So in iron, one Fe is replaced by a Sr. With NMR/ON, one can see what the hyperfine field is that this Sr nucleus feels in iron by looking at where we see a reduction, in other words, where we see a change in orientation of the ensemble. I think the sign of the hyperfine field can then be deduced by looking at the position of the peak in the spectrum and whether this is shifted left or right from the resonance peak you get in NMR. I’m not sure about this answer…