The retentivity of a material is its capacity to remain magnetized after the external magnetizing field has ceased to exist.
A material with high retentivity (i.e. iron) will keep some magnetic properties, it will become a permanent magnet, whereas a material with low or no retentivity will not keep the magnetic properties—it will lose its magnetization.
Coercivity of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. Coercivity is usually measured in oersted or ampere/meter units and is denoted HC.When the coercive field of a ferromagnet is large, the material is said to be a hard or permanent magnet. Permanent magnets find application in electric motors, magnetic recording media (e.g. hard drives, floppy disks, or magnetic tape) and magnetic separation. A ferromagnet with a low coercive field is said to be soft and may be used in microwave devices, magnetic shielding, transformers or recording heads.
