R. Barrué

Composite magnetic materials based on nanocrystalline powders for middle- and high-frequency applications up to 1 MHz

The preparation of mechanically resistant composite materials made of nanocrystalline powders is described. Core losses per cycle, magnetic induction versus frequency (up to 200 kHz), and real and imaginary parts of the complex permeability, up to 500 MHz, are given. These results show that such composites may ensure fair magnetic properties for middle and high-frequency power applications up to 1 MHz.

Anisotropy and domain patterns of flash stress-annealed soft amorphous and nanocrystalline alloys

Abstract Domain structures and stress-induced anisotropy in some soft ferromagnetic materials are reported. Stress-Joule heating performed on amorphous Co66Fe4B12Si16Mo2 induces an anisotropy perpendicular to the ribbon axis, as in nanocrystalline FeSiBCuNb alloys. However, in the case of FeZrB–Cu–Nb systems, large longitudinal domains are observed. We think that the origin of this anisotropy comes from the so-called back stress effect as previously proposed by Herzer. As magnetostrictions of both α-Fe and α-FeSi crystallites are negative, the frozen-in stress exerted on α-Fe nanograins is expected to be compressive-type. We suggest that this surprising behaviour is related to more pronounced creep because of the smaller hardness of α-Fe nanograins.

Chapter 3 – Soft amorphous and nanocrystalline magnetic materials

Publisher Summary This chapter discusses soft amorphous and nanocrystalline magnetic materials. During the past few years, considerable work has been done with the aim of producing new soft-magnetic materials by nanocrystallization of an amorphous precursor. The main drawback of these kinds of metallic glass-based materials lies in the restricted forming possibilities, which are mainly limited to toroidal or C-core shapes. In the coming years, the main challenge related to nanocrystalline materials will probably lie in finding technological solutions to produce soft magnetic materials from ferromagnetic particles by powder metallurgy techniques. Such a material should be adaptable to integrated devices and planar technology. The main difficulty remains the insulation of nanoparticles that have to be perfectly effective but thin enough to allow full exchange coupling among particles. If such a technological barrier could be passed, the working frequency of high-permeability magnetic materials will dramatically increase and the use of new magnetic parts in the very-high-frequency bands will be possible.

Permeability of soft magnetic composites elaborated from flakes of nanocrystalline ribbons

Summary form only given. Finemet composites were made by cold pressing powder with resin . The initial permeability of samples was measured by means of an impedance analyzer up to 1.8 GHz. The permeability of powder cores is usually modeled using the nonmagnetic grain boundary model (NMGB) which is improved by assuming the distribution of particles on a flat body centered-like lattice.