where γ is the gyromagnetic ratio and B 0 the magnetic field. The electron spins are characterized by two quantum mechanical states, one parallel and one antiparallel to B 0. Because of the lower energy of the parallel state more electron spins can be found in this state according to the Boltzmann distribution. This imbalanced population results in a net magnetization, which is the vector sum of all magnetic moments in the sample, parallel to the z-axis and the magnetic field. To better comprehend the effects of the microwave field B 1 it is easier to move to the rotating frame.