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R. Grössinger:
"Properties, Benefits and Application of Nanocrystalline Structures in Magnetic Materials ";
Vortrag: 2nd International Conference on Nanomaterials by Severe Plastic Deformation, Wien, Österreich (eingeladen); 09.12.2002 - 11.12.2002.

Modern permanent magnet materials are based on 3d-4f intermetallics. High quality magnets based on SmCo5 on Sm2Co17 and on Nd2Fe14B are in use. Recently also hard magnetic nanocrystalline materials were developed. In hard magnetic nanocrystalline materials the magnetic exchange length is smaller than the grain size but still comparable. This leads to a remanence enhancement which makes these materials interesting for applications. The remanence enhancement was first observed in Nd-Fe-B-Si and later also in many other compounds. Exchange coupled hard magnetic materials can be divided into two main groups:
· single phased nanocrystalline
· nanocomposites, also known as spring magnets (contains a hard + soft magnetic phase)
Those materials consist of isotropic hard and soft magnetic grains with diameters ranging from 20 to 50 nm. Many investigations on this field showed that when the grain sizes decrease under ~50 nm they exhibit magnetic properties which are different from those of microcrystalline magnetic materials. An overview of the production methods for nanocrystalline materials will be given. Additional the magnetic properties as well as micro-structure of nanocrystalline materials is shown.
For applications the most interesting feature is an enhanced remanence for isotropic grains with uniaxial easy axis. Here the ratio Mr/Ms is higher than the expected 0.5 for noninteracting uniaxial grains. This leads to a high energy product even for isotropic material.
Beside the metallic nanocrystalline material also ferrite based nano-crystalline magnets were investigated. It seems that in the case of localised 3d-electrons no exchange enhancement is possible.

Nanocrystalline materials form also a family of new soft magnetic compounds. There amorphous ribbons with the approximate composition Fe-Cu-Nb-B-Si are heat treated in order to achieving a nanocrystalline state. The composition is mainly Fe73.5(Si,B)22.5 where the addition of Cu and Nb hinders the grain growing thus leading to crystals of about 10 - 15 nm. This material was developed in Hitachi Metals company and is under the name "Finemet" on the market. The magnetic behaviour is strongly determined by the ratio of crystalline to amorphous phase, which can be adjusted by the heat treatment.
The second nanocrystalline soft magnetic system of importance is based on a Fe-Zr-Cu-B amorphous alloy which also transforms after a heat treatment of about 600oC for 1h into the nanocrystalline state. In this material nanocrystals of a-Fe are embedded in an amorphous matrix. In both cases the exchange coupling between the nanocrystals leads to excellent soft magnetic properties at room temperature. The physical properties of all these systems will be surveyed.