利用中子繞射探討層狀硫屬氧化物BiOCuX (X=S,Se)的 自旋、電荷及晶體結構間的交互作用

Spin, Charge and Lattice Couplings in Layered Oxychalcogenide BiOCuX (X= S, Se) Compounds Studied by Neutron Diffraction

卡術尼(Sunil Kumar)

指導教授 ： 李文献

國立中央大學/理學院/物理學系/博士(2011年)

摘要

利用中子繞射、交流磁化率、磁化強度和電阻率進行BiOCu0.94S樣品之磁性及電性研究。樣品BiOCu0.94S的晶體結構為方P4/nmm對稱，室溫下晶格常數為 a = b =3.8645（1）和c=8.5493（3）。四方晶系的BiOCu0.94S原子結構包括BiO層以及CuS層，且層與層之間藉由弱離子鍵連接。在T=250 K以下銅的自旋以鐵磁性排列。反鐵磁性在低於180 K時始發展，同時發生晶格常數的激烈變化。傾斜鐵磁自旋在低溫下飽和磁矩大小為<μZ> =0.61 μB。在磁有序態下的電傳導可以用三維不定域跳躍傳導來描述。外加磁場可以有效地減少跳躍傳導之能障。當溫度高於 250 K，銅自旋方向進入無序態，此時電阻率隨溫度變化關係背離跳躍傳導的形式，而成為隨溫度的升高而變大，顯示出自旋與電荷之間的偶合。

另外，我們也研究四方晶系BiOCu0.98Se的晶體結構和磁結構。銅的鐵磁性自旋有序於 TC =300K以下開始發展。在260 K以下單位晶格發生負的熱膨脹，並產生了傾斜鐵磁自旋排列，銅自旋在低溫之飽和磁矩大小為<μZ> =0.5μB。這些觀察到現象清楚地揭示了晶格與磁結構之間的相互作用，這些罕見現象背後的機制肯定是有趣的。

關鍵字

中子繞射 ； 硫屬氧化物 ； 硫屬氧化物 ； 磁結構 ； 中子繞射 ； 磁結構

並列摘要

The electrical and magnetic properties of slightly Cu-deficient BiOCu0.94S are investigated using neutron diffraction, ac magnetic susceptibility, magnetization and electric resistivity measurements. The sample BiOCu0.94S crystallizes into a tetragonal P4/nmm symmetry with cell parameters of a=b=3.8645(1) Å and c=8.5493(3) Å at 293 K. Tetragonal BiOCu0.94S consists of BiO and CuS layers that are interconnected through weak ionic bindings. The Cu spins order with a ferromagnetic arrangement below TC =250 K. An antiferromagnetic component develops below 180 K when the crystalline unit cell experiences a sharp thermal contraction upon cooling, resulting in a canted ferromagnetic spin arrangement at low temperatures with saturated magnetic moment <μZ> = 0.61μB. In the magnetically ordered state the electrical transport can be described by three-dimensional (3D) variable range hopping conduction. An applied magnetic field can effectively reduce the hopping barrier. Spin-charge couplings are clearly revealed as the resistivity departs from the hopping conduction to become increases with increasing temperature, when the Cu spins become disordered above 250 K.
The crystalline and magnetic structures of tetragonal BiOCu0.98Se are also studied. The ferromagnetic ordering of Cu spins develops below TC = 300K. An antiferromagnetic component develops below 260 K when the crystalline unit cell experiences negative thermal expansion upon cooling resulting in a canted ferromagnetic spin arrangement for the Cu spins at low temperatures with a saturated magnetic moment <μZ> = 0.5μB. These observations clearly reveal the appearance of interplay between lattice and magnetic structures, which is rarely seen and the mechanism behind is certainly interesting.

並列關鍵字

Magnetic structure ； Oxychalcogenides ； Magnetic structure ； Neutron diffraction ； Oxychalcogenides ； Neutron diffraction

參考文獻

Chapter 1

1Y. Kamihara, T. Watanabe, M. Hirano & H. Hosono, J. Am. Chem. Soc. 130, 3296 (2008).

3G. F. Chen, Z. Li, G. Li, J. Zhou, D. Wu, J. Dong, W. Z. Hu, P. Zheng, Z. J. Chen, H. Q. Yuan, J. Singleton, J. L. Luo, and N. L. Wang, Phys. Rev. Lett. 101, 057007 (2008).

8J. Dong, H. J. Zhang, G. Xu, Z. Li, G. Li, W. Z. Hu, D. Wu, G. F. Chen, X. Dai, J. L. Luo, Z. Fang, and N. L. Wang. Preprint at http://arxiv.org/abs/0803.3426v1 (2008).

13F. J. Ma & Z. Y. Lu, Phys. Rev. B 78, 033111 (2008).