Nanostructured Magnetic Materials

Mossbauer spectroscopy has been extensively used for studies of nanostructured materials and several reviews on magnetic nanoparticles have been published, see e.g. [6-8, 46 8]. The magnetic properties of nanoparticles may differ from those of bulk materials for several reasons. The most dramatic effect of a small particle size is that the magnetization direction is not stable at finite temperatures, but fluctuates. 

There is considerable interest in developing new types of magnetic materials, with a particular hope that ferroelectric solids and polymers can be constructed— materials having spontaneous electric polarization that can be reversed by an electric field. Such materials could lead to new low-cost memory devices for computers. The fine control of dispersed magnetic nanostructures will take the storage and tunability of magnetic media to new levels, and novel tunneling microscopy approaches allow measurement of microscopic hysteresis effects in iron nanowires. 

Debra Rolison (right) was born in Sioux City, Iowa in 1954. She received a B.S. in Chemistry from Florida Atlantic University in 1975 and a Ph.D. in Chemistry from the University of North Carolina at Chapel Hill in 1980 under the direction of Prof. Royce W. Murray. She joined the Naval Research Laboratory as a research chemist in 1980 and currently heads the Advanced Electrochemical Materials section. She is also an Adjunct Professor of Chemistry at the University of Utah. Her research at the NRL focuses on multifunctional nanoarchitectures, with special emphasis on new nanostructured materials for catalytic chemistries, energy storage and conversion, biomolecular composites, porous magnets, and sensors. 

Accurate control of microstructure on nanometric scale makes it possible to control magnetic and mechanical properties to a hitherto unattainable degree. In particular, magnetic nanostructures have recently become the subject of an increasing number of experimental and theoretical studies. The materials are made of alternating layers, around 10 A thick, of magnetic (e.g., cobalt) and nonmagnetic metals (e.g., copper). 

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 broad variety of magnetic nanostructures corresponds to a diverse range of processing methods. The suitability of individual methods depends on the length scale and geometry of the nanostructures. In addition, each method is usually restricted to a relatively narrow class of magnetic materials. 

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. 

Abstract The focus of this chapter is primarily directed towards nanocrystalline soft magnetic materials prepared by crystallization of amorphous precursors. The key elements involved in the development of this class of materials are three-fold (i) theoretical models for magnetic softness in nanostructures (ii) nanostructure-property relationships and (iii) nanostructural formation mechanisms. This chapter surveys recent research on these three areas with emphasis placed on the principles underlying alloy design in soft magnetic nanostructures. 

The behavior of magnetic nanostructures reflects both nanoscale features, such as particle size and geometry, and the intrinsic properties of the magnetic substances. For example, the magnetization reversal in nanodots crucially depends on the anisotropy of the dot material. Furthermore, nanostructures are often used as bulk materials, so that their extrinsic properties must be evaluated from the point of view of bulk materials. This appendix summarizes the characteristics of some important classes of magnetic materials and provides exemplary data. 

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