High Frequency Characterisation of Magnetic Nanoparticles

There has been a renewed interest in magnetic hyperthermia therapies. These provide a low side effect, high precision therapy for malignant and non-malignant tumours. An issue preventing more widespread application of hyperthermia therapies is the inconsistency in heating rate. While the mechanisms of hyperthermia are known their proportions depend on nanoparticle size and shape. The mechanisms of import at the frequencies used are rotational or viscous heating, in which particles physically spin to follow an applied field, and hysteretic heating in which the moment of the particle switches to follow the field. Hysteretic heating is measurable and can be predicted based on a known sample whereas rotational heating is significantly less consistent. This work has three main sections. The first provides further context to the physics around ferrofluids, before describing the equipment and techniques for their characterisation. The second is the presentation of a high frequency B-H looper along with the design process. The looper is the first in its class to use soft ferrites to amplify the field in the sample space by up to a factor 6. This allows for low currents to achieve fields of 420 Oe at frequencies between 47 kHz and 111 kHz. The looper was used to characterise ferrofluid samples at frequencies common in hyperthermia therapies. The third is a comparison of regular colloid based ferrofluid with a new sample in which the nanoparticles have been immobilised in micrometer scale polymer spheres. The immobilised particles cannot rotate to follow the field, providing minimal rotational heating. The hysteresis heating measured in the B-H looper and the specific absorption rate were compared for two regular ferrofluids and one immobilised fluid. For both of the regular fluids, hysteresis heating accounted for 55% of the total heating, whereas for the immobilised nanoparticles this increased to 98%.