Schematic representation of the coherences
When the sample is in a field Bo we create a coherence at the thermal equilibrium (Fig 2). We create therefore also a magnetization along the Z axis.
This figure represents the spins precession movement which is induced by the ﬁeld Bo. For example, we may consider the proton as a magnetic dipole whose component is either parallel or antiparallel to the z axis. The angular speed of the precession moment is