Is It Possible To Get High TC Magnets with Prussian Blue Analogues? A Theoretical Prospect

Theoretical methods based on density functional theory have been employed to search for Prussian blue analogues with Curie temperatures higher than the ones reached today. Our study suggests several possible cyano-bridged compounds as candidates to present stronger exchange coupling and higher ordering temperatures than the well known Cr(III)V(II) derivatives.     

Structure and Magnetic Properties of Gadolinium Hexacyanoferrate Prussian Blue Analogue

The structure of gadolinium hexacyanoferrate Prussian blue analogue Gd[Fe(CN)₆]·4H₂O was investigated by powder x-ray diffraction technique using the Rietveld structure refinement method. Obtained results have confirmed pseudohexagonal symmetry of the compound with lattice parameters a = 7.394(4), b = 12.836(6) and c = 13.666(6). The negative value of IS in Mössbauer spectrum is due to the presence of cyano groups. Magnetic susceptibility above T = 20K obeys Curie-Weiss law. Value of effective magnetic moment _eff = 7.19 _B is lower than the expected value for a free Gd³⁺ ion ( _eff = 7.94 _B). Differences between FC and ZFC behavior of magnetic susceptibility indicate a spin-glass like magnetic arrangement. Our results support prediction that the material orders ferrimagnetically below T _c = 2.3K.     

Molecular Spin Clusters: New Synthetic Approaches and Neutron Scattering Studies

We review our recent work in the field of molecular spin clusters and single-molecule magnets, showing how inelastic neutron scattering (INS) can be used to determine magnetic exchange interactions and anisotropy splittings. A general introduction to neutron scattering precedes selected examples, building upon the first determination of exchange coupling in a transition metal complex using INS, through anisotropic exchange in cobalt(II ) spin clusters to the determination of exchange interactions in a dodecanuclear nickel(II ) wheel. The strength of INS for the accurate determination of anisotropy splittings in single-molecule magnets is revealed. Not only can one determine the axial zero-field splitting parameter D, which plays a key role in single-molecule magnet behavior, but also higher-order terms important in understanding the quantum tunneling behavior. Finally, we review two of our synthetic approaches towards new single-molecule magnets based on nickel, manganese, and iron.     

Model simulations of ground-state and finite-temperature properties of disordered magnetic nanostructures

The properties of two-dimensional ensembles of magnetic nanoparticles that interact by magnetic dipole coupling are investigated. The low-temperature magnetic arrangements, the average binding energy E _dip due to dipolar interactions, and its scaling behavior with respect to the particle density C are calculated for different types of structural disorder and particle-size distributions. Many different metastable magnetic states are obtained, which exhibit strong noncollinearities and are reminiscent of a spin-glass behavior. For a given C, |E _dip| increases with increasing disorder of the particle positions. For random distributions at low particle densities C0.2, E _dip is dominated by the contributions of short interparticle distances. Thus, it scales as | E _dip| C with an unusually small exponent = 0.85–1. The straightforward scaling of the dipole interaction, 3/2, is obtained only for C0.5 or for nearly periodic ensembles. The finite temperature behavior of these disordered interacting nanomagnets is explored. The specific heat and magnetic susceptibility are calculated by performing Monte Carlo simulations. The onset of long-range magnetic order is discussed. In addition we determine hysteresis loops at finite temperatures and compare the results for different degrees of disorder.     

高温超导电缆终端的研究与开发

简单介绍了高温超导电缆的终端的基本结构特点与作用等 ;并主要介绍了工业化国家中美国、日本、丹麦等所研制的 HTS电缆终端以及目前我们所研制的终端情况。     

Photo-excitation from dia- to ferri-magnetism in a Rb-Co-hexacyanoferrate Prussian blue analogue

We studied the photo-excitation process, the relaxation of the photo-excited state towards the stable state, and the photo-induced magnetic properties of the Prussian blue analogue Rb_0.52Co [Fe(CN)₆]_0.84, 2.3 H₂O. Magnetic, M?ssbauer and reflectivity measurements have been performed during and after illumination. The efficiency of the photo-excitation device is maximum at nm. The process, however, is severely hindered by bulk absorption of the light; it is rapidly completed at the surface of the sample and then proceeds slowly in the bulk. Under the effect of photo-excitation the system turns from a dia- to a ferri- magnet, with a value K, indicative of the transformation of the material due to the following optical electron transfer: Thermal relaxation towards the stable electronic state is observed in the 95-110 K temperature interval, obeying a self-accelerated kinetics. At low temperature, a weak, non-exponential, relaxation is detected. These features are discussed in terms of co-operative effects in the frontal process of photo- excitation. The peculiarities of a photo-excited state created below the magnetic ordering temperature are discussed. A metastable magnetic state has been observed in low fields, denoted “Raw Photo-Induced State" (RPI), with a magnetization curve in-between the field-cooled and zero-field-cooled curves. Received 12 March 1999 and Received in final form 11 December 1999     

Study on cooling process of cryogenic system for superconducting magnets of BEPCⅡ

In the upgrade project of the Beijing Electron Positron Collider(BEPCⅡ),three superconducting magnets are employed to realize the goal of two orders of magnitude higher luminosity.A cryogenic system with a total capacity of 0.5 kW at 4.5 K was built at the Institute of High Energy Physics(IHEP)to support the operations of these superconducting devices.For preparing the commissioning of the system,the refrigeration process Was simulated and analyrzed numerically.The numerical model Was based on the latest engineering progress and focused on the normal operation mode.The pressure and temperature profiles of the cryogenic system are achieved with the simulation.The influence of the helium mass flow rates to cool superconducting magnets on the thermodynamic parameters of their normal operation is also studied and discussed in this paper.