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Magnetism of chemically modified ultrathin metal films and nanorods studied by magneto-optical methods
https://ir.soken.ac.jp/records/248
https://ir.soken.ac.jp/records/2489c438ec8-cf1b-4b6b-8523-3687e257e775
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要旨・審査要旨 (349.9 kB)
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本文 (8.6 MB)
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| Item type | 学位論文 / Thesis or Dissertation(1) | |||||
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| 公開日 | 2010-02-22 | |||||
| タイトル | ||||||
| タイトル | Magnetism of chemically modified ultrathin metal films and nanorods studied by magneto-optical methods | |||||
| タイトル | ||||||
| タイトル | Magnetism of chemically modified ultrathin metal films and nanorods studied by magneto-optical methods | |||||
| 言語 | en | |||||
| 言語 | ||||||
| 言語 | eng | |||||
| 資源タイプ | ||||||
| 資源タイプ識別子 | http://purl.org/coar/resource_type/c_46ec | |||||
| 資源タイプ | thesis | |||||
| 著者名 |
馬, 暁東
× 馬, 暁東 |
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| フリガナ |
マア, ショウドン
× マア, ショウドン |
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| 著者 |
MA, Xiao-Dong
× MA, Xiao-Dong |
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| 学位授与機関 | ||||||
| 学位授与機関名 | 総合研究大学院大学 | |||||
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| 学位名 | 博士（理学） | |||||
| 学位記番号 | ||||||
| 内容記述タイプ | Other | |||||
| 内容記述 | 総研大甲第1026号 | |||||
| 研究科 | ||||||
| 値 | 物理科学研究科 | |||||
| 専攻 | ||||||
| 値 | 07 構造分子科学専攻 | |||||
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| 学位授与年月日 | 2007-03-23 | |||||
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| 値 | 2006 | |||||
| 要旨 | ||||||
| 内容記述タイプ | Other | |||||
| 内容記述 | Magnetism of low-dimensional nanoscale materials is not of the most attractive current issues because of technological requirements for higher density recording media and nano-spintronics as well as a fundamental interest in the particle size effect on the magnetic properties. As the dimensionality and size of a physical system is reduced, magnetic ordering tends to become more complicated as the fluctuations become relatively more pronounced. For instance, a magnetic nanorod array with the diameter of 30nm buried in a self-organized porous alumina nanohole template shows an easy axis along the rod, indicating the shape anisotropy is dominating. However, the study of atomic Co chains on Pt(997) revealed perpendicular magnetization with respect to the chain axis. This shows that the spin-orbit interaction rather than shape anisotropy has become dominating for magnetic anisotropy when the size is ultimately reduced. On the other hand, the critical behavior in low dimensional materials represents another fascinating topic. The value of the critical exponent <b><i>β</i></b> of one-layer ribbons on stepped W(110) has been reported to be close to the two-dimensional Ising model while quite less than that of the three-dimensional one. However, so far there is no report concerning magnetic anisotropy and critical behaviors of one-dimensional nanostructure within several atoms width and several atoms height. This is mainly because the magnetic layers in these previous studies grow on step edges of the substrate surfaces in a layer-by-layer fashion and the one-layer ribbon structure is eventually merged before the second or higher layer grows.<br /> In this thesis, He has succeeded in the investigation of the critical behavior and magnetic anisotropy of Co nanorods within several atoms range by cooperating with a STM group at University of Missouri, Kansas city. The novel results from this research is expected to fill the gap between magnetic properties of atomoc-chain and large scale nanorods(>20nm). The Co nanorods with 1.5nm width and several atoms height is prepared on a N teminated Cu (110) surface. The nanorods extend along the [1-10] direction and are well isopated from each other up to nominal as much as 2 ML (monolayer) Co coverage with six times intervals along the [001] direction. The magnetization curves recorded by the magneto-optical Kerr effect (MOKE) clarify that the magnetic easy axis is perpendicular to the rod axis within the substrate plane, implying that the magnetic anisotropy is not dominated by the shape anisotropy. The x-ray magnetic circular dichroism measurement presents a significant enhancement of the orbital magnetic moment along the easy axis compared to the hard axes, confirming that the spin-orbit interaction determines the easy axis. The critical behaviors of the Co nanorods show that the critical exponent <b><i>β</i></b> is much larger than the value expected from the two-dimensional Ising model. By performing the Monte-Carlo simulations based on single domain model and multiple spin segment model, the staggered suppression of the magnetization around the critical temperature is undoubtfully ascribed to the finite length of he Co nanorods.<br /> Meanwhile, the chemisorption effect on the magnetic anisotropy of Fe ultrathin film on Ag(001)surface is also studied systemically in this thesis. It is well known that the variation in surroundings such as chemical adsorption can have a significant effect on the morphology in thin film, which accordingly alters the magnetic anisotropies, resulting in the spin reorientation toward the easy axis of the magnetization. Previous studies revealed a destabilization of perpendicular magnetization due to the oxygen adsorption but failed to provide a sound proof of the origin of this spin reorientation. The answer to this problem has been wrapped in mystery since then. It is therefore the purpose of this work to tackle this subject and probe what is decisive factor responsible for the observed destabilization of perpendicular magnetization after oxygen adsorption. He has performed a systematic investigation on the effect from surface chemisorption. By way of comparisons, three different gases were adsorbed on the Fe thin film at the temperature 100 K. It is found by the MOKE that adsorption of O<small>2</small> and NO induces the shift of the critical thickness for the transitions to a thinner side, together with the suppression of the remanent magnetization and the coercive field of the Fe film. This implies destabilization of the perpendicular magnetic anisotropy. On the other hand, H<small>2</small> adsorption is found not to change the magnetic anisotropy, though the enhancement of the coercive field is observed. The XMCD reveals that although both the spin and orbital magnetic moments along the surface normal are noticeably reduced upon O<small>2</small> and NO adsorption, the reduction of the orbital magnetic moments are more significant. This indicates that the destabilization of the perpendicular magnetic anisotropy upon chemisorption of O<small>2</small>and NO originates from the change of the spin-orbit interaction at the surface. <br /> The last part of this thesis contains the determination of local structure and electronic state of a novel molecular photomagnets of Cu-Mo cyanides. This photomagnet is unique for many reasons. The photoinduced phase is essentially different in this sample because there is no high temperature phase in this sample while it usually can be found in all the other phtomagnet cyanides. Moreover, these compounds, which show ferromagnetism, consist only of magnetic ions of Cu and Mo, which seldom exhibit ordered magnetism. No typical magnetic ions such as Mn, Fe, Co, or Ni are present. In addition, the change of local structures around Cu in a redox cycle is an important issue in catalysis and biology. The results from this study are expected to provide a deep insight into these regards. A direct evidence for the reduction of divalent Cu to monovalent is detected by Cu <i>K</i>-edge x-ray-absorption near-edge structure. The extended x-ray-absorption fine-structure confirms that the interatomic distances around Cu and Mo in the photoinduced phase are almost identical to those of the initial low temperature phase. The higher-nearest neighbor coordination numbers, however, are apparently reduced. He has interpreted this as the result of the bending of the Mo-CN bond due to the tetrahedral distortion around the monovalent Cu(Ⅰ) site. | |||||
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| 値 | 有 | |||||
