Introduction to the Soft Magnetic Materials

Nanocrystalline materials, obtained by devitrification of the precursor amorphous alloy, displaying soft magnetic character (high magnetic permeability and low coercivity), have been the subject of increasing attention from the scientific community, not only because of their potential use in technical applications but also because they provide an excellent setting in which to study basic problems in nano-structures formation and magnetism [1-8]. In fact, these materials provide a crucial point in opening up new fields of research in materials science, magnetism, and technology, such as metastable crystalline phases and structures, extended solid solubilities of solutes with associated improvements of mechanical and physical properties, nano-crystalline, nanocomposite and amorphous materials that, in some cases, have unique combinations of properties (magnetic, mechanical, corrosion, etc.). Technological development of the fabrication technique of the amorphous precursor material and studies of the structure, glass formation ability, and thermodynamics, and magnetism of amorphous alloys were intensively performed in 1960s and 1970s. These aspects have been analyzed extensively in few review papers and books [9-11].

Most commercial and technological interests have been paid to soft amorphous and nanocrystalline magnetic materials. Initially, it was believed that ferromagnetism could not exist in amorphous solids because of a lack of atomic ordering. The possibility of ferromagnetism in amorphous metallic alloys was theoretically predicted by Gubanov [12] and the experimental confirmation of this improbable prediction was the main cause of the sudden acceleration of research on amorphous alloys from about 1970 onward, this onrush of activity was due both to the intrinsic scientific interest of a novel and unexpected form of ferromagnetism and also to the gradual recognition that this is the key to the industrial exploitation of amorphous ferromagnetic alloys. The amorphous alloy ribbons obtained by the melt-spinning technique have been introduced widely as the soft magnetic materials in the 1970s. Their excellent magnetic softness and high wear and corrosion resistance made them very attractive in the recording head and microtransformer industries. In contrast with the flood of work on magnetic behavior, the study of electrical transport (i.e., magnetoimpedance effect) is very recent and is making significant progress.

Conventional physical metallurgy approaches to improving soft ferromagnetic properties involve tailoring the chemistry and optimizing the microstructure. Significant in the optimizing of the microstructure is recognition that a measure of the magnetic hardness (the coercivity, Hc) is roughly inversely proportional to the grain size (D) for a grain size exceeding ~0.1 to 1 ¡m (where the grain size exceeds the domain wall thickness). In such cases, grain boundaries act as impediments to domain wall motion, and, thus, finegrained materials usually are magnetically harder than large grain materials. Significant developments in the understanding of magnetic coercivity mechanisms have lead to the realization that for very small grain size D < ~100 nm [13-21], Hc decreases rapidly with increasing grain size. This can be understood by the fact that the domain wall, whose thickness exceeds the grain size, now samples several (or many) grains so that fluctuations in magnetic anisotropy on the grain-size length scale are irrelevant to domain wall pinning. This important concept suggests that nanocrystalline and amorphous alloys have significant potential as soft magnetic materials.

In this section, we explore issues that are pertinent to the general understanding of the magnetic properties of amorphous and nanocrystalline materials. As the state of the art for amorphous magnetic materials is well developed and much of which has been thoroughly reviewed [11, 22-24], we will concentrate on highlights and recent developments. The development of nanocrystalline materials for soft magnetic applications is an emerging field for which we will try to offer a current perspective that may well evolve further with time.

The development of soft magnetic materials for applications requires the study of a variety of intrinsic magnetic properties as well as development of extrinsic magnetic properties through an appropriate optimization of the microstructure. As intrinsic properties, we mean microstructure insensitive properties. Among the fundamental intrinsic properties (which depend on alloy composition and crystal structure), the saturation magnetization, Curie temperature, magnetic anisotropy, and magnetostriction coefficient are all important. In a broader sense, magnetic anisotropy and magnetostriction can be considered as extrinsic in that, for a two-phase material (in aggregate), they depend on the microstructure.

A vast literature exists on the variation of intrinsic magnetic properties with alloy composition. Although new discoveries continue to be made in this area, it can be safely stated that a more wide open area in the development of magnetic materials for applications is the fundamental understanding and exploitation of the influence of the microstructure on the extrinsic magnetic properties. Important microstructural features include grain size, shape, and orientation; defect concentrations; compositional inho-mogeneities; magnetic domains; and domain walls. The interaction of magnetic domain walls with microstructural impediments to their motion is of particular importance to the understanding of soft magnetic behavior. Extrinsic magnetic properties important in soft magnetic materials include magnetic permeability and coercivity, which typically have an inverse relationship. Thorough discussions of soft magnetic materials are available [25-28, 29].

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