A ferromagnetic material is made of countless microscopic magnetic domains — regions where atomic magnetic moments already point the same way. With no applied field, these domains point in random directions and cancel out. As an external field H is applied, domains aligned with it grow at the expense of others, producing a net magnetization M. Because domain walls have to overcome friction-like pinning at defects, the material "remembers" its magnetic history — it lags behind the field, tracing a looped B–H curve instead of a single reversible line.
Hard ferromagnets like Alnico and neodymium (high Hc, high Mr) are used for permanent magnets, while soft ferromagnets like transformer-core iron (low Hc) are chosen specifically because a thin loop means less wasted energy — the loop's enclosed area is the energy dissipated as heat each cycle.
An oscillating magnetic field sweeps across a 3D ferromagnetic block while its microscopic domains flip into alignment, and the resulting magnetization is traced live as a B–H hysteresis loop.
Why magnetization lags the applied field: domain walls resist reversing until the field overcomes coercivity, leaving remanent magnetization at zero field and tracing a looped, history-dependent curve rather than a straight line.
Adjust field amplitude and sweep speed to drive the cycle, then change coercivity to widen or narrow the loop and remanence to raise or lower it. Watch the domain arrows flip as the loop plots on the right.
The area enclosed by a hysteresis loop equals the energy dissipated as heat per magnetization cycle — engineers pick "soft" low-coercivity alloys for transformer cores specifically to shrink that loop and cut losses.