Ferrite (magnet)

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A stack of ferrite magnets, with magnetic household items stuck to it. Ceramic magnets.jpg
A stack of ferrite magnets, with magnetic household items stuck to it.

A ferrite is an iron oxide-containing magnetic ceramic material. They are ferrimagnetic, meaning they are attracted by magnetic fields and can be magnetized to become permanent magnets. Unlike many ferromagnetic materials, most ferrites are not electrically conductive, making them useful in applications like magnetic cores for transformers to suppress eddy currents. [1]

Contents

Ferrites can be divided into two families based on their magnetic coercivity, their resistance to being demagnetized: [2]
"Hard" ferrites have high coercivity, so are difficult to demagnetize. They are used to make permanent magnets for applications such as refrigerator magnets, loudspeakers, and small electric motors.
"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 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. [3]

Composition, structure, and properties

Ferrites are usually ferrimagnetic ceramic compounds derived from iron oxides, with either a body-centered cubic or hexagonal crystal structure. [4] Like most of the other ceramics, ferrites are hard, brittle, and poor conductors of electricity.

They are typically composed of α-iron(III) oxide (e.g. hematite Fe2O3) with one, or more additional, metallic element oxides, usually with an approximately stochiometric formula of MO·Fe2O3 such as Fe(II) such as in the common mineral magnetite composed of Fe(II)-Fe(III)2O4. [5] Above 585 °C Fe(II)-Fe(III)2O4 transforms into the non-magnetic gamma phase. [6] Fe(II)-Fe(III)2O4 is commonly seen as the black iron(II) oxide coating the surface of cast-iron cookware). The other pattern is M·Fe(III)2O3, where M is another metallic element. Common, naturally occurring ferrites (typically members of the spinel group) include those with nickel (NiFe2O4) which occurs as the mineral trevorite, magnesium containing magnesioferrite (MgFe2O4), [7] [8] cobalt (cobalt ferrite), [9] or manganese (MnFe2O4) which occurs naturally as the mineral jacobsite. Less often bismuth, [10] strontium, zinc as found in franklinite, [11] aluminum,yittrium, or barium ferrites are used [12] [13] In addition, more complex synthetic alloys are often used for specific applications. [14] [15]

Many ferrites adopt the spinel chemical structure with the formula A B
2O
4, 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 2−) with A cations occupying one eighth of the tetrahedral holes, and B cations occupying half of the octahedral holes, i.e., A2+
B3+
2O2−
4. An exception exists for ɣ-Fe2O3 which has a spinel crystalline form and is widely used a magnetic recording substrate. [16] [17]

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 [M2+
(1−δ) Fe3+
δ ] [M2+
δ Fe3+
(2−δ) ] O
4, where δ is the degree of inversion.[ example needed ][ clarification needed ]

The magnetic material known as "Zn Fe" has the formula Zn Fe
2O
4, with Fe3+
 occupying the octahedral sites and Zn2+
 occupying the tetrahedral sites, it is an example of normal structure spinel ferrite. [18] [ page needed ]

Some ferrites adopt hexagonal crystal structure, like barium and strontium ferrites BaFe
12O
19 (BaO : 6 Fe
2O
3) and SrFe
12O
19 (Sr O : 6 Fe
2O
3). [19]

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, as follows.

Soft ferrites

Various ferrite cores used to make small transformers and inductors Ferrite cores.jpg
Various ferrite cores used to make small transformers and inductors
A ferrite AM loopstick antenna in a portable radio, consisting of a wire wound around a ferrite core Panasonic RQ-NX60V - controller board - Ferrite rod antenna-9553.jpg
A ferrite AM loopstick antenna in a portable radio, consisting of a wire wound around a ferrite core
A variety of small ferrite core inductors and transformers Aplikimi i feriteve.png
A variety of small ferrite core inductors and transformers

Ferrites that are used in transformer or electromagnetic cores contain nickel, zinc, and/or manganese [20] 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. The low coercivity also means the material's magnetization 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: [19]

Manganese-zinc ferrite
"Mn Zn", with the formula Mn
δ Zn
(1−δ) Fe
2O
4. Mn Zn have higher permeability and saturation induction than Ni Zn.
Nickel-zinc ferrite
"Ni Zn", with the formula Ni
δ Zn
(1−δ) Fe
2O
4. Ni Zn ferrites exhibit higher resistivity than Mn Zn, and are therefore more suitable for frequencies above 1 MHz. [21]

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. [22]

Semi-hard ferrites

Cobalt ferrite
Co Fe
2O
4Co O·Fe
2O
3 , is in between soft and hard magnetic material and is usually classified as a semi-hard material. [23] It is mainly used for its magnetostrictive applications like sensors and actuators [24] thanks to its high saturation magnetostriction (~200 ppm). Co Fe
2O
4 has also the benefits to be rare-earth free, which makes it a good substitute for terfenol-D. [25]

Moreover, cobalt ferrite's magnetostrictive properties can be tuned by inducing a magnetic uniaxial anisotropy. [26] This can be done by magnetic annealing, [27] magnetic field assisted compaction, [28] or reaction under uniaxial pressure. [29] 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. [30]

Hard ferrites

A small permanent magnet electric motor disassembled, showing the two crescent-shaped ferrite magnets in the stator assembly (lower left) Motor disassembeled.JPG
A small permanent magnet electric motor disassembled, showing the two crescent-shaped ferrite magnets in the stator assembly (lower left)

In contrast, permanent ferrite magnets are made of hard ferrites, which have a high coercivity and high remanence after magnetization. Iron oxide and barium carbonate or strontium carbonate are used in manufacturing of hard ferrite magnets. [31] [32] 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 B is about 0.35  tesla and the magnetic field strength H is about 30–160 kiloampere turns per meter (400–2000  oersteds). [33] The density of ferrite magnets is about 5 g/cm3.

The most common hard ferrites are:

Strontium ferrite
Sr Fe
12O
19 (Sr O · 6 Fe
2O
3), used in small electric motors, micro-wave devices, recording media, magneto-optic media, telecommunication, and electronics industry. [19] Strontium hexaferrite (Sr Fe
12O
19) is well known for its high coercivity 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. [34]
Barium ferrite
Ba Fe
12O
19 (Ba O · 6 Fe
2O
3), 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, e.g. on magnetic stripe cards.

Production

Ferrites are produced by heating a mixture of the oxides of the constituent metals at high temperatures, as shown in this idealized equation: [35]

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:

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.

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.

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 electromagnets where the high electrical resistance 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. [36]

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, they passed it on to the magnetic research group. [37] 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. [38] The low price and good performance led to a rapid increase in the use of permanent magnets. [39]

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. [40] and Ba2ZnFe18O23 came in 1991. [41]

See also

Related Research Articles

<span class="mw-page-title-main">Ferromagnetism</span> Mechanism by which materials form into and are attracted to magnets

Ferromagnetism is a property of certain materials that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagnetic materials are noticeably attracted to a magnet, which is a consequence of their substantial magnetic permeability.

<span class="mw-page-title-main">Magnet</span> Object that has a magnetic field

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, cobalt, etc. and attracts or repels other magnets.

Magnetostriction 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 changes the magnetostrictive strain until reaching its saturation value, λ. The effect was first identified in 1842 by James Joule when observing a sample of iron.

<span class="mw-page-title-main">Coercivity</span> Resistance of a ferromagnetic material to demagnetization by an external magnetic field

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 oersted or ampere/meter units and is denoted HC.

<span class="mw-page-title-main">Neodymium magnet</span> Strongest type of permanent magnet from an alloy of neodymium, iron and boron

A neodymium magnet (also known as NdFeB, NIB or Neo magnet) is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B tetragonal crystalline structure. They are the most widely used type of rare-earth magnet.

<span class="mw-page-title-main">Alnico</span> Family of iron alloys

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, and are used to make permanent magnets. Before the development of rare-earth magnets in the 1970s, they were the strongest type of permanent magnet. Other trade names for alloys in this family are: Alni, Alcomax, Hycomax, Columax, and Ticonal.

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<span class="mw-page-title-main">Spark plasma sintering</span>

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<span class="mw-page-title-main">Barium ferrite</span> Chemical compound

Barium ferrite, abbreviated BaFe, BaM, is the chemical compound with the formula BaFe
12O
19. This and related ferrite materials are components in magnetic stripe cards and loudspeaker magnets.

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 coupled with low electrical conductivity. Moreover, because of their comparatively low 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 ferrite loopstick antennas for AM radio receivers.

The article Ferromagnetic material properties is intended to contain a glossary of terms used to describe ferromagnetic materials, and magnetic cores.

Zinc ferrites are a series of synthetic inorganic compounds of zinc and iron (ferrite) with the general formula of ZnxFe3−xO4. Zinc ferrite compounds can be prepared by aging solutions of Zn(NO3)2, Fe(NO3)3, and triethanolamine in the presence and in the absence of hydrazine, or reacting iron oxides and zinc oxide at high temperature. Spinel (Zn, Fe) Fe2O4 appears as a tan-colored solid that is insoluble in water, acids, or diluted alkali. Because of their high opacity, zinc ferrites can be used as pigments, especially in applications requiring heat stability. For example, zinc ferrite prepared from yellow iron oxide can be used as a substitute for applications in temperatures above 350 °F (177 °C). When added to high corrosion-resistant coatings, the corrosion protection increases with an increase in the concentration of zinc ferrite.

Cuprospinel is a mineral. Cuprospinel is an inverse spinel with the chemical formula CuFe2O4, where copper substitutes some of the iron cations in the structure. Its structure is similar to that of magnetite, Fe3O4, yet with slightly different chemical and physical properties due to the presence of copper.

The spinels are any of a class of minerals of general formulation AB
2X
4 which crystallise in the cubic (isometric) crystal system, with the X anions arranged in a cubic close-packed lattice and the cations A and B occupying some or all of the octahedral and tetrahedral sites in the lattice. Although the charges of A and B in the prototypical spinel structure are +2 and +3, respectively, other combinations incorporating divalent, trivalent, or tetravalent cations, including magnesium, zinc, iron, manganese, aluminium, chromium, titanium, and silicon, are also possible. The anion is normally oxygen; when other chalcogenides constitute the anion sublattice the structure is referred to as a thiospinel.

A complex oxide is a chemical compound that contains oxygen and at least two other elements. Complex oxide materials are notable for their wide range of magnetic and electronic properties, such as ferromagnetism, ferroelectricity, and high-temperature superconductivity. These properties often come from their strongly correlated electrons in d or f orbitals.

<span class="mw-page-title-main">Permanent magnet motor</span> Type of electric motor

A permanent magnet motor is a type of electric motor that uses permanent magnets for the field excitation and a wound armature. The permanent magnets can either be stationary or rotating; interior or exterior to the armature for a radial flux machine or layered with the armature for an axial flux topology. The schematic shows a permanent magnet motor with stationary magnets outside of a brushed armature.

<span class="mw-page-title-main">High entropy oxide</span> Complex oxide molecules that contain five or more metal ions

High-entropy oxides (HEOs) are complex oxides that contain five or more principal metal cations and have a single-phase crystal structure. The first HEO, (MgNiCuCoZn)0.2O in a rock salt structure, was reported in 2015 by Rost et al. HEOs have been successfully synthesized in many structures, including fluorites, perovskites, and spinels. HEOs are currently being investigated for applications as functional materials.

Cobalt ferrite is a semi-hard ferrite with the chemical formula of CoFe2O4 (CoO·Fe2O3). The substance can be considered as between soft and hard magnetic material and is usually classified as a semi-hard material.

Hexagonal ferrites or hexaferrites are a family of ferrites with hexagonal crystal structure. The most common member is BaFe12O19, also called barium ferrite, BaM, etc. BaM is a strong room-temperature ferrimagnetic material with high anisotropy along the c axis. All the hexaferrite members are constructed by stacking a few building blocks in a certain order.

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  35. ↑ M. Wittenauer; P. Wang; P. Metcalf; Z. Ka̧kol; J. M. Honig (1995). "Growth and Characterization of Single Crystals of Zinc Ferrites, Fe 3-X Zn x O 4". Inorganic Syntheses. Vol. 30. pp. 124–132. doi:10.1002/9780470132616.ch27. ISBN   9780470132616.{{cite book}}: |journal= ignored (help)
  36. ↑ Pullar, Robert C. (September 2012). "Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics". Progress in Materials Science. 57 (7): 1191–1334. doi:10.1016/j.pmatsci.2012.04.001.
  37. ↑ Marc de Vries, 80 Years of Research at the Philips Natuurkundig Laboratorium (1914-1994), p. 95, Amsterdam University Press, 2005 ISBN   9085550513.
  38. ↑ Raul Valenzuela, Magnetic Ceramics, p. 76, Cambridge University Press, 2005 ISBN   0521018439.
  39. ↑ R. Gerber, C.D. Wright, G. Asti, Applied Magnetism, p. 335, Springer, 2013 ISBN   9401582637
  40. ↑ Lotgering, F. K.; Vromans, P. H. G. M.; Huyberts, M. A. H. (1980). "Permanent-magnet material obtained by sintering the hexagonal ferriteW=BaFe18O27". Journal of Applied Physics. 51 (11): 5913–5918. Bibcode:1980JAP....51.5913L. doi:10.1063/1.327493.
  41. ↑ Raul Valenzuela, Magnetic Ceramics, p. 76-77, Cambridge University Press, 2005 ISBN   0521018439.

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