Magnetic Hysteresis: Domains and the B-H Loop

A ferromagnet driven by an oscillating applied field H, modelled with the Jiles-Atherton description of domain-wall motion. Because the magnetic domains flip toward the field but lag it (domain walls pin and unpin irreversibly), the magnetization B is not a single-valued function of H but traces a closed hysteresis loop. The scene shows a lattice of domains reorienting while the side panel draws the B-H loop and shades its enclosed area, which equals the energy dissipated as heat per cycle. Soft iron gives a thin lossy-but-easily-switched loop, hard steel a wide loop with large remanence and coercivity (a permanent magnet), and ferrite an intermediate case; sliders set coercivity, saturation and drive amplitude.