Magnetization and magnetostriction studies of TbFeCo/YFeCo multilayers

Exchange-spring TbFeCo/YFeCo multilayers exhibit interesting magnetic and magnetostrictive properties that are rather promising for application in microsystems. In this paper, we present a study of the effect of the exchange coupling on the magnetic properties of these magnetostrictive multilayers. An exchange bias phenomenon, revealed by a shift of the minor hysteresis loop along the applied field axis, is found as the result of the creation of interfacial domain walls. This effect strongly depends on the magnetic properties of the soft YFeCo layer, and becomes more pronounced at low temperatures due to the enhancement of the magnitude of the exchange coupling between the layers.     

Temperature Dependence of Structural Relaxation in Glass-Forming Liquids and Polymers

Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature T_g, whether there is an underlying thermodynamic transition at some finite temperature below T_g, what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section discusses the specific case of polymeric glass-forming liquids, which usually have extremely high fragility. We emphasize the apparent difference between the glass transitions in polymers and small molecules. We also discuss the possible role of quantum effects in the glass transition of light molecules and highlight the recent discovery of the unusually low fragility of water. At the end, we formulate the major challenges and questions remaining in this field.     

侧链基团对聚合物薄膜瞬态发光性能的影响

采用飞秒激光光谱技术比较研究了两种卟啉侧链聚合物:卟啉丙烯酸酯-苯乙烯共聚物P[(por)A-S]和卟啉铁(Ⅲ)丙烯酸酯-苯乙烯共聚物P[(por)FeA-S]的瞬态发光性能。并采用纳秒激光光谱技术测量了小分子卟啉(TPP)的荧光动力学过程。结果表明:P[(por)FeA-S]具有比P[(por)A-S]快得多的荧光弛豫过程,而P[(por)A-S]的荧光寿命远小于小分子卟啉的荧光寿命。对上述过程进行了分析,P[(por)A-S]的荧光衰变主要来源于聚合物分子链间的相互作用;而P[(por)FeA-S]的荧光衰变除了来源于聚合物分子链间的相互作用外,中央金属离子与配位体之间的电荷转移也对卟啉发色基团的激发态超快无辐射弛豫具有重要的影响,对上述过程的产生机理进行了讨论。     

Energy Transfer and Trapping in Rigid Solutions of Aromatic Polymers

Energy transfer by excitons is now well established in films and solutions of aromatic polymers. Excitons in disordered systems can be considered as electronically excited states, which are transferred between the light absorbing groups in a random hopping process (the same is true in molecular crystals if, because of thermal vibrations, the coherence length of the exciton waves approaches the lattice constant and consequently the band model of the excitons breaks down).     

The kinetic limitation of anti-stokes luminescence of Er3+ and Yb3+ doped infrared upconversion materials

Infrared-to-visible wave-length conversion in the Yb³⁺−Er³⁺ doped phosphors system has been described by a simple three level model based on two ions mechanism. The excitation in the range of 900–1000 nm of an IR-photon is first absorbed by Yb³⁺ ion as a sensitizer attributed to the resonant energy transition in Er³⁺ ion from ⁴ I _3/2 → ⁴ S _15/2 and ¹ F _9/2 → ⁴ I _15/2, respectively for green and red emission. The essential energy transfer processes in this system i.e. upconversion from ⁴ I _11/2 and ¹ I _13/2, cross-relaxation from ⁴ S _3/2 and ¹ F _9/2 are taken into account. The limitations of the rate-equation approach are examined with a focus on the underlying dynamics of this rare-earth system.     

Synthesis and Characterization of Polybenzobisoxazole Polymers Containing Trifluoromethyl or Sulfone Groups

Two series of polybenzoxazoles (PBOs) containing trifluoromethyl or sulfone groups were synthesized via solution condensation polymerization. The polymers were characterized by FT-IR, ¹H-NMR, thermogravimetric analysis (TGA), UV-vis absorption, and photoluminescence (PL) spectroscopy. The solubility of these polymers was greatly enhanced over that of pure PBO and some of them could dissolve well in polar solvents, such as N-methyl-2-pyrrolidone (NMP) and 2,5-Dimethylfuran (DMF). Due to the introduction of trifluoromethyl and sulfone groups, the UV absorption peaks and the emission peaks of all the polymers blue shifted, compared with those of pure PBO. The polymers derived from 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (6FAP) had a lower dielectric constant, which is due to the strong electron-withdrawal inductive effect, compared to that of the polymers derived from 3,3-diamino-4,4-dihydroxydiphenylsulfone (6SAP).     

铁磁/反铁磁/铁磁三层膜系统中反铁磁自旋结构及其交换各向异性

研究铁磁/反铁磁/铁磁三层膜中界面存在二次以及双二次交换耦合下反铁磁磁矩转动及其交换各向异性.结果表明,其反铁磁膜中的磁矩转动存在可逆"恢复行为"、不可逆"连续倒转行为"以及不可逆"中断倒转行为"三种情形,三种情形的出现强烈地依赖于两界面处的线性耦合和双二次耦合.钉扎界面的交换耦合与旋转界面的交换耦合相互竞争,当钉扎界面耦合占主导时,反铁磁磁矩发生可逆"恢复行为",系统出现交换偏置.在旋转界面耦合占主导情形下,其线性耦合与双二次耦合也相互竞争,分别导致反铁磁磁矩发生不可逆"连续倒转行为"和不可逆"中断倒转行为",系统都不出现交换偏置,但矫顽场都得以增强.相关结论为实验上观测的磁滞能耗以及界面垂直耦合提供了可能的解释.     

侧链结构对PPV/C60组合体系的激发传递和电荷转移过程的影响

本文考察了不同的侧链长度对聚对苯乙炔（PPV）衍生物的光学性质等的影响,以及PPV／C60的组合薄膜中的激发传递和电荷转移过程,结果表明随着连接在PPV的侧链长度的增长,其π-π跃迁带隙变窄,使得吸收和发射峰的位置逐渐红移;同时观测到PPV／C60间存在着极有效的激发传递过程,从而使得PPV衍生物的荧光在很大程度上被C60分子所猝灭;MD－PPV／C60组合膜中观测了C60的吸收峰,这表明该体系存在着基态电荷转移过程,同时也表明PPV衍生物与C60分子间的相互作用和PPV的侧链长短密切相关。     

基于CEST机制的pH成像方法、原理和应用

化学交换饱和转移（Chemical Exchange Saturation Transfer,CEST）技术作为一种新型的磁共振成像（Magnetic Resonance Imaging,MRI）技术.它的原理为溶质池中被激发饱和的质子与游离水中未被饱和的质子间的化学交换,能够引起水质子磁共振信号的下降,从而获得组织内生物分子的相关信息.由于质子间的交换速率k_ex与组织微环境的pH值之间存在直接联系,因而可以通过溶质质子的CEST信号对活体组织进行pH成像.目前用于pH成像的溶质分子既包括内源性游离的蛋白质、多肽分子,还包括一系列的外源性小分子和金属螯合物.通过不同类型的比率法、内源性胺和酰胺浓度-独立检测（Amine and Amide Concentration-independent Detection,AACID）等成像方法,能够获得肾脏、中风脑组织以及肿瘤组织的pH图谱.本文详细总结了2000年以来利用CEST技术进行pH成像方面的研究进展,包括对比剂、成像方法和相关应用,展望了活体组织pH成像的发展趋势和应用前景.     

Dynamics of the generalized Glauber-Ising chain in a magnetic field

A one-dimensional kinetic Ising model with nearest neighbor interactionJ and magnetic fieldH 0 is treated in both linear and nonlinear response, using the most general single spin-flip transition probabilities that depend on nearest neighbor states only. The dynamics is reformulated in terms of kinetic equations for the concentration n_l ⁺(t) [@#@ n_l(t) of clusters containingl up- [or down-] spins, which is exact in the homogeneous case. The initial relaxation time ^* of the magnetization is obtained rigorously for arbitraryJ, H, and temperatureT. The relaxation function is found by numerical integration forJ/T < 2. It is shown that coagulation of minus-clusters becomes negligible for bothJ/T andH/T large, and the resulting set of equations is solved exactly in terms of an eigenvalue problem. A perturbation theory is developed to take into account the neglected coagulation terms. The relaxation function is found to be non-Lorentzian in general, in contrast to the Glauber results atH = 0, which are recovered as a special case. In addition, nonlinear and linear relaxation functions differ forH 0. Consequences for the application to biopolymers are briefly mentioned.Supported in part by the Deutsche Forschungsgemeinschaft (SFB 130).     

The Role of a Specific Response of Media in the Vavilov-Cherenkov Effect

It is shown that the condition υ > υ_p (ω), which is necessary in order to trigger the Vavilov-Cherenkov effect, determines the interval of radiated frequencies and it alone cannot establish a strong restriction to the velocity of the particle. It is exhibited that it is possible to define a general lower bound for the velocity of the particle, which does not depend on the frequency when both, a specific response of the medium is taken into account and the mentioned condition is considered. The minimum value of the phase velocity of light in the medium determines the existence of such general lower bound.     

Luminescence of an erbium- and ytterbium-containing complex in porous silicon films

Er³⁺ and Yb³⁺ ions are introduced into porous silicon films, stabilized by oxidation in an oxygen plasma, in the form of a gadolinium oxychloride-based luminophor by means of thermal diffusion. An investigation is made of the luminescence and photoexcitation spectra of samples with Er³⁺ (10 and 30 wt.%), Yb³⁺ (10 wt.%), and Er³, Yb³⁺⁺ (10 wt.% each). It is shown that the intense IR luminescence (1.00 and 1.54 μm) is caused by cross-relaxation effects. The most effective excitation of the luminescence has been observed in the UV absorption band of the porous silicon. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 428–433, May–June, 1999.     

侧链结构对PPV/C_(60)组合体系的激发传递和电荷转移过程的影响

本文考察不同的侧链长度对聚对苯乙炔 (PPV)衍生物的光学性质等的影响 ,以及PPV/C60 的组合薄膜中的激发传递和电荷转移过程。结果表明随着连接在PPV的侧链长度的增长 ,其π π 跃迁带隙变窄 ,使得吸收和发射峰的位置逐渐红移 ;同时观测到PPV/C60 间存在着极有效的激发传递过程 ,从而使得PPV衍生物的荧光在很大程度上被C60 分子所猝灭 ;MD PPV/C60 组合膜中观测到了C-60 的吸收峰 ,这表明该体系存在着基态电荷转移过程 ,同时也表明PPV衍生物与C60 分子间的相互作用和PPV的侧链长短密切相关。     

A theoretical and numerical consideration of the longitudinal and transverse relaxations in the rotating frame

We previously derived a simple equation for solving time-dependent Bloch equations by a matrix operation. The purpose of this study was to present a theoretical and numerical consideration of the longitudinal (R_1ρ = 1/T_1ρ) and transverse relaxation rates in the rotating frame (R_2ρ = 1/T_2ρ), based on this method. First, we derived an equation describing the time evolution of the magnetization vector (M(t)) by expanding the matrix exponential into the eigenvalues and the corresponding eigenvectors using diagonalization. Second, we obtained the longitudinal magnetization vector in the rotating frame (M_1ρ(t)) by taking the inner product of M(t) and the eigenvector with the smallest eigenvalue in modulus, and then we obtained the transverse magnetization vector in the rotating frame (M_2ρ(t)) by subtracting M_1ρ(t) from M(t). For comparison, we also computed the spin-locked magnetization vector. We derived the exact solutions for R_1ρ and R_2ρ from the eigenvalues, and compared them with those obtained numerically from M_1ρ(t) and M_2ρ(t), respectively. There was excellent agreement between them. From the exact solutions for R_1ρ and R_2ρ, R_2ρ was found to be given by R_2ρ = (2R₂ + R₁)/2 − R_1ρ/2, where R₁ and R₂ denote the conventional longitudinal and transverse relaxation rates, respectively. We also derived M_1ρ(t) and M_2ρ(t) for bulk water protons, in which the effect of chemical exchange was taken into account using a 2-pool chemical exchange model, and we compared the R_1ρ and R_2ρ values obtained from the eigenvalues and those obtained numerically from M_1ρ(t) and M_2ρ(t). There was also excellent agreement between them. In conclusion, this study will be useful for better understanding of the longitudinal and transverse relaxations in the rotating frame and for analyzing the contrast mechanisms in T_1ρ- and T_2ρ-weighted MRI.     

Optimization of the irradiation power in chemical exchange dependent saturation transfer experiments

In chemical exchange dependent saturation transfer imaging experiments, exchangeable solute protons are saturated and the transfer of saturation to water is subsequently detected. When the applied irradiation power is comparable to the resonance frequency difference between the water protons and saturated solute protons, the proton transfer (PT) efficiency is reduced due to concomitant direct saturation effects. In this study, the PT process is modeled using a two-pool system. An empirical general proton transfer ratio (PTR) equation for arbitrary RF irradiation power is derived, and its optimal power to maximize the PTR is analyzed. The results are confirmed experimentally on 4.7 T using a poly-L-lysine solution. The theory provides a useful tool for optimizing the irradiation power of the PT sequences in the presence of direct saturation effects.     

分子链间距离对于卟啉侧链聚合物稳态及瞬态发光性能的影响

采用荧光光谱技术,测量了一种卟啉侧链聚合物:卟啉丙烯酸酯-苯乙烯共聚物(P[(por)A-S])的稳态和瞬态的发光性能.通过将聚合物P[(por)A-S]掺杂到有机玻璃(PMMA)中,研究了链间距离对于P[(por)A-S]发光性能的影响.结果表明:随着聚合物分子浓度增大,即链间距离减小,导致卟啉侧链基团之间的相互间作用增加,使其激发态无辐射跃迁几率增加,聚合物荧光强度减弱,激发态寿命缩短.讨论了上述过程产生的原因.     

NaYF4 微晶与纳米晶中晶相对Tb3+ -Er3+ 耦合对下转换的影响

实验研究了晶相对下转换的影响。Tb³⁺ -Er³⁺ 耦合对将一个紫外光子(Tb³⁺ 的⁷F₆→⁵L₁)294 nm 剪裁成800 nm (Er³⁺ 的⁴I_9/2→⁴I_15/2) 和467 nm (Tb³⁺ 的⁵D₄→⁷F₆) 两个都能被GaAs 太阳能电池吸收的低能光子。采用水热法制备了NaYF₄六角相微晶和立方相纳米晶粒子,六角相由于具有热力学稳定性和有序的排列结构而更有利于量子剪裁过程的发生,相反在立方相结构中没有发现量子剪裁现象。分别采用294 nm 和355 nm波长的光对六角相NaYF₄进行激发,从发射光谱可以看出,下转换的实现是通过一个交叉弛豫过程完成的。实验结果表明,与熔融法相比,用水热法制备的NaYF₄ 的量子产率明显降低。     

The Role of Coherence Transfer Efficiency in Design of TROSY-Type Multidimensional NMR Experiments

An improved method for TROSY-type (Pervushin et al., Proc. Natl. Acad. Sci. USA 94, 12366-12371 (1997)) heteronuclear two-dimensional correlation involving protons of negligible CSA is presented. Rather than applying a simple INEPT sequence for back-transfer to protons (Pervushin et al., J. Am. Chem. Soc. 120, 6394-6400 (1998)), we replace the pi/2 proton pulse in INEPT by a spin-state-selective coherence transfer element (Sorensen et al., J. Biomol. NMR 10, 181-186 (1997)) and maintain broadband decoupling during acquisition. Theoretically that results in a sensitivity enhancement of a factor of 2. The new method is demonstrated using a (13)C,(15)N-labeled protein sample, RAP 18-112 (N-terminal domain of alpha(2)-macroglobulin receptor associated protein), at 750 MHz.     

Influence of Ga+ ion irradiation on thermal relaxation of exchange bias field in exchange-coupled CoFe/IrMn bilayers

This paper reports that the CoFe/IrMn bilayers are deposited by magnetron sputtering on the surfaces of thermallyoxidized Si substrates.It investigates the thermal relaxations of both non-irradiated and Ga + ion irradiated CoFe/IrMn bilayers by means of holding the bilayers in a negative saturation field.The results show that exchange bias field decreases with the increase of holding time period for both non-irradiated and Ga + ion irradiated CoFe/IrMn bilayers.Exchange bias field is also found to be smaller upon irradiation at higher ion dose.This reduction of exchange bias field is attributed to the change of energy barrier induced by ion-radiation.     

Homogeneous Detection of Specific DNA Sequences by Fluorescence Quenching and Energy Transfer

The use of fluorescence quenching and energy transfer in DNA hybridization assays is reviewed. Placement of DNA probe labels within interacting distances by hybridization of DNA probes to target DNA or to one another allows rapid homogeneous analysis of specific DNA sequences. Due to the inherently lower sensitivity relative to heterogeneous assays, the fluorescence assays have been coupled with DNA amplification methods such as PCR to provide highly sensitive, clinically relevant homogeneous assays which can be performed in closed systems.