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 Co(2)Z. Y-type fenites (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, V. 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 7-11 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. (C) 2012 Elsevier Ltd. All rights reserved.
keywords
SOL-GEL METHOD; U-TYPE HEXAFERRITES; M-TYPE BARIUM; MICROWAVE ABSORBING PROPERTIES; LOW-TEMPERATURE FORMATION; SUBSTITUTED STRONTIUM HEXAFERRITE; FREQUENCY MAGNETIC-PROPERTIES; MODIFIED CO(2)Z HEXAFERRITE; RARE-EARTH SUBSTITUTIONS; CITRATE PRECURSOR METHOD
subject category
Materials Science
authors
Pullar, RC
our authors
Rob Pullar
Collaboratoracknowledgements
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 (Fundacao para a Ciencia e a Tecnologia in Portugal) and the FCT Ciencia 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 figures from previous sources used in this article, which have been referenced in the relevant figure caption.