Under what situation a magnet suspended in a uniform magnetic field will be (a) in stable equilibrium and (b) in unstable equilibrium?

Explanation

If a bar magnet is placed in such a manner that its magnetic dipole moment is parallel to the external magnetic field intensity, then it is said to be in a state of stable equilibrium. If you slightly rotate the bar magnet from this state then it starts oscillating due to restoring action of the magnetic torque. And when energy is lost in the form of heat then the bar magnet will again be in an equilibrium state. When the magnetic dipole moment vector of a bar magnet is antiparallel to the applied magnetic field intensity then also it can remain in equilibrium, but this time the nature of equilibrium is of an unstable nature. As you slightly rotate the bar magnet from this orientation, the magnetic torque further rotates it in the same sense and the bar magnet does not return back to its original state. After heat loss it will stay in stable equilibrium with the magnetic moment parallel to the magnetic field.