  Magnetic nanoparticle based hyperthermia emerged as a potential tool for
treating malignant tumours. The efficiency of the method relies on the
knowledge of magnetic properties of the samples; in particular, knowledge of
the frequency dependent complex magnetic susceptibility is vital to optimize
the irradiation conditions and to provide feedback for material science
developments. We study the frequency-dependent magnetic susceptibility of an
aqueous ferrite suspension for the first time using non-resonant and resonant
radiofrequency reflectometry. We identify the optimal measurement conditions
using a standard solenoid coil, which is capable of providing the complex
magnetic susceptibility up to 150 MHz. The result matches those obtained from a
radiofrequency resonator for a few discrete frequencies. The agreement between
the two different methods validates our approach. Surprisingly, the dynamic
magnetic susceptibility cannot be explained by an exponential magnetic
relaxation behavior even when we consider a particle size-dependent
distribution of the relaxation parameter.
