The synthesis and exchange bias effect of monodisperse NiO nanocrystals

Monodisperse NiO nanocrystals with an average particle size of 3 ± 0.4 nm are successfully synthesized by the thermal decomposition of Ni-oleylamine complex in an organic solvent under a continuous O2 flux. The crystalline structure and the morphology of the product are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Magnetization and alternating-current (ac) susceptibility measurements indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core and a spin-glass-like surface shell. In addition, both the exchange bias field and the vertical magnetization shift can be observed in this system at 10 K after field cooling. This observed exchange bias effect is explained in terms of the exchange interaction between the antiferromagnetic core and the spin-glass-like shell.     

Improved multiferroic properties of La-doped 0.6BiFeO3—0.4SrTiO3 solid solution ceramics

The 0.6（Bi1-xLax）FeO 3-0.4SrTiO 3（x = 0,0.1） multiferroic ceramics are prepared by a modified Pechini method to study the effect of substitution of SrTiO3 and La in BiFeO3.The X-ray diffraction patterns confirm the single phase characteristics of all the compositions each with a rhombohedral structure.The magnetic properties of the ceramics are significantly improved by a solid solution with SrTiO3 and substitution of La.The values of the dielectric constant ε r and loss tangent tan δ of all the samples decrease with increasing frequency and become constant at room temperature.The La-doped 0.6BiFeO3-0.4SrTiO3 ceramics exhibit improved dielectric and ferroelectric properties,with higher dielectric constant enhanced remnant polarization（Pr） and lower leakage current at room temperature.Compared with a anti-ferromagnetic BiFeO3 compound,the 0.6（Bi0.9La0.1）FeO3-0.4SrTiO3 sample shows the optimal ferromagnetism with remnant magnetization M r ～ 0.135 emμ/g and ferroelectricity with Pr ～ 5.94 μC/cm 2 at room temperature.     

Improved multiferroic properties of La-doped 0.6BiFeO₃-0.4SrTiO₃ solid solution ceramics

The 0.6(Bi1-xLax)FeO 3-0.4SrTiO 3(x = 0,0.1) multiferroic ceramics are prepared by a modified Pechini method to study the effect of substitution of SrTiO3 and La in BiFeO3.The X-ray diffraction patterns confirm the single phase characteristics of all the compositions each with a rhombohedral structure.The magnetic properties of the ceramics are significantly improved by a solid solution with SrTiO3 and substitution of La.The values of the dielectric constant ε r and loss tangent tan δ of all the samples decrease with increasing frequency and become constant at room temperature.The La-doped 0.6BiFeO3-0.4SrTiO3 ceramics exhibit improved dielectric and ferroelectric properties,with higher dielectric constant enhanced remnant polarization(Pr) and lower leakage current at room temperature.Compared with a anti-ferromagnetic BiFeO3 compound,the 0.6(Bi0.9La0.1)FeO3-0.4SrTiO3 sample shows the optimal ferromagnetism with remnant magnetization M r ～ 0.135 emμ/g and ferroelectricity with Pr ～ 5.94 μC/cm 2 at room temperature.     

自旋转换配合物[Fe（C4H4N2）｛Pt（CN4）｝]晶体结构及低温相的研究

本文对自旋转换配合物[Fe(C4H4N2){Pt(CN4)}]进行了晶体结构的理论研究,分析表明XRD测量给出的结构只是一个理想化的结构,不适用于理论计算.我们通过第一性原理计算给出的晶格结构空间群为P2/m,相比较XRD的理想结构的空间群P4/m,其对称性降低,该体系实际属于单斜晶系,而不是测量中所给出的四方晶系的结构.通过对比实验数据特别是对低温相的分析,看出对该配合物的理论计算得出的结构常数及键长与实验数据相吻合.用局域密度近似和广义梯度近似算得的低温相的能量分裂分别为2.7824eV和2.5617eV,故低自旋态为低温相的稳态,这也与实验结论很好的吻合.     

La4/3Ca5/3Mn2O7的结构、电输运及磁性质研究

采用溶胶-凝胶法,在不同温度烧结制备了名义组分为La4/3Ca5/3Mn2O7的多晶样品.X射线结构分析以及电、磁输运性质的测量表明:La4/3Ca5/3Mn2O7形成的是La2/3Ca1/3MnO3和CaO的混合物.     

La2/3Ca1/3MnO3/TE(TE=Cu、Fe、Zn)复合颗粒系统的磁输运行为比较性研究

用化学方法制备出La2/3Ca1/3MnO3/TE(TE=Cu、Fe和Zn)复合颗粒系统,并通过零场和外加低磁场(0.3 T)下电阻率(ρ)随温度(T)变化关系测量对这些复合系统的输运和低场磁电阻行为进行了比较性研究.相对于纯La2/3Ca1/3MnO3颗粒系统,Cu引入后,绝缘体-金属转变温度(TIM)降低,ρ减小,ρ(T)曲线表现出明显的热滞豫现象,磁电阻(MR)在转变温度附近表现出峰行为且峰值处的MR值高达～35%.引入Fe后,TIM升高,电阻率减小,但MR随T降低而单调增加.引入Zn后,TIM降低,ρ增加,在TIM附近也出现MR峰但峰值较小.同时,也注意到,在含Fe和Zn的系统中都没有观察到明显的热滞豫现象.     

Multiferroic properties and exchange bias in Bi1-xSrxFeO3 （x = 0-0.6） ceramics

Multiferroic properties and exchange bias(EB) in Bi1-xSrxFeO3(x = 0–0.6) ceramics synthesized by a modified Pechini method are investigated. Sr concentration dependence of structure distorting, ferroelectric properties, and dielectric properties were studied at room temperature. Appropriate Sr doping(x = 0.05–0.2) has been found to decrease the conductivity, enhance ferroelectric properties and give rise to high dielectric constant. Compared with antiferromagnetic BiFeO3 compound, BSFO-x(0 ≤ x ≤ 0.4) ceramics show weak ferromagnetism at room temperature, and their exchange bias field and vertical magnetization shift are observed and exhibit a strong dependence on the content of Sr. This observed EB effect which keeps stable in BSFO ceramics at 10 K tend to vanish at room temperature with Sr concentration over 0.4.     

First-Principles Based Model of Spin-state Phase Transition

The nature of spin-state phase transition is investigated with [Fe（C4H4N2）（Pt（CN）4}] that is＂ a novel 3D Hofmann-like compound. The bistability of this system is obtained by the first-principles calculation. It is demonstrated that thermal expansion is the intrinsic force involved in spin-state transition. Based on these results, we suggest a thermal exciting bistable model of spin-state transition with a temperature dependent crystal-field splitting （CFS）. Experimental evidence of spin-state phase transition coincides with our theoretical model. This model approaches something fundamental in the mechanism leading to the transition, and it is important in developing new and practical controllable quantum devices.