According to their magnetic susceptibility, x, materials are classified as diamagnetic, paramagnetic and ferromagnetic. Diamagnetic materials (x , 0) are repelled from magnetic fields, i.e. they are forced towards minima of magnetic field strength. Most materials are weakly diamagnetic, including water, proteins, DNA, cells, polymers, wood and glass. Often such materials are simply called non-magnetic. Paramagnetic materials (x . 0) align in a magnetic field and experience a small force towards magnetic field maxima, i.e. they are attracted to magnetic fields. Examples of paramagnetic mater- ials include oxygen, platinum and manganese(II) salts. Ferromagnetic materials such as iron, cobalt and nickel have x & 0 and are strongly attracted to magnetic fields. Another special case of paramagnetism is superparamagnetism. Super- paramagnetic particles have a core of small iron oxide crystals encased by a polymer shell. The particles are magnetised in a magnetic field. However, they have no magnetic memory. Once the external field is removed, the particles redisperse and behave like a non-magnetic material.