Under an external magnetic field, the direction of magnetic dipoles of a magnetic substance tends to be along the direction of the external magnetic field.  If the external magnetic field is an alternative-current (AC) magnetic field with low AC frequency (generally lower than microwave frequency), the magnetic dipole oscillates with this AC magnetic field with phase delay. The phase delay drivers the susceptibility into a complex number. XacQuan V is a good device to measure the AC susceptibility and usually is also a good device to measure the magnetic concentration of magnetic materials.