7 - Magnetic Resonance and Relaxation

Summary

Electromagnetic (EM) radiation consists of coupled electric and magnetic fields oscillating in directions perpendicular to each other and the direction of propagation of radiation. EM radiation can be an interesting probe to study materials’ properties. It is the electric field component of EM radiation that interacts with molecules and solids in most cases. Two conditions need to be fulfilled for the absorption of EM radiation during such interaction: (i) the energy of a quantum of EM radiation must be equal to the separation between energy levels in the atom/molecule, (ii) the oscillating electric field component must be able to stimulate an oscillating electric dipole in the atom/molecule. EM radiation in the microwave region of the EM spectrum can interact with molecules having a permanent electric dipole moment created by molecular rotation. On the other hand, infrared radiation would interact with molecules in vibrational modes giving rise to a change in the electric dipole moment.

Similarly, a solid or molecule containing magnetic dipoles is expected to interact with the magnetic component of EM radiation. EM irradiation of a molecule over a wide range of spectral frequencies does not normally result in absorption attributable to magnetic interaction. The absorption of EM radiation attributable to magnetic dipole transitions may, however, occur at one or more characteristic frequencies if the material of interest is additionally subjected to a static magnetic field. The application of a magnetic field B can cause precession of a magnetic moment at an angular frequency of |γB|, where γ is the gyromagnetic ratio. A material with magnetic moments placed in a magnetic field can absorb energy at this frequency. It is thus possible to observe a resonant absorption of energy from an EM wave tuned to an appropriate frequency. This phenomenon is known as “magnetic resonance”, and can be studied with a number of different experimental techniques, depending upon the type of magnetic moment involved in the resonance.

The presence of a static magnetic field is a crucial requirement for magnetic dipolar transitions. If there is no static magnetic field, the energy levels will be coincident. The permanent magnetic moments in a material are associated either with electrons or with nuclei. The magnetic dipoles arise from net electronic or nuclear angular momentum.