Abstract: Disclosed is a pole piece for use in a Halbach-type magnet configuration, the pole piece having an elongated body adapted for insertion into the Halbach-type magnet configuration and comprising at least two parts operably and removably connected to each other. The at least two parts of the pole piece comprise a top iron having a front face and a rear face, a bottom iron having a front face and a rear face, and an interstitial shim layer adapted to be inserted in a shim cavity defined by a depression formed in at least one of: the front face of the top iron and the rear face of the bottom iron. A method for shimming is also disclosed which includes modifying a material content of the shim cavity and inserting the pole piece into the central cavity of the Halbach-type magnet configuration for shimming the magnetic field generated.