Once we’ve measured the radiofrequency in which the anisotropy is minimum, we can calculate the magnetic hyperfine field. We just need to know how the hyperfine splitting is related with the magnetic field (ΔE=g*µ*Bhf). If magnetic moment µ and g factor are known, we can equal the last expresion with the energy of the radifrequency found in the spectrum (E=hv), and calculate Bhf!
AS we can see, higher magnetic fields lead to lower radifrequencies, which means that the energy splitting decreases. This can only be explained if the external magnetic field has an opposite direction with respect to the hyerfine field, in that case, an increase will lower the total magnetic field recived for the atoms, leading to a lower sliting.