# Ferrite resistivity

Discussion in 'Electronic Design' started by George Herold, Sep 14, 2012.

1. ### George HeroldGuest

So following a comment by Mikko that ferrites stop working when cooled
to liq. helium temperatures. I ‘discovered’ that ferrites have a
phase change somewhere near 120 K. (at least magnetite does.)
Wiki is mostly silent on the subject though there is a bit here.
http://en.wikipedia.org/wiki/Charge_ordering

So first I’m a ferrite novice. I’ve wound some RF transformers in the
deep past, but that’s about it.
I found that I could measure the resistivity of some ferrite beads
with just my DMM. I’ve got two types of bead the 43 material and the
73 material. The reported resistivity’s are 43 = 1E5 ohm-cm and 73 is
100 ohm-cm.

I made a little jig to squeeze the beads in. The brass screw has a
cone turned on the end.

http://bayimg.com/FAcKEAAeE

So first off there seems to be a huge variation in the piece to piece
resistivity. At least an order of magnitude in the few pieces I
looked at.
Second the resistivity was (most of the time) much higher than the
reported numbers.

1E6 to 1E7 for the 43 material and 3E3 to 2E2 for the 73. (The 200
Ohm-cm for one piece of 73 was about right.)

And finally the 43 material was some older stuff in my parts box. I’m
not quite sure of it’s provenance. So I got some new pieces out of
stock. For the new material I couldn’t measure the resistance with my
setup. I even biased the bead from a 30 volt supply and used the
10Meg of the DMM as a voltage divider... I could measure a 1 G-ohm
resistor that way, but not the beads! resistance greater than 10 G
ohm or os.

I’m wondering if anyone has some more in depth knowledge they might
share. The ferrites look like they might be a ‘model system’ for some
new solid state experiments. Besides looking at the Curie
temperature the phase transition at 120K shows a peak in the heat
capacity, change in resistivity and magnetic properties. What could
be better!

Thanks,

George H.

2. ### whit3rdGuest

[and on resistivity of the ferrite material]
Probably the resistivity is quoted mainly because it affects inductor Q.
Ferrites are, alas, composite materials; the magnetic grains (of hopefully
uniform size, big enough to magnetize) are in a matrix of fired ceramic
like kaolin clay. So, electric conduction depends on percolation through
the ceramic matrix, or through contact between grains, or in the boundary
layer where the kaolin is in contact with the hematite/magnetite.

It is dubious that resistivity repeats from batch to batch, or from one contact
point to another. A bit of internal fracture might not change the magnetic
properties much, but would kill a resistivity measurement. I've done
impedance measurement on ferrites that had LOTS of internal movement,
at some frequencies the internal fractures rang like a lossy bell.

3. ### Bill SlomanGuest

Zebra strip?

http://www.fujipoly.com/design-guidelines/89.html

Farnell don't seem to stock it any more, so you may find it difficult
get hold of.
The Kelvin connection scheme is always attractive.

4. ### George HeroldGuest

I did stick the extreme pieces of 73 on an SRS RCL meter. (1.2k and
25k) (wire through the bead) The 25k did show less resistance and
higher Q consitent with the resistivity... but only a 10-20%
difference.

So if most of the resistance is because of some random path... I still
should be able to see any phase change in the resistivity.

George H.

5. ### George HeroldGuest

Sure, the resistance changed as I cranked on the screw. But only by
maybe a factor of 2 or 3. I could tighten things up to within ~20% of
the cracking point.... after I'd cracked a few beads.
Oh yeah this was very crude... the spinning of the screw was the worst
part.

I tried using some 'gummy' copper tape as a 'gasket', but it spun off
into a crumple ball.
Hmmm.. those beads aint so big, making two more (ring?) terminals
would be real work.

George H.

6. ### leggGuest

Although this is a useful method to differentiate the two materials,
it's not going to give meaningful bulk resistivity information.
The higher resistivity material is sintered in a fairly crystaline
structure, with insulator/semiconductor type relationships between
domain groups. You can't count on it functioning as an insulator
in-circuit, because this characteristic is uncontrolled - but you sure
can count on the low resistivity stuff causing shorts, if misapplied
in a higher voltage circuit.

Some materials are not formed as individual beads, but are machined
from tubular structures, or finished into their final dimensions by
grinding, The machined parts provide a more reliable contact to the
bulk material, but their function is also marginally affected due to
the reduced bulk impedance at the machined surface.

It used to be that low resistivity, low frequency parts also had
noticably lower currie temperatures. This restricts uses of parts in
places where significant power loss is expected - they become
loss-self-regulating, much like an NTC resistor - which is no good if
a loss must be absorbed and the site of application isn't heatsunk in
some way.

RL

7. ### George HeroldGuest

Thanks, If this works I can try looking for better samples.
The ferrite beads are probably the 'low end' of the production run.
A low Curie temperature would be attactive. (easier to reach)
I like this melamine foam which is good to?? (at least 140 C)

George H.

8. ### George HeroldGuest

Hmm, I think the first thing I need to get rid of is the twisting
motion of the screw. I don't quite see how to do that. (And keep the

Now here's a question for your four point measurement suggestion.

If I've got some random walk conduction through the sample...
(to explain the large resistance)
Then when I put on my two voltage probes,
do they make random walks through the sample too?
In which case the position of the volatge probes becomes only an
approximate measure of the distance along the conduction channel.

Hey, can I do some sort of AC measurement that probes shorter range
conduction?
Or is that at a very high frequency?

George H.

My ferrite knowledge is about 40 years old, from the Philips plant where
I worked as ferrite lab engineeer, so there are probably newer
compositions on the market.

Ferrites are solid solutions of either manganese or nickel and zinc and
iron oxides, sintered to form micro crystals around 1 micron size. Each
crystal is intended to be one magnetic domain, for lowest losses. There
are minor but well controlled additions of silica and/or calcium oxides
which act as glassy insulators between the ferrite grains. In the
manganese-zince ferrites there is also a critical amount of FeO in the
inter-grain spaces, whereas the grains themselves are MnO2-ZnO-Fe2O3. (
Magnetite, Fe3O4 is really a solid solution of FeO in Fe2O3, ie its a
'ferrite' without the manganese, nickel or zinc components ).

Basically there are two formulations:

Manganese zinc ferrites
For low frequency, say 1 KHz to 200 KHz.
High permeability, around 2000 and up, low resistivity, around 100-1
kohm-cm or so.
Curie temp around 150-200 degC, depending on formulation (
manganese-zinc ratio ).

Nickel zinc ferrites
For high frequency, say 500 KHz to 50 MHz.
Permeability around 50-500, high resistivity, around 1e9 ohm-cm or higher.
Curie temp 200-500 degC, again depending on formulation ( nickel-zinc
ratio ).

Resistivity is not well controlled, and only has a small part in ferrite
losses. It also has very high negative, and very uncontrolled, temp coeff.

Ferrites are optimised for work around -20 to about +100 deg C. Below
that, the permeability falls rapidly, and there may well be phase

10. ### Mr StonebeachGuest

phenomenon. The microscopic mechanism behing Verwey transition is
quite intriguiging, and its existence does not seem to be widely known
- at least I had never heard about it before (well, if its covered in
Kittel that is just my ignorance, we followed Aschroft-Mermin...). The
ferrites I have tested in LHe have all been the standard MnZn or NiZn
spinel types (it seems hard to find other types, like garnets, sold in
small quantities) and it sounds likely that it was exactly the Verwey
transition which killed them - assuming the transition also happens in
mixed spinels, the papers I've found only describe pure magnetite.

Anyone knows an easy small-quantity source for microwave ferrites,
by the way?

Regards,
Mikko

11. ### Robert MacyGuest

I read a paper where someone 'measured' the precession rate of the
'available magnetic molecules and it was in the range of 1 to 2 GHz,
which the author used to explain why there are no microwave ferrites.

In my experience, 1 GHz was about the upper limit, air core above that
was far more effective, and for EMI graphite..

12. ### Mr StonebeachGuest

Hi Robert, thanks for your insight. Do you happen to remember
a reference to the paper?

To me it looks that Mini-Circuits transformers do use magnetic
cores,
see e.g.
http://217.34.103.131/pdfs/TC1.5-1X+.pdf
http://217.34.103.131/pdfs/TCM2-33X+.pdf
http://217.34.103.131/pdfs/TC4-14+.pdf

There are also all sorts of tuneable microvawe thingies which make
use of YIG garnets.

Then there are circulators available for a wide variety of
frequencies,
those must utilize ferrite cores, no?

Regards,
Mikko

13. ### Mr StonebeachGuest

Minicircuits makes both TLT:s and ordinary ones. Eg.
http://217.34.103.131/pdfs/TC1.5-1X+.pdf is an ordinary transformer
(judging by the 'Config D' in the data sheet), and the in the
photograph
it looks to me like having a magnetic core.

An example of their TLT's is
http://217.34.103.131/pdfs/TC4-25+.pdf
which is clearly indicated as 'Config. H' in the datasheet.

There are a large variety of companies like
but I still haven't found one with a small-quantity webstore. My first
guess was RELL but I wasn't able to find the stuff there.

Regards,
Mikko

14. ### leggGuest

Adrian - great to hear something from the horses mouth for a change.
Copied to my magnetics article index, with your permission, I hope.

RL

15. ### Mr StonebeachGuest

Yes, I have made superconducting TLTs in the past using the now-
obsolete
Amidon #6 (yellow-grey) and I'd expect their other iron powder cores
to work, too.
But their ferrites freeze, I've tried. Coincidentally, your colleague
R. Koch has
also constructed superconducting TLTs, but I don't know which core
material he
used. Amidon, I would guess.

But as you say, something else is needed for higher frequencies.

Must go now, the fridge just reached the 8mK base ...

Regards,
Mikko

16. ### Spehro PefhanyGuest

About what temperature do they work down to?

Best regards,
Spehro Pefhany

17. ### whit3rdGuest

Well, if you have a square-section bead... make a jig with a central pin (to 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-array
technique that's similar) but it should work well enough to track any
change in properties.

18. ### Mr StonebeachGuest

I have used them in LHe only.

Regards,
Mikko

19. ### Mr StonebeachGuest

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

20. ### George HeroldGuest

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

George H.