Magnetic moment is a vecotr quantity

The resultant magnetic moment is dependent on the orientation in which the identical magnets were joined. If they are joined in opposite direction, the net magnetic moment will go to zero.

The Spin magnetic moment i approximately the same as the angular magnetic dipole moment. What then do you men by grater magnitude?

Yes, the magnetic moment can be calculated from the diamagnetic substances.

Since, a steel wire is bent into a semi circular arc ,the new magnetic moment will be M