化合物HoMn6Sn6的磁晶各向异性及自旋重取向相变研究

采用交换相互作用的分子场理论模型对金属间化合物HoMn₆Sn₆的自旋重取向相变进行了研究. 从理论上计算了HoMn₆Sn₆的易磁化方向以及Ho和Mn离子磁矩与c轴夹角随温度的变化. 基于单离子模型计算了Ho离子的一阶和二阶磁晶各向异性常数K_1R和K_2R随温度的变化. 研究表明，为了很好描述该化合物的自旋重取向相变，必须考虑Ho离子的四阶晶场项及相应的二阶磁晶各向异性常数K_2R，K_2R与K_1R和Mn离子磁晶各向异性常数K_1t之间的相互竞争是导致HoMn₆Sn₆自旋重取向相变的重要因素. 关键词： 稀土-过渡族金属间化合物 自旋重取向 磁晶各向异性     

Ho2(Co,Si)17化合物的结构与磁晶各向异性

用三弧Czochralski法和真空电弧熔炼法制备了Ho2Co17-xSix化合物.通过X射线衍射和磁性测量手段研究了化合物的结构与内禀磁性.重点讨论了磁晶各向异性和自旋重取向转变.实验结果表明,Ho2Co17为Th2Ni17型六角结构,在0.5≤x≤3的化合物均为Th2Zn17型菱方结构,能够获得单相2:17型化合物的最大Si含量是x=3.在x≤2的浓度范围,化合物的易磁化方向垂直于c轴.随Si含量增加,化合物的居里温度和Co原子平均磁矩单调减少.根据Ho2Co17-xSix化合物的居里温度和自旋重取向温度,获得了磁相图.根据热磁曲线,确定了温度补偿点.在Ho2Co17化合物中观察到了在1005 K发生自旋重取向转变.     

HoFe_(0.95)Mn_(0.05)O_3单晶的自旋重取向相变和磁热效应研究

本文采用光学浮区法生长了HoFe_(0.95)Mn_(0.05)O_3单晶样品,对其磁性和磁热效应进行了研究.研究表明,微量Mn的掺杂使HoFeO_3的自旋重取向温度由58K升高到102K.磁化强度随温度变化和自旋重取向相变过程中的热滞现象共同说明,自旋重取向由原来HoFeO_3中的Γ_4(G_x,F_z)→Γ_2(G_z,F_x)自旋重取向变为HoFe_(0.95)Mn_(0.05)O_3中的Γ4(G_x,F_z)→Γ_1(C_z,A_x)的自旋重取向.磁熵计算表明,虽然在a轴方向得到了-16.7J/kg·K的磁熵变,HoFe_(0.95)Mn_(0.05)O_3的磁熵变显示出明显的各向异性,但是微量Mn的掺杂并未对HoFeO_3的磁熵造成明显的影响.     

Ho2(Co,Si)17化合物的结构与磁晶各向异性

用三弧Czochralski法和真空电弧熔炼法制备了Ho2Co17-xSix化合物.通过X射线衍射和磁性测量手段研究了化合物的结构与内禀磁性.重点讨论了磁晶各向异性和自旋重取向转变.实验结果表明,Ho2Co17为Th2Ni17型六角结构,在05≤x≤3的化合物均为Th2Zn17型菱方结构,能够获得单相2∶17型化合物的最大Si含量是x=3.在x≤2的浓度范围,化合物的易磁化方向垂直于c轴.随Si含量增加,化合物的居里温度和Co原子平均磁矩单调减少.根据Ho2Co17-xSix化合物的居里温度和自旋重取向温 关键词：     

金属间化合物Ho2Co17-xSix的结构和磁性研究

研究了非磁性原子Si替代Co对Ho2Co17金属间化合物结构和磁性的影响。X射线衍射结果表明,所有Ho2Co17-xSix（x=0.5,1.0,1.5,2.0,2.5,3.0）化合物都为Th2Ni17型六角结构;化合物的晶格常数和单胞体积都随Si含量的增加而呈线性下降。磁性测量结果表明,Ho2Co17-xSix化合物的饱和磁化强度随Si含量的增加而呈线性下降。从热磁曲线测量观察到,Ho2Co17-xSix化合物在x=0.5时可能呈面各向异性,当0.5≤x≤3.0时出现由易面到易轴的自旋重取向,自旋重取向温度Tsr随Si原子含量的增加先下降,而后双上升,在x=2.5处出现最低点。     

Nd_2Fe_14_B化合物磁晶各向异性起源的理论研究

本文根据Nd_2Fe_14_B化合物中存有少量巡游电子的基本实验事实, 提出了在Nd_2Fe_14_B 中配位子所产生的晶场和巡游申子所产生的电场共同造成了N d 离子磁犯各向异性的理沦模型.提出并解决了计算巡游电子同中心Nd 离子相互作用的理论方法.在同时考虑Nd 离子所受配位子的晶场和巡游电子的电场的作用情况下, 用单离子模型计算了Nd_2Fe_14_B中Nd 离子所产生的磁晶各向异性及其随温度的变化. 所得的K-T , K-T 关系与实验符合得很好. 这一结果还同时解释了这种材料在低温下所存在的自旋重取向现象.人此可以认为, 这一模型本上揭示了Nd_2Fe_14_B 中Nd 离子磁品各向异性的起源。 关键词：     

R_2Fe_(14)B(R=Ce,Pr,Gd)磁晶各向异性常数K_1,K_2随温度的变化

本文在1.5—300K温度范围内测量了R_2Fe_(14)B(R=Ce,Pr,Gd)各向异性常数K_1,K_2和各向异性场H_A随温度的变化。同时用单离子模型计算了Pr~(3 )离子对Pr_2Fe_(14)B磁晶各向异性的贡献,得到与实验值半定量符合的结果。     

金属间化合物RMn2Ge2(R=La,Pr,Nd,Sm,Gd,Tb和Y)中的自发磁相变 及相变时的磁弹性异常

在10—800K的温度范围内用X射线衍射方法测量了RMn₂Ge₂(R=La,Pr ,Nd,Sm,Gd,Tb和Y)的晶格常数与温度的变化关系.在各种类型的自发磁相变观察到晶格常数 的磁弹性异常现象.实验得出,自发磁相变时的磁弹性异常主要由Mn次晶格引起,并且Mn-Mn 交换相互作用能不仅与晶格常数a有关,而且与晶格常数c有关.用Kittle的交换反转模型讨 论了低温时的铁磁—反铁磁一阶相变. 关键词： 稀土金属间化合物 磁相变 磁弹性     

金属间化合物RMn_2Ge_2(R=La,Pr,Nd,Sm,Gd,Tb和Y)中的自发磁相变及相变时的磁弹性异常

在 10— 80 0K的温度范围内用X射线衍射方法测量了RMn2 Ge2 (R =La ,Pr,Nd ,Sm ,Gd ,Tb和Y)的晶格常数与温度的变化关系 .在各种类型的自发磁相变观察到晶格常数的磁弹性异常现象 .实验得出 ,自发磁相变时的磁弹性异常主要由Mn次晶格引起 ,并且Mn Mn交换相互作用能不仅与晶格常数a有关 ,而且与晶格常数c有关 .用Kittle的交换反转模型讨论了低温时的铁磁—反铁磁一阶相变 .     

稀土合金RCo5中稀土离子的磁晶各向异性

利用单粒子点电荷模型研究了RCo5中一系列稀土离子,包括轻离子Pr3+,Nd3+和重离子Tb3+,Dy3+,Er3+的磁晶各向异性.计算分析表明过渡族元素对晶场的贡献和晶场高阶项不可忽略,虑及过渡族元素和晶场高阶项对晶场的贡献后,得到的各稀土离子的磁晶各向异性常数及其随温度变化都与实验一致的结果,此结果还特别解释了以前不能解释的轻稀土离子Pr3+的易锥向取向.     

Magnetic structure and anisotropy of YFe 6 Ga 6 and HoFe 6 Ga 6

The magnetic structure of RFe₆Ga₆ intermetallic compounds with R = Y, Ho have been determined by neutron powder diffraction, ⁵⁷Fe M?ssbauer spectroscopy, AC susceptibility, TGA (Thermo-Gravimetric Analysis) and magnetization measurements. Both compounds crystallize in the tetragonal ThMn₁₂ structure (space group I4/mmm) with the magnetic structure of YFe₆Ga₆ consisting of a simple ferromagnetic alignment of Fe moments in the basal plane with a Curie temperature of 475(5) K. Gallium atoms are found to fully occupy the 8i site, with Fe and Ga atoms equally distributed over the 8j site, whilst Fe atoms fully occupy the 8f site. The average Fe moments are 1.68(10) and 1.46(10) at 15 and 293 K, respectively. The average room temperature Fe magnetic moments determined by neutron diffraction are in overall agreement with the average Fe moment deduced from M?ssbauer spectroscopy and bulk magnetization measurements on this compound. The magnetic anisotropy of the compound HoFe₆Ga₆ is also planar in the temperature range 6-290 K, with Ho magnetic moments of 9.28(20) and 2.50(20) at 6 K and 290 K, respectively, coupled anti-ferromagnetically to the Fe sublattice and a Curie temperature of 460(10) K. The magneto-crystalline anisotropies of both compounds are comparable at low temperatures. Received 8 March 2001 and Received in final form 18 June 2001     

Hyperfine coupling constants of muonium-substituted cyclohexadienyl radicals in the gas phase: C6H6Mu, C6D6Mu, C6F6Mu

Muon spin rotation (μSR) and avoided level crossing resonance (ALCR) have been used to determine the hyperfine coupling constants (hfcs) of the muonium-substituted cyclohexadienyl radicals C₆H₆Mu, C₆D₆Mu and C₆F₆Mu in the gas phase, at pressures ～1 and 15 atm and temperatures in the range 40–80°C. Equivalent studies of polyatomic free radicals in gases, by electron spin resonance (ESR) spectroscopy, are generally not possible in this pressure range. The present gas phase results support the findings of earlier studies of cyclohexadienyl radicals in the condensed phase, by both μSR and ESR. Minor but not insignificant (～1%) effects on the hfcs are observed, which can be qualitatively understood for such nonpolar media in terms of their differing polarizabilities. This is the first time that comparisons of this nature have been possible between different phases at the same temperatures. These μSR/ALCR gas-phase results provide a valuable benchmark for computational studies on radicals, free from possible effects of solvent or matrix environments.     

Crystal fields in PrB6 and NdB6

The electron-phonon relaxation times of the crystal-field levels of the metastable state⁵D₄ of Tb³⁺ were measured between 2 and 4.2K. The relaxation of the lowest pseudodoublet (E=1.25 cm^–1) shows an Orbach process via higher crystal-field levels. The other levels show direct relaxation on a nanosecond time scale. The higher crystal-field levels were used to look for propagation of high-energy (4THz) nonequilibrium phonons by phonon-induced fluorescence. No ballistic propagation of high-energy phonons could be found.     

Intercalant-driven superconductivity in YbC6 and CaC6

Recently discovered superconductivity in YbC6 and CaC6, at temperatures substantially higher than previously known for intercalated graphites, raises several new questions. (1) Is the mechanism considerably different from that of previously known intercalated graphites? (2) If superconductivity is conventional, what are the relevant phonons? (3) Given the extreme similarity between YbC6 and CaC6, why are their critical temperatures so different? We address these questions on the basis of first-principles calculations and conclude that coupling with intercalant phonons is likely to be the main force for superconductivity in YbC6 and CaC6, but not in alkaline-intercalated compounds, and explain the difference in T(c) by the "isotope effect" due to the difference in Yb and Ca atomic masses.     

CARS spectroscopy of SF6 and UF6

CARS spectra of thev ₁-band of SF₆ and UF₆ performed under (static) cell conditions and under supersonic jet expansion have been recorded. The cooling behavior in supersonic jet expansions has been demonstrated for both gases.Work supported by BMFT-funds     

Magnetic-field-induced spin-reorientational transition in TbMn6Sn6

Magnetic state of the intermetallic TbMn₆Sn₆ compound was studied by neutron diffraction analysis in external magnetic fields of H=0, 4, and 5 kOe. Magnetic moments and their orientations were determined for the Tb and Mn sublattices. Magnetic fields higher than 4 kOe were shown to induce spin-reorientational transition at room temperature from the c axis to the basal plane.     

6-S-Alkyl-6-thiohexonolactones and 6-S-Alkyl-6-thiohexitols: Efficient synthesis and new smectic liquid crystal phases

The mesophasic properties of S-alkylthiohexonolactones (d-galactono- and d-mannono-) and the corresponding itol derivatives with general formula Su-SR (R = C₁₁H₂₃) have been investigated. The observation that the synthesized compounds of this series has been studied using crossed polarized optical microscopy. This study involves the discovery of new chiral Smectic A liquid crystal phase through direct comparison with the defective texture of known Smectic A phase of available commercially biphenyls 8CB and 10CB.