Mu-metal Shielding

Can changing magnetic fields also be shielded by high permeability ferromagnetic materials? Let's imagine having a magnet that is moving all around a ferromagnetic surface, is the magnetic field being shielded? (Ignoring Eddy Currents since most ferromagnets are conductors).
- Moe (age 20)

That's the mu-metal shielding to which we referred. So long as it's arranged in the right pattern, it can shield fields. It was used to cover phonograph cartridges to reduce magnetic pick-up. It's used for low-noise signal transformers for the same reason. We just encased a low-noise experiment in a large mu-metal chamber for shielding.

Mike W.

(published on 08/26/2013)

I understand that some soft iron ferromagnetic material's are used to re-direct/shield/block a magnetic field from sensitive area/materials/objects etc... But those shields would require to have some specific properties. One, is the magnetic permeability, I was wondering... If a thin sheet of ferromagnetic object of high saturation(2.1Tesla) and relative permeability of 7000 can shield a magnetic field of 1.2Tesla with it being as 1-4mm thick! Is that possible? I assume yes, because the magnetic field would rather pass through such an object since it an easier "path" than air, and also it will not saturate until it's near 2.1 Tesla Just wanted to confirm this. Thank you!
- Moha

I believe that depends on the geometry.  For simplicity, let's consider the case where the external field is homogeneous over the whole volume of interest. If you need only a small region to be shielded (compared with your 4mm maximum thicknes of mu-metal) Then sheets of mu metal along the field lines will do a good job of shielding the space between them. Having the sheets make as much contact as possible, sealing off a volume, helps even more.

If, however, you need to shield a large volume the mu-metal generally won't work, unless the volume has special geometry.  Say you want to shield a big sphere, radius R. The flux through your shield will only go like its thickness times R times the saturation magnetization.  The flux you need to divert from the volume goes like R². So for big R, it won't work. 

So it really depends on what you're trying to shield.

Mike W.

(published on 08/27/2013)