A nonuniform magnetic field B pointing in the z direction is applied to a sample of...

A nonuniform magnetic field B pointing in the z direction is applied to a sample of protons. The field B (in tesla) varies as a function of z (in cm):

The magnetization vector M precesses around the z-axis. Suppose at time t = 0, all magnetization vectors have the same phase. At what time will the magnetization vector M at z = 1 cm and that at z = 0 have the same phase again?