| Summary: | 博士 === 國立清華大學 === 物理學系 === 92 === Domain wall and spin resonance of magnetic thin films and nanoparticles are investigated in this dissertation. Two main thrusts are: (1) electron spin resonance studies of quantum phase transition in magnetic nanoparticles, and (2) magnetic force microscopic studies of domain walls on thin nickel films.
Sol-gel glass embedded with iron nanoparticles provides fascinating features inheriting with paramagnetic, ferromagnetic, and superparamagnetic resonance properties under various compositional weight ratios and annealing conditions. Two spectra arising from paramagnetic Fe3+ ions and ferrimagnetic Fe2O3 particles both centered at ge =2.0 compete with intensities as the annealing temperature TA increases. The asymmetric and rather broad line shape can be elucidated by the ferromagnetic resonance of the single domain Fe2O3 nanoparticles. The half linewidth simulation agrees well with the super paramagnetic resonance theory of particle size ~10.5 nm.
The classical, thermally driven transition from ferrimagnets to superparamagnets in Fe3O4 nanoparticles can be converted into another quantum phase by a strong internal anisotropic field. The field induced by crystal anisotropy, which is perpendicular to the Ising axis can destroy the magnetic long-range order to quantum paramagnets by exceeding some critical values. Electron spin resonance (ESR) spectrometer, a very sensitive instrument with fast detecting window to explore quantum phase transition for magnetic nanoparticles, was exploited to study the fascinating interplay between thermal and quantum fluctuation in the vicinity of a quantum critical point. We have determined the static spin susceptibility and critical exponent g, which is power-law dependent spanning the quantum critical point and investigated the effects of various microwave fields, particle sizes, and temperature dependence on the magnetic states of single domain spinel ferrite nanoparticles. As temperature decreases, the spectrum behaves from superparamagnetic (SPR) to ferromagnetic resonance (FMR) and retrieves to quantum superparamagnetic as the temperature lowers down further. On account of the highly anisotropy field of Co-ferrite, the quantum critical behavior of CoFe2O4 nanoparticles is prominently observed. We have combined magnetization measurement and electron spin resonance to study the fascinating quantum tunneling of magnetization in Co-ferrite nanoparticles at low temperatures. ESR spectra are shifted to low fields (1600G) at 35 K accompanying with the intensity increasing and linewidth narrowing. The increasing of magnetic moment at low-temperature (below 11 K) and the extraordinarily small coercive field (1.3 kOe) at 2K of Co-ferrite NPs are investigated by magnetization measurements. A new approach, Heisenberg model with strong easy-plane anisotropy, is proposed to attribute these to the conversion of ferrimagnets to quantum superparamagnets (short-range ordered). The quantum superparamagnetic size is estimated about~1.5nm corresponding to the short-range ferromagnetically correlated volumes existing in MNPs.
Magnetic force microscopy images with a resolution as high as 3 nm are examined for thin nickel films deposited by argon ion sputtering method. Samples grown at 100∘C with grain sizes of near 60nm exhibits coexistence of Bloch and Neel walls whereas samples grown at room temperature with grain size of ~10nm display an alternative Bloch line with a cap switch. The relationship between domain size and film thickness follows a law (where d is the film thickness) as predicted by Kittel.
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