Date of Award
Open Access Dissertation
Physics and Astronomy
Thomas M Crawford
A novel manufacturing technology that offers a low-cost alternative for creating more complex optical materials that are assembled with single-nanometer precision is demonstrated. Using the enormous magnetic field gradients present near the surface of magnetic recording media, colloidally suspended superparamagnetic nanopartilces are self-assembled into patterned microstructures.
The position and shape of these microstructures are precisely controlled by magnetic patterns on the template. The template that can be reprogrammed and reused is magnetically recorded using commercial magnetic recording technology. These microstructures consisting entirely of self-assembled magnetic nanoparticles are then transferred to flexible polymer thin films with patterns maintained. In particular, all-nanoparticle diffraction gratings are fabricated by employing this technology and extensively studied.
Based on the nanomanufacturing technology, a versatile measurement technique is developed to study magnetic nanoparticle self-assembly dynamics. The self-assembly dynamics is monitored in real-time by detecting optical diffraction from an all-nanoparticle grating as it self-assembles. It is demonstrated the nanoparticle self-assembly not only strongly depends on the nanoparticle concentration and size, but also shows a dramatic change in the diffracted intensity as a result of the suspension pH that is not observed with static light scattering. Further, the diffracted signal not only has high sensitivity to the particle aggregation, but also detects different time dependence that depends on the colloidal stability of particles. The diffraction efficiency can be strongly enchanted by mixing the nanoparticle suspension with a small amount of phosphate buffer saline (PBS). While common dynamic light scattering and Zeta potential measurements do not show such a dramatic dependence on the PBS volume as this optical diffraction measurement shows. This demonstrates not only the nanoparticle self-assembly process is highly tunable, but also the optical diffraction is more sensitive to subtle changes in colloidal stability of particle suspensions than commonly used light scattering. This metrology has a strong potential as a complementary metrology for commonly used dynamic light scattering measurements.
In a second study, ultrafast magnetization dynamics are investigated by employing a pulse shaping scheme consisting of two ultrashort (30 ps) spin-transfer torque (STT) pulses with variable delay, amplitudes and polarities. A coherent control of the magnetization dynamics is demonstrated to reliably manipulate the magnetization dynamics. Magnetic dynamics show strong asymmetrical dependence on the inter-pulse delay for oppositely polarized pulses. Experimental measurements suggest that appropriately-shaped spin transfer can be used to efficiently manipulate the orientation of a free layer nanomagnet, thus providing an alternative for spin torque driven spintronic devices. An additional 5 ns STT pulse with variable amplitudes are combined and precisely timed with the pair of picosecond pulses to cancel the magnetic damping.
Although partial damping cancellation is possible with dc currents, the resulting trajectories are completely dephased, demonstrating that precisely-timed pulses are required to observe nearly complete damping cancellation with time-domain sampling experiments. Partial experimental work that attempts to uncover ultrafast demagnetization process by combining 1ps STT pulses with femtosecond optical pulses has also been performed.
Ye, L.(2013). Nanoparticle Self-Assembly and Ultrafast Nanomagnet Switching Dynamics. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/2509