低維度過渡金屬氧化物Bi2Sr2CoO6+δ 與LiCu2O2的單晶成長與物理特性研究

Crystal growth and physical property studies of low dimensional transition metal oxide materials Bi2Sr2CoO6+δ and LiCu2O2

徐泓璋(Hung Chang Hsu)

指導教授 ： 劉祥麟 ； 周方正

國立臺灣師範大學/理學院/物理學系/博士(2010年)

摘要

磁自旋與晶格、磁自旋與墊偶極交互作用力在強關連電子材料系統被廣泛研究。單晶的樣品Bi2Sr2CoO6+delta與LiCu2O2可以藉由光學聚焦式熔化液體懸浮區方式成長。在低維度強關連材料Bi2Sr2CoO6+delta與LiCu2O2分別發現了磁自旋與聲子交互作用力與無磁性粒子參雜誘發新的短程磁有序排列。
在Bi2Sr2CoO6+delta的樣品中，可以藉由變溫的遠紅外光譜與x光粉末繞射研究，在反鐵磁有序相轉變溫度下可以發現磁自旋與聲子有交互作用。而此交互作用令聲子在溫度低於反鐵磁相轉變溫度下有頻率位置紅移的現象。
在Zn參雜的LiCu2O2樣品中，低濃度(Zn < 5%)的參雜使得樣品的磁有序溫度降至18 K，但是高濃度(Zn > 5 %)的樣品中，配合磁性、電性、晶格與比熱的量測，一個新的磁有序性被發現，並且與理論預測吻合。

關鍵字

多鐵電材料 ； 鋅參雜

並列摘要

The spin-lattice and spin-electric dipole interactions have been the focus on the study of strongly correlated electron materials. Herein, spin-phonon coupling constant and non-magnetic ion substitution effects in the strong correlated low dimensional materials Bi2Sr2CoO6+delta and LiCu2O2 have been studied. For low-dimensional and spin 1/2 system, the impact of quantum fluctuations becomes important for the ground state of the system. The quantum fluctuations is even more pronounced if the system under consideration exhibits strongly frustrated interactions. Single crystal samples were grown by using traveling solvent floating zone method and physical properties have been studied thoroughly.
Bi2Sr2CoO6+delta shows the antiferromagnetic (AFM) transition (T_N) from 100 K to 280 K depending on the ratio of Co2+ and Co3+. In a Co3+ rich region, where delta ~ 0.5, T_N is around 280 K. While delta ~ 0.25, where crystal has the equal number of Co2+ and Co3+, except T_N reduces to around 100 K and also exhibits ferromagnetic (FM) behavior. For 0.25 < delta < 0.5, both ferromagnetism and antiferromagnetism coexist in the system. The ab-plane infrared and optical reflectance of Bi2Sr2CoO6+delta (delta ~ 0.3 and 0.4 < delta < 0.5) single crystals has been measured over a wide frequency range (50-55000 cm^-1) and at temperatures between 20 and 330 K. The room-temperature infrared spectrum displays an insulating character. The optical gap determined from the infrared conductivity (~ 0.53 eV) is consistent with thermal activation energy from dc transport measurements. Upon passing through the 150 and 265 K antiferromagnetic ordering transition, a softening of the phonon mode near 205 cm^-1 correlates well with the temperature dependent normalized square of the sublattice magnetization. Furthermore, the magnetic-ordering induced splitting of the phonon modes at about 238 and 386 cm^-1 is observed. Additionally, the phonon mode at about 588 cm^-1 exhibits a Fano-like line shape. Since no appreciable structural change was detected at low temperatures in x-ray diffraction studies, all of these observations support the suggestion of a complex nature of spin-phonon coupling in these materials. Floating-zone growth of untwinned single crystal of LixCu2-zZnzO2 is reported. Li content of Zn free LixCu2O2 has been determined accurately through combined iodometric titration and thermogravimetric methods, which also ruled out the speculation of chemical disorder between Li and Cu ions. The morphology and physical properties of single crystals obtained from slowing-cooling (SL) and floating-zone (FZ) methods are compared. The floating-zone growth under Ar/O2=7:1 gas mixture at 0.64 MPa produces large area of untwinned crystal with highest Li content of x ~ 0.99 , which has the lowest helimagnetic ordering temperature ~ 19 K in the LixCu2O2 system. Helical ordering transition temperature (T_h) of the original LiCu2O2 follows nite size scaling for less than ~ 5.5% Zn substitution, which implies that the existence of finite helimagnetic domains with domain boundaries formed with nearly isolated spins. Higher Zn substitution > 5.5% quenches the long range helical ordering and introduces an intriguing Zn level dependent magnetic phase transition with slight thermal hysteresis and a universal quadratic eld dependence for Tc(Zn > 0.055,H). The magnetic coupling constants of nearest-neighbor (nn) J1 and next-nearest-neighbor (nnn) J2 (alpha=J2/J1) are extracted from high temperature series expansion (HTSE) fitting and N=16 finite chain exact diagnolization simulation. We have provided evidence of direct correlation between long range helical spin ordering and the magnitude of electric polarization in this spin driven multiferroic material also.

並列關鍵字

LiCu2O2 ； Bi2Sr2CoO6+delta ； finite size effect ； multiferroic ； isolated dimer

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