Spin-lattice relaxation:

This process involves the transfer of energy from the nuclear spin in the high energy state to the molecular lattice. The random motion of the adjacent nuclei in the lattice set up fluctuating magnetic fields at the nucleus leading to its interaction with the magnetic dipole of the excited nucleus. This output within the transfer of energy from the excited nucleus to the neighbouring atoms. This is also called longitudinal relaxation. Therefore, a nucleus returns to its original low energy state from the excited state and excess of nuclei are maintained in the low energy state which is an essential condition for the energy to be absorbed.