Soft Magnetic Nanostructures and Applications

School of Physics and Materials Engineering Monash University Clayton, VIC 3800, Australia 

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. 

Two major alloy systems in the family of soft magnetic nanostructures are Fe-Si-B-Nb-Cu [1, 4-6] and Fe-M-B-(Cu) (M= Zr, Hf or Nb) [3, 7-9], commercially known as FINEMET (or VITROPERM) and NANOPERM, respectively. They are produced by primary crystallization of amorphous ribbons. Hence, the nanostructural formation upon crystallization as well as the mechanism of magnetic softening has been actively studied for these two 

Following this introductory section, we will overview the development of random anisotropy models and discuss the origin of the magnetic softness in nanostructures. The nanostructural formation process and alloy development in the Fe-M-B-(Cu) alloys to which less attention has been addressed in the previous reviews, will be another focal point in this chapter. 

Magnetically soft materials are used as magnetic cores for the purpose of magnetic flux multiplication. Major requirements for the materials include high saturation magnetization (Js = pi0Ms,), small coercivity (Hc) and high permeability pi). 

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The effect of alloying elements on K for Fe-based crystalline thin films has been reviewed by Hayashi et al. [37], It was summarized that K = 0 is expected in Fe-Ga-Si, Fe-Al-Ge, Fe-Co-Ga-Si and Fe-Co-Al-Ge, along with classical Fe-Ni and Fe-Al-Si. Consequently, the addition of Al, Si, Ga and Ge to Fe-based magnetic nanostructures may result in a further reduction of . However, a substantial amount of these additives is accompanied by a serious decrease in the saturation magnetization [38], making the Fe-based soft magnetic nanostructures featureless from the viewpoint of technological applications. 

The deposition of Fe-Ni alloys is of industrial interest because these materials find applications in electronic devices (e.g. PC hard disk). The most popular alloys are Permalloy (soft magnetic properties) and Invar (veiy low thermal expansion). The magnetic and mechanical properties of Fe-Ni alloy can be designed by nanostructuring. Natter and Hempelmann (2003) used an electrolyte containing 40g/l NiS04, 20g/l (NH lCl, 20g/l Na citrate, 5g/l citric acid, lg/1 saccharin, 45 g/1 boric acid and a variable content of iron (II) ammonium sulfate. The pulse parameters used were t 2 ms, 48 ms and 250mA/cm2. For different concentrations of iron salts, alloys (crystallite size, 16-19 nm) with iron content between 0 and 71 mol% conld be obtained. 

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],... 

In Volume 6, various topics related to nanotechnology and nanostructured materials are discussed. The topics include electrochemically self-assembled ordered nanostructured arrays (quantum dots, dashes and wires), mechanical spectroscopy of nanostructured metallic systems, soft amorphous and nanociystalline magnetic materials, nanoporous materials for microlasers and microresonators, nanoporous materials for optical applications, optical properties and impurity states in nanostructured materials, and confined systems and nanostructured materials. 

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