Elsevier

Progress in Materials Science

Volume 57, Issue 7, September 2012, Pages 1191-1334
Progress in Materials Science

Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

https://doi.org/10.1016/j.pmatsci.2012.04.001Get rights and content

Abstract

Since their discovery in the 1950s there has been an increasing degree of interest in the hexagonal ferrites, also know as hexaferrites, which is still growing exponentially today. These have become massively important materials commercially and technologically, accounting for the bulk of the total magnetic materials manufactured globally, and they have a multitude of uses and applications. As well as their use as permanent magnets, common applications are as magnetic recording and data storage materials, and as components in electrical devices, particularly those operating at microwave/GHz frequencies. The important members of the hexaferrite family are shown below, where Me = a small 2+ ion such as cobalt, nickel or zinc, and Ba can be substituted by Sr:

  • M-type ferrites, such as BaFe12O19 (BaM or barium ferrite), SrFe12O19 (SrM or strontium ferrite), and cobalt–titanium substituted M ferrite, Sr- or BaFe12−2xCoxTixO19 (CoTiM).

  • Z-type ferrites (Ba3Me2Fe24O41) such as Ba3Co2Fe24O41, or Co2Z.

  • Y-type ferrites (Ba2Me2Fe12O22), such as Ba2Co2Fe12O22, or Co2Y.

  • W-type ferrites (BaMe2Fe16O27), such as BaCo2Fe16O27, or Co2W.

  • X-type ferrites (Ba2Me2Fe28O46), such as Ba2Co2Fe28O46, or Co2X.

  • U-type ferrites (Ba4Me2Fe36O60), such as Ba4Co2Fe36O60, or Co2U .

The best known hexagonal ferrites are those containing barium and cobalt as divalent cations, but many variations of these and hexaferrites containing other cations (substituted or doped) will also be discussed, especially M, W, Z and Y ferrites containing strontium, zinc, nickel and magnesium. The hexagonal ferrites are all ferrimagnetic materials, and their magnetic properties are intrinsically linked to their crystalline structures. They all have a magnetocrystalline anisotropy (MCA), that is the induced magnetisation has a preferred orientation within the crystal structure. They can be divided into two main groups: those with an easy axis of magnetisation, the uniaxial hexaferrites, and those with an easy plane (or cone) of magnetisation, known as the ferroxplana or hexaplana ferrites. The structure, synthesis, solid state chemistry and magnetic properties of the ferrites shall be discussed here. This review will focus on the synthesis and properties of bulk ceramic ferrites. This is because the depth of research into thin film hexaferrites is enough for a review of its own.

There has been an explosion of interest in hexaferrites in the last decade for more exotic applications. This is particularly true as electronic components for mobile and wireless communications at microwave/GHz frequencies, electromagnetic wave absorbers for EMC, RAM and stealth technologies (especially the X and U ferrites), and as composite materials. There is also a clear recent interest in nanotechnology, the development of nanofibres and fibre orientation and alignment effects in hexaferrite fibres, and composites with carbon nanotubes (CNT). One of the most exciting developments has been the discovery of single phase magnetoelectric/multiferroic hexaferrites, firstly Ba2Mg2Fe12O22 Y ferrite at cryogenic temperatures, and now Sr3Co2Fe24O41 Z ferrite at room temperature. Several M, Y, Z and U ferrites have now been characterised as room temperature multiferroics, and are discussed here. Current developments in all these key areas will be discussed in detail in Sections 7 The microwave properties of hexagonal ferrites, 8 Magnetoelectric (ME), multiferroic (MF) and dielectric properties of hexaferrites, 9 Hexaferrite composites, 10 Hexagonal ferrite fibres, 11 Nanoscale hexagonal ferrite particles, ceramics and powders of this review, and for this reason now is the appropriate time for a fresh and critical appraisal of the synthesis, properties and applications of hexagonal ferrites.

Section snippets

Note on magnetic units, conversion factors and hysteresis loops

There exists a real confusion in the literature, with several different systems of units being used even today. The most common system in the literature is CGS (centimetre-gram-second system), and this is still used by many authors today, even though the accepted international standard has been the SI system for decades now. All values in this text have been converted to SI units, but some figures used may have CGS units.

QuantitySymbolCGS unitConversion factor to SISI unit
Magnetic

The discovery, composition and characteristics of the hexagonal ferrites

The magnetic mineral magnetoplumbite was first described in 1925 [2], and in 1938 the crystal structure was deduced as being hexagonal with the composition PbFe7.5Mn3.5Al0.5Ti0.5O19 [3]. The synthetic form of magnetoplumbite was found to be PbFe12O19, or pure PbM, and a number of isomorphous compounds were suggested including BaFe12O19, although this material was not structurally investigated until after the Second World War, when Philips Laboratories led the way in developing ferrites under

The structure of the hexagonal ferrites

All of the hexagonal ferrites have closely related, highly complex crystal structures [18], which can be interpreted in various ways as summarised in Table 3. At a simple level, they can all be seen as molecular combinations of the three ferrite compounds S (spinel, MeFe2O4), M (BaFe12O19) and Y (Ba2Me2Fe12O22). W ferrite, BaMe2Fe16O27, can be considered as M + 2S, X ferrite (Ba2Me2Fe28O46) = W + M = 2M + 2S, Z ferrite (Ba3Me2Fe24O41) = M + Y, and U ferrite (Ba4Me2Fe36O60) = Z + M = 2M + Y. HRTEM images of the

Synthesis methods for hexagonal ferrites

The formation of the hexagonal ferrites is an extremely complicated process, and the mechanisms involved are not fully understood despite having been investigated by many researchers for over 50 years [21], [45], [46], [47], [48]. If a non-stoichiometric mixture of BaO·Fe2O3·CoO is heated the following products generally form and decompose in this order:

Empty CellMajor productsMinor products
500 °Cα-Fe2O3, Co3O4, BaO
600 °Cα-Fe2O3, Co3O4, BaOCoFe2O4, BaFe2O4
700 °Cα-Fe2O3, CoFe2O4, BaFe2O4BaM, BaO
800 °CBaM, BaFe2

The solid state chemistry of the hexagonal ferrites

Many studies have been made of the solid state reactions of the BaO·Fe3O3·MeO system using standard ceramic preparations from oxides and BaCO3. M.A. Vinnik constructed several phase diagrams (Fig. 16) and obtained the X-ray spacings of BaM, Y, Z and W for samples containing more than 50% Fe2O3. In samples which were heated to 1200 °C/2 h, pressed and then annealed at 1250 °C/4 h, no Co2X or Co2U could be detected even at their stoichiometric compositions in polycrystalline samples [46]. Also

Magnetism in hexagonal ferrites

All hexagonal ferrites contain at least one large metal 2+ ion (usually Ba2+ or Sr2+), which causes a slight perturbation in the lattice due to size differences, and is responsible for the magnetocrystalline anisotropy (MCA) in hexaferrites. The most common hexagonal ferrites have a preferred axis of magnetisation along the c-axis, so loose crystals in an applied field will align themselves with the c-axis parallel to the field, showing a different XRD pattern to randomly oriented samples. The

A brief theory of microwave resonance and losses in ferrites

Dielectric losses occur in materials due to the damping of the vibrations of electrical dipoles, and as well as intrinsic losses due to crystal structure, extrinsic losses due to impurities, porosity, and grain boundaries in polycrystalline materials, dominate at higher frequencies, causing a great decrease of permittivity in most materials at MW frequencies. However, the effective magnetic permeability, μeff, is virtually independent of frequency, but there is a critical frequency, fc, above

Dielectric properties

The dielectric properties as well as magnetic properties are very important for many high frequency applications of hexaferrites, particularly if they are to become integrated chip components. The key properties of interest for many applications are resistivity and permittivity (dielectric constant, ε, or relative permittivity, εr, is a measure of how easy it is to establish, or “permit”, an electric flux in a material), which in general should both be as high as possible at as higher frequency

Multiphase ceramic composites

To lower the coercivity of BaM while maintaining the high saturation magnetisation for use in magnetic recording applications, composite ferrites have been made, in which BaM is coated with 36 wt.% nanocrystalline superparamagnetic particles of iron oxide, which have zero coercivity. The original M ferrite had a coercivity of 271 kA m−1 and Ms = 70.3 A m2 kg−1, and after particles of δ-FeOOH (Ms = 19.7 e A m2 kg−1) were added to the surface the composite had an Hc of 182 kA m−1 but a much lower Ms of 45.5 A m2 

Hexagonal ferrite fibres

As shown in the previous section, it has been predicted that properties such as thermal and electrical conductivity, and magnetic, electrical and optical behaviour could be enhanced in material in fibrous form. This is because a continuous fine fibre can be considered as effectively one-dimensional, and it does not behave as a homogeneously distributed powder or sintered monolith. Although the intrinsic magnetisation of the material is unaffected, the effective magnetisation of an aligned fibre

Nanoscale hexagonal ferrite particles, ceramics and powders

For the purposes of this section we will consider nanoparticles (NPs)/nanocrystals (NCs)/nanoscale artefacts to be below 100 nm in dimension, and disregard thin films, as this subject would be deserving of a review all of its own. When at the sub-100 nm scale, the NPs are below the critical minimum domain size for maximum coercivity, and Hc will decrease greatly with decreasing grain size as the domain size at which the hexaferrites become superparamagnetic is being approached. Normally, any

Applications of hexagonal ferrites

Magnets are used in a multitude of applications, for example motors, generators, transformers, actuators and sensors, information storage, mobile communications, transport, security, defence and aerospace, diagnostic devices and to focus electron beams. The most used magnetic materials are ferromagnetic metals and alloys or ferrimagnetic ceramics. Of the ceramics, by far the most used are hexagonal ferrites, and some of their multitude of applications are shown in Fig. 64. The total number of

Acknowledgments

The author would firstly like to thank Prof. T. Massalski for being a very patient and understanding editor. Thanks also to D.V. Karpinsky and A.L. Kholkin at CICECO/Aveiro University for providing the MFM measurements in Fig. 42. The FCT (Fundação para a Ciência e a Tecnologia in Portugal) and the FCT Ciência 2008 program are acknowledged for funding the author during the writing and publication of this paper. The author would also like to thank the publishers and copy write holders of all

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