Interface coupling transition in a thin epitaxial antiferromagnetic film interacting with a ferromagnetic substrate

We report experimental evidence for a transition in the interface coupling between an antiferromagnetic film and a ferromagnetic substrate. The transition is observed in a thin epitaxial NiO film grown on top of Fe(001) as the film thickness is increased. Photoemission electron microscopy excited with linearly polarized x rays shows that the NiO film is antiferromagnetic at room temperature with in-plane uniaxial magnetic anisotropy. The anisotropy axis is perpendicular to the Fe substrate magnetization when the NiO thickness is less than about 15 A, but rapidly becomes parallel to the Fe magnetization for a NiO coverage higher than 25 A.     

MnFeP_(0.45)As_(0.55)材料中的相变研究

利用不同的测量方法,研究了MnFeP1-xAsx(0·32相似文献   

Effects of the anomaly on the two-flavor QCD chiral phase transition

We use strongly coupled lattice QED with two flavors of massless staggered fermions to model the chiral phase transition in two-flavor massless QCD. Our model allows us to vary the QCD anomaly and thus study its effects on the transition. Our study confirms the widely accepted viewpoint that the chiral phase transition is first order in the absence of the anomaly. Turning on the anomaly weakens the transition and turns it second order at a critical anomaly strength. The anomaly strength at the tricritical point is characterized using r=(M(eta')-M(pi))/rho(eta'), where M(eta'), M(pi) are the screening masses of the anomalous and regular pions and rho(eta') is the mass scale that governs the low energy fluctuations of the anomalous symmetry. We estimate that r ~ 7 in our model. This suggests that a strong anomaly at the two-flavor QCD chiral phase transition is necessary to wash out the first order transition.     

Percolation of immobile domains in supercooled thin polymeric films

We present an analysis of heterogeneous dynamics in molecular dynamics simulations of a polymeric film supported by an absorbing surface. Using a bead-spring model for polymers, we show that slow, immobile beads occur throughout the film, with the probability of their occurrence decreasing with distance from the substrate. Still, enough immobile beads are located near the free surface to cause them to percolate in the direction perpendicular to the substrate, at a temperature near the glass transition one. This result is consistent with a recent theoretical model of glass transition.     

Direct measurement of the low-temperature spin-state transition in LaCoO3

LaCoO3 exhibits an anomaly in its magnetic susceptibility around 80 K associated with a thermally excited transition of the Co3+-ion spin. We show that electron energy-loss spectroscopy is sensitive to this Co3+-ion spin-state transition, and that the O K edge prepeak provides a direct measure of the Co3+ spin state in LaCoO3 as a function of temperature. Our experimental results are confirmed by first-principles calculations, and we conclude that the thermally excited spin-state transition occurs from a low to an intermediate spin state, which can be distinguished from the high-spin state.     

Synthesis of powders and thin films of bismuth ferrite from solution: a magneto-electric study

Metalorganic decomposition and coprecipitation from aqueous solution have been used to synthesize thin film and bulk bismuth ferrite (BiFeO₃) with perovskite structure. X-ray diffraction, thermomagnetic analysis, and scanning electron microscopy were used to analyze the material and identify spurious phases. The magnetic characterization at room temperature revealed remarkable differences between bulk and film samples. The results show that in BiFeO₃ bulk ceramics, spontaneous magnetization is not observed at room temperature. Nevertheless, in thin films, a well-defined ferromagnetic behavior is observed at room temperature. Dielectric measurements taken on ceramic samples present an anomaly around 643 K that can be associated with the Néel temperature.     

Charge ordering in the TMTTF family of molecular conductors

Using one- and two-dimensional NMR spectroscopy applied to 13C spin-labeled (TMTTF)2AsF6 and (TMTTF)2PF6, we demonstrate the existence of an intermediate charge-ordered phase in the TMTTF family of charge-transfer salts. At ambient temperature, the spectra are characteristic of nuclei in equivalent molecules. Below a continuous charge-ordering transition temperature T(co), there is evidence for two inequivalent molecules with unequal electron densities. The absence of an associated magnetic anomaly indicates only the charge degrees of freedom are involved and the lack of evidence for a structural anomaly suggests that charge-lattice coupling is too weak to drive the transition.     

Microstructure evolution of Al-doped zinc oxide films prepared by in-line reactive mid-frequency magnetron sputtering

Aluminium-doped zinc oxide (ZnO:Al or AZO) thin films were deposited on glass substrates by reactive mid-frequency (MF) magnetron sputtering from Zn/Al metallic targets. Strong (002) preferred orientation was detected by X-ray diffraction (XRD). It was observed by plan-view transmission electron microscopy (TEM) that an AZO film deposited at low substrate temperature was composed of irregular large grains; but the film prepared at high temperature was composed of moderate sized grains with a regular shape. A secondary phase of ZnO₂ was also observed for the film deposited at low substrate temperature. The cross-sectional TEM study of the AZO film showed that prior to the well-aligned columnar growth an initial interfacial zone with nano crystallites were formed. The nano crystallites formed initially with a large tilt angle normal to the substrate surface and during the growth of the transition zone, the tilt angle decreased until it vanished. The evolution of the film structure is discussed in terms of evolutionary selection model and the dynamic deposition process. PACS 73.61.Ga; 68.37.Lp; 68.55.Jk     

VO_2薄膜Vis-NIR及NIR-MIR椭圆偏振光谱分析

采用射频磁控溅射在石英玻璃基底上反应溅射制备单斜相(M相)VO_2薄膜.利用V-VASE和IR-VASE椭圆偏振仪及变温附件分别在0.5—3.5 eV(350—2500 nm)和0.083—0.87 eV(1400—15000 nm)入射光能量范围内对相变前后的VO_2薄膜进行光谱测试,运用逐点拟合的方式,并通过薄膜的吸收峰的特征,在0.5—3.5 eV范围内添加3个Lorentz谐振子色散模型和0.083—0.87 eV范围内添加4个Gaussion振子模型对低温态半导体态的薄膜椭偏参数进行拟合,再对高温金属态的薄膜添加7个Lorentz谐振子色散模型对进行椭偏参数的拟合,得到了较为理想的拟合结果.结果发现:半导体态的VO_2薄膜的折射率在近红外-中红外基本保持在最大值3.27不变,且消光系数k在此波段接近于零,这是由于半导体态薄膜在可见光-近红外光范围内的吸收主要是自由载流子吸收,而半导体态薄膜的d//轨道内的电子态密度较小.高温金属态的VO_2薄膜的折射率n在近红外-中红外波段具有明显的增大趋势,且在入射光能量为0.45 eV时大于半导体态的折射率;消光系数k在近红外波段迅速增大,原因是在0.5—1.62 eV范围内,能带内的自由载流子浓度增加及电子在V_(3d)能带内发生带内的跃迁吸收,使k值迅速增加;当能量小于0.5 eV时k值变化平缓,是由于薄膜内自由载流子浓度和电子跃迁率趋于稳定所致.     

碳/碳化硅复合材料静止环境下氧化行为模拟

碳/碳化硅(C/SiC)复合材料是应用于临近空间高超声速飞行器热防护的一种新型防热材料.国内外通过性能测试较多地研究了材料不同制备工艺对抗烧蚀性的影响,提出的抗烧蚀分析理论模型均基于液态氧化膜.而近期开展的C/SiC复合材料管式炉加热实验和试样微观形貌电镜表征显示:常压下,当温度低于1696 K时,C/SiC复合材料氧...     

Anomalies of the phonon properties in multiferroic BiFeO3 thin films near the magnetic phase transition temperature

Size, substrate, doping and magnetic field effects on the phonon properties in multiferroic BiFeO₃ thin films are studied based on a microscopic model. We obtain an anomaly near the magnetic phase transition temperature TN which can be attributed to the magnetoelectric nature of BiFeO₃ and strong anharmonic spin-phonon interaction. It is shown that due to crystal lattice distortion for dopants with ionic radius smaller than that of the host ions the phonon energy decreases (for example Tb or Ti), whereas for the opposite case (larger radius of the doping ions, for example Co or Ni) it increases. The phonon damping is always enhanced compared to the undoped thin film.     

Ultrasonic attenuation by a thin film of high transition temperature Nb3Ge

The electron-phonon contribution to the ultrasonic attenuation coefficient of a 0.5 μm Nb₃Ge film has been measured using 1.2 GHz surface acoustic waves. Analysis of the attenuation data shows that the largest part of the film has a superconducting transition near the bulk transition temperature, 18 K, even though the total film has a high transition temperature, namely 21.5 K. The electron mean free path is calculated from the electron-phonon attenuation data and is found to be an order of magnitude higher than expected for high transition temperature Nb₃Ge films.     

MnFeP0.45As0.55材料中的相变研究

利用不同的测量方法，研究了MnFeP_１-xAs_x（0.32关键词： MnFePAs 磁致伸缩 巡游电子变磁性 一级磁相变     

Superconductor–insulator transition induced by over-deposited Ge in the insulating ultrathin a-Nb film

We have observed the superconductor–insulator (S–I) transition induced by over-deposited Ge in the insulating ultrathin amorphous Nb (a-Nb) film. The experiments are performed by depositing Ge onto the insulating a-Nb film with a thickness of 1.04 nm. For , where d_Ge was a thickness for Ge film, the reduction of electrical sheet resistances was seen at low temperature region, suggesting superconductor. The normal sheet resistance at 8 K decreased monotonically with increasing d_Ge. The simplest explanation of this S–I transition is that the electron localization effect is weakened due to the addition of Ge film.     

Reversible structural transition in epitaxial manganite film

A reversible structural transition of an epitaxial La(2/3)Sr(1/3)MnO3 film deposited on a LaAlO3 substrate has been investigated by means of in situ high-resolution transmission electron microscopy and electron diffraction, combined with image and diffraction calculations. We observe that the crystallographic symmetry of the film can be lowered via electron beam irradiation, leading to a rhombohedral-monoclinic transition. This transition can be attributed to the cooperating effect of the mismatch stress and the irradiation-induced thermal stress.     

掺钨VO_2薄膜的电致相变特性

采用直流磁控溅射与后退火工艺相结合的方法,在掺氟SnO_2(FTO)导电玻璃基底上制备了高质量的掺钨VO_2薄膜,对薄膜的结构、表面形貌和光电特性进行测试,分析了钨掺杂对其相变性能的影响.结果表明,室温下掺钨VO_2薄膜的阈值电压为4.2 V,观察到阈值电压下约有两个数量级的电流突变.随着温度升高,相变的阈值电压降低,且电流突变幅度减小.当施加8 V电压时,分别在不同温度下测试了掺钨VO_2薄膜的透过率.温度为20和50℃时,掺钨VO_2薄膜相变前后的红外透过率差量分别为23%和27%.与未掺杂的VO_2薄膜相比,掺钨VO_2薄膜具有相变温度低、阈值电压低和电阻率小的特点,在高速光电器件中有广阔的应用前景.     

Non-fermi-liquid behavior in quasi-one-dimensional Li0.9Mo6O17

We present temperature dependent scanning tunneling spectroscopy data of the quasi-one-dimensional conductor Li0.9Mo6O17. The differential tunneling current in our low-temperature spectra shows a power-law behavior around the Fermi energy, which is expected for a clean Luttinger liquid. The power-law exponent is found to be 0.6. Spectra for a temperature range of 5 to 55 K can be fitted fairly well with a model for tunneling into a Luttinger liquid at the appropriate temperature. A fit with a model based on a zero bias anomaly is significantly worse compared to the Luttinger liquid model. No signature of a phase transition at T = 24 K is observed in our temperature dependent data.     

Reflectivity oscillations in superconducting superlattices Nb/SiO2

The reflectivity of superconducting superlattices Nb/SiO₂ was experimentally studied at room temperature in a spectral range 450-5000 cm^-1. It was found that the reflectivity as a function of SiO₂ layer thickness oscillates with a period 3.5 Å. This value coincides with the period of oscillations of the superconducting transition temperature found earlier on these superlattices. We suppose that in both cases the oscillations are due to changes of electron state density. So it is possible to considerably influence the superconducting transition temperature of a superconducting film by making sandwich-like structures using thin dielectric layers.     

Effective interactions between parallel-spin electrons in two-dimensional jellium approaching the magnetic phase transition

We evaluate the effective interactions in a fluid of electrons moving in a plane, on the approach to the quantum phase transition from the paramagnetic to the fully spin-polarized phase that has been reported from Quantum Monte Carlo runs. We use the approach of Kukkonen and Overhauser to treat exchange and correlations under close constraints imposed by sum rules. We show that, as the paramagnetic fluid approaches the phase transition, the effective interactions at low momenta develop an attractive region between parallel-spin electrons and a corresponding repulsive region for antiparallel-spin electron pairs. A connection with the Hubbard model is made and used to estimate the magnetic energy gap and hence the temperature at which the phase transition may become observable with varying electron density in a semiconductor quantum well.     

Angular dependence of the photothermally modulated magnetic resonance in Gd thin films

In this paper we use electron spin resonance and photothermally modulated magnetic resonance techniques to investigate gadolinium thin films as a function of the orientation of the film surface with respect to the external magnetic field and of the temperature, around the magnetic phase transition temperature. We observe that, in the ferromagnetic phase, the resonance line is shifted up to higher external magnetic fields when the angle between the film surface and the field increases, revealing the magnetic anisotropy of the sample. At the same time, when the temperature is augmented to values higher than the phase transition temperature, the external field of the resonance collapses back to the expected value in the paramagnetic phase for all orientations. We also demonstrated that, even for the perpendicular orientation (magnetic field perpendicular to the sample surface), the photothermally modulated magnetic resonance signal is maximized near the magnetic phase transition temperature. Furthermore, in the ferromagnetic phase the photothermally modulated magnetic resonance intensity is very sensitive to the orientation, showing a significant enhancement in the perpendicular direction.