Ferromagnetic materials remember their magnetic history. As the applied field H cycles, magnetization M traces a hysteresis loop — the enclosed area is energy dissipated per cycle.
Domains are simulated via a 2D Ising-like model with applied field. The hysteresis loop shows M vs H. Coercive field Hc (where M=0) and remanent magnetization Mr (where H=0) characterize the material's "hardness" as a magnet.