Surface Plasmons And Magneto-optical Effects In Ordered Magnetic Nanostructures

Since the first magneto-optical effect (MOE), Faraday effect, was experimentally observed by M. Faraday in1845, a bunch of new phenomena related to the interaction between light and magnetism, such as Kerr effect (KE), Zeeman effect, were discovered, together with the numerous applications of these effect, for example, fiber isolator, Kerr microscopy, etc. In the latest several decades, for the sake of the development of plasmonics and the technology breakthrough of nano-science, a new inter-discipline which is focusing on the relationship between surface plasmons (SPs) and magnetism in magneto-plasmonic crystals, called Magneto-plasmonics, have come into sight. We focus our research on the ordered cobalt structure, namely the2D square-ordered Co anti-dots film (CoADF) and2D square-ordered Co double layer film (CoDLF). Not only the traditional optical and magnetic properties were investigated, but also the anisotropy and asymmetry of the systems were addressed. However, one of the preconditions of the experimental research on magneto-plasmonics is a suitable MO measurement system. Thus, our work is mainly covering three parts as following.1. MOKE measurement setupThe setup of the MOKE measurement system includes calculation design, hardware organization, and software implementation. According to the idea of splitting the light into signal light and reference light, we calculate the Kerr angle with two optimizations:(1) place the beamsplitter on the path of reflective light (instead of incident light), to avoid the reflectance fluctuation of the sample under different magnetic field, lending to more accurate results.(2) instead of the small angle approximation of the trigonometric functions, we keep the trigonometric functions in the calculations. By this way, we can still keep the accuracy under the situations of giant Kerr angles. The hardware system consists of power source, electromagnet, laser, optical parameter oscillator (OPO), probers, energy meter, PC and a suit of optics, which are properly organized. We programmed the utility software, named MOKESOFT, which can control and collect data from power source, OPO and energy meter. MOKESOFT was designed to be able to automatically scan the magnetic field or light wavelength, and analyze data and output data in real time.2. Research on CoADFWe fabricated CoADF by combining the interference lithography and magnetron sputtering. A Lloyd’s Mirror interference lithography system with a325nm wavelength He-Cd laser was used to obtain periodical negative photoresist (NPR) antidots pattern. After magnetron sputtering, we transferred the NPR pattern to CoADF. The period is412nm, the hole diameter is175nm and the height of Co is60nm. We measured the reflectance spectra of CoADF by an ellipsometer in p-light configuration. In the reflectance spectra, there are two reflectance minima, namely Wood’s anomalies, which are changing positions with changing azimuthal angles. We attributed this anisotropic phenomenon to the excitation of SPPs with different diffraction orders by the theoretical calculation of the relation between SPP and incident light, also by a series of COMSOL simulations. Utilizing our MOKE measurement setup, we get the Kerr angle spectra and Kerr angle loops at the45degree incient angle. The Kerr spectra are also show anisotropic effect, the origin of which is the same as the anisotropy of reflectance spectra. When incident light wavelength is523nm and azimuthal angle is45degree, the maximum Kerr angle reaches to10minutes approximately, which is3times of that of Co film. Besides, the anisotropies of coercivity and remanences in the Kerr angle loops are consistent with the M-H loops which are measured with VSM. These anisotropies have a magnetical origin, that is, the pinning effects of hole upon the magnetic domains are different under the different azimuthal angles.3. Research on CoDLFWe produced dots array PR pattern by aforementioned interference lithography only with the differences of the positive PR instead of NPR, and incident light wavelength of442nm instead of325nm. The changing parameters are not occasionally but deliberately chose because of the different properties of PR under illumination of lights with different wavelengths. We split the PR-pattern substrate into4pieces, three of which undergo the reactive-ion etching process with different etching times; afterwards we have four PR patterns with different dot heights, namely102nm/90nm/55nm/40nm.After magnetron-sputtering Co on the PR patterns, we get4different CoDLFs. The period of CoDLF is400nm, the dot diameter is285nm, the height of Co is30nm. We measured their reflectance spectra and Kerr angle spectra. The results show that both height, incident angle, azimuthal angle have effects on the spectra. Similar to the spectra of CoADF, we attribute these effects to the SPPs with different diffraction orders, together with relatively weak effects from LSPs. Moreover, we find the asymmetry phenomena in the Kerr angle spectra, that is, the spectra are different at the opposite azimuthal angles along the symmetric axes of the structure. The cause of these phenomena is originated from the axial vector nature of magnetic field:the Lorentz forces upon the electron oscillations of SPPs have different directions at the opposite azimuthal angles, while the forces upon LSPs stay the same.