Characterization Of Magneto-optical Properties Of Nanoscale Magnetic Film

Recently, nanotechnology has been perceived as one of the leading techniques in industry revolution by the whole world. In21th century, many countries have treated nanotechnology as one of the key technologies in the domain of the economy and politics of the future world. Over the years, with the development of electron device miniaturization and nano-technique, a new area striding across semiconductor and magnetic material has appear. This new area is called spintronics. New devices based on spintronic such as high density nonvolatile memory, magnetic sensor and electrical isolator have been used in many areas. Spin transistor and spin quantum computer under development have promising prospects in the future too.In the future, the basic material of spin transistor and spin field-effect tube is magnetic film and semiconductor hybrid system. Fe/GaAs film has some advantages such as low miss match rate, easily fabricated and the injection of spin polarized current from magnetic film to semiconductor. So Fe/GaAs film becomes the representation of magnetic film and semiconductor system. When the thickness of Fe film reduces to nanometer size, the structure and magnetic will change. The overlayer will change the magnetic strength of Fe film. Till now, research on the scale effect of the nanoscale magnetic film magnetic anisotropy, especially the influence of nonmagnetic surface layer on the magneto-optical property is still not clear. The overlayer is intended to protect the magnetic material and avoid it from oxidation. But the Madami group in Italy found that the overlayer Cu has strong constraint on the magnetic anisotropy of nanoscale magnetic film, even the surface layer is only0.2nm. The Falco group in America also found that the Al overlayer can reduce the magnetic anisotropy of Fe nano-film. Researches above show that the overlayer has affected the magnetic property of magnetic film and semiconductor system. Inevitably, the spin polarized current transport will also be affected. Solving this problem has strong significance on the film growth and the realization of spin transistor. Except the surface layer, the substrate also has strong influence on the magneto-optical property of Fe ultrathin film. In film growth process, different substrates are chosen to satisfy the lattice matching condition. When the substrate is optical anisotropy, the reflection on the surface is quite complex. When the light passes through magnetic film and reaches the substrate, there will be two refracted lights, ordinary light and extraordinary light. These two lights have different polarizations and light paths. This make it difficult to detect the polarization change of the reflected light. By studying the magneto-optical property of magnetic film-anisotropic substrate system, the action mechanism of substrate constraint can be known. Meanwhile, this research can expand the application of magneto-optical effect and provide a reference to spin-electron device application.Recently, with the development of electron device miniaturization, the IT industry has rapidly development. But now it is still quite difficult to reduce the excessive heat caused by the device miniaturization. This status restricts the development of IT industry. But the application of topological insulator（TI） is expected to solve this problem. Topological insulator is different from the traditional semiconductor and conductor. It has metallic surface and insulator bulk. Topological insulators （Bi2Te3, Bi2Se3, and Sb2Te3） have insulating bulks and metallic surfaces with an odd number of Dirac cones that are responsible for most unique TIs properties. When a light incidents to the magnetic film surface, there will be two lights, reflected light and transmitted light. The polarization change of reflected light is called Kerr effect and that of the refractive light is called Faraday effect. The Kerr and Faraday magneto-optical （MO） effects allow light to directly realize time-reversal symmetry （TRS） breaking in a solid. When TRS is weakly broken, a band gap is induced at the Dirac points of a topological insulator and the surface states exhibit strong magnetoelectric effects. So the research in magneto-optical property of topological insulator is quite important for its application in spin electron device. The film thickness, incident angle and magnetic coupling strength are all quite important in determining the magneto-optical property of TI. This research can provide the proper film thickness and magnetic coupling strength in industry application.With the development of film preparation technique, nanoscale film preparation can be realized, In nanoscale magnetic film research, besides the film property characterization, the characterization technique investigation is also quite important. Magneto-optical ellipsometry combines magneto-optical Kerr effect and ellipsometry technique. It realizes the detection of optical property and magneto-optical property in one single experiment. When the magneto-optical ellipsometry is applied in ultrathin film measurement, the polarization change of the reflected light is quite tiny. So it is difficult to detect the magnetic induced signal. So the application of magneto optical ellipsometry faces two key factor, tiny signal detection and signal to noise ratio enhancement. Solving these two problems can raise the measurement ability and complete the nanoscale magnetic film characterization technique.As is shown above, magnetic film and semiconductor system and topological insulator will play important roles in future electronics area, and research on magnetic film characterization will promote the development of spin electronics devices. In this dissertation, the magnetic film and semiconductor system, topological insulator and magneto-optical ellipsometry technique are investigated. The influence of surface layer, substrate material and incident angle on magneto-optical property of magnetic layer is investigated. After measurement on nanoscale Al/Fe/GaAs film, it can be found that magneto-optical property can be constrained by the surface layer. After that, the magneto-optical property of topological insulator is studied. Results show that the film thickness, incident angle and external magnetic coupling strength play important role in determining the magneto optical property of topological insulator Bi2Te₃与and Bi2Se3. At last, a magneto-optical ellipsometry is developed which can realize the measurement of multilayer film. Simulations show that the incident accuracy has strong influence on magneto optical coupling constant accuracy. Using this device, the Ta/Fe/GaAs is investigated. Results show that the magneto optical coupling constant is independent, it will not change with the film thickness. The work of this dissertation will be described in the following aspects:1. The magneto-optical theory of three layers nanoscale film is investigated. Based on this model, the reflection matrix and the longitudinal magneto-optical Kerr rotation formula can be obtained. Using this formula, simulations were done. Results showe that the magneto-optical Kerr rotation can be affected by incident angle and overlayer material. Then the magneto-optical coupling constants can be obtained by numerical inversion. After that, using magneto-optical Kerr rotation instrument, the Al/Fe/GaAs films prepared by MBE with different overlayer thickness were measured. Then the relation between Kerr rotation and film thickness can be given. Results show that the surface layer Al has strong constraint on the magneto-optical property of the magnetic layer, and this constraint increases with the surface layer thickness. This conclusion gives a reference to get a huge Kerr rotation in film growth and experiment.2. The magneto-optical theory of two-layer system with optical anisotropic substrate is investigated. Using this theory, we found that the three dielectric parameters ε_x ε_v ε_z play important roles in determining the relation between Kerr rotation and incident angle. After that, simulations were done on Fe film with TiO2anisotropic crystal as substrate. Then the Kerr rotation was calculated with TiO2crystal treated as anisotropic and isotropic substrate, respectively. Results show that the influence on Kerr rotation depend on the difference between three dielectric parameter. The substrate anisotropic has strong impact on the magneto-optical property of Fe-substrate system.3. The macroscopic magneto-optical effects of new material topological insulator were investigated. Simulations were done on3D topological insulators Bi2Te3and Bi2Se3. Results show that the Faraday rotation peak position will change with magnetic coupling strength. For Kerr rotation, the spectral range reduces with increasing film thickness, magnetic coupling strength and incident angle. There are two giant Kerr non-low-frequency area. For the Bi2Se3film, the giant Kerr rotation in the non-low-frequency area shows an obvious red shift. In addition, it is observed that the Faraday rotation peak position of Bi2Se3is the same as that of Bi2Te3, so the Faraday rotation can be only determined by magnetic coupling strength. The giant Kerr rotation spectral range in the non-low frequency area is larger than that of Bi2Te3.4. A magneto-optical ellipsometry instrument was established and the key problems in this system which are reducing the noise of the light source and realizing the detection of tiny signal were solved. By introducing in the multilayer reflective matrix, magneto-optical ellipsometry can be extended to measurement of multilayer film. Then using this instrument, the Ta/Fe/GaAs multilayer films prepared by magnetron sputtering were measured. Results show that the magneto-optical coupling constant Q is independent on the film thickness.5. The influence of incident angle on magneto-optical coupling constant was investigated. Taking incident angle with different errors into simulation, results show that the little error of incident angle will cause a great change of magneto-optical coupling parameter. When the incident angle error reaches1°, the change of Q reaches9%. So the incident angle accuracy is quite important to this instrument. To improve the system accuracy, a modified spectrometer is introduced into this system to give a precise incident angle.In this dissertation, the magneto-optical properties of nanoscale films are investigated. The innovations are shown as below:1. A magneto-optical ellipsometry instrument was established which contains the light source system, optical system, electromagnet system, detection system and controlling and data processing system. Using the lock-in amplifier, the noise of the system can be successfully reduced and the tiny useful signal can be detected. Through the software, the integral time and number of excitations can be controlled by the computer. Through this, the noise can be reduced. After that, the instrument was used to measure the Ta/Fe/GaAs multilayer films to investigate the relation between magneto-optical coupling constant and film thickness.2. The magneto-optical theory of two-layer system with anisotropic crystal as substrate was investigated. Using this theory, we found that the three dielectric parameter ε_v ε_z, play important roles in determining the relation between Kerr rotation and incident angle. After that, the relation between kerr rotation and dielectric parameter is discussed. Results show that ε_y determines the peak position of kerr rotation and ε_x ε_z. can slightly change it.3. The magneto-optical theory of three layers nanoscale film was investigated. Results show that the magneto-optical Kerr rotation can be affected by incident angle, overlayer material and overlayer thickness. This means that the surface layer has strong impact on the magnetic-semiconductor system, this can not be neglected in film preparation and property characterization.