Magnetic properties of substances
Substances that become magnetised when placed in a magnetic field are called magnetic materials (magnetics). The induction inside a magnetic material is B = μ·B₀, where B₀ is the induction in vacuum (with no substance) and μ = B/B₀ is the magnetic permeability of the medium, showing by what factor the substance changes the field. Diamagnets have μ < 1 (gold, silver, copper, zinc, some gases): they weaken the field slightly and are pushed out of it. Paramagnets have μ slightly above 1 (platinum, aluminium, chromium, manganese, oxygen): they strengthen the field a little. Ferromagnets have μ ≫ 1 (iron, nickel, cobalt and some alloys): they strengthen the field hundreds of times and are strongly attracted to it; their μ is not constant but depends on the field. Ferromagnetic cores are used in transformers, generators, electric motors and electromagnets; special alloys make permanent magnets. Ferromagnetic properties disappear above a certain temperature (the Curie point).
A safe home experiment: test various objects with a permanent magnet (a steel paper clip, a copper coin, aluminium foil, plastic) and make a table of which are attracted. Keep the magnet away from cards, phones and medical devices such as pacemakers, and put small magnets away so young children cannot swallow them.