Changyeon Won | Kyung Hee University (original) (raw)
Papers by Changyeon Won
Journal of Applied Physics, 2007
Cr-dosed FePt films with total thicknesses of 1.8 and 3.6 nm were grown in an ultrahigh vacuum at... more Cr-dosed FePt films with total thicknesses of 1.8 and 3.6 nm were grown in an ultrahigh vacuum at room temperature and studied by means of the surface magneto-optic Kerr effect, scanning electron microscopy, and x-ray magnetic circular dichroism. After annealing at 550 °C, the L10 and A1 phases are spatially separated. In the L10 phase, round nanoparticles form that are 15+/-4.8 nm in diameter and possess out-of-plane magnetic easy axes. An addition of <0.01 nm Cr enhances the coercivity (~0.9 T), which then decreases with additional Cr, and a spin reorientation transition occurs near 0.3 nm Cr dosage as the L10 islands disappear. Segregation of Cr during annealing is believed to be responsible for the formation of round FePt islands and the separation of the L10 islands and planar A1 phase.
The dissertation addresses how the interaction between two magnetic layers changes the magnetic p... more The dissertation addresses how the interaction between two magnetic layers changes the magnetic properties. The dissertation is divided into three sections, chapters 1,2 and 3 introduce the magnetic films, basic theories and experimental methods. Second part, Chapters 4,5 and 6 describe the projects related with magnetic phase transitions and the effect of coupling on them. The third part, Chapters 7,8 and 9 describe the projects related with magnetization direction; spin reorientation transition in Ni/Cu(001), the effect of interlayer coupling on the magnetization direction, and the exchange bias effect in the coupled ferromagnetic system. In the magnetic phase transition projects, the effect of anisotropy and long-range dipole interaction on the Curie temperature is studied in the Fe/Ni/Cu(001)system first. Curie temperature is measured around stripe domain forming spin reorientation transition regions, in which it is found the anisotropy and the long range dipole interaction is important to build magnetic order. Secondly, the effect of indirect exchange coupling on the Curie temperature is studied; there are three kinds of phase transitions. When one film is in the ferromagnetic state, the coupling increases the Curie temperature of the other film. Both films can have simultaneous phase transitions when the Curie temperatures are similar. Thirdly, the Neel temperature and Curie temperature are discussed when the antiferromagnetic layer and ferromagnetic layers are coupled. The randomly uncompensated magnetic moments induced by antiferromagnetic material decrease the Curie temperature of the ferromagnetic film. In the magnetization direction studies, first, the spin reorientation behaviors of the Ni/Cu(001) system is discussed. The evidence of oscillatory behavior of the anisotropy of the system is found. Secondly, the effect of coupling on the magnetization direction is discussed in the Ni/Fe/Co/Cu(001) system. It is found that only collinear part of magnetization is coupled in the region of a slowly rotating spin reorientation transition. Lastly, the exchange bias in the coupled ferromagnetic system is studied.
Optics Communications, 1999
The near-field of a hollow-core optical fiber is calculated by means of the Huygens-Fresnel integ... more The near-field of a hollow-core optical fiber is calculated by means of the Huygens-Fresnel integral and it is shown how the near-field develops into a far-field. The diffracted patterns are experimentally measured by a novel imaging technique, and are in good agreement with the numerical results. A possible application to atom optics is also presented. q 1999 Elsevier Science B.V. All rights reserved.
Journal of Optics B-quantum and Semiclassical Optics, 1999
The diffracted field of a hollow-core optical fibre is studied and it is shown how the near field... more The diffracted field of a hollow-core optical fibre is studied and it is shown how the near field of a guided mode develops into the far field. We then describe our method of producing a diffracted output in the shape of a diverging dark hollow beam with a minimum dark spot of a few m, which is comparable to the hollow-core diameter, by superposing two LP11 modes which have node lines and polarizations orthogonal to each other. Its possible application to an efficient, novel atomic funnel which guides cold atoms into the hollow fibre is also discussed.
Journal of Optics-nouvelle Revue D Optique, 1999
The diffracted field of a hollow-core optical fibre is studied and it is shown how the near field... more The diffracted field of a hollow-core optical fibre is studied and it is shown how the near field of a guided mode develops into the far field. We then describe our method of producing a diffracted output in the shape of a diverging dark hollow beam with a minimum dark spot of a few µm, which is comparable to the hollow-core diameter, by superposing two LP11 modes which have node lines and polarizations orthogonal to each other. Its possible application to an efficient, novel atomic funnel which guides cold atoms into the hollow fibre is also discussed.
We show the realization of 2D condensation of trapped Bose atoms directly by obtaining the stable... more We show the realization of 2D condensation of trapped Bose atoms directly by obtaining the stable solutions for the condensate wave function from the 2D Gross-Pitaevskii equation. We calculate the ground-state energy of the 2D system, and also the wave function of the 2D vortex state. In particular, the 2D energy state becomes less stable than the 3D case with the number of trapped atoms. The results of the 2D Bose condensation are also compared with those of the well-known 3D case.
Growth-induced uniaxial magnetic anisotropy and the surface morphology of Co/Cu(100) films were i... more Growth-induced uniaxial magnetic anisotropy and the surface morphology of Co/Cu(100) films were investigated using Surface Magneto-Optic Kerr Effect (SMOKE), Low Energy Electron Diffractions (LEED), and Scanning Tunneling Microscopy (STM). We found that the Co film grown at low temperature (230K) at 63 degrees of off-normal growth direction processes an in-plane uniaxial magnetic anisotropy with the easy magnetization axis perpendicular to the growth incident plane. LEED measurement reveals a rougher Co surface than room temperature grown film. STM measurements show that the low temperature grown Co film consists of small islands without obvious anisotropic roughness or elongated islands. This result rules out the dipolar origin of the uniaxial magnetic anisotropy. Cu and CO absorption experiments suggest that the uniaxial magnetic anisotropy originates from the magneto crystalline anisotropy of the Co film surface.
The spin reorientation transition (SRT) effect of thin Ni films grown on Cu(100) is known to resu... more The spin reorientation transition (SRT) effect of thin Ni films grown on Cu(100) is known to result from the competition between the volume and surface anisotropies. A capping layer on the top of the Ni film may change the surface anisotropy, and hence influences the SRT thickness. We recently demonstrated that Cu capping layers on top of the Ni film modify the Ni magnetic anisotropy thus to increase the SRT thickness. It was shown that it takes about 5ML Cu capping layer to finish the modification of the Ni magnetic anisotropy. No oscillatory behavior was found in the Ni magnetic anisotropy as a function of the Cu capping layer thickness. The relationship between the magnetic anisotropy and quantum well states in Cu/Ni/Cu(100) has been discussed.
Journal of Applied Physics, 2007
Cr-dosed FePt films with total thicknesses of 1.8 and 3.6 nm were grown in an ultrahigh vacuum at... more Cr-dosed FePt films with total thicknesses of 1.8 and 3.6 nm were grown in an ultrahigh vacuum at room temperature and studied by means of the surface magneto-optic Kerr effect, scanning electron microscopy, and x-ray magnetic circular dichroism. After annealing at 550 °C, the L10 and A1 phases are spatially separated. In the L10 phase, round nanoparticles form that are 15+/-4.8 nm in diameter and possess out-of-plane magnetic easy axes. An addition of <0.01 nm Cr enhances the coercivity (~0.9 T), which then decreases with additional Cr, and a spin reorientation transition occurs near 0.3 nm Cr dosage as the L10 islands disappear. Segregation of Cr during annealing is believed to be responsible for the formation of round FePt islands and the separation of the L10 islands and planar A1 phase.
The dissertation addresses how the interaction between two magnetic layers changes the magnetic p... more The dissertation addresses how the interaction between two magnetic layers changes the magnetic properties. The dissertation is divided into three sections, chapters 1,2 and 3 introduce the magnetic films, basic theories and experimental methods. Second part, Chapters 4,5 and 6 describe the projects related with magnetic phase transitions and the effect of coupling on them. The third part, Chapters 7,8 and 9 describe the projects related with magnetization direction; spin reorientation transition in Ni/Cu(001), the effect of interlayer coupling on the magnetization direction, and the exchange bias effect in the coupled ferromagnetic system. In the magnetic phase transition projects, the effect of anisotropy and long-range dipole interaction on the Curie temperature is studied in the Fe/Ni/Cu(001)system first. Curie temperature is measured around stripe domain forming spin reorientation transition regions, in which it is found the anisotropy and the long range dipole interaction is important to build magnetic order. Secondly, the effect of indirect exchange coupling on the Curie temperature is studied; there are three kinds of phase transitions. When one film is in the ferromagnetic state, the coupling increases the Curie temperature of the other film. Both films can have simultaneous phase transitions when the Curie temperatures are similar. Thirdly, the Neel temperature and Curie temperature are discussed when the antiferromagnetic layer and ferromagnetic layers are coupled. The randomly uncompensated magnetic moments induced by antiferromagnetic material decrease the Curie temperature of the ferromagnetic film. In the magnetization direction studies, first, the spin reorientation behaviors of the Ni/Cu(001) system is discussed. The evidence of oscillatory behavior of the anisotropy of the system is found. Secondly, the effect of coupling on the magnetization direction is discussed in the Ni/Fe/Co/Cu(001) system. It is found that only collinear part of magnetization is coupled in the region of a slowly rotating spin reorientation transition. Lastly, the exchange bias in the coupled ferromagnetic system is studied.
Optics Communications, 1999
The near-field of a hollow-core optical fiber is calculated by means of the Huygens-Fresnel integ... more The near-field of a hollow-core optical fiber is calculated by means of the Huygens-Fresnel integral and it is shown how the near-field develops into a far-field. The diffracted patterns are experimentally measured by a novel imaging technique, and are in good agreement with the numerical results. A possible application to atom optics is also presented. q 1999 Elsevier Science B.V. All rights reserved.
Journal of Optics B-quantum and Semiclassical Optics, 1999
The diffracted field of a hollow-core optical fibre is studied and it is shown how the near field... more The diffracted field of a hollow-core optical fibre is studied and it is shown how the near field of a guided mode develops into the far field. We then describe our method of producing a diffracted output in the shape of a diverging dark hollow beam with a minimum dark spot of a few m, which is comparable to the hollow-core diameter, by superposing two LP11 modes which have node lines and polarizations orthogonal to each other. Its possible application to an efficient, novel atomic funnel which guides cold atoms into the hollow fibre is also discussed.
Journal of Optics-nouvelle Revue D Optique, 1999
The diffracted field of a hollow-core optical fibre is studied and it is shown how the near field... more The diffracted field of a hollow-core optical fibre is studied and it is shown how the near field of a guided mode develops into the far field. We then describe our method of producing a diffracted output in the shape of a diverging dark hollow beam with a minimum dark spot of a few µm, which is comparable to the hollow-core diameter, by superposing two LP11 modes which have node lines and polarizations orthogonal to each other. Its possible application to an efficient, novel atomic funnel which guides cold atoms into the hollow fibre is also discussed.
We show the realization of 2D condensation of trapped Bose atoms directly by obtaining the stable... more We show the realization of 2D condensation of trapped Bose atoms directly by obtaining the stable solutions for the condensate wave function from the 2D Gross-Pitaevskii equation. We calculate the ground-state energy of the 2D system, and also the wave function of the 2D vortex state. In particular, the 2D energy state becomes less stable than the 3D case with the number of trapped atoms. The results of the 2D Bose condensation are also compared with those of the well-known 3D case.
Growth-induced uniaxial magnetic anisotropy and the surface morphology of Co/Cu(100) films were i... more Growth-induced uniaxial magnetic anisotropy and the surface morphology of Co/Cu(100) films were investigated using Surface Magneto-Optic Kerr Effect (SMOKE), Low Energy Electron Diffractions (LEED), and Scanning Tunneling Microscopy (STM). We found that the Co film grown at low temperature (230K) at 63 degrees of off-normal growth direction processes an in-plane uniaxial magnetic anisotropy with the easy magnetization axis perpendicular to the growth incident plane. LEED measurement reveals a rougher Co surface than room temperature grown film. STM measurements show that the low temperature grown Co film consists of small islands without obvious anisotropic roughness or elongated islands. This result rules out the dipolar origin of the uniaxial magnetic anisotropy. Cu and CO absorption experiments suggest that the uniaxial magnetic anisotropy originates from the magneto crystalline anisotropy of the Co film surface.
The spin reorientation transition (SRT) effect of thin Ni films grown on Cu(100) is known to resu... more The spin reorientation transition (SRT) effect of thin Ni films grown on Cu(100) is known to result from the competition between the volume and surface anisotropies. A capping layer on the top of the Ni film may change the surface anisotropy, and hence influences the SRT thickness. We recently demonstrated that Cu capping layers on top of the Ni film modify the Ni magnetic anisotropy thus to increase the SRT thickness. It was shown that it takes about 5ML Cu capping layer to finish the modification of the Ni magnetic anisotropy. No oscillatory behavior was found in the Ni magnetic anisotropy as a function of the Cu capping layer thickness. The relationship between the magnetic anisotropy and quantum well states in Cu/Ni/Cu(100) has been discussed.