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Diamagnetism
Diamagnetism can be regarded as originating from shielding currents induced by an applied
field in the filled electron shells of ions. These currents are equivalent to an induced moment
present on each of the atoms. The diamagnetism is a consequence of Lenz’s law stating that
if the magnetic flux enclosed by a current loop is changed by the application of a magnetic
field, a current is induced in such a direction that the corresponding magnetic field opposes
the applied field.
For obtaining expressions by means of which the diamagnetism of a sample can be
described quantitatively, we will follow Martin (1967) and consider the perturbation of the
when moving in a magnetic field. For a conductor element
orbital motion of electrons in the sample due to the force which each electron experiences
carrying a current I in the
presence of a magnetic field, this so-called Lorentz force is given by
and hence in free space
If we consider the motion of a single charge e with velocity we obtain
The effect of this force on an electron moving in a classical orbit around a single nucleus
is easy to work out. It provides a picture that is not greatly changed in a quantum-
mechanical treatment and is sufficient for our purpose. Let us assume that the field H
is applied in a direction perpendicular to the plane of a circular orbit. The force F will
act either away from the center of the orbit or toward it, depending on whether the elec
tron is moving clockwise or anticlockwise with respect to the field. In either case, the
change in the radius of the orbit can be neglected in comparison with the associated
increase or decrease in the orbital angular velocity We will define the sign
of as positive for clockwise orbital motion with respect to the field and negative for
anticlockwise motion. Noting that the applied fields considered here are so small that they
produce only small changes in (and and denoting such small incremental changes
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