RETENTIVITY AND COERCIVITY

Retentivity or Remanence or Remanent Magnetization -  It is a measure of how well the substance will memorize the magnetism, and become a permanent magnet. 

Example: If a piece of soft iron is magnetized by induction, and later the inducing magnet is removed, it will loose its induced magnetism almost immediately.

But if instead of soft iron piece, there were a piece of hard steel, it would be seen that when the inducing magnet is removed, the steel piece will continue to behave like a magnet for sometime. In other words, steel may be said to have retained magnetism whereas, the soft iron piece did not.

This power of magnetism after the inducing agency is removed, is called Retentivity of a substance.

Retentivity is used for remanence measured in units of magnetic flux density.

Materials which are required to retain their magnetism will have a fairly high retentivity and as such are used to make permanent magnets. Permanent magnets find application in electric motors, magnetic recording media (e.g. hard drives, floppy disks, or magnetic tape) and magnetic separation. While those materials required to loose their magnetism quickly such as soft iron cores for relays and solenoids will have a very low retentivity.

Coercivity or Coercive Force or Coercive Field - The amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic flux return to zero.

It is a measure of a ferromagnetic or ferroelectric material to withstand an external magnetic or electric field.

Coercivity is usually measured in oersted or ampere/meter units and is denoted H_c. It can be measured using a B-H Analyzer or magnetometer.

Ferromagnetic materials with high coercivity are called magnetically hard materials, and are used to make permanent magnets. Permanent magnets find application in electric motors, magnetic recording media (e.g. hard drives, floppy disks, or magnetic tape) and magnetic separation.

Materials with low coercivity are said to be magnetically soft. They are used in transformer and inductor cores, recording heads, microwave devices, and magnetic shielding.

Hysteresis Loop  (B-H Graph) showing Retentivity and Coercivity