How does diamagnetism originate?

[Robert100]

For AP Physics or Chemistry, how do we explain the origin of magnetism, and diamagnetism?
Saying that "Well, it's a quantum mechanical effect" or "it is a relativistic effect" isn't much help in explaining it's origin. Are there semi-classical explanations or analogies, that high school seniors could grasp?

Many responses I have received stated that the actual origin is so deeply rooted in QM and relativity that there are no useful analogies that can be made to high school students, as all analogies are highly misleading. Is that really so, or were these responses a bit cranky?

Can you suggest diagrams that clearly show the difference between: ferromagnetism, antiferromagnetism, and ferrimagnetism?

 

[Henryk]

Hmm, let me try
Diamagnetism occurs to certain degree in all materials and is a rather weak effect. Essentially, when you apply a magnetic field, you induce eddy currents that oppose the applied field. Thus, the net magnetic field inside diamagnetic material is somewhat smaller than the applied field (susceptibility < 0)
Paramagnetism occurs in materials where electrons have magnetic moments. The magnetic moments comes from electron spin and orbital angular momentum (i.e. little current loops).
In the presence of an applied field, these moments tend to orient in the direction of the applied field thus enhance it (susceptibility > 0). At the same time, thermal motion tends to de-stabilize the ordering and the paramagnetic effect is also weak and temperature dependent (Curie law).
Ferromagnets, ferrimagnets and antiferromagnets are materials where there is an interaction between the neighbouring magnetic moments strong enough to make them ordered, kind of ordering of atoms in a crystal.
In ferromagnetic material, all the neighbouring little magnetic moments are oriented in the same direction. Antiferromagnets are the materials where the ordering of magnetic moments are oriented in opposite direction. Ferrimagnets are materials in-between ferro and antiferromagnets.
The key to this class of materials is that the there are thousands or millions of little current loops ordered and thermal fluctuations can't de-stabilize the order.
In the presence of an applied field, all the little current loops react the same way to the applied field and magnetic susceptibilities of these materials is much, much larger than that of paramagnetic materials.
However, if the temperature is high enough (above Curie temperature), the ordering between the neighbouring current loops is destroyed (kind of melting, but not exactly) and ferromagnetic materials become paramagnetic.

Hope it helped
H.

 

[DrDu]

Feynman gave a relatively easy classical explanation:

http://www.feynmanlectures.caltech.edu/II_34.html