Here is what is called finemet core

FINEMET® is registered trade mark of Hitachi Metals Ltd.

FINEMET® is a Nanocrystalline amorphous soft magnetic metal. It's phase characteristics is far better than conventional core material such as Ni and Co-based amorphous metal. Amorphous metals do not have a crystalline structure like other magnetic materials. All the atoms in an amorphous metal are randomly arranged, thus giving it a higher resistivity (about three times) value than that for crystalline counterparts. Amorphous alloys are prepared by cooling the melt at a rate of about million degrees per second. This fast cooling does not give the atoms enough time to rearrange into a stable crystalline form. As a result one gets metastable amorphous structure. Because of the absence of crystalline structure amorphous alloys are magnetically soft (lower coercivity, lower core loss, higher permeability). High resistivity gives lower loss at higher frequencies. The losses are among the lowest of any known magnetic materials.

Features:

1) Satisfy both high saturation magnetic flux density and high permeability

High saturation magnetic flux density comparable to Fe-based amorphous metal. High permeability comparable to Co-based amorphous metal.

2) Low core loss.

1/5th the core loss of Fe based amorphous metal and approximately the same core loss as Co-based amorphous metal.

3) Low magnetostriction. Less affected by mechanical stress. Very low audio noise emission.

4) Excellent temperature characteristics and small aging effects. Small permeability variation (less than ±10%) at a temperature range of -50°C~150°C. Unlike Co-based amorphous metals, aging effects are very small.

5) Excellent high frequency characteristics. High permeability and low core loss over wide frequency range, which is equivalent to Co-based amorphous metal.

6) Flexibility to control magnetic properties “B-H curve shape” during annealing Three types of B-H curve squareness, high, middle and low remanence ratio, corresponding to various applications.