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Mu metal

Hebrewhammer8

Cast Iron
Joined
May 14, 2009
Location
Bellingham, Wa
Looking for some high permiability Mu metal.
anyone know where i could get a small sample of this.
the only kind i can find is .005" thick.
looking for something in the .050 to .1 area
 
Like in the previous post, the sheet metal around the CRT tubes in CRO's is Mu Metal and also the metal cans around small cylindrical audio transformers in PA equipment. It has to be heat treated and annealed after any kind of bending and machining to have the full magnetic properties too which is an involved process.

Edit: I suspect the supermagnets in modern hard drives are attached to a thick base of Mu metal.
 
Mu-metal should have range of nickel-iron alloys, it should also notable for their high magnetic permeability. Normally it is composed of 76% nickel, 16% iron, 6% copper and 2% chromium. The high permeability makes mu-metal useful for shielding against static or low-frequency magnetic fields.
 
The permeability is reduced not just by bending, but also can be reduced by just hitting the stuff without deforming it.

It is good shielding, but paradoxically, the high permeability lets it saturate easily, and it less effective as a shield unless combined with other shielding or physical spacing that reduces the fields drastically BEFORE they can reach the mumetal.
 
Hence the setup where you have nested mu-metal cans.

Fields attenuate exponentially as a function of depth in the cans.
If you put the opening for the second can one diameter into the first,
the fields "start" decreasing from the attenuated level. Do that three
times and you can achieve hundreds of microgauss field levels in the
third can.

Degauss the outer one in-situ, and the fields inside the final shield
are measured in terms of one or two microguass.
 
Hence the setup where you have nested mu-metal cans...

Degauss the outer one in-situ, and the fields inside the final shield
are measured in terms of one or two microguass.
In principle, you're right, but in practice some magnetic flux lines get pinned to sites in the mu metal making it very difficult to get below the milligauss level. Degaussing the outermost can is done with an AC field that decreases in amplitude with time, "shaking" the flux lines loose from pinning centers and allowing them to enter the mu metal (i.e. taking them out of region in the middle of the can where you want the low field). However, because the outer can shields the inner cans, the degaussing coil doesn't work on them, leaving some of the flux lines trapped, which is why there is a limit to how much shielding is possible no matter how many cans are used. Before someone suggests it, no, degaussing the inner can, then adding a second can and degaussing it, then adding another can, etc. doesn't solve the problem, because when the subsequent cans are added they carry with them flux lines, some of which are not perfectly shielded by the innermost can and that get trapped by that can.

To do better requires can in the center made of a superconductor. First the mu metal reduces the field as much as it can, then the inner shield is cooled below the superconducting transition temperature, at which point "all" remaining flux lines are expelled ("the Meissner Effect"). Once again, though, pinning rears its ugly head, and even superconductivity doesn't reduce the field to truly zero in the center. Some superconductors are better than others at having minimal (but, unfortunately, still not zero) pinning. None of which helps the OP find the 0.05" mu metal he's looking for...
 
When I design a shield for our detectors I typically will heat treat the MU Metal to get rid of any concentration.

Mu Shield has lasered some parts for me. Nice work.
 
"To do better requires can in the center made of a superconductor. "

In practice this does not work. The superconducting inner sheild will simply pin the
existing ambient field so you can never ever get below what the mu-metal sheilding
provides.

One exception to this is a technique pioneered by blas cabrera where you cool a sealed
lead bag which has been accordioned shut. By expanding it after cooling (mechanically)
you increase its volume and therby reduce the internal field.

One issue with superconducting sheilding that nobody really appreciates much, is that
it is very common to get thermally induced currents that create HUGE fields during a cooldown.
These are again, trapped in superconduting sheilding. Reducing the cooling rate
drastically just before going through the critical temperature can help with this.

The late stuart bermon actually invented a scanning flux vortex microscope - to investigate
things like this that were impeding progress on the josephson computer project here. There
were some tricks invented to solve problems like this, which could only be solved once you
knew what was going on.

The flux scanner was pretty cool - used a flip-coil magnetomer hooked to an rf squid - and the
flip-coil could be flipped, and traversed in X and Y to scan across assemblies all at 4K.

He developed some of the best low-field shielding setups I've ever seen. Just about everything
I know about magnetic sheilding, I learned from him.
 
"To do better requires can in the center made of a superconductor. "

In practice this does not work. The superconducting inner sheild will simply pin the
existing ambient field so you can never ever get below what the mu-metal sheilding
provides.
This is getting pretty far afield, so to speak. Actually, in practice a superconductor does work, although the rest of your post indeed correctly highlights some of the problems. Two of my patents wouldn't have been possible if an appropriate superconducting shield in side nested mu-metal shields, appropriately and very slowly cooled through its superconducting transition, did not exclude some (although not all) magnetic flux, reducing the field below that of the mu metal alone. But, let's put this discussion aside, because it has taken us off into an area a long way from machining.
 








 
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