Magnetism is a class of physical phenomena that are mediated by
magnetic fields. Electric currents and the
magnetic moments of elementary particles give rise to a magnetic field, which acts on other currents and magnetic moments. Every material is influenced to some extent by a magnetic field. The most familiar effect is on permanent
magnets, which have persistent magnetic moments caused by
ferromagnetism. Most materials do not have permanent moments. Some are attracted to a magnetic field (
paramagnetism); others are repulsed by a magnetic field (
diamagnetism); others have a more complex relationship with an applied magnetic field (
spin glass behavior and
antiferromagnetism). Substances that are negligibly affected by magnetic fields are known as
non-magnetic substances. These include
copper,
aluminium,
gases, and
plastic. Pure
oxygen exhibits magnetic properties when cooled to a
liquid state.
