What metal is used to concentrate magnetic fields in this video?

[ANarwhal]

I was watching this video here: http://www.youtube.com/watch?v=bzRee_yHeOs&feature=g-hist

and was wondering what kind of metal they used to concentrate the magnetic field?

 

[mfb]

My guess: Mu-metal or something similar.
Note the orientation of the magnet: The magnetic field inside is parallel to the metal block surface, it goes through the small plates and returns in the big metal block.