There is provided a magnetic field generator for MRI 10 applicable to a variety of magnetic field generators, and capable of preventing separation of permanent magnets 20 which constitute permanent magnet groups 14a, 14b. The photochromic polymer has a glass transition temperature ranging from 30\xb0 C. A second plurality of reference bursts are written along a second radius of the disk circumferentially offset from the first radius, the second plurality of reference bursts comprising a second reference burst. A first layer of a semiconductor material is formed between the electrically conductive feature and the layer of electrically insulating material.