@misc{oai:ir.soken.ac.jp:00000193,
 author = {Nath, Krishna Gopal and ナットゥ, クリシュナ・ゴッパール and NATH, Krishna Gopal},
 month = {2016-02-17, 2016-02-17},
 note = {In the present thesis, the results of the photoemission spectroscopy for some rare earth compounds and thin film systems of 3d transition metal have been described. Photoemission spectroscopy is very useful and direct method for studying both the electronic and magnetic structures of the systems. Synchrotron radiation plays an important role in the photoemission study where the higher photon flux, light polarization and selection of particular photon energy are advantageous. 
    In order to perform the present experiments, the VGESCALAB220i-XL spectromicroscopy system was mainly used. During the present study, the improvement of the system has been performed as follows. First, a chamber for sample preparation and epitaxial growth of thin film was developed. The LEED optics, water-cooled evaporators, thickness monitor and magnetic coil were installed in that preparation chamber. The manipulator was also improved by constructing a He-cryostat for performing temperature dependent photoemission experiments. Several experiments were thus successfully carried out after finishing all those improvements. The performance of the cryostat was checked by measuring the core level photoemission in Eu-compound. The result will be shown in appendix part of the thesis. 

Core level photoemission studies of TmX compounds: 
    One of the main parts of this thesis is to describe the study of the core level photoemission for rare earth TmX (X=S, Se and Te) compounds. Rare earth compounds are known to show their remarkably different physical, magnetic and electronic properties. Among these mixed-valent systems, TmS is known to be mostly trivalent, TmTe is mostly divalent and TmSe is intermediate valent. In the rare earth system, the well localized 4f electrons are strongly interacted with the core hole in the final state of photoemission. The effect of this interaction shows multiplet splitting, lifetime broadening effect, exchange splitting and configuration interaction in the final state of photoemission. Interpretation of the core level photoemission through those final state effects gives the information of interaction between core hole and 4f electrons. This interaction also depends on the nature of the wave function of core level and the valence shell. For TmX system, no systematic experiment of different core levels (Tm4d, 4p, 5p, 3d) has been performed so far. In the present study, photoemission experiments including resonant effects were performed for these core levels in all TmX compounds. A rich information about the final state effects (mentioned above) and valence fluctuation in core level photoemission was obtained. Comparison with the calculation is also given to get better understanding of the experimental results. The results are summarized below. 
　　1. Tm4d shows multiplet structures and each multiplet shows different resonance enhancement depending on the excitation conditions around the Tm 3d-4f absorption edges (hν=1450-1550eV). The lifetime broadening effect depending on the binding energy is present in both the on- and off-resonant photoemission. Through this experiment, the divalent and trivalent components in Tm4d were successfully separated and identified. 
　　2. In case of 5p level, the values of spin-orbit splitting of two valences were estimated by the help of resonant effect and calculation. The values obtained from the present experiment are considered more reasonable than those of the previous reports.
　　3. In case of 4p XPS, the complex spectral features are explained by the configuration interaction phenomenon.
　　4. Tm3d level was also measured for studying the effect of 3d-4f interaction. It was found that the spin-orbit peaks are locally broadened due to the existence of multiplet structures.

Photoemission study of magnetic thin films:
     It is now well known that the ultrathin films in nanometer range of 3d-materials often show very interesting and striking magnetic properties different from the bulk. Magnetic dichroism in photoemission, where two spectra are taken for two different magnetized states or light polarization states, is considered to be one of the most powerful methods to study thin film magnetism.
  1. Ni/Co system: The reasons why the Ni/Co system was studied are as the followings. At first, in order to get the information of pure Ni3d states without any interaction with the substrate, Co substrate was suitable rather than Cu. In the previous result of thinner Ni film on Cu(001), the Ni 6eV satellite structures are overlapped with the tail of the broad Cu3d emission. Secondly, it is interesting to know whether the resonant effect (around the Ni 3p-3d excitation region) on the magnetic dichroism signal for the valence band of Ni-thin film system occurs or not. Finally, it is very curious to study the capability of epitaxial growth and corresponding magnetic phases of Ni on ferromagnetic Co.
     The dichroism measurements of the Ni valence band photoemission conclude that the resonance effect is present on magnetic linear dichroism (MLD) for "6eV satellite". The present result is comparable with the previous MCDAD (Magnetic circular dichroism in angular distribution) results for bulk Ni (110) sample, but not MLDAD (L, linear) result. The MLD result of core level further shows that the Ni and Co are ferromagnetically coupled. The present result indicates that it is possible to magnetize the Ni film of 2.3, 8 and 11ML with in the surface plane (in-plane magnetization).
  2. Oxidized Co film: The effect of oxidization for magnetic surface is very important in magnetism study. In order to study the modification of electronic and magnetic structures of oxidized Co film, the Co2p core level photoemission spectra were measured. It was found that the behavior of the early oxidization followed by saturated CoO formation was different depending on the Co-thickness. Thinner films are more reactive and show an earlier formation of Co0 than that of thicker films. The present result is different from the previous one where thickness independent oxidization in the early stage was reported. In the present study, two types of CoO phase depending on the thickness were found. The CoO phases are two-dimensional (2D) and three- dimensional (3D). It is predicted that the generally accepted CoO<SUB>6</SUB> model for 3D CoO phase might not be suitable for describing the core level spectra in case of 2D CoO phase . 
    Magnetic state of the oxidized Co film was studied by the MUDAD (U, unpolarized) measurement for Co2p level. MUDAD signal of Co2p level was found from the oxidized film even with higher exposure (40L). The result is different from the previous one. The present result concludes that the MD signal for O(4OL)/Co(5ML)  system probably is coming from the deeper and unaffected Co-site., application/pdf, 総研大甲第377号},
 title = {Photoemission experiments in rare earthcompounds and magnetic thin films:Study of the electron correlation effects},
 year = {}
}