YBaCuFeO5 單晶成長與磁性特性研究

Crystal Growth and Magnetic Property Studies of YBaCuFeO5

賴彥仲(Yen-Chung Lai)

指導教授 ： 杜昭宏

淡江大學/理學院/物理學系博士班/博士(2015年)

https://doi.org/10.6846/TKU.2015.00574

摘要

使用改良式光學移動熔區法(traveling solvent floating zone, TSFZ)成功成長的高品質單晶樣品在磁化率量測上分別於455K與170K呈現兩個反鐵磁相。使用中子單晶及粉末繞射，在高溫的反鐵磁相中，(455K~170K)，發現Fe與Cu的磁性結構為一個等量的反鐵磁有序排列結構，但此反鐵磁相在170K以下則變成一個增殖向量為ki=(0.5 0.5 0.5±δ)的非等向性沿著c軸的螺旋磁結構，並且經由單晶磁化率量測確定Cu與Fe的自旋磁矩在170 K下，排列在ab平面上。
在外加磁場的單晶中子繞射實驗中，低溫的磁結構(IC)藉由外加磁場轉換至高溫的反鐵磁相(C)之前會經過一個C-IC的混合相；而在移除磁場的過程中，發現了一個全新的亞穩相(metastable phase)。
最後本論文也對於不同比例的Cu與Fe的粉末樣品進行晶體結構與磁性特性的研究，發現在不同比例的樣品中，磁性特性有非常明顯的差異，這個實驗能夠解釋在以前的報導中不一致的轉換溫度。

關鍵字

YBaCuFeO5 ； 懸浮帶區法 ； 中子繞射 ； 磁性結構

並列摘要

This thesis reports the single crystal growth and magnetic properties of YBaCuFeO5 (YBCFO) using magnetic susceptibility and neutron diffraction techniques. Using the modified traveling solvent floating zone (TSFZ) technique, we are able to grow the high quality and sizable single crystal of YBCFO for the further studies. The crystal displayes two antiferromagnetic transitions at TN1 ~455 K and TN2 ~ 170 K as indicated by the spin susceptibility anomalies. Using single crystal neutron diffraction measurement, the antiferromagnetic phase between 170 and 455 K is revealed to be a commensurate phase with a q-wavevector qN1=(1/2 1/2 1/2), and this phase undergoes a phase transition to form an incommensurate phase with a q-wavvector qN2=(1/2 1/2 1/2±δ) at TN2, where the incommensurabity δ is dependent on temperature. Further, using high resolution neutron powder diffraction, the incommensurate magnetic phase is confirmed to be a spiral magnetic phase in which the magnetic moments of Fe and Cu ions are laid on the ab-plane to be consistent to that observed by the magnetic susceptibility measurement result.
A detailed H-T phase diagram has been mapped for the YBCFO. By sweeping the temperature and magnetic fields perpendicular to the c-axis, an irreversible commensurate-incommensurate phase crossover with intermediate metastable phases are observed.
This thesis also reports the study of the nonstoichiometry effect of Cu and Fe in YBa(Cu1-xFex)2O5 system. Both of the transition temperatures TN1 and TN2 are sensitively varied from the Cu:Fe=1:1 ratio as little as 1%, which explains the inconsistency of TN1 and TN2 as reported in literatures.

並列關鍵字

YBaCuFeO5 ； Traveling solvent floating zone ； Neutron diffraction ； Magnetic structure

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