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Hexagonal Ferrite Fibres
Keywords: hexagonal ferrites, fibres, magnetic matereials
The hexagonal ferrites, also know as hexaferrites, 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, and as GHz electromagnetic wave absorbers for EMC, RAM and stealth technologies. The hexagonal ferrites are all ferrimagnetic materials, and their magnetic properties are intrinsically linked to their crystalline structures. They all have a magnetocrystalline anisotropy; 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. There is currently increasing interest in composite materials containing hexaferrite fibers. It had been predicted that properties such as thermal and electrical conductivity, and magnetic, electrical and optical behavior will be enhanced in material in fibrous form. This is because a continuous fine fiber can be considered as effectively one-dimensional, and it does not behave as a homogeneously distributed solid. Although the intrinsic magnetisation of the material is unaffected, the effective magnetisation of an aligned fiber sample should be greater when a field is applied parallel with fiber alignment compared to when applied perpendicularly to fiber alignment. This feature was first demonstrated by the author for aligned hexaferrite fibers in 2006, who was the first to synthesise fibres of BaM, SrM, Co2Y, Co2Z, Co2W, Co2X and Co2U hexaferrites [1], as micron-scale (3-10 um) continuous fibers in both random and aligned blankets. The latest results for such hexaferrite microfibres will be presented.
Robert C. Pullar, Investigador
University of Aveiro
Aveiro, Aveiro

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