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    Title: 利用中子散射探討多鐵材料Co3TeO6暨普魯士藍Rb-Co-Fe@K-Ni-Cr奈米核殼結構的複雜磁相變;Neutron scattering investigation of complex magnetic transitions in multiferroic Co3TeO6 and core@shell Prussian blue analogue Rb-Co-Fe@K-Ni-Cr nano-cubes
    Authors: 李其紘;Lee,Chi-Hung
    Contributors: 物理學系
    Keywords: 多鐵材料;中子散射;普魯士藍;multiferroic;Neutron scattering;Prussian blue analogue
    Date: 2015-07-15
    Issue Date: 2015-09-23 11:33:41 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 本論文有兩部分。第一部分探討多鐵系統Co3TeO6的複雜磁相變。第二部分為奈米核殼結構普魯士藍Rb-Co-Fe@K-Ni-Cr的磁交互作用探討。
    在第一部分中,利用中子繞射、磁化率、比熱、介電常數探討單晶Co3TeO6系統的鐵電與commensurate及incommensurate磁有序的關係。在磁化率、比熱的實驗中發現在低溫有四個磁相變,並利用中子繞射探討這四個磁相變的磁性行為。溫度在26、18、16 K為incommensurate磁有序的相變,19.5 K為commensurate磁有序的產生。其中在18 K的相轉變中發現了鐵電的產生,並且在鐵電產生後有負熱膨脹的行為。從中子繞射圖解出磁結構,並發現磁結構可被電場或磁場所調控。Co3TeO6應為第二類多鐵材料。
    第二部分為探討奈米核殼結構普魯士藍Rb-Co-Fe@K-Ni-Cr的磁交互作用。核為250 奈米的RbCoFe普魯士藍,殼為45奈米的KNiCr普魯士藍。在86、69與67 K中觀察到磁相變。從中子繞射數據中可看出K-Ni-Cr為鐵磁性物質。Ni與Ni離子之間有兩種交互作用路徑,一種經由C與N離子透過Cr做超交互作用,另一種沿著晶體[110]方向的direct exchange。而Cr與Cr離子之間僅有一種交互作用路徑,為經由C與N離子透過Ni做超交互作用。因此認為69 K為Ni離子的磁有序溫度,而67 K為Cr離子的磁有序溫度。在Rb-Co-Fe相中並沒有看到照光的反應,認為是CoN6與FeC6八面體之間距離較遠導致電荷較難從Fe離子轉移到Co離子,導致照光沒有磁性變化。
    ;There are two parts in my thesis. The first part focuses on the complex magnetic couplings in Co3TeO6. The second part focuses on the magnetic phases in core/shell Prussian blue analogue Rb-Co-Fe@K-Ni-Cr nano-cubes.
    In the first part, neutron diffraction, magnetic susceptibility, specific heat, and dielectric constant of single crystal cobalt tellurate Co3TeO6 have all been measured to study the interplay between the ferroelectricity, commensurate and incommensurate magnetic ordering developed in the compound. Four critical temperatures are identified. A non-collinear arrangement of the Co spins is found. A negative thermal expansion of the crystalline unit cell is identified when electric polarization develops. Both applied magnetic field and electric field significantly affect the magnetic and electric order parameters as well. These behaviors characterize Co3TeO6 to be a type-II multiferroics.
    Four magnetic phases have been identified in nano-sized core/shell Prussian blue analogue cubes, with a 250 nm Rb-Co-Fe phase in the core coated by a 45 nm K-Ni-Cr phase on the shell. The stress preserved in the core results in separated CoN6 and FeC6 octahedra, which weaken the magnetism and photo-sensitivity of the core. Three separated magnetic phase transitions at 86, 69 and 67 K are found in the K-Ni-Cr phase on the shell. Two magnetic exchange paths are identified. One propagates along the three crystallographic axis directions. The other propagates along the [110] crystallographic direction for the Ni-Ni ions, but not for the Cr-Cr ions. The severe Cr-deficiency and the appearance of direct Ni-Ni exchange are used to understand the appearance of multiple magnetic phases. The formation of compact CoN6 and FeC6 octahedra weakens the charge transfer between the Fe and Co ions, which results in the development of weak magnetic moments in the core.
    Appears in Collections:[Graduate Institute of Physics] Electronic Thesis & Dissertation

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