🧲 Magnetic Hysteresis Loop
Iron remembers. When you magnetise a ferromagnet and then reduce the field, the domains don't fully unalign — remanence Br stays behind. The area of the B-H loop equals the energy dissipated as heat per cycle — critical for transformer core design.
⚡ The Physics of Hysteresis
Inside a ferromagnet, magnetic domains — regions of uniform magnetisation — tend to align with an applied field H. As H increases, domains grow and rotate until saturation Bs is reached. Reducing H doesn't reverse this fully: domains are pinned by defects, leaving a permanent magnetisation called remanence Br.
A field of -Hc (coercivity) is needed to demagnetise the material. Soft materials (low Hc) are used in transformer cores to minimise loop area and hence heat loss. Hard materials (high Hc) make permanent magnets.