Magnetism in the complex cobaltates Y1−xSrxCoO3−δ (0.7 ≤ x ≤ 0.95) and Ca3Co2O6

The magnetic phases in the complex cobaltates Y1−xSrxCoO3−δ (0.7 ≤ x ≤ 0.95) and Ca3Co2O6 have been investigated by susceptibility, heat capacity, X-ray and neutron scattering techniques. These measurements have shown that the super- structure ordering in the perovskite cobaltate Y1−xSrxCoO3−δ which evolves as a function of temperature heavily influences the ferrimagnetic behaviour of this mate- rial. Neutron scattering has also been used to probe the unusual time and magnetic field dependent behaviour of the spin-chain compound Ca3Co2O6, and to further our understanding of the magnetic phase diagram of this system. Both polycrystalline and single crystal samples have been used in this study. High quality single crystals of the A-site (Sr/Y) and oxygen vacancy ordered form of the perovskite Y1−xSrxCoO3−δ have been produced using the floating zone technique and characterised using EDAX and TGA. The single crystals produced were large enough to perform polarised and inelastic neutron scattering experiments on this compound for the first time, revealing anisotropic quasi-elastic scattering above the magnetic transition temperature. In addition, diffraction experiments on these samples found evidence of coincident structural and magnetic transitions in Y1−xSrxCoO3−δ at both 370 and 280 K. Neutron diffraction measurements were also performed on the geometrically frustrated compound Ca3Co2O6. The low temperature magnetisation process was found to be accompanied by clearly visible steps in the intensity of the ferromagnetic and antiferromagnetic Bragg peaks. Detailed measurements have shown that the presence of short-range correlations cannot account for the reduction in intensity of the antiferromagnetic Bragg peaks at low temperatures. Instead, the origin of this drop in intensity was found to be a slow time-dependent magnetic transition from one long-range ordered antiferromagnetic state to another. This transition occurs over a timescale of hours and is never complete. The experimental work detailed in this thesis provides new information about the phase diagrams of Y1−xSrxCoO3−δ and Ca3Co2O6 and contributes to our overall understanding of the physics of these complex cobaltate compounds

Spin correlations in Ca3Co2O6: A polarised-neutron diffraction and Monte Carlo study

We present polarised-neutron diffraction measurements of the Ising-like spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature TN. Below TN, a clear evolution from a single-phase spin-density wave (SDW) structure to a mixture of SDW and commensurate antiferromagnet (CAFM) structures is observed on cooling. For a rapidly-cooled sample, the majority phase at low temperature is the SDW, while if the cooling is performed sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5 and 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse scattering data. We show that both intra- and inter-chain correlations persist above TN, but are essentially decoupled. Intra-chain correlations resemble the ferromagnetic Ising model, while inter-chain correlations resemble the frustrated triangular-lattice antiferromagnet. Using previously-published bulk property measurements and our neutron diffraction data, we obtain values of the ferromagnetic and antiferromagnetic exchange interactions and the single-ion anisotropy.Comment: 10 pages, 7 figure

Research and Design of a Routing Protocol in Large-Scale Wireless Sensor Networks

无线传感器网络，作为全球未来十大技术之一，集成了传感器技术、嵌入式计算技术、分布式信息处理和自组织网技术，可实时感知、采集、处理、传输网络分布区域内的各种信息数据，在军事国防、生物医疗、环境监测、抢险救灾、防恐反恐、危险区域远程控制等领域具有十分广阔的应用前景。 本文研究分析了无线传感器网络的已有路由协议，并针对大规模的无线传感器网络设计了一种树状路由协议，它根据节点地址信息来形成路由，从而简化了复杂繁冗的路由表查找和维护，节省了不必要的开销，提高了路由效率，实现了快速有效的数据传输。 为支持此路由协议本文提出了一种自适应动态地址分配算——ADAR（AdaptiveDynamicAddre...As one of the ten high technologies in the future, wireless sensor network, which is the integration of micro-sensors, embedded computing, modern network and Ad Hoc technologies, can apperceive, collect, process and transmit various information data within the region. It can be used in military defense, biomedical, environmental monitoring, disaster relief, counter-terrorism, remote control of haz...学位：工学硕士院系专业：信息科学与技术学院通信工程系_通信与信息系统学号：2332007115216

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

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