Magnetocrystalline anisotropy and spin-wave stiffness in tensile-strained La0.67Ba0.33MnO3 films: An investigation via ferromagnetic resonance

Tensile-strained epitaxial La_0.67Ba_0.33MnO₃ (LBMO) film has been prepared by magnetron sputtering technique on (001) oriented spinel MgAl₂O₄ substrate. The transport and magnetic measurements give an insulator-metal transition and paramagnetic-ferromagnetic transition occurring at ～150 K and 250 K respectively, which implies the phase separation in such a tensile-strained film. By analyzing the angular and temperature dependences of the ferromagnetic resonance (FMR), we determine the magnetocrystalline anisotropy of the film. It is found that the tensile-strained film is dominated by an easy-axis corresponding to the compressive out-of-plane direction, though the magnitudes of anisotropy constants are relatively small and their temperature dependences are some complex. Furthermore, the FMR spectra show additional spin wave resonance (SWR), and the field positions can be indexed to follow a linear dependence on the square of index n. The scaling gives a spin-wave exchange stiffness D of 20.7 meV Ų at low temperature, which is less than half of that in strain-free LBMO films, implying that the double exchange interaction is remarkably suppressed in the tensile-strained LBMO films.     

Phase field simulation of monotectic transformation for liquid Ni-Cu-Pb alloys

Based on the subregular solution model, the liquid phase separation of ternary (Ni _x Cu_100−x )₅₀Pb₅₀ monotectic alloys is simulated by the phase field method. It is found that if the surface segregation potential is not incorporated, the dynamic morphologies of alloy melt show a transition from disperse microstructure into bicontinuous microstructure with the increase of fluidity parameter. When the surface segregation potential is coupled, Pb-rich phase migrates preferentially to the surface of the liquid alloy, and the Ni-rich phase depends on the Pb-rich phase to nucleate. With the extension of the phase separation time, the surface layer is formed through coagulation and growth, and its thickness gradually increases. The Ni-rich phase migrates to the central part, and finally a two-layer core-shell microstructure is produced. The concentration in the surface layer fluctuates more conspicuously than that inside the bulk phase, which subsequently transfers from the surface to the interior by a wave. The fluid field near the liquid-liquid interface is strong at the beginning of phase separation, and reduces later on. The surface segregation is essential to the formation of the surface layer, concentration profile variation, fluid field distribution and phase separation morphology. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101, 50395105)     

Change of proton motive force across thylakoid membrane in soybean leaf during state transitions

Change of proton gradient across thylakoid membrane in soybean leaves was studied with millisecond delayed light emission(ms-DLE) during the coures of state transitions which were indicated by the cholrophyll fluores-cence at room temperature and 77K.When dark-adapted leaves were induced to state Ⅰ with far-red light,Fm/F0,F685/F735 and the intensity of fast phase of ms-DLE were af-fected slightly.However,during the induction to state Ⅱ with red light,both Fm/F0 and F685/F735 decreased immedi-ately and the former were quicker than the latter.In this interval,the intesity of fast phase of ms-DLE increased to a maximum and then decreased to a lower valus during the transition to stateⅡ.Nigericin,an uncoupler which elimi-nates the proton gradient across thylakoid membrane,inhib-ited the increase in the intensity of fast phase of ms-DLE during the transition to state Ⅱ.Another uncoupler,valino-mycin,which eliminates the membrane potential,did not affect the changes of the intensity of fast phase.These results suggest that the prompt increase in the intensity of fast phase of ms-DLE at the beginning of transitions to state Ⅱ is cor-related mainly with the proton gradient released from water oxidation in photosystemⅡ.     

Imaging the vortex-lattice melting process in the presence of disorder

General arguments suggest that first-order phase transitions become less sharp in the presence of weak disorder, while extensive disorder can transform them into second-order transitions; but the atomic level details of this process are not clear. The vortex lattice in superconductors provides a unique system in which to study the first-order transition on an inter-particle scale, as well as over a wide range of particle densities. Here we use a differential magneto-optical technique to obtain direct experimental visualization of the melting process in a disordered superconductor. The images reveal complex behaviour in nucleation, pattern formation, and solid-liquid interface coarsening and pinning. Although the local melting is found to be first-order, a global rounding of the transition is observed; this results from a disorder-induced broad distribution of local melting temperatures, at scales down to the mesoscopic level. We also resolve local hysteretic supercooling of microscopic liquid domains, a non-equilibrium process that occurs only at selected sites where the disorder-modified melting temperature has a local maximum. By revealing the nucleation process, we are able to experimentally evaluate the solid-liquid surface tension, which we find to be extremely small.     

Critical electronic structures controlling phase transitions induced by lithium ion intercalation in molybdenum disulphide

We report on first-principles studies of lithium-intercalation-induced structural phase transitions in molybdenum disulphide (MoS2 ), a promising material for energy storage in lithium ion batteries. It is demonstrated that the inversion-symmetry-related Mo-S p-d covalence interaction and the anisotropy of d-band hybridization are the critical factors influencing the structural phase transitions upon Li ion intercalation. Li ion intercalation in 2H-MoS2 leads to two competing effects, i.e. the 2H-to-1T transition due to the weakening of Mo-S p-d interaction and the D 6h crystal field, and the charge-density-wave transition due to the Peierls instability in Li-intercalated 2H phase. The stabilization of charge density wave in Li-intercalated MoS2 originates from the enhanced electron correlation due to nearest-neighbor Mo-Mo d-d covalence interaction, conforming to the extended Hubbard model. The magnitude of charge density wave is affected by Mo-S p-d covalence interaction and the anisotropy of d-band hybridization. In 1T phase of Li-intercalated MoS2 , the strong anisotropy of d-band hybridization contributes to the strong Fermi surface nesting while the d-band nonbonding with S-p facilities effective electron injection.     

Morphologic characteristics and growth interface stability of nano-micron FeS2 whiskers

Micromorphology is further studied on the basis of our previous researches concerned with the nano-micron FeS2 whisker. There are obvious differences in the intensive degree, diameter and micro- morphology among the FeS2 whiskers growing in different stages. From the early to late stage, the intensive degree increases, the diameter decreases, and the surface micro-morphology changes following the regularity： protrusive nodulation → coarse → smooth → flat. According to the theory of crystal growth, the geological setting and processes of whisker formation, we discuss the stability and evolution of crystal growth interface of FeS2 whisker occurring in Gengzhuang gold deposit （Shanxi Province, China）. The results suggest that the negative temperature gradient and the supercooling appear in the early stage of the whisker growth, whereas the positive temperature gradient of reposeful state appears in the late stage. In the whisker growth stage, the component concentration changes through the three stages： severely nonhomogeneous in the early stage, relatively homogeneous in the middle stage, more homogeneous in the late stage. The general changing process of the interfacial state is from unstable to stable. Micromorphology of FeS2 whisker in Gengzhuang is the result of synergism of temperature, component concentration and stability of crystal interface phase in hydrothermal system. The micromorphology not only reflects the physical and chemical characteristics of the hydrothermal system during the whisker growth, but also indicates the stability characteristics of the interface phase and records the changing process of the whisker growth.     

Zr50Cu50金属玻璃形成过程中自由体积与玻璃态转变温度关系的分子动力学模拟

利用分子动力学模拟了Zr50Cu50金属玻璃的形成过程,并获得了不同温度下合金的原子构型.借助金属玻璃中自由体积量等于金属玻璃与对应晶体的体积差理论提出一种自由体积湮没速度法,对Zr50Cu50金属玻璃形成过程中的临界玻璃态转变温度Tc以及热力学玻璃态转变温度Tg进行预测.用该方法确定出的Tc(969.5K)与利用模式耦合理论计算获得的Tc(978.4K)接近;Tg(731K)与利用平均原子体积随温度变化关系曲线确定的Tg(725K)相近.运用自由体积湮没速度法计算的Tc和Tg无需计算各温度下的原子扩散系数,节省了计算时间.     

Synthesis of antiferroelectric (Bi0.534Na0.5)0.94Ba0.06TiO3 ceramics with high phase transition temperature and broad temperature range by a solid-state reaction method

Antiferroelectric 0.94(Bi 0.534 Na 0.5 )TiO 3 -0.06BaTiO 3 ceramics were prepared using a solid-state reaction method, involving the addition of excessive amounts of Bi2O3 . The resulting ceramics featured a very high phase transition temperature (Tm～330°C), from the antiferroelectric to the paraelectric phase, and a low depolarization temperature (Td<25°C). The broad temperature range, within which antiferroelectric properties are retained, of the prepared materials indicates their higher potential over lead-based antiferroelectric ceramics such as PZT-based materials that exhibit a lower T m ≤170°C. The lower Td and higher T m obtained val- ues, relative to those reported in the literature, are believed to be due to the formation of A-site vacancies originating from the incorporation of excess Bi into the perovskite structure of the studied sample. In addition, the synthesized sample shows a high dielectric constant of ～1460, in a temperature range of 50-150°C at 1 kHz, and a high energy storage density of 0.71 J/cm 3 , which is an asset in energy storage capacitor applications.     

The evolution of local structure of Mo6S8 during Li+ electrochemical storage studied by in-situ tender X-ray absorption spectroscopy

Chevrel phase Mo₆S₈ has attracted great interest because of its capability to accommodate different types of small cations through reversible topotactic redox reactions. However, the intercalation mechanism of Mo₆S₈ is still far from being fully understood owing to the complexity of the crystal structure and limitation of the probe tools. In our study, the reaction mechanism of Li ?⁺ ?intercalation into Mo₆S₈ was studied as a model by in-situ tender X-ray absorption spectroscopy (itXAS) in the viewpoint of local structure. According to the isobestic points of the 1st derivative, the discharge process is divided into five regions while the charge process is divided into four regions, where most of the regions are assigned to the two-phase transition reactions. The intensity change of the two spectroscopic features of the 1st derivative is found to be correlated, implying that the intercluster bond length directly affects the localization of the unoccupied states of the intracluster bond. The asymmetric evolution of the intensity of a’ and b’ as well as the asymmetric partition of reaction regions demonstrate the asymmetry of the reaction path in discharge and charge process. The results help to clarify the remaining debates on the phase transitions. Besides, itXAS is demonstrated to be a powerful tool to study the reaction mechanism of Mo₆S_8, which is complementary to NMR, XRD and so on.     

The hidden disintegration of cluster heterogeneity in Fe-based glass-forming alloy melt

The evolution of liquid metal at high temperature is known much less than their solid states. This is partially due to that the message concerning clusters, metastable phase or heterogeneity in liquid is usually too slight to be traced. Here, we shed some light on the nature of structural evolution of Fe-based glass-forming alloy during overheating process by the investigation of high-temperature melt viscometry and first principles simulations. It was found that a structural transition around 1400 ℃ occurred in the melts of initial homogeneous ingot, heterogeneous ingot and amorphous ribbons jointly, and was confirmed by the results from differential scanning calorimeter(DSC), and ab initio molecular dynamics(AIMD). Combining these results with Fe-Si-B ternary phase diagram and the melting characteristics of Fe-B compounds, it is safe to conclude that the disintegration of Fe_2B type clusters to Fe_3B-type clusters leads to the observed transition. These results offer a significant reference for the preparation and property control of Fe-based amorphous alloys.     

Phase analysis of AlFe2B2 by synchrotron X-ray diffraction, magnetic and Mössbauer studies

The structural and magnetic properties of essentially phase pure AlFe_2B_2 prepared by arc melting were compared with a sample containing impurities. Analysis was carried out by means of synchrotron X-ray diffraction, magnetic measurements and M?ssbauer spectroscopy. The analysis of synchrotron X-ray diffraction data confirmed the orthorhombic structure of space group Cmmm for AlFe_2B_2 with Al_(13)Fe_4 as main impurity phase. The magnetic measurements revealed an unsaturated ferromagnetic state in AlFe_2B_2 with a transition temperature(T_c) of 286 K. Isothermal magnetization measurment at 5 K gave a saturation magnetization of 0.7 μB per Fe atom while Arrott plots establish the second order nature of ferromagnetic transition. The thermal evolution of M?ssbauer spectra confirmed the ferromagnetic nature of this material revealing an hyperfine field of 73 kOe and an isomer shift of 0.46 mm/s at 100 K. The M?ssbauer spectra of a phase pure sample was compared with the one containing impurity phases. It is suggested that the low temperature paramagnetic contribution in M?ssbauer spectra of phase pure material may have its origin in some intrinsic phenomena arising from defects or inhomogeneities in crystal structure.     

Bonding characteristics of the Al2O3-metal composite coating fabricated onto carbon steel by combustion synthesis

The fabrication of an alumina-metal composite coating onto a carbon steel substrate by using a self-propagating high-temperature synthesis technique was demonstrated. The effects of the type and thickness of the pre-coated layer on the binding structure and surface quality of the coating were systematically investigated. The macrostructure, phase composition, and bonding interface between the coating and the substrate were investigated by scanning electronic microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectrometry (EDS). The diffraction patterns indicated that the coating essentially consisted of α-Al₂O₃, Fe(Cr), and FeO·Al₂O₃. With an increase in the thickness of the pre-coated working layer, the coating became more smooth and compact. The transition layer played an important role in enhancing the binding between the coating and the substrate. When the pre-coated working layer was 10 mm and the pre-coated transition layer was 1 mm, a compact structure and metallurgical bonding with the substrate were obtained. Thermal shock test results indicated that the ceramic coating exhibited good thermal shock resistance when the sample was rapidly quenched from 800°C to room temperature by plunging into water.     

SnO2中空球和纳米粒子结构的光谱对比分析

以SnCl4.5H2O作为锡源,采用共沉淀法制备SnO_2纳米粒子,以碳球为模板制备了SnO_2中空球。对产物进行了TGA,XRD,TEM,EDX表征,以检测产物的微观结构和形貌为纯SnO_2样品金红石结构。与SnO_2纳米粒子相比,SnO_2中空球结构具有较大的比表面积,因此在表面产生大量的氧空位,造成中空球结构的禁带宽度较小,拉曼光谱中四方金红石的固有模式Eg和A1g发生蓝移现象,SnO_2中空球的PL光谱强度比纳米粒子要强。     

Decompression-induced disorder to order phase transition in low-melting ionic liquid [OMIM][PF6]

In situ pressure-induced Raman spectral changes of 1-octyl-3-methyl imidazolium hexafluorophosphate （[OMIM][PF6]） have been investigated under the pressure up to 5.86 GPa at room temperature. The results indicated that [OMIM][PF6] experienced a phase transition at about 4.12 GPa during compression, and it was thought as a phase transition of liquid to a superpressurized glass. Upon decompression, from the obvious change of Raman spectra of [OMIM][PF6] at about 0.48 GPa, it could be inferred that a decompression-induced disorder to order phase transition in [OMIM][PF6] occurred. The phase behavior of [OMIM] [PF6] at low temperature under atmospheric pressure was also investigated in detail. The result showed that Raman spectra of [OMIM][PF6] varied slightly and no crystallization occurred upon cooling. These facts suggested that a disorder to order phase transition was induced by decompression in [OMIM][PF6], and [OMIM][PF6] served as a superpressurized glass under the pressure above 4.12 GPa, which was similar to the glassy state at low temperature.     

Folding of a single polymer chain and phase transition

Using an ultra-sensitive differential scanning calorimetry （US-DSC）, we have investigated the folding and aggregation behaviors of poly（N-isopropylacrylamide） （PNIPAM） chains in dilute and semidilute solutions. In the heating process, the intrachain folding and interchain aggregation simultaneously occur in the dilute solutions, and the ratio of intrachain folding increases with decreasing concentra- tion. In the semidilute solutions, PNIPAM chains show limited interchain aggregation with elevated temperature, because most of the PNIPAM chains have been collapsed at lower temperature. In an ex- tremely dilute solution, PNIPAM chains undergo a single folding transition in the heating process. By extrapolating heating rate and concentration to zero, we have obtained the phase transition tempera- ture （Ts） and enthalpy change （Arts） of the single chain folding. AHs is higher than that for a phase transition involving intrachain collapse and interchain aggregation, indicating that a single chain fold- ing can not be taken to be a macroscopic phase transition.     

Effects of Sr2+ substitution on the structural,dielectric, and piezoelectric properties of PZT-PMN ceramics

This study described the structural, dielectric, and piezoelectric behavior of Pb_1?x Sr _x [(Zr_0.52Ti_0.48)_0.95(Mn_1/3Nb_2/3)_0.05]O₃ ceramics (PSZT-PMN, x = 0, 0.025, 0.050, and 0.075), prepared by a semi-wet route. X-ray diffraction, dielectric, and piezoelectric investigations were carried out to analyze the crystal structure. The relative dielectric constant and dielectric loss were both calculated as the functions of temperature. The room-temperature dielectric constant reaches a maximum for a Sr²⁺-modified PZT-PMN ceramic with an x value of 0.050, which corresponds to the morphotropic phase boundary (MPB). Raman spectroscopy studies also confirm the existence of this MPB for x = 0.050. The piezoelectric strain coefficients (d ₃₃) value shows a maximum response for this composition. In addition, the phase transition temperature decreases significantly when the Sr²⁺ concentration increases in the PZT-PMN ceramics.     

Low-temperature magnetic properties of horse spleen ferritin

The magnetic properties of the antiferromagnetic cores in ferritin are of importance in the construction and improvement of ferritin-based magnetic resonance imaging systems and their application to environmental magnetism. In this study, we carry out integrated magnetic and transmission electron microscopy analyses of horse spleen ferritin (HoSF) to understand the relationships between the magnetic behavior of HoSF and temperature, applied field and grain-size distributions. The R-value from the Wohlfarth-Cisowski test for the investigated sample at 5 K was 0.46, indicating very weak magnetostatic interactions among the nanoparticles of HoSF. The nanoparticles of HoSF show superparamagnetic properties at room temperature, while below the blocking temperature of T _b ≈ 12 K it has a net magnetic moment that comes from the uncompensated spins of the nanoparticle surface or spin-canting. The thermal relaxation process of HoSF follows the Néel-Arrhenius expression. From low-temperature AC susceptibility data, we calculated the effective magnetic anisotropy energy E _a=(5.52±0.16)×10^?21 J; the effective magnetic anisotropy energy constant K _eff =(4.65±0.14)×10⁴ J/m³ and the pre-exponential frequency factor f ₀=(4.52±2.93)×10¹¹ Hz. These values are useful in understanding the magnetic behavior of the antiferromagnetic nanoparticles and their potential application in biomedical technology.     

Pyroelectric spectrum in Pb(Zr,Sn,Ti)O3 antiferroelectric-ferroelectric ceramics

The pyroelectric effect of phase transition induced with temperature in Nb-modified Pb(Zr,Sn,Ti)O₃ antiferroelectric-ferroelectric ceramics is studied. Experimental results reveal that the phase transitions are accompanied with marked pyroelectric peaks, there exists the close relation between the type of phase transition and the shape of pyroelectric peak. Because of the variations of phase transition, various pyroelectric spectra result. The pyroelectric spectrum can display the polarization effect and some inferior phase transitions with temperature variations, such as antiferroelectric AFE_A-AFE_B or ferroelectric FE_L-FE_H transition, which are not detected by the conventional dielectric measurement.     

Structure and properties of NASICON synthesized by two different zirconium salts

ZrOCl₂·8H₂O and ZrO(NO₃)₂·2H₂O were used respectively to synthesize a NASICON solid electrolyte by a sol-gel method. The structure and properties of two samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). The crystal structure was investigated by the Rietveld refinement. It is found that both the samples contain a monoclinic C2/c phase as the main conductive phase with the lattice parameters of a=1.56312 nm, b=0.90784 nm and c=0.92203 nm, though a small amount of rhombohedral phase is also detected in the final product. The sample synthesized by ZrO(NO₃)2·2H₂O contains more monoclinic phase (89.48wt%) than that synthesized by ZrOCl₂·8H₂O (74.91wt%). As expected, the ionic conductivity of the latter is higher than that of the former; however, the activation energy of the latter (0.37 eV) is slightly higher than that of the former (0.35 eV).