A phenomenon observed in ferromagnetic materials below the Curie point (see ferromagnetism) where the magnetization of the material varies nonlinearly with the magnetic field strength and also lags behind it. The magnetic susceptibility of such materials is large and positive, a large value of magnetization being produced for comparatively small fields. A characteristic plot of either magnetization, M, or magnetic flux density, B, against magnetic field strength, H, demonstrates the hysteresis effect and is termed a hysteresis loop (see diagram). If an initially unmagnetized sample of iron is subjected to an increasing magnetic field the magnetization follows the curve shown by the dotted line OAS. This is known as the magnetization curve. If the specimen is then subjected to a complete magnetizing cycle with magnetic field varying symmetrically between +H and −H the curve shown by the solid line is followed.

The value of B at zero field is termed the remanence. The value of H at the point at which B (or M) falls to zero is known as the coercive force, and is the reverse field required to demagnetize the sample. The area enclosed by the hysteresis loop is proportional to the energy dissipated in each complete cycle when ferromagnetic material is subjected to an alternating magnetic field. This is known as hysteresis loss.

The effect of hysteresis in a magnetic core is to cause an increase in the effective resistance of the coil surrounding the core; the hysteresis factor is the increase in the effective resistance of a coil carrying a current of one ampere at a specified frequency.

The general form of the hysteresis curve is shown in the diagram. The area enclosed by the curve depends on the nature of the ferromagnetic sample: minimum area (and minimum coercive force) occur with soft iron, rising to a value some twenty times greater with tungsten steel. Any complete magnetizing cycle (say between the values of H + h and H − h) will give rise to a hysteresis loop. See also ferromagnetism.

Magnetic hysteresis loop