Magnetic Resonance in an Iron Oxide Nanaoparticle Suspension at RF and Microwave Frequencies

We measured the complex permeability of an iron oxide nanoparticle suspension at RF/microwave frequencies in zero-field and in static magnetic fields up to 50 mT. The measurements were made using a toroidal loop-gap resonator that can be modelled as an LRC circuit. With the bore of the resonator partially filled with the nanosuspension, the change in resonant frequency and quality factor were used to determine the real and imaginary parts of the suspension's permeability. By filling the gap of the resonator with various low-loss dielectrics, we were able to make these measurements over a frequency range that spanned 600 to 1300 MHz. The zero-field measurements, surprisingly, suggest a weak diamagnetic response that is suppressed as frequency is lowered. As the static magnetic field was scanned, the real part of the permeability increased sharply and then saturated above a frequency-dependent critical field. At this same critical field, the imaginary part of the permeability exhibited a peak. The value of the critical static magnetic field was found to be proportional to the frequency of the applied RF/microwave fields.