Coercivity and retentivity using BH curve

When a ferromagnetic material is magnetised and then demagnetised by an alternating field, the flux density B does not retrace its path. The result is a closed B–H (hysteresis) loop, and two of its intercepts give the properties asked for.

              B
              |      .------- +Bs
              |    /
          Br -+---/---.
              |  /    |
  ------------+-/-----+-------- H
            -Hc /     +Hc
     -Bs ------'      |
              |

Reading the loop: start from the unmagnetised state at the origin and raise H until the material saturates (+Bs). Reducing H to zero leaves the flux density Br. Reversing H to −Hc brings B to zero. Further reversal saturates the material in the opposite direction, and the loop closes in the same way.

Retentivity (residual magnetism)

• It is the flux density Br that remains in the material when the magnetising field H is reduced to zero after the material has been saturated.
• On the loop, it is the intercept on the B-axis.
• High retentivity is needed in permanent magnets. It is also the reason a self-excited dynamo can build up its voltage (from the residual flux of its poles).

Coercivity (coercive force)

• It is the reverse magnetising force Hc (in A/m) needed to reduce the residual flux density Br to zero.
• On the loop, it is the intercept on the H-axis.
• A large Hc (hard materials such as steel and alnico) is used for permanent magnets. A small Hc (soft iron, silicon steel) is used for the cores of transformers and machines, because the loop is narrow and the hysteresis loss is small.