Pole piece

A method for shimming a magnetic field generated by a Halbach-type magnet configuration, the method comprising:

• a) providing a pole piece, the pole piece comprising a top iron having a front face and a rear face;
• a bottom iron having a front face and a rear face, the bottom iron being adapted to receive and/or be received by the top iron to form an assembled pole piece such that the front face of the top iron faces the rear face of the bottom iron;
• a depression formed in at least one of:
• the front face of the top iron and the rear face of the bottom iron, such that an interstitial shim cavity is formed by the depression when the top iron and the bottom iron are in an assembled position;
• and an interstitial shim layer provided in the interstitial shim cavity;
• b) modifying a material content of the interstitial shim cavity by one or more of:
• removing material from the interstitial shim layer, adding material above or underneath the interstitial shim layer in the interstitial shim cavity, and moving material within the interstitial shim layer;
• and
• c) inserting the pole piece into a central cavity of the Halbach-type magnet configuration for shimming the magnetic field generated by the Halbach-type magnet configuration.

A method for shimming a magnetic field generated by a Halbach-type magnet configuration at least partly enclosing a sample volume, the method comprising:

• a) identifying a magnetic field inhomogeneity in the sample volume of the Halbach-type magnet configuration, the inhomogeneity being generated by a magnetic field gradient;
• b) providing a multipartite pole piece, the multipartite pole piece comprising a top iron, a bottom iron, and an interstitial shim layer, and the top iron comprising a cut-away region operable to receive a centerpiece of non-magnetic material;
• c) forming a modified pole piece by performing one or more of:
• removing material from, adding material to, moving material within at least one part of the multipartite pole piece based on the identified magnetic field inhomogeneity;
• and
• d) inserting the modified multipartite pole piece into the Halbach-type magnet configuration to shim the magnetic field.