part 1 - Why are things magnetic?   current loops and atomic  moments
Fig. 1.4.a Fig. 1.4.b The current loop shown in Fig. 1.2.a seems rather esoteric since there are no wires supplying current to the loop, so it is hard to maintain a current. Current loops, however, are a good pre-quantum mechanical analog to atoms. In the Bohr model negatively charged electrons orbit the positively charged nucleus similar to planets orbiting the sun (Fig. 1.4a). Such an electron motion can be considered as an atomic-scale current loop generating its own (very small) magnetic field similar to the one shown in Fig. 1.2.b.

Such an atomic field can interact with other magnetic fields, and these interactions can be described by a magnetic moment. The magnetic moment is a vector and in a macroscopic description it can be written as:
where I is the current in the loop and A is the area defined by the loop itself (see Fig. 1.4.b).

For our purposes it is enough to consider that every atom can act like a (very) small bar magnet or compass needle, and the strength of the small magnet is given by its magnetic moment μ. Magnetic moments interact with other magnetic fields and have a tendency to align themselves parallel to any other magnetic fields just as a compass needle aligns itself with the earth's magnetic field. They are vector quantities, so when you want to find the sum of several magnetic moments you have to use vector algebra to add them up.

One comment on the direction of μ: The direction of m is given by the right hand rule, where you wrap your hand around the moment vector so that your fingers follow the electric current. If Fig. 1.4.b seems to have it backwards, consider that the direction of I is defined through positive charges. Since electrons have negative charge the direction of the current I is opposite to the motion of the electron.

Since all atoms have electrons orbiting the nucleus, which gives rise to atomic magnetic moments, why aren't all materials magnetic?

next - diamagnetic materials
part 1 - index 1.1 currents 1.2 solenoids 1.3 electromagnets 1.4 mag. moment 1.5 diamagnets 1.6 paramagnets 1.7 mag. ordering