It is safe to say that from 200 to 50K a magnetic field / hyperfine field is present. I am not quite sure as to why it wasn’t there before. Is this because the nucleus exceeded the Curie temperature? If the field were to become stronger or weaker it would result in a shift of the energy levels but this does not seem to be the case. Considering that temperature is considered an average thermal energy higher temperature nuclei will vibrate more. Because of this the Doppler broadening will be more intense for the warmer nuclei and thus the exact energy at which the resonant absorption happens is less well defined / more spread out and thus more difficult to pin down.