The overall magnetic behavior of a material can be predicted from its electron configuration, which dictates the magnetic properties observed when a material is placed in a magnetic field. The origin of magnetism lies in the orbital and spin motions of electrons and how the electrons interact with one another.
Each electron in an atom moving about the nucleus while spinning about its own axis gives rise to an electric field and a consequent magnetic field. A magnetic moment is assigned to each electron to describe the magnetic field produced. The total magnetic moment of the substance is the vector sum of individual magnetic moments of individual electrons. If two electrons are oriented in the opposite direction, their vectors are in equal in magnitude but opposite direction; therefore their vector sum will be zero.
All matter exhibits magnetic properties when placed in an external magnetic field, and matter can be classified according to this interaction. Application of an external magnetic field, to a substance with unpaired electron spins causes the random orientation of unpaired electron spins to become aligned with the external field. This alignment causes a change in the magnetic moment that results in a slight attraction to the external field. These substances are termed paramagnetic.
On the other hand, application of an external magnetic field to a substance whose electron spins are all paired causes a change in the magnetic moment that results in a magnetic field that is in opposition to the external field and a slight repulsion to the external field. These substances are termed diamagnetic.