Materials whose rheological properties may be varied by application of magnetic fields belong to a specific class of so-called smart materials because they can respond, via solid-state electronics and modern control algorithms, to changes in their environment.  Such electroactive elastomers are composites made of solid particles embedded in an elastomeric network whose mechanical or optical properties can be changed by the application of an electric or a magnetic field. 
An applied current or field aligns the particles and provides a structure to the doped materials.  More specifically, magneto-rheological fluids (MRF) and magnetically controlled elastomers (MCE) are compounds that respond to a magnetic field.  The response exhibited is immediate and reversible with a change in rheological behavior, for (MRFs) or elastic behavior, for MCEs.  In both cases, ferromagnetic particles, such as iron, are suspended in a carrier liquid.  The carrier liquid in MRFs may be mineral oil and may be a rubber matrix in an MCE.  In each, the iron particles may be present in sizes ranging from 3-10 micron in diameter.  Additional types of filled elastomers include those based on carbonyl iron particles and silica particles, for example.