First answer

When an external magnetic field is applied this splits all levels of the system. So when the correct frequency photon hits a La atom, that is not yet in the highest of thse split states, it can transition to the next level. This corresponds with a gradual change in orientation of the magnetic moment of the electron cloud each time it absorbs a photon. Relaxation effects could return the electron cloud back to the previous state, again in gradual steps changing the orientation of the magnetic moment of the electron cloud.