Amorphous and nanocrystalline magnetic materials

All "soft" ferromagnets, whether crystalline or not, are essential components of almost every electrical or electronic device in modern civilization. Their main role is to amplify and/or channel the magnetic flux produced by a current or a magnet. The current market for these materials can be divided into four major families:

• (a) electrical steels (mainly Fe-3%Si alloys);

• (b) iron-nickel and iron-cobalt alloys;

• (c) soft ferrites ;

• (d) amorphous and nanocrystalline metal alloys.

The economic interest of these materials reflects their worldwide annual production. For example, family (a) can be split into two: grain-oriented Fe-Si sheets (GO), with a worldwide production of 2 Mt/year, used mainly in the manufacture of distribution transformers, and non-oriented sheets (NO), with a production of 11 Mt/year, used mainly in the manufacture of electrical machines. Production of materials in families (b) and (c) is of the order of a few dozen kt/year and 1 to 2 Mt/year, with significant use in electronics. A reasonable estimate of world tonnage for materials in family (d), which are of interest to us here, was 20 kt/year in 1997 and would be in the order of 300 to 600 kt/year in 2023 for nanocrystalline materials, with a forecast growth rate of 10% per year, i.e. around 1 Mt/year in 2033, and around 1 Mt/year for amorphous materials, with a forecast growth rate of 6%, i.e. just under 2 Mt/year in 2033.

These materials (d) can be obtained by various processes, but here we'll focus on those prepared in ribbon form by rapid solidification of a liquid. The product is amorphous, i.e. its constituent atoms do not exhibit long-range order. For certain specific atomic compositions, controlled crystallization of the amorphous alloy can produce two-phase materials with a nanocrystalline phase embedded in a matrix that remains amorphous. Amorphous and nanocrystalline materials feature low magnetic anisotropy, which can be adapted to the applications targeted by post-quench heat treatments. Their magnetic properties are remarkable: a low coercive field, high permeabilities (impedance or initial), low electromagnetic losses and ease of property control through heat treatments. Depending on their composition, these alloys have saturation magnetizations of between 0.5 and 1.7 T, with magnetostriction values ranging from close to zero to as high as 35 × 10 ^-6 .

This wide range of magnetic properties means that, for certain applications, these materials can compete with the other three categories of soft ferromagnets (a, b, c). They can be used in a wide frequency...