Magnetic hysteresis

In a ferromagnetic material, an applied magnetic field changes the arrangement of magnetic domains. As the field increases, domains favorably aligned with it grow and the material approaches magnetic saturation. When the external field is reduced, the material’s magnetic state does not generally return along the same path: some magnetization remains. Reversing the field and cycling it back completes a hysteresis loop. The lag and irreversibility between field and magnetization give the phenomenon its name.

A loop is commonly plotted as magnetic flux density B against applied magnetizing field H, or as magnetization against field. After a specimen is driven toward saturation, reducing the applied field to zero leaves remanent magnetization (or remanent flux density, often denoted Br). A reverse field is needed to bring the net magnetization or flux density to zero; the magnitude of that reverse field is coercivity (often Hc). A sufficiently strong field in the reverse direction produces saturation of opposite polarity. These quantities depend on how the material was magnetized and on the chosen loop convention.

Soft magnetic materials are relatively easy to magnetize and demagnetize and have low coercivity; they are useful where a magnetic state must change readily, such as in transformer cores. Hard magnetic materials resist demagnetization and generally have greater coercivity and remanence, properties useful in permanent magnets. The width and shape of a hysteresis loop therefore help indicate how a material may serve in magnetic devices.

A hysteresis loop captures both useful magnetic retention and the costs of repeatedly changing magnetization. In alternating-field applications, the energy dissipated per cycle per unit volume is represented by the area enclosed by the B–H loop. This loss appears as heat, so engineers select magnetic materials with loop characteristics suited to the device: a narrow loop and low coercivity can reduce losses in frequently cycled cores, while substantial remanence and coercivity are desirable for permanent magnets.