Ac susceptibility measures the magnetic response of material upon application of external AC magnetic fields. The real part can be considered in phase with the applied magnetic field, while imaginary part represents part that is out of phase with the applied magnetic field. This translates to the magnetic response characteristics of materials
The notion of "phase separation" in this context appears ambiguous to me, to say the least. Dear OP, please let us know whether the previous answer got to the point of what you wanted to know. If not, I stand by the advice of Behnam Farid.