You say even mu-metal wouldn't work, but RFEngineer says even steel is good enough to work. One of you is clearly wrong (or both). However, I think that you are probably correct. A high-current pulse with a very short pulse-length is bound to produce a very high magnetic field.
“One of you is clearly wrong (or both). “
The military usually uses steel for fixed facilities.
“A high-current pulse with a very short pulse-length is bound to produce a very high magnetic field.”
This would create a broad-spectrum pulse with frequency content up to the inverse of the pulse-length.
At low frequencies magnetic fields dominate, at higher frequencies electric fields dominate.
Electric fields at all frequencies can be shielded with foil-thickness shielding. magnetic fields are different. The lower the frequency the more it will penetrate. Think about it...the earths magnetic field goes through the entire earth. you never have to worry about the earths electric field- it never makes it anywhere close to the surface.
The most energetic part of an EMP does not have low frequency components - though the later stage EMP DOES have a geomagnetic component - but it is not highly energetic - so you need a very large collector for it to impact (e.g. power transmission lines, or pipelines).