利用X光能譜和中子散射技術研究單晶材料鍶鐵氧和鎳錳鈦氧之電子和磁結構

由同步輻射光所衍生出來的X光相關能譜實驗技術提供了具備元素針對性的電子、原子結構資訊於基礎科學研究、材料研發及產業應用上，包括針對晶格結構的X射線繞射分析(X-ray Diffraction, XRD)、電子或軌域結構的X光吸收近邊結構(X-ray Absorption Near Edge Structure, XANES)和X光線偏振二向性能譜術(X-ray Linear Dichroism, XLD)、佔據態及其交互作用的X光發射能譜術(X-ray Emission Spectroscopy, XES)和共振非彈性X光散射能譜術(Resonant Inelastic X-ray Scattering, RIXS)及局部原子結構的衍生X光吸收精細結構(Extended X-ray Absorption Fine Structure, EXAFS)等；另外，中子相關散射實驗技術能對分析磁結構提供最直接的形貌和證據，包括針對自旋磁矩間交互作用和動態的彈性和非彈性中子散射(Elastic and Inelastic Neutron Scattering, ENS and INS)及磁簇間交互作用的小角度中子散射(Small Angle Neutron Scattering, SANS)等。這些實驗技術被是為探討具備特殊磁結構之新穎單晶材料複雜耦合機制的有力工具。本文第一部分針對龐磁阻單晶材料SrFeO3-δ的巨觀物理現象和微觀電子與原子結構的關係做探討。SrFeO3-δ因為其特殊的磁結構而在電性量測上有熱滯現象，但最近發現其熱滯現象在材料為單晶型態時有異向性的行為。為了探究其物理機制，利用XANES、XLD、EXAFS來探討SrFeO3-δ在不同方向上的軌域優先態、鐵和氧之間的鍵長、結構有序度。實驗結果顯示，在升降溫過程其鐵氧八面體扭曲之行為有所不同，造成了在升降溫過程時鐵的軌域優先態3d3z2-r2轉變為3dx2-y2，進而導致了熱滯現象異向性的發生。此外，我們將價電帶光電子能譜(Valence-Band Photoemission Spectroscopy, VB-PES)和X光吸收光譜(X-ray Absorption Spectroscopy, XAS)的結果做結合更進一步確認了SrFeO3-δ的相對能隙在不同方向及不同升降溫過程時有所改變，為SrFeO3-δ有電荷密度波的假說提供了最直接的證據。我們完整地揭開SrFeO3-δ局部電子、原子與能帶結構之間的物理機制，期望拓展其潛在應用價值。第二部分針對XY-like自旋玻璃單晶材料Ni0.4Mn0.6TiO3(NMTO)之特殊磁結構做詳盡的分析。NMTO在近年被認為其磁結構應是準二維的XY-like自旋玻璃態，而非以往所認為的Heisenberg自旋玻璃態。利用ENS和INS探討其自旋磁矩的空間動態和交互作用，將其在自旋玻璃態時不同溫度和方向之自旋相干長度、生命週期計算出來。其結果為NMTO是XY-like自旋玻璃態一說提供了直接的證據。最後，近年更發現NMTO其自旋玻璃態可以利用外加電場或磁場來調控並成為新的硬碟和記憶體潛力材料，其調控機制被用toroidal glass模型解釋。我們利用SANS揭開NMTO外加磁場下磁結構的形貌和相圖，為toroidal glass一說提供了直接的證據。並利用XANES、XLD、EXAFS、RIXS針對NMTO的電子和原子結構做完整的分析，期許能開發NMTO在更多方面的應用可能性。實驗結果顯示，在自旋玻璃態時，NMTO在ab平面上有局部晶格對稱性降低的現象發生，進而造成鎳的軌域優先態從3d3z2-r2轉變為3dx2-y2。

關鍵字

X光吸收近邊結構能譜術； X光線偏振二向性能譜術； X光發射能譜術；共振非彈性X光散射；衍生X光吸收精細結構；彈性和非彈性中子散射；小角度中子散射

並列摘要

The synchrotron radiation based spectroscopic techniques provide element-specific information of electronic and atomic structures for fundamental researches, material studies and industrial applications. Techniques include X-ray diffraction (XRD) for lattice structure, X-ray absorption near edge structure (XANES) and X-ray linear dichroism (XLD) for electronic and orbital structures, X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) for information of the occupied states and inherent elementary excitations, extended X-ray absorption fine structure (EXAFS) for local atomic structure. In addition, neutron scattering related techniques provide direct evidence and texture of magnetic structure, such as elastic and inelastic neutron scattering (ENS and INS) for spatial correlations and dynamics of spin moments, small angle neutron scattering (SANS) for spatial correlations of magnetic clusters. These techniques are considerable and powerful techniques for the study of various complex couplings in novel single crystal materials with special magnetic structure. In the first part of this thesis, the local electronic and atomic structures of the high-quality single crystal of SrFeO3-δ were studied using temperature-dependent x-ray absorption (XAS) and valence-band photoemission spectroscopy (VB-PES) to investigate the origin of anisotropic resistivity in the ab-plane and along the c-axis close to the region of thermal hysteresis (near temperature for susceptibility maximum, Tm~78 K). All experiments herein were conducted during warming and cooling processes. The Fe L3,2-edge XLD results show that during cooling from room temperature to below the transition temperature, the unoccupied Fe 3d eg states remain in persistently out-of-plane 3d3z2-r2 orbitals. In contrast, in the warming process below the transition temperature, they change from 3d3z2-r2 to in-plane 3dx2-y2 orbitals. The nearest-neighbor (NN) Fe-O bond lengths also exhibit anisotropic behavior in the ab-plane and along the c-axis below Tm. The anisotropic NN Fe-O bond lengths and Debye-Waller factors stabilize the in-plane Fe 3dx2-y2 and out-of-plane 3d3z2-r2 orbitals during warming and cooling, respectively. Additionally, a VB-PES study further confirms that a relative band gap opens at low temperature in both the ab-plane and along the c-axis, providing the clear evidence of the charge-density-wave nature of SrFeO3-δ single crystal. In the second part, ENS and INS experiments were performed on a single crystal of Ni0.4Mn0.6TiO3 (NMTO) to study the spatial correlations and dynamics of spins in the XY-like spin-glass (SG) state. Magnetization measurements reveal signatures of SG behavior in NMTO with a freezing temperature of TSG ~ 9.1 K. The ENS experiments indicated that the intensity of magnetic diffuse scattering starts to increase around 12 K, which is close to TSG. Also, spin-spin correlation lengths (ξ) at 1.5 K are approximately 21.05±0.6 and 72.99±1.6 Å in the interlayer and the in-plane directions, respectively, demonstrating that magnetic correlations in NMTO exhibit quasi two-dimensional-like antiferromagnetic order. INS results show quasi-elastic neutron scattering (QENS) profiles below TSG. The life-time of dynamic correlations (τ), obtained from the QENS profiles, are approximately 16.27±0.8 and 15.88±1.9 psec at 10 K for two positions (0.00, 0.00, 1.52) and (0.01, 0.01, 1.50), respectively. Therefore, our experimental findings demonstrate that short-range-ordered antiferromagnetic clusters with short-lived spin correlations are present in the XY-like SG state of NMTO at a temperature of approximately TSG. In final part, SANS, RIXS, and XAS experiments on the single crystal of NMTO have been carried out to study the role of electronic and atomic properties in the XY-like SG state. SANS presents the texture of toroidal glass and phase diagram of NMTO. RIXS experiments provide the evidence of crystal field (d-d) excitations at Ni and Mn L3-edge. However, spin-flip excitations are only observable which are prominent at Ni L3-edge RIXS. Using temperature dependent XANES, XLD and EXAFS studies along with RIXS, it has been shown that symmetry breaking/ lattice distortion occurs at low temperature near SG phase transition due to the local disorder in Ni-O bond lengths, and further induced the orbital preference occupation charge from out-of-plane state (3d3z2-y2) to in-plane state (3dx2-y2). We therefore believe that XY-like SG state of NMTO is associated with the unoccupied in-plane (3dx2-y2) states.

並列關鍵字

XANES ； XLD ； XES ； RIXS ； EXAFS ； ENS ； INS ； SANS

參考文獻

[1] Y. Yamaguchi, T. Nakano, Y. Nozue, and T. Kimura, Physical review letters 108, 057203 (2012).

Google Scholar

[2] Y. Yamaguchi and T. Kimura, Nature communications 4, 2063 (2013).

Google Scholar

[3] A. Lebon, P. Adler, C. Bernhard, A. V. Boris, A. V. Pimenov, A. Maljuk, C. T. Lin, C. Ulrich, and B. Keimer, Physical review letters 92, 037202 (2004).