The Magnetic Memory
Hysteresis — from the Greek 'to lag behind' — is the defining characteristic of ferromagnetic materials. When you apply a magnetic field to iron, its magnetization does not simply follow the field; it lags, creating a loop when the field cycles. This loop encodes the material's magnetic history and determines whether it serves as a permanent magnet, a transformer core, or a recording medium. The shape of the B-H curve is the fingerprint of a magnetic material.
Anatomy of the Loop
The hysteresis loop reveals four critical parameters. Saturation magnetization (M_s) is the maximum magnetization when all atomic moments are aligned — an intrinsic property of the material's chemistry and crystal structure. Remanence (B_r) is the flux density remaining after the field is removed. Coercivity (H_c) is the reverse field needed to demagnetize the material. Squareness (S = B_r/B_s) measures how abruptly domains switch, ranging from gradual (low S) to snap-like (high S).
Hard vs. Soft
The width of the hysteresis loop separates the magnetic world into two classes. Soft magnetic materials — silicon steel, ferrite, permalloy — have razor-thin loops (coercivity under 1 kA/m), switching easily with minimal energy loss. They are essential for transformers, inductors, and magnetic shielding. Hard magnetic materials — NdFeB, SmCo, ferrite magnets — have wide loops (coercivity above 100 kA/m), resisting demagnetization tenaciously. They power electric motors, headphones, and MRI machines.
Energy and Loss
The area enclosed by the hysteresis loop equals the energy dissipated per unit volume per magnetization cycle. In a transformer operating at 60 Hz, this loss occurs 60 times per second and must be minimized — hence the use of grain-oriented silicon steel with extremely thin loops. Conversely, permanent magnets benefit from wide loops, as the maximum energy product (BH)_max — proportional to the area of the largest rectangle inscribable in the second quadrant — determines the magnet's strength per unit volume.