Iron and to a lesser extent Cobalt and Nickel respond to a
magnetic field by becoming themselves magnetized with a flux
density many times that of the applied field.
Permanent magnets
with strong magnetic fields below the Curie temperature (~600°C)
are made from alloys; typically ALNICO (field ~1 T) and more
recently, rare earth alloys with iron cobalt and nickel (field
~5 T).
Coins from India, China and Europe are ferromagnetic (due
to a high % of Nickle??).
Coins from Thailand, USA,
and NZ are not magnetic.
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Pure nickel is weakly ferromagnetic
The rare earth alloy magnet has a 5 Tesla field
at the pole face. It supports several kilograms of iron but will
only just pick up the nickel plate.
Magnetic domains
Ferromagnetic materials may become strongly magnetized because
the material is made up of small regions known as magnetic domains.
(The atoms of many other metals have similar magnetic moments
but no domains). In each domain, all the atomic dipoles are coupled
together in a single direction. Ferromagnetic materials become
magnetized when the magnetic domains within the material are
aligned. This can be done by placing the material in a strong
external magnetic field.
magnets
with strong magnetic fields below the Curie temperature (~600°C)
are made from alloys; typically ALNICO (field ~1 T) and more
recently, rare earth alloys with iron cobalt and nickel (field
~5 T).