Micromagnetic analysis of magnetic nanoparticles for hyperthermia cancer treatment and of transition characteristics for recording media

Abstract

Research has recently focused on magnetic nanoparticles due to fascinating properties that could see great potential employment in biomedicine as well as data storage devices. Micromagnetic analysis was utilized in order to predict the dynamic motion for the magnetization vector of magnetic nanoparticles in biomedical application (hyperthermia cancer therapy) and magnetic information storage (hard disk drive). In this dissertation, the heating properties of magnetic nanoparticles for hyperthermia and the characteristics of magnetic recording media (both conventional perpendicular media and exchange coupled composite media) with a soft underlayer and an antiferromagnetic soft underlayer are presented. Magnetic nanoparticles have great potential as heating elements for use in hyperthermia. One of the critical issues with widely used iron-oxide compounds such as magnetite and mag-hematite nanoparticles is the relatively low magnetic moment, which results in low heating efficiency. To overcome this demerit, nonoxide high moment Fe70Co30 nanoparticles were considered. The mean size of particles was 12nm with 13.6% standard deviation. Micromagnetic simulation of particles’ experimental hysteresis loop suggests that their behavior is dominated by a uniaxial anisotropy. In order to understand the source of energy loss in hyperthermia, magnetic anisotropy and applied field have been optimized for iron cobalt nanocrystalline particles using numerical micromagnetics. The optimized anisotropy energy is 7.6 kBT at 500 kHz and the hysteresis loss at this optimized energy is approximately 120 x 106 ergs/s/g for a very small oscillating field of magnitude 10 Oe. We have also investigated the effects of varying the applied field and find that the addition of a 20 Oe static field applied perpendicular to the oscillating field approximately doubles the energy loss without subjecting the patient to additional radiation. This is an important benefit for magnetic hyperthermia. To achieve higher areal density in magnetic recording media, the general method is to reduce and make more uniform the grain size, while augmenting the media anisotropy in order to maintain stability. Transition jitter and shape have been studied for “soft” exchange coupled composite (ECC) media and conventional perpendicular media at equal grain size using micromagnetic simulation. A realistic medium having nonuniform grain size has been employed. Media anisotropies are optimized to reduce the high density jitter for ECC and conventional media. Surprisingly, jitter is slightly decreased at high temperature for both media types. Eye diagrams show that short bit length amplitude is higher for ECC by approximately 10 % at room temperature. This indicates that sharper transitions were obtained for ECC media particularly at 300 K where the thermal stability of ECC media presumably aids the write process. A key component of perpendicular recording has long been the soft underlayer. Conventional perpendicular media and “soft” exchange coupled composite (ECC) media with a conventional soft underlayer (SUL) and an antiferromagnetic soft underlayer (AF-SUL) have been investigated using micromagnetic simulation. The fast Fourier transform (FFT) technique and graphics processing unit (GPU) based computing have been used to reduce the intensive computation time for magnetostatic interactions between the head, SUL, and recording layer. Interestingly, the jitter is always less dependent on reader offset from track center with the AF-SUL. Jitter for ECC media is also shown to depend less strongly on reader offset than for conventional media. The transition center deviation at the optimal anisotropy for both recording layers is lower with the AF-SUL at both linear densities considered. We further find that the track center moves alternately with direction of fringing field as expected from magnetostatic considerations.