Integrated passive iron shims in silicon

A magnetic apparatus, comprising:

• a support structure configured to support two permanent magnets in defined relative orientation and location, said support structure comprising non-magnetic material, said support structure having an aperture defined therein for placement of a measurement element and a material of interest between said two permanent magnets;
• two permanent magnets supported by said support structure, each of said two permanent magnets having a first face of a first magnetic polarity and a second face of a second magnetic polarity, said two permanent magnets supported in defined relative orientation and location so as to provide a magnetic field between a respective first face of a first of said two permanent magnets and a second face of the other of said two permanent magnets;
• a magnetic material coupled between said second face of said first of said two permanent magnets and said first face of the other of said two permanent magnets;
• and
• at least one magnetic shim situated between said first face of said first of said two permanent magnets and said second face of the other of said two permanent magnets, said magnetic shim having all of its dimensions smaller than a smallest dimension of either of said first face of said first of said two permanent magnets and said second face of the other of said two permanent magnets.