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

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 BaFe 12O 19 (BaM or barium ferrite), SrFe 12O 19 (SrM or strontium ferrite), and cobalt-titanium substituted M ferrite, Sr- or BaFe 12-2xCo xTi xO 19 (CoTiM).Z-type ferrites (Ba 3Me 2Fe 24O 41) such as Ba 3Co 2Fe 24O 41, or Co 2Z.Y-type ferrites (Ba 2Me 2Fe 12O 22), such as Ba 2Co 2Fe 12O 22, or Co 2Y.W-type ferrites (BaMe 2Fe 16O 27), such as BaCo 2Fe 16O 27, or Co 2W.X-type ferrites (Ba 2Me 2Fe 28O 46), such as Ba 2Co 2Fe 28O 46, or Co 2X.U-type ferrites (Ba 4Me 2Fe 36O 60), such as Ba 4Co 2Fe 36O 60, or Co 2U. 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 Ba 2Mg 2Fe 12O 22 Y ferrite at cryogenic temperatures, and now Sr 3Co 2Fe 24O 41 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 7-11 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. © 2012 Elsevier Ltd. All rights reserved.