Ferrite resistivity

About what temperature do they work down to?

Best regards, Spehro Pefhany

I know a bunch of Roger Koch's guys, so I could ask them if they know.

Great guy, Roger--really one of the white hats, and one of the best physicists I've ever met. RIP.

Cheers

Phil Hobbs

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Well, if you have a square-section bead... make a jig with a central pin (t= o fit the central hole of the toroid) and four pogo pins (spring-loaded contacts) that engage the flat face of the bead at 0/90/180/270 degrees. Inject current at 0 and 90, sense voltage at 180 and 270.

It's not the usual four-point measurement (for sheets, there's a square-arr= ay technique that's similar) but it should work well enough to track any change in properties.

I have used them in LHe only.

Regards, Mikko

Thanks for the offer. However, I dug out their paper and the material *is* mentioned there. They used T30-6 carbonyl-iron cores from Micrometals Inc. Actually, this sounds suspiciously similar to Amidon #6, I wonder if there is an universal numbering system for core materials? For instance Amidon #43 ferrite looks quite similar to Fair-Rite #43 (from memory, the Fair-Rite web site is down so I cannot check).

I mis-rememberd the # of the yellow-gray material, by the way, the now- obsolete stuff I've used was #4, not #6.

Regards, Mikko

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Thank you, There's nothing wrong with old knowledge.

George H.

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OK this is a pure speculation. But I'd guess they all change somewhere near 120 K. As soon as we get some more LN2 in I'll try some tests and post results. It's apparently a phase change in the Fe2O3 that happens in magnetite. And all the ferrites have Fe2O3 in them... so....

George H.

"The Journey is the reward"

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Amidon is a Fair-Rite distributor, so that similarity isn't accidental.

Cheers

Phil Hobbs

Also Magnetics Inc. and Micrometals, so if you ever want all the data you can find, check around. To designate material, Fair-Rite uses two digit numbers, Micrometals uses 1-2 digits and color codes, and Magnetics uses letters for their ferrites (e.g., W is mu_r = 10k).

Most of these are also available from other distributors, like Adams Magnetic and Elna Magnetics; they have an online searchable inventory, but weak or no catalog; they don't have online ordering, but they're responsive and nice to deal with.

I have never seen a powder core that wasn't recognizably Micrometals or Magnetics (who also bought Arnold, whose cores are designated with 6-7 digits). They probably all buy from the same Chinese manufacturer.

There are more ferrite manufacturers than powdered iron, but most OEMs either get something from Fair-Rite, Mag-Inc, Ferroxcube or TDK, or something very similar to their products.

Tim

Thanks, so surely useless at 4K

Best regards, Spehro Pefhany

Thanks Tim, Phil, this is interesting. So the materials list

Regards, Mikko

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Hi Adrian, first I wanted to thank you again. (SED is an amazing repository of knowledge.) And second there is a reference in Kittel's Solid state physics to a Philips technical review.

J.J. Went and E.W. Gorter, "Magnetic and electrical properties of Ferroxcube materials," Philips Tech. Rev. 13, 181 (1952)

The local Uni. library seems to have this in deep storage and it's not clear that I (as an alumnus) will be able to get access. But if anyone has a copy of this old journal I'd really like to know if they cover the electrical and magnetic properties at low temperatures (down to liquid nitrogen.) If so then I'll put more effort into digging up a copy. (After all a few hours in the library can save days in the lab!)

George H.

Yeah that's according to Mikko. I think he's looking for one of the iron garnets (YIG) that look like they will work at 4K. (Though getting them in toroids or some other convenient shape may be hard.)

George H.

"The Journey is the reward"

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Nice!

I was reading on the Jack Smith's website (clifton labs) that there may have been some recent change in the way the 43 material is made.

Which may explain the very high resistivity I measured (or couldn't measure :^) for the newer beads.

Though I didn't report it in my first post, I also found that the shorter beads (1/8" length vs 1/4") had a resistivity that was closer to what was listed on the spec sheets. This sort of fits with a model of a random conduction path through the material. So if I grind down thinner samples I may get resistivities that are closer to what I expect.

George H.

There was a very good, but very theoretical, book put out by Philips in the 1950s ( or maybe even earlier ). Cant give you the title, or even the authors, but that paper probably refers to it. Lots of good physics, and certainly much more than just the manufacturers specifications etc. Extreme low temps were discussed, certainly there were plots of permeability over the full temp range, and how they related to the compositions, but I dont remember any details.

Interesting. Maybe "Ferrites" by Smit & Wijn (1959)? There's the full text on archive.org,

(Merci, Monsieur Google.)

Cheers

Phil Hobbs

There are a few plots going down to about 77K in Snelling's "Soft Ferrites", but nothing lower that I could find.

Jeroen Belleman

Smit & Wijn was the one I remember cutting my teeth on.

Amazing that its available on-line.