Ferrite (magnet)

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A stack of ferrite magnets Ceramic magnets.jpg
A stack of ferrite magnets

A ferrite is a ceramic material made by mixing and firing large proportions iron(III) oxide (Fe2O3, rust) blended with small proportions of one or more additional metallic elements, such as barium, manganese, nickel, and zinc. [1] They are both electrically non-conductive, meaning that they are insulators, and ferrimagnetic, meaning they can easily be magnetized or attracted to a magnet. Ferrites can be divided into two families based on their resistance to being demagnetized (magnetic coercivity).

Ceramic inorganic, nonmetallic solid prepared by the action of heat

A ceramic is a solid material comprising an inorganic compound of metal, non-metal or metalloid atoms primarily held in ionic and covalent bonds. Common examples are earthenware, porcelain, and brick.

Iron(III) oxide chemical compound

Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main source of iron for the steel industry. Fe2O3 is readily attacked by acids. Iron(III) oxide is often called rust, and to some extent this label is useful, because rust shares several properties and has a similar composition. To a chemist, rust is considered an ill-defined material, described as hydrated ferric oxide.

Rust type of iron oxide

Rust is an iron oxide, a usually red oxide formed by the redox reaction of iron and oxygen in the presence of water or air moisture. Several forms of rust are distinguishable both visually and by spectroscopy, and form under different circumstances. Rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3).

Contents

Hard ferrites have high coercivity, so are difficult to demagnetize. They are used to make permanent magnets for refrigerator magnets, loudspeakers, small electric motors, and so on.

Coercivity measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized

In electrical engineering and materials science, the 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. An analogous property, electric coercivity, is the ability of a ferroelectric material to withstand an external electric field without becoming depolarized.

Magnet material or object that produces 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, and attracts or repels other magnets.

Refrigerator magnet

A refrigerator magnet or fridge magnet is an ornament, often whimsical, attached to a small magnet, which is used to post items such as shopping lists, child art or reminders on a refrigerator door, or which simply serves as decoration. Refrigerator magnets come in a wide variety of shapes and sizes, and may have promotional messages placed on them. Refrigerator magnets are popular souvenir and collectible objects.

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 and transformers, and in various microwave components.

A magnetic core is a piece of magnetic material with a high magnetic permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, inductors, magnetic recording heads, and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such as ferrites. The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated in the core material. The magnetic field is often created by a current-carrying coil of wire around the 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 coupled with low electrical conductivity. 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.

Inductor passive two-terminal electrical component that stores energy in its magnetic field

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 coil around a core.

Ferrite compounds have extremely low cost, being made of mostly rusted iron (iron oxide), and also have excellent corrosion resistance. They are very stable and difficult to demagnetize, and can be made with both high and low coercive forces. Yogoro Kato and Takeshi Takei of the Tokyo Institute of Technology synthesized the first ferrite compounds in 1930. [2]

Tokyo Institute of Technology national top-tier research university located in Greater Tokyo Area, Japan

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, and is generally considered to be one of the most prestigious universities in Japan.

Composition, structure, and properties

Ferrites are usually ferrimagnetic ceramic compounds derived from iron oxides. Magnetite (Fe3O4) is a famous example. Like most of the other ceramics, ferrites are hard, brittle, and poor conductors of electricity.

Ferrimagnetism

In physics, a ferrimagnetic material is one that has populations of atoms with opposing magnetic moments, as in antiferromagnetism; however, in ferrimagnetic materials, the opposing moments are unequal and a spontaneous magnetization remains. This happens when the populations consist of different materials or ions (such as Fe2+ and Fe3+).

Iron oxide chemical compound composed of iron and oxygen

Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.

Magnetite iron ore mineral

Magnetite is a rock mineral and one of the main iron ores, with the chemical formula Fe3O4. 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. It is the most magnetic of all the naturally-occurring minerals on Earth. Naturally-magnetized pieces of magnetite, called lodestone, will attract small pieces of iron, which is how ancient peoples first discovered the property of magnetism. Today it is mined as iron ore.

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., A2+
B3+
2
O2−
4
.

The spinels are any of a class of minerals of general formulation AB
2
X
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 chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and plus (+) and minus (−) signs. These are limited to a single typographic line of symbols, which may include subscripts and superscripts. A chemical formula is not a chemical name, and it contains no words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula. Chemical formulas can fully specify the structure of only the simplest of molecules and chemical substances, and are generally more limited in power than are chemical names and structural formulas.

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 [M2+1−δFe3+δ][M2+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. [3] [ page needed ]

Some ferrites adopt hexagonal crystal structure, like barium and strontium ferrites BaFe12O19 (BaO:6Fe2O3) and SrFe12O19 (SrO:6Fe2O3). [4]

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

Ferrites that are used in transformer or electromagnetic cores contain nickel, zinc, and/or manganese compounds. They have a low coercivity and are called soft ferrites. The low coercivity 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 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: [4]

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. [5]

Semi-hard ferrites

Hard ferrites

In contrast, permanent ferrite magnets are made of hard ferrites, which have a high coercivity and high remanence after magnetization. Iron oxide and barium or strontium carbonate are used in manufacturing of hard ferrite magnets. [14] [15] 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 to 160 kiloampere turns per meter (400 to 2000 oersteds). [16] The density of ferrite magnets is about 5 g/cm3.

The most common hard ferrites are:

Production

Ferrites are produced by heating a mixture of the oxides of the constituent metals at high temperatures, as shown in this idealized equation: [17]

Fe2O3 + ZnO → ZnFe2O4

In some cases, the mixture of finely-powdered precursors is pressed into a mold. For barium and strontium ferrites, these metals 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. They are commonly seen 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.

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. One such type of material is iron (III) oxide.

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.

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 (BaFe12O19) 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. [18] 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. [19] The low price and good performance led to a rapid increase in the use permanent magnets. [20]

In the 1960s Philips developed strontium hexaferrite (SrFe12O19), with better properties than barium hexaferrite. Barium and strontium hexaferrite dominate the market due to their low costs. However other materials have been found with improved properties. BaFe2+2Fe3+16O27 came in 1980. [21] and Ba2ZnFe18O23 came in 1991. [22]

See also

Related Research Articles

Magnetostriction is a property of ferromagnetic 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.

Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence. The remanence of magnetic materials provides the magnetic memory in magnetic storage devices, and is used as a source of information on the past Earth's magnetic field in paleomagnetism.

Neodymium magnet type of magnet

A neodymium magnet (also known as NdFeB, NIB or Neo magnet), the most widely used type of rare-earth magnet, is a permanent magnet made from an alloy of neodymium, iron and boron to form the Nd2Fe14B tetragonal crystalline structure. Developed independently in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest type of permanent magnet commercially available. They have replaced other types of magnets in many applications in modern products that require strong permanent magnets, such as motors in cordless tools, hard disk drives and magnetic fasteners.

In physics, a ferromagnetic material is said to have magnetocrystalline anisotropy if it takes more energy to magnetize it in certain directions than in others. These directions are usually related to the principal axes of its crystal lattice. It is a special case of magnetic anisotropy.

Alnico family of iron alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni) and cobalt (Co)

Alnico is an acronym referring to a family of iron alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni) and cobalt (Co), hence al-ni-co. They also include copper, and sometimes titanium. Alnico alloys are ferromagnetic, with low coercivity-its resistance to loss of magnetism varies by grade from 630–1,880 oersteds (50–150 kA/m)-and it is 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.

A samarium–cobalt (SmCo) magnet, a type of rare earth magnet, is a strong permanent magnet made of an alloy of samarium and cobalt. They were developed in the early 1960s based on work done by Karl Strnat and Alden Ray at Wright-Patterson Air Force Base and the University of Dayton, respectively. In particular, Strnat and Ray developed the first formulation of SmCo5. They are generally ranked similarly in strength to neodymium magnets, but have higher temperature ratings and higher coercivity. They are brittle, and prone to cracking and chipping. Samarium–cobalt magnets have maximum energy products (BHmax) that range from 16 megagauss-oersteds (MG·Oe) to 33 MG·Oe, that is approx. 128 kJ/m3 to 264 kJ/m3; their theoretical limit is 34 MG·Oe, about 272 kJ/m3. They are available in two "series", namely Series 1:5 and Series 2:17.

Electroceramics is a class of ceramic materials used primarily for their electrical properties.

Rare-earth magnet permanent magnets made from alloys of rare earth elements

Rare-earth magnets are strong permanent magnets made from alloys of rare-earth elements. Developed in the 1970s and 1980s, rare-earth magnets are the strongest type of permanent magnets made, producing significantly stronger magnetic fields than other types such as ferrite or alnico magnets. The magnetic field typically produced by rare-earth magnets can exceed 1.4 teslas, whereas ferrite or ceramic magnets typically exhibit fields of 0.5 to 1 tesla. There are two types: neodymium magnets and samarium–cobalt magnets. Magnetostrictive rare-earth magnets such as Terfenol-D also have applications, e.g. in loudspeakers. Rare-earth magnets are extremely brittle and also vulnerable to corrosion, so they are usually plated or coated to protect them from breaking, chipping, or crumbling into powder.

Barium ferrite, abbreviated BaFe, BaM, is the chemical compound with the formula BaFe12O19. This and related ferrite materials are components in magnetic stripe cards and loudspeaker magnets. BaFe is described as Ba2+(Fe3+)12(O2−)19. The Fe3+ centers are ferromagnetically coupled. This area of technology is usually considered to be an application of the related fields of materials science and solid state chemistry.

Chromium(IV) oxide chemical compound

Chromium dioxide or chromium(IV) oxide is an inorganic compound with the formula CrO2. It is a black synthetic magnetic solid. It once was widely used in magnetic tape emulsion. With the increasing popularity of CDs and DVDs, the use of chromium(IV) oxide has declined. However, it is still used in data tape applications for enterprise-class storage systems. It is still considered by many oxide and tape manufacturers to have been one of the best magnetic recording particulates ever invented.

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. A recent investigation shows that the zinc ferrite, which is paramagnetic in the bulk form, becomes ferrimagnetic in nanocrystalline thin film format. A large room temperature magnetization and narrow ferromagnetic resonance linewidth have been achieved by controlling thin films growth conditions.

The Stoner–Wohlfarth model is a widely used model for the magnetization of single-domain ferromagnets. It is a simple example of magnetic hysteresis and is useful for modeling small magnetic particles in magnetic storage, biomagnetism, rock magnetism and paleomagnetism.

Cuprospinel is a mineral that occurs naturally in Baie Verte, Newfoundland, Canada. The mineral was found in an exposed ore dump, on the property of Consolidated Rambler Mines Limited near Baie Verte, Newfoundland. The mineral was first characterized by Ernest Henry Nickel, a mineralogist with the Department of Energy, Mines and Resources in Australia, in 1973.

Exchange spring magnet

An Exchange Spring Magnet is a magnetic material with high coercivity and high saturation properties derived from the exchange interaction between a hard magnetic material and a soft magnetic material, respectively. Coehoorn et al. were the first able to observe an actual exchange spring magnet. Exchange spring magnets are cheaper than many magnets containing rare earth/transition metals, as the hard phase of the magnet can be less than 15% of the overall magnet by volume.

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.

Magnadur is a sintered barium ferrite, specifically BaFe12O19 in an anisotropic form. It is used for making permanent magnets. The material was invented by Mullard and was used initially particularly for focussing rings on cathode ray tubes. Magnadur magnets retain their magnetism well, and are often used in education. Magnadur can also be used in DC motors.

References

  1. Carter, C. Barry; Norton, M. Grant (2007). Ceramic Materials: Science and Engineering. Springer. pp. 212–15. ISBN   978-0-387-46270-7.
  2. Okamoto, A. (2009). "The Invention of Ferrites and Their Contribution to the Miniaturization of Radios". 2009 IEEE Globecom Workshops. pp. 1–42. doi:10.1109/GLOCOMW.2009.5360693. ISBN   978-1-4244-5626-0.
  3. Shriver, D.F.; et al. (2006). Inorganic Chemistry. New York: W.H. Freeman. ISBN   978-0-7167-4878-6.
  4. 1 2 3 Ullah, Zaka; Atiq, Shahid; Naseem, Shahzad (2013). "Influence of Pb doping on structural, electrical and magnetic properties of Sr-hexaferrites". Journal of Alloys and Compounds. 555: 263–267. doi:10.1016/j.jallcom.2012.12.061.
  5. http://www.mag-inc.com/products/ferrite-cores/learn-more-about-ferrites
  6. Hosni (2016). "Semi-hard magnetic properties of nanoparticles of cobalt ferrite synthesized by the co-precipitation process".
  7. Olabi (2008). "Design and application of magnetostrictive materials".
  8. Sato Turtelli; et al. (2014). "Co-ferrite – A material with interesting magnetic properties". Iop Conference Series: Materials Science and Engineering. 60: 012020. doi:10.1088/1757-899X/60/1/012020.
  9. J. C. Slonczewski (1958). "Origin of Magnetic Anisotropy in Cobalt-Substituted Magnetite". Physical Review. 110 (6): 1341–1348. doi:10.1103/PhysRev.110.1341.
  10. Lo (2005). "Improvement of magnetomechanical properties of cobalt ferrite by magnetic annealing".
  11. Wang (2015). "Magnetostriction properties of oriented polycrystalline CoFe2O4".
  12. Aubert, A. (2017). "Uniaxial anisotropy and enhanced magnetostriction of CoFe2O4 induced by reaction under uniaxial pressure with SPS". Journal of European Ceramic Society. 37 (9): 3101–3105. arXiv: 1803.09656 . doi:10.1016/j.jeurceramsoc.2017.03.036.
  13. Aubert, A. (2017). "Enhancement of the Magnetoelectric Effect in Multiferroic CoFe2O4/PZT Bilayer by Induced Uniaxial Magnetic Anisotropy". IEEE Transactions on Magnetics. 53 (11): 1–5. arXiv: 1803.09677 . doi:10.1109/TMAG.2017.2696162.
  14. "Ferrite Permanent Magnets". Arnold Magnetic Technologies. Archived from the original on 14 May 2012. Retrieved 18 January 2014.
  15. "Barium Carbonate". Chemical Products Corporation. Archived from the original on 1 February 2014. Retrieved 18 January 2014.
  16. "Amorphous Magnetic Cores". Hill Technical Sales. 2006. Retrieved 18 January 2014.
  17. M. Wittenauer, P. Wang, P. Metcalf, Z. Ka̧kol, J. M. Honig (2007). Growth and Characterization of Single Crystals of Zinc Ferrites, Fe3−xZnxO4. Inorg. Synth. Inorganic Syntheses. pp. 124–132. doi:10.1002/9780470132616.ch27. ISBN   9780470132616.CS1 maint: Uses authors parameter (link)
  18. Marc de Vries, 80 Years of Research at the Philips Natuurkundig Laboratorium (1914-1994), p. 95, Amsterdam University Press, 2005 ISBN   9085550513.
  19. Raul Valenzuela, Magnetic Ceramics, p. 76, Cambridge University Press, 2005 ISBN   0521018439.
  20. R. Gerber, C.D. Wright, G. Asti, Applied Magnetism, p. 335, Springer, 2013 ISBN   9401582637
  21. F. K. Lotgering, P. H. G. M. Vromans, M. A. H. Huyberts, "Permanent‐magnet material obtained by sintering the hexagonal ferrite W=BaFe2Fe16O27", Journal of Applied Physics, vol. 51, pp. 5913-5918, 1980
  22. Raul Valenzuela, Magnetic Ceramics, p. 76-77, Cambridge University Press, 2005 ISBN   0521018439.

Sources