Eddy Currents are killing me.

My recollection (seemingly verified by a quick wikipedia check) is that mu-metal has some unique characteristics at low frequencies, but not at "high" frequencies, including in the 100's of kHz.

Have you considered insulated/laminated shheets of AL, just like iron lamibations in an isolationtransformer core ?

No comparison. At least, from what I know of mu-metal.

Permalloy and related alloys (same thing), at extremely fine thicknesses (1-3 mils), are useful in tape cores. They begin to roll off in the single digit kHz, and are strongly dispersive beyond that (more or less, mu inversely proportional to frequency). So while they might be mu=100k in the sub-kHz, they're below ferrite in the 100s of kHz and beyond.

Amorphous and nanocrystalline go a little bit further, being even thinner and a little more resistive (you can make extremely power-dense transformers this way, up to 50kHz or so). They typically outperform ferrites by a modest factor. But, the material is impossible to handle, being only a little flexible (enough to roll up into toroids or bond into 'C' shapes), and completely brittle (oh, the splinters and shards you get when it snaps!). Obviously, being in sheets, it's also anisotropic, so it doesn't make a good shunt or shield.

Ferrite has rated mu until ~100kHz at the lowest (for mu=10k materials), up to 10s of MHz for low mu NiZn types. The GBW product, so to speak, is relatively constant across ferrite materials and grades.

(There's also a mu peak in the MHz to GHz, due to standing waves in the material, dependent on physical size and shape. This is more obvious on ferrite beads, where you usually see a Z(f) curve. So if you were ever pondering the mu', mu'' curve of a given material, and trying to reconcile that with the Z curve of a bead of the same material, this is why.)

So of the applicable materials, ferrite really is a good winner here!

Tim

The magnetic field reaches nearby components. Smaller coil, shielded magnetic circuit, or larger spacing are possible answers. Have you looked at the Q of self-shielded inductors? For 100 kHz, there's ferrite-box cemented assemblies like this

The magnetic field reaches nearby components. Smaller coil, shielded magnetic circuit, or larger spacing are possible answers. Have you looked at the Q of self-shielded inductors? For 100 kHz, there's ferrite-box cemented assemblies like this

Good points. A shielded magnet circuit is my only escape route.

Thanks, Harry