A permanent magnet is made from a material that is
magnetised and creates its own persistent magnetic field. An
everyday example is a refrigerator magnet used to hold notes on a
refrigerator door. Materials that can be magnetised, which are also
the ones that are strongly attracted to a magnet, are called
ferromagnetic (or ferrimagnetic). These include iron, nickel,
cobalt, some alloys of rare earth metals, and some naturally
occurring minerals such as lodestone. Although ferromagnetic (and
ferrimagnetic) materials are the only ones attracted to a magnet
strongly enough to be commonly considered magnetic, all other
substances respond weakly to a magnetic field, by one of several
other types of magnetism.
Ferromagnetic materials can be divided into magnetically "soft"
materials like annealed iron, which can be magnetised but do not
tend to stay magnetised, and magnetically "hard" materials, which
do. Permanent magnets are made from "hard" ferromagnetic materials
such as alnico and ferrite that are subjected to special processing
in a powerful magnetic field during manufacture, to align their
internal microcrystalline structure, making them very hard to
demagnetize. To demagnetise a saturated magnet, a certain magnetic
field must be applied, and this threshold depends on coercivity of
the respective material. "Hard" materials have high coercivity,
whereas "soft" materials have low coercivity.