TYPES OF MAGNETIC MATERIALS
All materials are affected by magnetic fields. Matter is made up of atoms with a positively charged nucleus surrounded by a field or cloud of negatively charged electrons. The electron field is in continual motion, spinning around the nucleus. When the material is subjected to a magnetic field, the electron orbits are slightly distorted. The amount of this distortion ( or the corresponding change in magnetic characteristics) when subjected to an external magnetic field provides a means of classifying materials into three groups:Â diamagnetic, paramagnetic or ferromagnetic.
Diamagnetic Materials
The term diamagnetic refers to a substance with magnetic permeability slightly less than that of a vacuum. When placed in a strong magnetic field, their induced magnetism is in a direction opposite to that of iron. Diamagnetic materials include mercury, gold, bismuth and zinc.
Paramagnetic Materials
Paramagnetic denotes a substance with permeability slightly greater than that of air. When such materials are placed in a strong magnetic field, there is a slight alignment of the electron spin in the direction of the magnetic flux flow. This alignment exists only as long as the paramagnetic material is in the external magnetic field. Aluminum, platinum, copper and wood are paramagnetic materials.
Ferromagnetic Materials
Ferromagnetic materials have a permeability greater than that of air. When placed in an external magnetic field, the magnetic domains align parallel with the external field and remain aligned for some period of time after removal from the field. This continued alignment after removal from the external field is called retentivity. It can be an important property in some magnetic particle testing procedures.