for ɣ-Fe
2O
3 which has a spinel crystalline form and is widely used a magnetic recording substrate.
However the structure is not an 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 [] [] , where is the degree of inversion.
The magnetic material known as "Zn Fe" has the formula , with occupying the octahedral sites and occupying the tetrahedral sites, it is an example of normal structure spinel ferrite.
Some ferrites adopt hexagonal crystal structure, like
barium and
strontium ferrites () and ().
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 Magnetization, demagnetized. Coercivity is usual ...
, as follows.
Soft ferrites
Ferrites that are used in
transformer or
electromagnetic
In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...
cores contain
nickel,
zinc
Zinc is a chemical element; it has symbol Zn and atomic number 30. It 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 periodic tabl ...
, and/or
manganese
Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...
compounds. Soft ferrites are not suitable to make permanent magnets. They have high
magnetic permeability so they conduct magnetic fields and are attracted to magnets, but when the external magnetic field is removed, the
remanent magnetization does not tend to persist. This is due to their 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 Magnetization, demagnetized. Coercivity is usual ...
. The low coercivity also means the material's
magnetization
In classical electromagnetism, magnetization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. Accordingly, physicists and engineers usually define magnetization as the quanti ...
can easily reverse direction without dissipating much energy (
hysteresis losses), while the material's high
resistivity prevents
eddy currents in the core, another source of energy loss. Because of their comparatively low
core losses at high frequencies, they are extensively used in the cores of
RF transformers and
inductors in applications such as
switched-mode power supplies and
loopstick antennas used in AM radios.
The most common soft ferrites are:
[
; Manganese-zinc ferrite: "Mn Zn", with the formula . Mn Zn have higher permeability and saturation induction than Ni Zn.
; Nickel-zinc ferrite: "Ni Zn", with the formula . Ni Zn ferrites exhibit higher resistivity than Mn Zn, and are therefore more suitable for frequencies above 1 MHz.
For use with frequencies above 0.5 MHz but below 5 MHz, Mn Zn ferrites are used; above that, Ni Zn 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 : 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 (~200 ppm). has also the benefits to be rare-earth free, which makes it a good substitute for terfenol-D.
Moreover, cobalt ferrite's 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. The induced magnetic anisotropy in cobalt ferrite is also beneficial to enhance the magnetoelectric effect in composite.
Hard ferrites
In contrast, permanent ferrite magnets 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 Magnetization, demagnetized. Coercivity is usual ...
and high remanence after magnetization. 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 magnets. The maximum magnetic field is about 0.35 tesla and the magnetic field strength is about 30–160 kiloampere turns per meter (400–2000 oersteds). The density of ferrite magnets is about 5 g/cm3.
The most common hard ferrites are:
; Strontium ferrite : (), used in small electric motors, micro-wave devices, recording media, magneto-optic media, telecommunication, and electronics industry.[ Strontium hexaferrite () 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 Magnetization, demagnetized. Coercivity is usual ...
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: (), 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 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 ...
, e.g. on magnetic stripe cards.
Iron oxide
An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust.
Iron ...
and ( barium carbonate or strontium carbonate) are used in manufacturing of hard ferrite magnets.
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 is 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), generally f ...
:
:MCO3 → MO + CO2
After this step, the two oxides combine to give the ferrite. The resulting mixture of oxides undergoes sintering.
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).
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.
Ferrite cores for electromagnets can be 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 a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
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 cores are used in electronic inductors, transformers, 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 (likely copper) wound into a electromagnetic coil, coil. A current through the wire creates a magnetic ...
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 paral ...
of the ferrite leads to very low eddy current 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) 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''. 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 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 magnets find applications in electric power steering systems and automotive sensors due to their cost-effectiveness and corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
resistance. Ferrite magnets are known for their high magnetic permeability and low electrical conductivity
Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
, making them suitable for high-frequency applications. In electric power steering systems, they provide the necessary magnetic field for efficient motor operation, contributing to the system's overall performance and reliability. Automotive sensors utilize ferrite magnets for accurate detection and measurement of various parameters, such as position, speed, and fluid levels.
Due to ceramic ferrite magnet’s weaker magnetic fields compared to superconducting magnets, they are sometimes used in low-field or open MRI systems. These magnets are favored in certain cases due to their lower cost, stable magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
, and ability to function without the need for complex cooling systems.
Ferrite nanoparticles exhibit superparamagnetic properties.
History
Yogoro Kato and Takeshi Takei of the Tokyo Institute of Technology synthesized the first ferrite compounds in 1930. This led to the founding of TDK Corporation in 1935, to manufacture the material.
Barium hexaferrite (BaO•6Fe2O3) was discovered in 1950 at the Philips Natuurkundig Laboratorium (''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 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 of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
, 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 Industries (Netherlands) and from 1952 was marketed under the trade name ''Ferroxdure''. Also Mullard's '' Magnadur''. 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