AC-susceptibility studies of magnetic relaxation mechanisms in superparamagnetic nanoparticles
A system of superparamagnetic (SPM) nanoparticles dissipates heat when it is subjected to an alternating magnetic field. The amount of heat generated is several times that of what would be produced in a system of ferromagnetic particles exposed to the same field. This remarkable fact has significant implications in the treatment of cancer using localized hyperthermia. It is known that artificially induced hyperthermia can be used to kill cancerous tissue while normal tissue is spared. A very promising method of producing localized hyperthermia is to deliver magnetic particles to the site of the tumor and then cause them to dissipate heat. In the past, ferromagnetic particles were used for this purpose. However, therapeutically useful rates of heat generation could not be attained in these particles when the applied magnetic field is limited to values that are suitable for human exposure. Superparamagnetic nanoparticles promise to overcome this limitation through much higher rates of heat generation which have been shown to be as much as three times higher than that of ferromagnetic particles. However, much work needs to be done to better understand the relaxation mechanisms responsible for the heat dissipation, particularly in relation to the particle composition, in order to make this approach feasible. ^ In this project we used AC magnetic susceptibility measurements to investigate the microscopic relaxation mechanisms (including relevant parameters) responsible for heat dissipation in systems of SPM particles, both immobilized in solid matrices and suspended in carrier liquids. In a first set of experiments we followed the dependence of the blocking temperature T B on the measuring frequency f (observation time) for ensembles of zinc doped nickel ferrite (Ni1-xZn xFe2O4) immobilized nanoparticles. At each frequency, TB values were determined from the peak of the imaginary component of the AC susceptibility χ'' measured vs. temperature within the 2–300 K range. An Arrhenius type f vs. TB dependency was probed, which allowed an accurate determination of the energy barrier to magnetization reversal Ueff related to the Néel relaxation of this system. We found that this barrier depends on the doping concentration and has a pronounced maximum when half of the nickel atoms are replaced by zinc. ^ A second set of experiments addressed the behavior of SPM ferrofluids consisting of cobalt doped iron oxide nanoparticles immersed in either kerosene or isopar M. As the freezing point of these carrier fluids are well within the 2–300 K range, we were able to observe the manifestation of both the Néel and the Brown relaxation as a double peak in the χ'' vs T dependence. ^
Physics, Electricity and Magnetism
Arora, Jitin, "AC-susceptibility studies of magnetic relaxation mechanisms in superparamagnetic nanoparticles" (2007). ETD Collection for University of Texas, El Paso. AAI1445682.