Ferrite (magnet)
A ferrite is one of a family of iron-oxide-containing magnetic ceramic materials. 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.
Ferrites can be divided into two groups based on their magnetic coercivity, their resistance to being demagnetized:
"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.
Composition, structure, and properties
Ferrites are usually ferrimagnetic ceramic compounds derived from iron oxides, with either a body-centered cubic or hexagonal crystal structure. Like most of the other ceramics, ferrites are hard, brittle, and poor conductors of electricity.They are typically composed of α-iron oxide with one, or more additional, metallic element oxides, usually with an approximately stoichiometric formula of MO·Fe2O3 such as Fe such as in the common mineral magnetite composed of Fe-Fe2O4. Above 585 °C Fe-Fe2O4 transforms into the non-magnetic gamma phase. Fe-Fe2O4 is commonly seen as the black iron. The other pattern is M·Fe2O3, where M is another metallic element. Common, naturally occurring ferrites include those with nickel which occurs as the mineral trevorite, magnesium containing magnesioferrite, cobalt, or manganese which occurs naturally as the mineral jacobsite. Less often bismuth, strontium, zinc as found in franklinite, aluminum,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, where A and B represent various metal cations, one of which is usually iron. Spinel ferrites usually adopt a crystal motif consisting of cubic close-packed oxides 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-Fe2O3 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, as follows.
Soft ferrites
Ferrites that are used in transformer or electromagnetic cores contain nickel, zinc, and/or manganese 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, 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. 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 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 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. 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 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, e.g. on magnetic stripe cards.
Iron oxide and are used in manufacturing of hard ferrite magnets.
Production
Ferrites are mainly produced by two main processes co-precipitation and by the sol-gel Both processes start with an aqueous solution, and by adding base/acid or a fuel/chelate, they initiate the transformation of the solution. In co-precipitation, metallic ions form hydroxide/oxide precipitates; after filtering, washing, drying, and grinding, the powder is calcined to obtain the ferrite phase. The sol-gel process needs an increase in temperature to change the homogeneous solution from liquid to a gel; after the gel is formed there will be a self-combustion/auto-ignition reaction that creates fine oxide particles, which after grinding and calcination are transformed into the final ferrite powder. An idealized equation of the reaction is shown: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:
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.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, 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.