空穴掺杂Sr2FeMoO6的晶体结构及磁性研究

采用传统高温固相反应法制备了空穴掺杂的系列样品Sr2xKxFeMoO6（0≤x≤0．04）,研究了其晶体结构和磁性质．X射线粉末衍射结果表明该系列样品均为单相,四方晶系,空间群为I4／m．碱金属K的含量可以调控反位缺陷的浓度．未掺杂样品Sr2FeM006在280K时的原胞磁矩为1．12μB．掺杂量为x=0．04样品的原胞磁矩为1．26μB．阳离子有序、晶格畸变是影响Sr2-xKxFeMoO6磁性的重要因素．     

具有人工晶界的Sr2FeMoO6粉末磁电阻的温度特性

利用固相反应合成了纯相的Sr2FeMoO6多晶块体，并通过机械球磨方法引入了人工晶界，研究了具有此种晶界的Sr2FeMoO6粉末磁电阻的温度特性．X射线衍射分析表明，机械球磨过程没有改变Sr2FeMoO6的晶体结构，但却在晶粒间界处引入了SrMoO4绝缘相，其量随着球磨时间的增加而增加．不同磁场下的磁电阻测量结果表明，由于一定量SrMoO4绝缘相的存在，晶粒间的绝缘隧穿势垒得到加强，更有利于自旋极化电子在晶粒间的隧穿，从而提高了Sr2FeMoO6多晶粉末的低温磁电阻值．然而随着温度的升高，磁电阻值迅速下降，表现出较强的温度依赖关系．这种现象是由于随着温度的升高，电子在晶界局域态间的非弹性跳跃逐渐增强引起的，而晶界局域态是由在晶界附近的大量缺陷构成．分析表明，晶界状态对Sr2FeMoO6多晶粉末磁电阻的温度特性有十分重要的影响．     

多晶Ba2FeMoO6的溶胶-凝胶制备及其结构、磁性和磁电阻效应研究

采用一种特殊的溶胶－凝胶方法制备出具有纳米尺寸晶粒的双钙钛矿结构Ba2FeMoO6多晶材料，样品的晶粒尺寸和相纯度可以通过改变烧结温度和不同的混合流动气体Ar/H2比例加以控制，对样品的电磁输运性质测量表明，在约316K处出现顺磁到铁磁的转变，并且样品从室温到低温的一个很宽的温区出现了低场磁电阻效应。     

Fe位Al掺杂对Sr2FeMoO6磁结构和磁输运性质的影响

研究了Sr2Fe1-xAlxMoO6(0≤x≤0.30)系列多晶样品的磁学和输运性质.室温X射线衍射谱图的精修结果显示Al3+掺杂没有改变样品的晶格结构,但提高了Sr2FeMoO6晶格的阳离子有序度.5K时样品的磁化曲线说明平均单位分子饱和磁矩随着Al含量的增加而下降,但平均单位Fe离子磁矩却逐渐提高.磁化曲线的拟合结果显示样品内反铁磁相互作用对饱和磁矩的贡献随着Al含量的增加而下降,说明一定量的Fe离子被Al替代后,抑制了样品内Fe-O-Fe反相边界的形成,从而提高了Sr2FeMoO6晶格的阳离子有序度和平均单位Fe离子磁矩.对饱和磁矩的分析表明非磁性Al3+离子掺杂会形成无磁相互作用的Mo-O-Al-O-Mo区,可以将原来较大的Mo-O-Fe亚铁磁区分割成许多小的区域,并且使这些亚铁磁区间的磁耦合作用变弱,从而提高了低场磁电阻效应.阳离子有序度的提高使来源于自旋相关电子在反相边界处散射的高场磁电阻明显降低,导致了样品的磁电阻在x=0.15时达到了最大值.     

Effect of preparation procedure on the magnetic and transport properties of double perovskite Sr2FeMoO6

Ordered and disordered double perovskite Sr2FeMoO6 ceramics have been investigated by powder x-ray diffraction,magnetic and transport measurements, as well as Moessbauer spectroscopy. The heavily disordered sample can be acquired by annealing the ordered samples in argon. The annealing procedure affects not only the nature of grain boundaries but also the grain itself. The evidence of Moessbauer spectra performed at 77 and 300K indicates that there exist small oxygen deficient clusters of SrFeO3-y in the disordered sample. The paramagnetic Fe^4 and Fe^3 ions in the compound subsist down to 77K and the ratio of Fe^4 /Fe^3 increases with decreasing temperature.     

Bi2Sr2—xLaxCuO6薄膜的热激活磁通耗散行为

利用不同磁场下的Ｒ－Ｔ曲线，得到了Ｂｉ２Ｓｒ２－ｘＬａｘＣｕＯ６薄膜的上临界场Ｈｃ２，讨论了热激活磁通蠕动对上临界场Ｈｃ２的影响。用热激活磁通蠕动理论解释了磁场下Ｒ－Ｔ曲线的展宽效应，得到激活能对温度的线性变化关系，对磁场的指数变化关系。     

PTR技术测量材料的热扩散系数的线性化方法

报道了一种用光热辐射 (PTR)技术测量不透明材料热扩散系数的优化方法。从光热辐射理论出发 ,在一定条件下 ,推导出位相信号和调制频率的关系表现为线性关系 ,分析实验数据得出材料的热扩散系数。     

厚非线性介质瞬态热光非线性效应的研究

瞬态热光非线性效应是指由脉冲宽度可以和非线性效应的建立时间相比拟的纳秒脉冲激光入射产生的热光非线性效应.用Zscan法测量了不同厚度、不同束腰半径碘的酒精溶液的瞬态热光非线性效应曲线.从声波的传播方程和光波的非线性传播方程出发,用数值计算方法模拟了这一非线性过程.并对实验结果和数值模拟结果进行了比较 关键词： 瞬态热光非线性效应 非线性光学 数值计算     

一维异质结构的瞬态热整流效应

瞬态条件下的热整流有广阔的实际应用背景,本文建立了一维板状复合结构热整流器的瞬态热整流模型,并利用有限元方法研究了不同恒定热阻、不同界面间隙、周期性温度边界条件以及材料和几何参数对瞬态热整流效果的影响规律.研究结果表明,界面热阻的存在可以提高系统的瞬态热整流系数,而初始界面间隙的引入让瞬态热整流系数实现了量级的飞跃.通过几何以及材料参数的合理设置有利于优化结构的热整流效果,针对周期性高温边界条件,温差和频率的变化可进一步提升复合结构的热整流系数.本文所提出热整流机制可以指导瞬态下热整流器的优化设计.     

Hole doping effects in (Sr2-xNax)FeMoO6 double perovskite

Hole-doped double perovskite compounds (Sr,Na)₂FeMoO₆ with the Na content of x=0,0.02,0.03,0.07 and 0.17 have been synthesized by sol–gel method. Effects of hole doping on the crystal structure and magnetic properties of Sr₂FeMoO₆ have been investigated by means of X-ray powder diffraction (XRD) and magnetic measurements. The XRD pattern indicates that all the samples are of single phase and belong to the space group I4/m. Due to the smaller ionic radius of Na⁺ than that of Sr²⁺ ions, the lattice constants and unit cell volume of the compound decrease slightly with x. The degree of cation ordering in the Na-doped Sr₂FeMoO₆ compounds shows a non-monotonic variation with the doping level, increasing from x=0 to x=0.03 and decreasing slightly with further increase of the doping. In contrast to the composition dependence of the degree of ordering, the Curie temperature of the compound decreases at low doping level and increases at high doping level. The saturation magnetization of the compound increases with x for x<0.17. Similar to the electron-doped Sr₂FeMoO₆, provided that the doped hole enters selectively the spin-down band, the variation of the saturation magnetization can be explained in light of the ferrimagnetic model (FIM). PACS 61.72.Ww; 75.50.-y; 75.30.Cr; 75.50.Gg     

Electronic structure of Sr2FeMoO6

We have analyzed the unusual electronic structure of Sr2FeMoO6 combining ab initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intra-atomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength ( U(eff)) at the Mo site due to these renormalized interaction strengths.     

Magnetocaloric effect in Sr2TiMoO6 double perovskite

Environmental friendly double perovskite Sr₂TiMoO₆ has been studied by using the density functional theory with generalized gradient approximation (GGA+U) approaches, and the Monte Carlo simulation in the framework of Ising model. The parameter of the unit cell has been optimized. The value of the crystal field and the exchange coupling have been estimated from the partial electronic density of states and the energy calculations, respectively. The magnetic properties and the magnetocaloric effect of the compound have been also studied. The Relative cooling power has been performed. It is found that Sr₂TiMoO₆ is an interesting material for the magnetic refrigeration applications.     

Half-metallic ferromagnetic nature of the double perovskite Pb2FeMoO6 from first-principle calculations

The structural, electronic and magnetic properties of the double perovskite Pb₂FeMoO₆ have been studied by using the first-principle projector augmented wave (PAW) potential within the generalized gradient approximation (GGA) as well as taking into account the on-site Coulomb repulsive interaction (GGA+U). Similar to Sr₂FeMoO₆ and Pb₂FeReO₆, the optimized crystal structure of Pb₂FeMoO₆ is a body-centered tetragonal (BCT) with the lattice constants a=b=5.60 Å and c=7.94 Å. The two axial TM?O distances are slightly larger than the four equatorial TM?O distances, which shows that the Jahn–Teller structural distortion exists in FeO₆ and MoO₆ octahedra. The half-metallic ferromagnetic nature implies a potential application of this new compound in spintronics devices. The Fe³⁺ and Mo⁵⁺ ions are in the states (3d⁵, S=5/2) and (4d¹, S=1/2) with magnetic moments 3.87 and ?0.38μ_B respectively and thus there exists an antiferromagnetic coupling via oxygen between them.     

Critical test for Altshuler-Aronov theory: evolution of the density of states singularity in double perovskite Sr2FeMoO6 with controlled disorder

With high-resolution photoemission spectroscopy measurements, the density of states (DOS) near the Fermi level (E(F)) of double perovskite Sr(2)FeMoO(6) having different degrees of Fe/Mo antisite disorder has been investigated with varying temperature. The DOS near E(F) showed a systematic depletion with increasing degree of disorder, and recovered with increasing temperature. Altshuler-Aronov (AA) theory of disordered metals well explains the dependences of the experimental results. Scaling analysis of the spectra provides experimental indication for the functional form of the AA DOS singularity.     

Structural and magnetic properties of the double perovskite Sr2MnWO6

The nuclear and magnetic structure and the magnetic properties of the polycrystalline double perovskite Sr₂MnWO₆ have been studied. Rietveld analysis of neutron powder diffraction (NPD) data at T=295 K shows that the sample is tetragonal (space group P4₂/n, a=8.0119(4) Å, c=8.0141(8) Å). Some additional magnetic diffraction peaks were found in the NPD pattern at 10 K, which can be accounted for by antiferromagnetic ordering of spins at the Mn sites. The magnetic unit cell is doubled in all three unit axes directions (a=b=15.9984(8) Å, c=16.012(2) Å) and the manganese moments are coupled antiferromagnetically along the unit cell axes. The total magnetic moment of Mn²⁺ is found to be 2.27(7) μ_B. The antiferromagnetic behaviour was confirmed from magnetisation measurements. The transition from a paramagnetic to an antiferromagnetic state takes place at 13.0±0.1 K.     

Raman scattering in the magnetically frustrated double perovskite Sr2YRuO6

The spin correlations and excitations of the Sr₂YRuO₆ double perovskite are investigated by means of Raman scattering, complemented by synchrotron X‐ray diffraction measurements. Anomalous softening of a breathing mode of the oxygen octahedra is observed below ~200 K, much above the long‐range antiferromagnetic ordering temperature, T_N1 = 32 K, due to a spin‐phonon coupling mechanism in the presence of magnetic correlations. A diffusive Raman signal is also observed, possibly associated with spin excitations within magnetically correlated regions. Our results point to a characteristic energy and temperature scale of ~25 meV/200 K below which unusual behavior associated with magnetic correlations is observed in this material. Copyright © 2013 John Wiley & Sons, Ltd.     

用激光引发热反射栅方法测量固体热扩散率

阐述了用脉冲激光引发瞬态热反射栅的方法来测量固体材料热扩散率的理论、实验装置和实验技术．给出用该实验方法测得的纯硅和纯铜的热扩散率，并与文献数据进行了比较，结果表明该方法是一种适用于不透明固体材料热扩散率测量的无损、快速和准确的方法．     

Pulsed laser deposition of Sr2FeMoO6 thin films

The effect of various deposition conditions and after-growth protocols on the magnetic and transport properties of Sr₂FeMoO₆ films has been explored. It is found that the saturation magnetization and the magnetoresistance (MR) are dominated by the degree of cationic order, and the strain effects are clearly evidenced in a lower T_C. The after-growth annealing of the films and the deposition of a buffer layer has been found to relax the film strains. This translates into a clear increase of the measured low-field magnetoresistance ratios.