Hsia, Chih-Hao (2010-08). Studies of Optically Induced Magnetization Dynamics in Colloidal Iron Oxide Nanocrystals. Doctoral Dissertation. Thesis uri icon

abstract

  • Studying dynamics of magnetization relaxation in excited magnetic materials is important both for understanding the rates and pathways of magnetization relaxation and for the potential use in spin-based electronics and data storage devices in the future. Previous studies have demonstrated that the size of nanocrystals is an important factor for energy relaxation in quantum dots and metal nanoparticles. Since magnetization relaxation is one of energy relaxation pathways, the size of nanocrystals may be also an important factor for magnetization relaxation in nanoscale magnetic materials. The goal of this study is to have a better understanding of magnetization relaxation in nanoscale magnetic materials. In particular, we focused on the correlation between the nanocrystal size and the rates of spin-lattice relaxation (SLR), a magnetization relaxation pathway, in magnetic nanocrystals. The size-dependent magnetization relaxation rate after optically induced demagnetization in colloidal Fe3O4 nanocrystals was measured by using time-resolved Faraday rotation (FR). Fe3O4 nanocrystals were chosen as the model system to study the correlation between the size of nanocrystals and the rates of SLR due to the wellestablished synthetic procedure of making nanocrystals with various sizes and narrow size dispersion. Faster SLR rates were observed in smaller Fe3O4 nanocrystals. The results suggested the surface of nanocrystals have higher efficiency of SLR than the interior region by using a simple model to analyze the SLR rates of Fe3O4 nanocrystals with various sizes. Higher efficiency of SLR at the surface may be due to the stronger spin-orbit coupling at the surface relative to the interior region. In addition to magnetization dynamics studies, the effect of oxidation on static FR in iron oxide nanocrystals (between Fe3O4 and y-Fe2O3) was studied. The results indicated FR signal is linearly correlated to the strength of optical transition between Fe2 and Fe3 in Fe3O4 for a given size of nanocrystals.
  • Studying dynamics of magnetization relaxation in excited magnetic materials is
    important both for understanding the rates and pathways of magnetization relaxation and
    for the potential use in spin-based electronics and data storage devices in the future.
    Previous studies have demonstrated that the size of nanocrystals is an important factor
    for energy relaxation in quantum dots and metal nanoparticles. Since magnetization
    relaxation is one of energy relaxation pathways, the size of nanocrystals may be also an
    important factor for magnetization relaxation in nanoscale magnetic materials. The goal
    of this study is to have a better understanding of magnetization relaxation in nanoscale
    magnetic materials. In particular, we focused on the correlation between the nanocrystal
    size and the rates of spin-lattice relaxation (SLR), a magnetization relaxation pathway, in
    magnetic nanocrystals.
    The size-dependent magnetization relaxation rate after optically induced
    demagnetization in colloidal Fe3O4 nanocrystals was measured by using time-resolved
    Faraday rotation (FR). Fe3O4 nanocrystals were chosen as the model system to study the correlation between the size of nanocrystals and the rates of SLR due to the wellestablished
    synthetic procedure of making nanocrystals with various sizes and narrow
    size dispersion. Faster SLR rates were observed in smaller Fe3O4 nanocrystals. The
    results suggested the surface of nanocrystals have higher efficiency of SLR than the
    interior region by using a simple model to analyze the SLR rates of Fe3O4 nanocrystals
    with various sizes. Higher efficiency of SLR at the surface may be due to the stronger
    spin-orbit coupling at the surface relative to the interior region. In addition to
    magnetization dynamics studies, the effect of oxidation on static FR in iron oxide
    nanocrystals (between Fe3O4 and y-Fe2O3) was studied. The results indicated FR signal
    is linearly correlated to the strength of optical transition between Fe2 and Fe3 in Fe3O4
    for a given size of nanocrystals.

publication date

  • August 2010