Soft versus hard magnetic materials

Magnetic materials are divided into soft and hard materials according to the shape of their B-H (hysteresis) loop. Soft materials are easy to magnetise and demagnetise; hard materials keep their magnetism once magnetised.

B-H curves

 Soft material            Hard material
        B                        B
        |   __..--             __|______
        | /                   /  |     /
   -----|/-----> H      -----+---|----+---> H
       /|                   /    |   /
  --''  |                  /_____|__/
        |                        |
 narrow loop, small Hc    wide loop, large Hc, Br

• A soft material has a tall, narrow loop: small coercive force $H_c$, high permeability, small loop area.
• A hard material has a short, fat loop: large coercive force $H_c$, large remanence $B_r$ and large loop area (large $(BH)_{max}$, energy product).

Comparison

Examples

• Soft: pure (Armco) iron, low-carbon steel, silicon steel (hot rolled and CRGO), nickel-iron alloys (permalloy, mumetal), soft ferrites (Mn-Zn, Ni-Zn) and amorphous metal. Used in transformer cores, armature and stator cores, relays, chokes and high-frequency cores.
• Hard: carbon steel, tungsten steel, cobalt steel, Alnico, barium/strontium ferrite, samarium-cobalt and neodymium-iron-boron (NdFeB). Used in permanent-magnet DC motors, PM synchronous machines, measuring instruments, loudspeakers and magnetos.

Machine cores carry alternating or rotating flux, so the loop is traced many times per second. This is why they need soft materials with a small loop area. A permanent magnet must keep its flux against demagnetising fields, so it needs a hard material.