The magnetic moment in various types of
materials is a result of the following factors.
● Electron orbit. An electron in an
orbit around a nucleus is analogous to a small current loop, in which
the current is opposite to the direction of electron travel. This
factor is significant only for diamagnetic and paramagnetic
materials, where it is the same order of magnitude as the electron
spin magnetic moment.
The magnetic properties of most
materials (diamagnetic, paramagnetic, and antiferromagnetic) are so
weak that they are commonly considered to be nonmagnetic.
● Electron spin. The electron cannot
be accurately modeled as a small current loop. However, relativistic
quantum theory predicts a value for the spin magnetic moment (or Bohr
magneton b).
In an atom with many electrons, only
the spin of electrons in shells which are not completely filled
contribute to the magnetic moment. This factor is at least an order
of magnitude larger than the electron orbit magnetic moment for
ferromagnetic, antiferromagnetic, and superparamagnetic materials.
● Nuclear spin. This factor is
insignificant relative to the overall magnetic properties of
materials. However, it is the basis for nuclear magnetic resonance
imaging (MRI).
● Exchange force. The exchange force
is an interaction force (or coupling) between the spins of
neighboring electrons. This is a quantum effect related to the
indistinguishability of electrons, so that nothing changes if the two
electrons change places.
The exchange force can be positive or
negative, and in some materials the net spins of neighboring atoms
are strongly coupled. Chromium and manganese (in which each atom is
strongly magnetic) have a strong negative exchange coupling, which
forces the electron spins of neighboring atoms to be in opposite
directions and results in antiferromagnetic (very weak) magnetic
properties.
Iron, cobalt, and nickel have
unbalanced electron spins (so that each atom is strongly magnetic)
and have a strong positive exchange coupling.
Therefore, the spins of neighboring
atoms point in the same direction and produce a large macroscopic
magnetization.This large-scale atomic cooperation is called
ferromagnetism.
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