The sign of the hyperfine field of Sr in iron can be deduced from comparison of applied RF frequency – in which direction the applied frequency shifts with respect to applied external B field -, and the shape of the peak.
The stronger the external B field, the stronger global sample atomic orientation, which can lead to stronger influence on Sr (External field + iron atom fields).
The peak broadness shows how anisotropic the Sr is. As one can expect it’s most anisotropic state is in the lowest external B field.
But I’m not exactly sure if I understand the meaning behind “sign” here.