Hysteresis Loop — Ferromagnet Domain Wall Dynamics

Physics: A ferromagnet is composed of magnetic domains. An applied field H drives domain wall motion through a pinning landscape (disorder). The magnetization M(H) traces a hysteresis loop — energy is dissipated each cycle. Barkhausen noise (1919): domain walls jump discontinuously between pinning sites, producing avalanches. The coercive field H_c is where M=0 during reversal; remanence M_r is the magnetization at H=0. The area enclosed by the loop equals the energy dissipated per cycle. Higher disorder → larger coercivity → harder magnet.