MadSci Network: Physics

Re: this question is in regards to magnetic permeability

Date: Wed Sep 14 19:21:43 2005
Posted By: Zack Gainsforth, Undergraduate, Physics, U.C. Berkeley
Area of science: Physics
ID: 1120286876.Ph

Dear Shawn,

We would all love a material with a magnetic permeability > 1,000,000.  It
sounds as though your task is to magnetically shield an object, so you
might want to consider creating a multiple layer shield.  (Is this what you
meant by layering?)  Before delving into the multi-layer shield idea, I'll
answer your question about metglas.

Metglas is an amorphous material, which means it was formed at a high
temperature and then cooled rapidly enough so that long range crystal
structures couldn't form.  This means that large ferromagnetic domains
cannot form, which in turn leads to a high permeability in metglas.

Annealing would destroy the amorphous structure and allow it to fall back
into a crystalline solid with large ferromagnetic domains.  These domains
would then resist attempts by an outside magnetic field to reorient
themselves, and the permeability would drop.  Therefore, you are most
interested in maintaining the amorphous structure for a shielding application.

In addition, for shielding applications, high permeability is not the whole
story.  If you have a very strong magnetic field, it can quickly saturate
the shield.  That is, if you have a high permeability, then the density of
the flux lines will more rapidly reach the maximum value sustainable by the
shielding material.  When this happens, the external flux lines penetrate
through the material easily.  In this case, increasing the permeability
doesn't help you -- your material is already saturated.

However, if you want to create an excellent magnetic shield, you should use
multiple layers of shield material with an air gap or some other insulator
between them.  It works like this:  Assume you've built a shield to enclose
your object, and it reduces the field inside to 1% of the field outside. 
If you then place another shield inside the first one, it will reduce the
field inside both shields to 1% of 1%, or 0.01% of the field outside both
shields.  You can keep this up for any number of shields, budget and
physical space permitting.  If you use a high permittivity material like
metglas for each layer, well, all the better!  You can find more detail on
this including equations for creating multilayer shields at:

If you poke around the site, you will find a treasure trove of magnetic
shielding knowledge.

If you can make it to a library with scientific journals, you can get an
excellent and more in depth treatment via the following citation:

T.J. Sumner, J. M. Pendlebury, and K. F. Smith, Conventional Magnetic
Shielding, J. Physics
D. Applied Physics 20 (1987), Pg. 1095-1101.

The latter is what I have used in the past, and it is very informative.

I hope this helps you!

Zack Gainsforth

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