Classification by hysteresis loop and applications

Based on the shape and size of the hysteresis loop, magnetic materials are classified into soft and hard magnetic materials (with square-loop materials as a special group).

        B   soft                   B   hard
        ^   __                     ^   _______
        |  / /                     |  /      /
   -----+-/-/----> H       --------+-/------/----> H
       /-/  |  Hc small      -Hc  / |      /  Hc large
      /_/   |                    /__|_____/
   narrow, tall loop           wide, fat loop

1. Soft magnetic materials

• Narrow loop, low coercivity ($H_c$ < $10^3$ A/m), low retentivity, high permeability, small hysteresis loss.
• Easily magnetized and demagnetized; domain walls move freely.
• Examples: soft iron, silicon steel, permalloy, Mumetal, soft ferrites, amorphous alloys.

Applications

• Cores of power transformers, motors and generators (silicon steel, CRGO): low loss under alternating flux.
• Relays and electromagnets (soft iron): quick release after the coil is switched off.
• High-frequency transformers, SMPS, RF inductors, antenna rods (soft ferrites): high resistivity, low eddy loss.
• Magnetic shielding and current transformers (permalloy, Mumetal): very high permeability.
• Recording and reading heads (permalloy, ferrite).

2. Hard magnetic materials

• Wide loop, high coercivity ($H_c$ > $10^4$ A/m), high retentivity, large $(BH)_{max}$, large hysteresis loss.
• Hard to magnetize and demagnetize; domain walls are pinned by impurities and fine particles.
• Examples: carbon/tungsten steel, Alnico, NdFeB, SmCo, barium/strontium ferrite.

Applications

• Permanent magnets in loudspeakers, microphones and headphones.
• PMDC motors, small generators, magnetos, alternators in bikes.
• Moving-coil meters, energy meters (braking magnet).
• MRI and magnetic separators, magnetic holders and locks.

3. Square-loop (rectangular-loop) materials

• Nearly rectangular loop with two stable states $\pm B_r$ and sharp switching at $H_c$.
• Examples: γ-Fe₂O₃, CrO₂, Mg-Mn ferrites, Co-alloy thin films.
• Applications: magnetic tapes, hard disks, magnetic cards, old core memories and magnetic switching devices, where $+B_r$ and $-B_r$ represent binary 1 and 0.