An electron spin resonance study of Eu doping effect in La_(4/3)Sr_(5/3)Mn_2O_7 single crystal

The effect of Eu3+ion doping in the La sites of single-crystal La4/3Sr5/3Mn2O7was investigated. Electron spin resonance(ESR) was applied to La4/3Sr5/3Mn2O7and(La0.8Eu0.2)4/3Sr5/3Mn2O7single crystals. A phase separation and phase transitions were observed from the ESR spectra data. Between 350 K and 300 K, both paramagnetic resonance(PMR)and anisotropic ferromagnetic resonance(FMR) lines were observed in the ab plane and the c axis direction, suggesting a coexistence of the paramagnetic(PM) phase and the ferromagnetic(FM) phase. The magnetization measurement reveals a spin-glass-like behavior in single-crystal(La0.8Eu0.2)4/3Sr5/3Mn2O7below the temperature of spin freezing Tf(～ 29.5 K).     

太赫兹波段介质微腔光学特性研究

采用匀胶法制备了厚度在微米量级的 Si/[TiO₂/Al₂O₃]²TiO₂和Si/[TiO₂/MgO]²/TiO₂ 多层介质膜反射镜. 采用太赫兹(THz)时域透射光谱系统获得了多层膜的时域透射谱. 用传输矩阵法模拟了Si/[TiO₂/Al₂O₃]²TiO₂ 和Si/[TiO₂/MgO]²/TiO₂两种分布式布拉格反射镜 (DBR)的反射相移和相位穿透深度等光学特性. 设计了两种结构为 DBR/LT-GaAs/DBR的对称THz光学微腔结构并模拟了腔结构的辐射光谱. 结果表明:通过引入谐振腔, 两种DBR组成的微腔器件在谐振波长处的强度分别提高了19和14倍. 其中Si/[TiO₂/Al₂O₃]²TiO₂/LT-GaAs (12 μm)/ [TiO₂/Al₂O₃]²TiO₂腔的辐射光谱存在两个峰, 分别位于208和248 μm, 并分析了出现两个谐振峰的原因. 探讨了通过引入介质谐振腔实现对THz源的辐射特性进行调控的可行性. 关键词： 分布式布拉格反射镜 光子晶体 穿透深度 太赫兹微腔     

First-principles study of electronic structure and magnetic properties of Sr3Fe2O5 oxide

We investigate the electronic structure and magnetic properties of layered compound Sr₃Fe₂O₅ based on first-principles calculations in the framework of density functional theory with GGA+U method. Under high pressure, the ladder-type layered structure of Sr₃Fe₂O₅ is transformed into the infinite layered structure accompanied by a transition from G-type anti-ferromagnetic (AFM) insulator to ferromagnetic (FM) metal and a spin transition from S=2 to S=1. We reproduce these transformations in our calculations and give a clear physical interpretation.     

钙钛矿型锰氧化物(La$lt;sub$gt;0.8$lt;/sub$gt;Eu$lt;sub$gt;0.2$lt;/sub$gt;)$lt;sub$gt;4/3$lt;/sub$gt;Sr$lt;sub$gt;5/3$lt;/sub$gt;Mn$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;7$lt;/sub$gt;的磁性和电性研究

采用传统固相反应法制备钙钛矿型锰氧化物 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇多晶样品, X-射线衍射分析表明, 样品(La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇结构呈现良好的单相. 通过磁化强度随温度的变化曲线(M-T)、不同温度下磁化强度随磁场的变化曲线(M-H)和电子自旋共振谱发现: 在300 K以下, 随着温度的降低, 样品先后经历了二维短程铁磁有序转变 (T_C^2D ≈ 282 K)、三维长程铁磁有序转变(T_C^3D ≈ 259 K)、奈尔转变(T_N ≈ 208K)和电荷有序转变(T_CO ≈ 35 K); 样品 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇在T_N以下, 主要处于反铁磁态; 在T_C^3D达到370 K时, 样品处于铁磁-顺磁共存态, 在370 K以上时样品进入顺磁态. 此外, 分析电阻率随温度的变化曲线(ρ-T)得到: 样品在金属-绝缘转变温度(T_P ≈ 80 K)附近出现最大磁电阻值, 其位置远离T_C^3D, 表现出非本征磁电阻现象, 其磁电阻值约为61%. 在T_CO以下, 电阻率出现明显增长, 这是由于温度下降使原本在高温部分巡游的e_g电子开始自发局域化增强所致. 通过对 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇的ρ-T 曲线拟合, 发现样品在高温部分的导电方式基本遵循小极化子的导电方式. 关键词： 磁性 电性 金属-绝缘转变温度 电子自旋共振     

钙钛矿锰氧化物(La1-xGdx)4/3Sr5/3Mn2O7 (x=0, 0.025) 磁性和输运性质研究

采用传统的高温固相烧结法制备了双层钙钛矿锰氧化物(La_1-xGd_x)_4/3Sr_5/3Mn₂ O₇ (x=0, 0.025)多晶样品. 通过X射线衍射仪研究发现样品为Sr₃Ti₂O₇型四方结构, 空间群为I4/mmm; 磁性测量表明, Gd³⁺掺杂后的样品(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂O₇的三维磁有序转变温度(T_C1^3D)、磁化强度(M)均降低, 这是由于Gd³⁺的掺杂引起晶格的畸变, 从而使得晶格常数发生改变, 减弱了铁磁耦合而导致的; 通过电子自旋共振谱测量发现, 在T_C^3D<T<300 K温度范围内, 两样品在顺磁的基体上均有短程的铁磁团簇存在, 出现了相分离现象. 电性测量表明: 两样品分别在T_C1^3D (La_4/3Sr_5/3Mn₂O₇ 样品的三维磁有序转变温度, T_C0^3D)<T<300 K温度范围内均以三维变程跳跃的方式导电, 分析得出Gd³⁺的掺杂使得载流子局域长度的减小. 这表明载流子需要吸收更多的能量才能克服晶格的束缚进行跳跃, 因此(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂ O₇ 样品的电阻较高.     

γ-Fe_2O_3/NiO核-壳纳米花的合成、微结构与磁性

利用溶剂热/热分解的方法合成出微结构可控的γ-Fe_2O_3/NiO核-壳结构纳米花.分析表明NiO壳层是由单晶结构的纳米片构成,这些纳米片不规则地镶嵌在γ-Fe_2O_3核心的表面.Fe3O4/Ni(OH)_2前驱体的煅烧时间对γ-Fe_2O_3/NiO核-壳体系的晶粒生长、NiO相含量和壳层致密度均有很大的影响.振动样品磁强计和超导量子干涉仪的测试分析表明,尺寸效应、NiO相含量和铁磁-反铁磁界面耦合效应是决定γ-Fe_2O_3/NiO核-壳纳米花磁性能的重要因素.随着NiO相含量的增加,磁化强度减小,矫顽力增大.在5 K下,γ-Fe_2O_3/NiO核-壳纳米花表现出一定的交换偏置效应(H_E=46 Oe),这来自于(亚)铁磁性γ-Fe_2O_3和反铁磁性NiO之间的耦合相互作用.与此同时,这种交换耦合效应也进一步提高了样品的矫顽力(H_C=288 Oe).     

聚乙二醇在Al2O3/水界面吸附行为的ESR研究

采用电子自旋共振技术结合自旋标记研究了聚乙二醇在Al₂O₃/水界面的分子构型和运动行为.结果表明,聚乙二醇在Al₂O₃上的吸附等温线呈S型,吸附量随聚乙二醇分子量增加而减小,聚乙二醇在Al₂O₃表面是多点吸附,分子中的大部分链节平躺在Al₂O₃表面、少部分链节伸向溶液。     

尖晶石结构自旋有序CaTi2O4单晶生长和磁化率特性研究

通过助溶剂熔融法并在氩气气氛炉中成功生长出高质量大尺寸的CaTi₂O₄的单晶.X射线衍射实验及能量损失谱EDS证实,制备的CaTi₂O₄单晶晶胞参数a=9.781?,b=9.966?,c=3.148?,所有样品均为单相,且符合化学计量比,样品高纯.通过直流磁化率的测量,首次给出了晶体的Van-Vleck顺磁因子为6.85×10^-5cm³/mol,Cure-Weiss温度为-0.44 K,呈弱反铁磁性.同时,通过单晶各向磁化率的测量,进一步确认了CaTi₂O₄晶体中一维有序Ti-Ti反铁磁dimer链的形成,并明确了其方向. 关键词： 2O₄')" href="#">CaTi₂O₄ 磁化率 各向异性 反铁磁二聚化     

Nuclear magnetic resonance study of the electron-doped high-temperature superconducting cuprates

Nuclear magnetic resonance (NMR) measurements have been made on two of the electron-doped high-temperature superconducting cuprates (HTSCs), Pr_2−xCe_xCuO₄ and Sr_0.9La_0.1CuO₂ that represent the two known electron-doped structures. The results are compared with the more-studied hole-doped HTSCs. We show that the electron and hole-doped HTSCs probe a similar antiferromagnetic spin fluctuation spectrum in the normal state, which provides support for theories of superconductivity where the pairing is mediated by antiferromagnetic spin fluctuations and the superconducting order parameter has a d_x²−y² symmetry. Contrary to results from underdoped and hole-doped HTSCs, there is no evidence for a normal-state pseudogap in the NMR data from measurements on the electron-doped HTSCs. Therefore, the electron-doped HTSCs can be better compared with overdoped and hole-doped HTSCs where the normal-state pseudogap is absent. The antiferromagnetic spin fluctuation spectrum as probed by the Cu spin–lattice relaxation rate, is independent of the doped electrons per Cu. A similar effect is observed in the overdoped and hole-doped HTSC, Y_1−xCa_xBa₂Cu₃O_7−δ for a hole concentration range of 0.063. The anomalous Cu NMR linewidth anisotropy observed in the electron-doped HTSCs suggests a small and static spin variation for temperatures up to room temperature.     

Nuclear magnetic resonance studies of vortices in high temperature superconductors

The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin–lattice relaxation has been reported in cuprates, including YBa₂Cu₃O_7-x, Bi₂SrCa₂Cu₂O_8+δ, and Tl₂Ba₂CuO_6+δ. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi₂SrCa₂Cu₂O_8+δ. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.     

{Cu3}单分子磁体在磁场中的热纠缠

本文研究单分子磁体Na₉[Cu₃Na₃(H₂O)₉ (α-AsW₉O₃₃)₂]·26H₂O中三角自旋 环在磁场作用下的热纠缠性质, 利用数值计算求出任意两个Cu²⁺离子量子比特之间的配对纠缠度, 分别记为C₁₂, C₂₃和C₁₃. 研究结果表明, 磁场的方向和大小以及温度对配对纠缠度具有重要影响, 而且参数的变化对C₁₂, C₂₃和C₁₃的影响也是各不相同. 给出外加三个不同方向的磁场时, 配对纠缠度C₁₂, C₂₃和C₁₃各自对应的临界温度T_c随磁场强度的变化图, 由此可以得到单分子磁体三角自旋环中存在纠缠态的参数范围. 通过选择适当的磁场方向和大小以及温度等实验参数, 可以有效地调节和提高单分子磁体中的配对纠缠度. 关键词： 配对纠缠 单分子磁体 三角自旋环     

Terahertz spectroscopy and lattice vibrational analysis of pararealgar and orpiment

Terahertz time-domain spectroscopy (THz-TDS) is an effective nondestructive and noninvasive tool for investigating sulfur-containing pigments. Combined with Raman spectroscopy and vibrational mode analysis, it is significant for artifact identification and conservation. In this work, the terahertz absorption spectra of pararealgar (As₄S₄) and orpiment (As₂S₃) samples mixed with polytetrafluoroethylene (PTFE) are characterized in a range from 0.2 THz to 2.2 THz, and their distinctive peaks are observed, respectively. Meanwhile, qualitative analysis is also implemented by using Raman spectroscopy as a complementary technique. The lattice vibrations are simulated by using solid-state density functional theory (ss-DFT), illustrating different characteristic absorption peaks for specific crystalline structures and dynamic properties. This work provides a reliable database of sulfur-containing pigments for using the THz technology to actually analyze and diagnose cultural relics.     

Magnetoresistance of Fe3O4/Au/Fe3O4 and Fe3O4/Au/Fe spin-valve structures

A detailed study of the in-plane magnetotransport properties of spin valves with one and two Fe₃O₄ electrodes is presented. Fe₃O₄/Au/Fe₃O₄ spin valves exhibit a clear anisotropic magnetoresistance in small magnetic fields but no giant magnetoresistance (GMR). The absence of GMR in these structures is due to simultaneous magnetization reversal in the two Fe₃O₄ layers. By contrast, a negative GMR effect is measured on Fe₃O₄/Au/Fe spin valves. The negative GMR is attributed to an electron spin scattering asymmetry at the Fe₃O₄/Au interface or an induced spin scattering asymmetry in the Au interfacial layers.     

Observation of giant magnetocaloric effect under low magnetic fields in EuTi_(1-x)Co_xO_3

The magnetic properties and magnetocaloric effect(MCE) in EuTi_(1-x)Co_xO_3(x = 0, 0.025, 0.05, 0.075, 0.1) compounds have been investigated. When the Ti~(4+) ions were substituted by Co2+ions, the delicate balance was changed between antiferromagnetic(AFM) and ferromagnetic(FM) phases in the EuTiO_3 compound. In EuTi_(1-x)Co_xO_3 system, a giant reversible MCE and large refrigerant capacity(RC) were observed without hysteresis. The values of -?S_M~(max) were evaluated to be around 10 J·kg~(-1)·K~(-1) for EuTi_(0.95)Co_(0.05)O_3 under a magnetic field change of 10 kOe. The giant reversible MCE and large RC suggests that EuTi_(1-x)Co_xO_3 series could be considered as good candidate materials for low-temperature and low-field magnetic refrigerant.     

非晶锂离子导体B2O3-Li2O-LiCl一Al2O3中VO2+的电子自旋共振

两种非晶锂离子导体B₂O₃-0.7Li₂O-0.7LiCl-xAl₂O₃-0.1V₂O₅(x=0.05和0.15)的电子自旋共振谱研究表明:(i)ESR线型是高斯型,证实V₂O₅添加量适当;(ii)超精细结构来源于VO²⁺络离子,具有四角对称性,属C_4v群。越精细耦合张量的平行分量平均值A_//=0.0175cm^-1,垂直分量A_⊥=0.0063cm^-1。由g_//(g_⊥)求出其基态²B_2g与第一激发态²E_g的能级间距△₁=2.46×10⁴ cm^-1,基态与第二激发态²B_1g的能级间距△₂=3.03×10⁴ cm^-1;(iii)变温实验证实:Al₂O₃组分较少(x=0.05)的非晶ESR强度比x=0.15的非晶高3倍至2倍,而Al₂O₃组分越多则ESR强度随温升下降越小。     

Ab initio study of electronic structure and magnetic properties of copper (II) terephthalate

The first principles within the full potential linearized augmented plane wave (FP-LAPW) method was applied to study the compound of Cutp(OH₂)₂. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has a ferromagnetic interaction arising from the bridging water molecule. The spin magnetic moment 1.0μ_B per molecule mainly comes from the Cu ion with little contribution from O, C anion.     

碱性介质电催化体系活性氧的生成及其光谱学研究

亚熔盐液相氧化技术可在相对低温下实现难分解两性金属矿物的高效转化,基于此进一步提出了碱性介质电化学活性氧协同强化新方法。利用紫外-可见光光谱和电子自旋共振波谱等手段对电催化体系活性氧的生成与转化机理进行了系统解析。通过阴极电催化的作用,在电极表面定向进行两电子氧气还原反应,原位产生大量的活性氧组分。研究发现过渡金属离子与两电子氧气还原产物HO-₂之间的“自诱发”效应,可生成具有更高氧化活性的羟基自由基,大幅促进两性金属转化过程,实现了碱性介质电化学高级氧化过程。利用紫外-可见光光谱检测到电化学体系活性氧催化氧化低价两性金属氧化物(Cr₂O₃和V₂O₃)的转化过程。与此同时,根据标准自由能变化的热力学数据计算可知,V₂O₃比Cr₂O₃更易在活性氧存在的条件下发生溶出反应。采用电子自旋共振波谱(ESR)对电催化体系内的羟基自由基进行检测,结果表明由V₂O₃引发的电化学-类Fenton反应激发产生的羟基自由基ESR信号比Cr₂O₃信号强。利用猝灭剂实验验证了具有高氧化电位的羟基自由基可以对两性金属液相氧化起到促进作用。该研究为碱性介质电化学矿物溶出实际反应提供理论参考。     

The resonance peak in the high-Tc cuprates

I present a theory in which the resonance peak observed in inelastic neutron scattering experiments on YBa₂Cu₃O_6+x and Bi₂Sr₂CaCu₂O_8+x arises from a dispersing spin mode. I argue that it is heavily damped in the normal state and becomes visible in the superconducting state due to a drastic decrease in the spin damping. I show that a spin-fermion model correctly describes the doping dependence of the peak position and of its integrated intensity. Finally, I derive a criterion for the existence of the resonance peak in other cuprate superconductors.     

Enhancing the magnetoelectric coupling of Co4Nb2O9[100] by substituting Mg for Co

We report experimental studies on enhancing the magnetoelectric (ME) coupling of Co₄Nb₂O₉ by substituting the non-magnetic metal Mg for Co. A series of single crystal Co_4−xMg_xNb₂O₉ (x = 0, 1, 2, 3) with a single-phase corundum-type structure are synthesized using the optical floating zone method, and the good quality and crystallographic orientations of the synthesized samples are confirmed by the Laue spots and sharp XRD peaks. Although the Néel temperatures (T_N) of the Mg substituted crystals decrease slightly from 27 K for pure Co₄Nb₂O₉ to 19 K and 11 K for Co₃MgNb₂O₉ and Co₂Mg₂Nb₂O₉, respectively, the ME coupling is doubly enhanced by Mg substitution when x = 1. The ME coefficient α_ME of Co₃MgNb₂O₉ required for the magnetic field (electric field) control of electric polarization (magnetization) is measured to be 12.8 ps/m (13.7 ps/m). These results indicate that the Mg substituted Co_4−xMg_xNb₂O₉ (x = 1) could serve as a potential candidate material for applications in future logic spintronics and logic devices.     

Fe_3O_4单晶薄膜磁性电场调控的微磁学仿真研究

磁性薄膜磁学特性电场调控的相关研究对开发新型低功耗磁信息器件具有突出意义.本文基于电场调控磁性的基本理论,以OOMMF(Object Oriented Micro-Magnetic Frame)微磁学仿真软件为工具,研究了电场对生长于PZN-PT单晶衬底上Fe_3O_4单晶薄膜磁学特性的调控.研究结果显示:无外加电场时,薄膜表现出典型的软磁特性;沿衬底[001]方向施加的外加电场可以使得薄膜矫顽力、矩形比等磁学特性发生显著改变:当外加磁场沿[100]([010])时,施加正值(负值)电场强度可以显著增大薄膜的矫顽力与矩形比,当电场强度不小于0.6 MV/m时薄膜矩形比达到1.这是因为外加电场导致薄膜产生单轴应力各向异性,使得薄膜的等效磁各向异性发生了从无外电场下的面内四重磁晶各向异性向高电场下的近似单轴磁各向异性的过渡.外加1 MV/m与-1 MV/m的电场时等效易磁化轴分别沿[100]与[010]方向.另外,外加1 MV/m(-1 MV/m)的电场强度可以使得铁磁共振的频率增大(减小)接近1 GHz.