Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics
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.Quantity Symbol CGS unit Conversion factor to SI SI 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 Cell Major products Minor products 500 °C α-Fe2O3, Co3O4, BaO 600 °C α-Fe2O3, Co3O4, BaO CoFe2O4, BaFe2O4 700 °C α-Fe2O3, CoFe2O4, BaFe2O4 BaM, BaO 800 °C BaM, 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
References (558)
- et al.
J Alloys Compd
(2001) - et al.
J Non-Cryst Solids
(1988) - et al.
J Alloys Compd
(2008) - et al.
J Magn Magn Mater
(1989) - et al.
J Magn Magn Mater
(2003) - et al.
J Magn Magn Mater
(2001) - et al.
J Magn Magn Mater
(2003) - et al.
J Magn Magn Mater
(1999) - et al.
J Alloys Compd
(2004) - et al.
J Magn Magn Mater
(1992)