Aferrite is one of a family ofiron-oxide-containing magneticceramic materials. They areferrimagnetic, meaning they are attracted by magnetic fields and can bemagnetized to becomepermanent magnets. Unlike manyferromagnetic materials, most ferrites are not electrically-conductive, making them useful in applications likemagnetic cores fortransformers to suppresseddy currents.[1]
Ferrites can be divided into two groups based on their magneticcoercivity, their resistance to being demagnetized:[2]
"Hard" ferrites have highcoercivity, so are difficult to demagnetize. They are used to make permanent magnets for applications such asrefrigerator magnets,loudspeakers, and smallelectric 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 efficientmagnetic cores calledferrite cores for high-frequencyinductors,transformers andantennas, and in variousmicrowave components.
Ferrite compounds are extremely low cost, being made mostly of iron oxide, and have excellent corrosion resistance. Yogoro Kato and Takeshi Takei of theTokyo Institute of Technology synthesized the first ferrite compounds in 1930.[3]
Ferrites are usuallyferrimagnetic ceramic compounds derived fromiron oxides, with either a body-centered cubic or hexagonalcrystal structure.[4] Like most of the otherceramics, ferrites are hard,brittle, and poorconductors of electricity.
They are typically composed of α-iron(III) oxide (e.g.hematiteFe2O3) with one, or more additional,metallic element oxides, usually with an approximatelystoichiometric formula ofMO·Fe2O3 such asFe(II) such as in the common mineralmagnetite composed ofFe(II)-Fe(III)2O4.[5] Above 585 °CFe(II)-Fe(III)2O4 transforms into the non-magnetic gamma phase.[6] Fe(II)-Fe(III)2O4 is commonly seen as the blackiron(II) oxide coating the surface of cast-iron cookware). The other pattern isM·Fe(III)2O3, whereM is another metallic element. Common, naturally occurring ferrites (typically members of thespinel group) include those withnickel (NiFe2O4) which occurs as the mineraltrevorite,magnesium containingmagnesioferrite (MgFe2O4),[7][8]cobalt (cobalt ferrite),[9] ormanganese (MnFe2O4) which occurs naturally as the mineraljacobsite. Less oftenbismuth,[10]strontium,zinc as found infranklinite,[11]aluminum,yittrium, orbarium ferrites are used[12][13] In addition, more complex synthetic alloys are often used for specific applications.[14][15]
Many ferrites adopt thespinel chemical structure with theformulaA B
2O
4, whereA andB represent various metalcations, one of which is usually iron (Fe). Spinel ferrites usually adopt a crystal motif consisting of cubic close-packed (fcc) oxides (O2−) withA cations occupying one eighth of the tetrahedral holes, andB 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 ordinaryspinel structure, but rather the inverse spinel structure: One eighth of the tetrahedral holes are occupied byB cations, one fourth of the octahedral sites are occupied byA cations. and the other one fourth byB 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, whereδ is the degree of inversion.[example needed][clarification needed]
The magnetic material known as "Zn Fe" has the formulaZnFe
2O
4, withFe3+
occupying the octahedral sites andZn2+
occupying the tetrahedral sites, it is an example of normal structure spinel ferrite.[18][page needed]
Some ferrites adopt hexagonal crystal structure, likebarium andstrontium ferritesBaFe
12O
19 (BaO·6Fe
2O
3) andSrFe
12O
19 (SrO·6Fe
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 magneticcoercivity, as follows.
Ferrites that are used intransformer orelectromagneticcores containnickel,zinc, and/ormanganese[20] compounds. Soft ferrites are not suitable to make permanent magnets. They have highmagnetic permeability so they conduct magnetic fields and are attracted to magnets, but when the external magnetic field is removed, theremanent magnetization does not tend to persist. This is due to their lowcoercivity. The low coercivity also means the material'smagnetization can easily reverse direction without dissipating much energy (hysteresis losses), while the material's highresistivity preventseddy currents in the core, another source of energy loss. Because of their comparatively lowcore losses at high frequencies, they are extensively used in the cores ofRF transformers andinductors in applications such asswitched-mode power supplies andloopstick antennas used in AM radios.
The most common soft ferrites are:[19]
For use withfrequencies above 0.5 MHz but below 5 MHz, Mn Zn ferrites are used; above that, Ni Zn is the usual choice. The exception is withcommon mode inductors, where the threshold of choice is at 70 MHz.[22]
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 ofspark plasma sintering. The induced magnetic anisotropy in cobalt ferrite is also beneficial to enhance themagnetoelectric effect in composite.[30]
In contrast, permanent ferritemagnets are made ofhard ferrites, which have a highcoercivity and highremanence after magnetization. The high coercivity means the materials are very resistant to becoming demagnetized, an essential characteristic for a permanent magnet. They also have highmagnetic permeability. These so-calledceramic magnets are cheap, and are widely used in household products such asrefrigerator magnets. The maximum magnetic fieldB is about 0.35 tesla and the magnetic field strengthH is about 30–160 kiloampere turns per meter (400–2000 oersteds).[31] The density of ferrite magnets is about 5 g/cm3.
The most common hard ferrites are:
Iron oxide and (barium carbonate orstrontium carbonate) are used in manufacturing of hard ferrite magnets.[33][34]
Ferrites are produced by heating a mixture of the oxides of the constituent metals at high temperatures, as shown in this idealized equation:[35]
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 orSrCO3. During the heating process, these carbonates undergocalcination:
After this step, the two oxides combine to give the ferrite. The resulting mixture of oxides undergoessintering.
Having obtained the ferrite, the cooled product is milled to particles smaller than 2μm, sufficiently small that each particle consists of asingle 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 anoxygen 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.
Ferrite cores are used in electronicinductors,transformers, andelectromagnets where the highelectrical resistance of the ferrite leads to very loweddy current losses.
Ferrites are also found as a lump in a computer cable, called aferrite 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.
Earlycomputer memories stored data in the residual magnetic fields of hard ferrite cores, which were assembled into arrays ofcore memory. Ferrite powders are used in the coatings ofmagnetic recording tapes.
Ferrite particles are also used as a component of radar-absorbing materials or coatings used instealth aircraft and in the absorption tiles lining the rooms used forelectromagnetic compatibility measurements. Most common audio magnets, including those used in loudspeakers andelectromagnetic instrument pickups, are ferrite magnets. Except for certain "vintage" products, ferrite magnets have largely displaced the more expensiveAlnico 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 loudspeakers, small motors for cordless appliances, and in automobile applications.[36]
Ferrite magnets find applications in electric power steering systems and automotivesensors due to their cost-effectiveness andcorrosion resistance.[37] Ferrite magnets are known for their high magneticpermeability and lowelectrical conductivity, making them suitable for high-frequency applications.[38] In electric power steering systems, they provide the necessary magnetic field for efficient motor operation, contributing to the system's overall performance and reliability.[39] Automotive sensors utilize ferrite magnets for accurate detection and measurement of various parameters, such as position, speed, and fluid levels.[40]
Due to ceramic ferrite magnet's weaker magnetic fields compared tosuperconducting magnets, they are sometimes used in low-field or open MRI systems.[41][42] These magnets are favored in certain cases due to their lower cost, stablemagnetic field, and ability to function without the need for complex cooling systems.[43]
Ferrite nanoparticles exhibitsuperparamagnetic properties.
Yogoro Kato and Takeshi Takei of theTokyo Institute of Technology synthesized the first ferrite compounds in 1930. This led to the founding ofTDK Corporation in 1935, to manufacture the material.
Barium hexaferrite (BaO•6Fe2O3) was discovered in 1950 at thePhilips 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 hexagonallanthanum ferrite for a team investigating its use as a semiconductor material. On discovering that it was actually a magnetic material, and confirming its structure byX-ray crystallography, they passed it on to the magnetic research group.[44] Barium hexaferrite has both high coercivity (170 kA/m) and low raw material costs. It was developed as a product byPhilips Industries (Netherlands) and from 1952 was marketed under the trade nameFerroxdure.[45] Also Mullard'sMagnadur. The low price and good performance led to a rapid increase in the use of permanent magnets.[46]
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.[47] and Ba2ZnFe18O23 came in 1991.[48]
