Magnetic Materials and Demagnetisation

Only certain materials are affected by magnetic fields. Understanding which materials are magnetic and how to make or destroy magnets is essential.

Magnetic materials

Only three elements are magnetic (attracted to magnets at room temperature):

• Iron (and steel, which is mostly iron)
• Cobalt
• Nickel

All other common materials (copper, aluminium, wood, plastic, gold, silver) are non-magnetic.

Hard and soft magnetic materials

Soft magnetic materials (e.g. iron):

• Easy to magnetise.
• Easy to demagnetise (lose magnetism quickly).
• Used in electromagnets, transformer cores, relay cores.

Hard magnetic materials (e.g. steel, alnico, neodymium):

• Harder to magnetise.
• Retain their magnetism (permanent magnets).
• Used in bar magnets, compass needles, loudspeakers.

Methods of magnetisation

• Stroking repeatedly with a magnet in one direction.
• Placing inside a solenoid carrying direct current.

Methods of demagnetisation

• Heating above the Curie temperature (domains lose alignment).
• Hammering or dropping repeatedly (mechanical shock disrupts domain alignment).
• Placing in a solenoid carrying alternating current that is slowly reduced to zero (gradually randomises domain orientation).

Common errors and how to correct them

• Saying aluminium or copper are magnetic. They are not. They are non-magnetic metals.
• Confusing hard and soft magnetic materials. “Hard” means retains magnetism (permanent), not physically hard.

How to apply this in an exam

State whether the material is magnetic or non-magnetic. If magnetic, state whether it is hard or soft and explain why that makes it suitable for the given application.