In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis, the precessing magnetic moments are randomly oriented around the z-axis. This results in a net magnetization, M, along the z-axis, with no net contribution from the transverse components on the xy plane.
Upon excitation with radiofrequency radiation, the nuclei absorb energy, and the excited spins acquire some coherence. It follows that Mx and My are no longer zero, Mz decreases, and the net magnetization, M, tips toward the y-axis. Upon continued excitation, the population difference between the spin states can decrease, along with the signal intensity. This is called saturation.
Eventually, the excited nuclear spins return to the equilibrium state through a process called relaxation. During relaxation, the xy coherence disappears, and the net magnetization is restored to the equilibrium value along the z-axis.
