Study On Fabrication And Magneto-optical Properties Of Magnetic Nanocomposite Structure

In this dissertation, the magneto-optical (MO) properties of nanocomposite magnetic structure were investigated by experimental measurements as well as theoretical calculations. In respect of the theoretical method, the transfer matrix method based on solving Maxwell’s equation was adopted to investigate the MO phenomenon in multilayer structure. The scattering-matrix approach was employed to analyze the MO properties of periodic nanocomposite array. We calculated the distribution of electric field intensity and optical parameters of nanocomposite film employing the COMSOL Multiphysics4.3b software. In our experiment, using dielectric medium of hafnium oxide (HfO2) and ferromagnetic metal, the nanocomposite multilayer films were fabricated with multi-target magnetron sputtering. Combining photo-etching technique with magnetron sputtering, periodic nanocomposite films’ arrays were prepared. Scanning probe microscope (SPM) was used to characterize samples’ morphologies. The MO measurements were performed in a homemade magneto-optical Kerr effect (MOKE) system. The optical parameters were measured with ellipsometry (ELLIP-A). Main work includes three parts.1. Research on preparation and MO properties of multilayer nanocomposite filmIn order to improve the MO properties of Co film, we designed a quadrilayer structure HfO2/Co/HfO2/Al/silicon substrate including Fabry-Perot cavities. Based on the theory of the transfer matrix method, the MO properties’ parameters of quadrilayer were calculated. The physical mechanism that leads to MOKE enhancement in the experiment was investigated. It was found that the Kerr rotation, Kerr ellipticity and reflectivity of the quadrilayer vary with light wavelength, corresponding to the optimum figures of merit (FOMs). The optical cavity effect was predicted by theoretical calculations. In view of the theoretical calculations, the quadrilayers of HfO2/Co/HfO2/Al(Ag)/silicon were designed and fabricated with magnetron sputtering. The MO measurements were performed in a homemade MOKE system. The optical parameters were measured with ellipsometry (ELLIP-A). The measurements’results reveal that Kerr spectra of quadrilayer change with the thicknesses of intermediate HfO2layers when the thicknesses of Co layer and Al layer keep unchanged; Also, the Kerr spectra change with the position of the Co layer while the total thickness of two dielectric medium layers keep unchanged. We confirmed the existence of the cavity effect in the multilayer according to our experimental results. As is demonstrated that, the maximum polar Kerr rotation-7.92°at wavelength of496nm for the quadrilayer HfO2(11nm)/Co(15nm)/HfO2(30nm)/Al(40nm)/silicon under s-polarized incident light, which is very large Kerr rotation. As also can be seen, the maximum longitudinal Kerr rotation-1.04°occurs at wavelength of570nm in the quadrilayer HfO2(15nm)/Co(15nm)/HfO2(30nm)/Al(40nm)/silicon for s-polarized incident light. Therefore, our magneto-optical structure exhibits interesting MO properties and provides a feasible way to enhance the Kerr rotation in a broad spectral region. We calculated the distribution of electric field intensity in the incoming light field employing the COMSOL Multiphysics4.3b software. We proved that the magneto-optical enhancement in the multilayer is origin from cavity effect by the theoretical calculation. Furthermore, to confirm the physical mechanism of abovementioned analysis, we also investigated the relation between the reflectivity and the maximum value of Kerr rotation. It is revealed that there should be optical enhancement effects in the MO spectra near a minimum in the reflectivity spectrum, and the measured Kerr rotation spectra are thus directly associated with the excitation of the optical cavities’ effect. Hence, these measurements are in agreement with the theoretical analysis.2. Study on fabrication and MO properties of magnetic nanocomposite periodic arraysMagnetoplasmonic crystal exhibits novel MO performance. It is the research focus of frontier magnetics area. Combining photo-etching technique with magnetron sputtering, two kinds of ordered nanocomposite arrays were fabricated:striped array of magnetic nanocomposite films and square array of magnetic nanocomposite films. Magneto-optical and optical measurements show that the array of nanocomposite films presents a comparatively better MO performance due to adopting dielectric medium of HfO2. The MO properties on striped array of nanocomposite films can be tuned by the striped width, height, and separation of the neighboring stripe. This is because the geometry shape of the striped array can tune the excitation modes of the surface plasmon polariton (SPP) and localized surface plasmon (LSP). Especially, the MO properties of striped arrays can be controlled by the intermediate HfO2layer’s. thickness. Namely, the MO properties of the striped array also can be enhanced by optical cavity effect. Furthermore, it is found that the MO properties of square array of magnetic nanocomposite films show space anisotropy. The result is relevant to the modes of SPP and LSP which are tuned by space anisotropy. Therefore, we suggest that the MO properties of nanocomposite arrays can be modulated by three factors: SPP, LSP and optical cavity effect.3. Theoretical calculation on the MO properties of cobalt nanotube periodic array by utilizing generalized scattering-matrixThe extended Fourier components had been derived, and effective electric dipole model allowing for the proper treatment of activity is presented. The scattering-matrix approach was employed to analyze the MO properties of hexagonal array of magnetic metallic nanotubes embedded in a dielectric medium in the presence of magnetic field. It was found that the MO properties of the nanotube array are more prominent than those of the nanodisk array for the same value of the cobalt filling ratio in a broad wavelength range. The MO properties of nanotube array associate with the length and wall thickness of the nanotubes, the filling medium and the neighboring nanotubes separation. Large enhancement in the MO signal was shown at the corresponding minima of the reflectivity. The excitation of surface plasmon corresponds to the position of the minima in reflectivity. Therefore, the interaction between the surface charges and the incident light accumulates large surface plasmon energy at the nanotube surface, which leads to the light reflectivity decrease. The MO enhancement is attributed to the result of light re-emitted by magnetoplasmons. Because the polarizability is determined by electric plasmon excitation and magnetization, it is suggested that the concerted action of nanoplasmonics and magnetization can control the sign of rotation of the reflected light’s polarization in ferromagnetic nanostructure.