Magnetic field correction using magnetized shims

The field dependence of magnetization, which is a serious handicap for a straightforward application of magnetizable materials in the process of basic field correction, is discussed. The principles for compensation of this dependence by an appropriate choice of magnetic material, the operating point on the hysteresis curve and the shapes of shims are specified. A design that eliminates undesired harmonics through the use of proper azimuthal configuration and symmetries of shim positions, as well as the choice of block parameters when using several shim blocks simultaneously, is described.