Hard-Nanostructure Magnetism

In the section 2 of this chapter, an introduction is given to the main concepts required for the understanding of coercivity in hard magnetic materials. The various types of hard nanostructures and the procedures used for their preparation are described in section 3. The specific properties of hard nanostructure magnetism are described in section 4. In section 5, the properties of industrial materials are presented and some applications are briefly summarised. 

The original magnetic properties of hard nanostructures, described in the above sections, are direct consequences of the crystallite particle size reduction, when this size approaches the domain wall thickness, S. In this section, we apply and extend the findings of Chapters 3 and 4 to hard-magnetic nanostructures. We discuss (i) systems of exchange decoupled particles, and (ii) systems of exchange-coupled particles, themselves divided into single-phase systems and nanocomposites. 

In hard nanostructures, the magnetization is by nature highly heterogeneous and the validity of the usual demagnetizing field corrections may thus be questioned [129], Let separate the demagnetizing field Hn acting on a given nanoparticle, (/), into three contributions ... 

The main application of hard nanostructures is for the preparation of bonded magnets. Although economically marginal today, MEMS constitute a specific domain of applications, since the magnetic properties of nano-structured materials are maintained at small magnet dimensions. Finally it is realised that hard nanostructured materials could find applications as high density recording media. 

Electronic storage media Hard-disks based on nanostructured magnetic multilayers currently dominate the market. 

Furthermore, this technique allows variation of the grain size [30-33] this is important because many chemical and physical properties of nanostructured materials depend on the grain size. Only by variation of the crystallite size - this is a novd aspect in materials sdence and technology [34] - is it possible to tune and hopefully improve certain physical properties of one and the same material for example, the enhanced hardness of nano-Au, the toughness of nano-Ni/P alloys [35], the soft magnetic properties of nano-Ni [36] and the resistance of nanostructured materials [37, 38] promise industrial applications [39-41],... 

The description of nanomagnets requires new approaches. First, nanostructures are not periodic and tend to have large surface-to-volume ratios. Because of this the magnetization is not uniform across the nanostructure, local magnetic moments differ from site to site, exchange coupling varies throughout the nanostructure, and the anisotropy can be quite different from bulk or surface anisotropies. Second, it is hard to define properties in the similar fashion as in the bulk or as in case of molecules. 

Hard-Magnetic Nanostructures 3.3.3. FePt-hased nanocomposites... 

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