Macroscopic magnetism is most
common in ferrous materials, but everything has magnetic fields. At the atomic
scale, electrons have something analogous to angular momentum called spin. In
addition, the electrons are moving around the atom. Both spin and electron
orbits generate a small magnetic dipole for every atom. However, electrons fill
orbitals in pairs, one up and one down. When outer electron orbitals are
unpaired, it is ferromagnetic. When the orbitals are full, the net dipole
moment of the electrons cancels out. When an atom with unpaired electrons is
placed in a magnetic field, the atom experiences a force and lines up. An
object is magnetic at the macroscopic scale when the constituent atoms have
their poles aligned by a magnetic field. The tiny net magnetic field of each
atom being aligned in the same direction generates the magnets we are familiar
with.
There is no special magnet for
non-ferrous materials. There is only one kind of magnetism.
In order to be affected by a static magnetic field, an
object needs to have a magnetic moment (i.e., N and S poles). When a
ferromagnetic material is placed in a magnetic field, it develops these poles.
We say that it is easily "polarized".
Other materials can be polarized, but the response is
much weaker. One can, for example, levitate a frog in a strong magnetic field.
In
the presence of magnetic fields, the non-magnetic material magnetize.
There are
three types of magnetization: ferromagnetism, paramagnetism and diamagnetism.
The most interesting example is ferromagnetism. Non-magnetized iron (latine
ferrum) is composed of small domains - domains are little parts of
material that act like small magnets, but since domains' magnetic fields are
oriented in different directions, net magnetism of non-magnetized iron bar is
about zero. In presence of magnetic field, these domains align themselves in
the direction of outer magnetic field, therefore, non-magnetized material
becomes itself a little magnet. And two magnets attract each other with
opposite poles.
