In-plane structural order of domain engineered La0.7Sr0.3MnO3 thin films

We present a detailed structural study of tensile-strained La_0.7Sr_0.3MnO₃ thin films. We use the substrate miscut to control the number of rhombohedral variants in the films and study the in-plane order and structural distortions. Using high-resolution X-ray diffraction, we demonstrate that step-edge induced lattice modulations occur in 4-variant films, whereas periodic twinning is the dominant in-plane order for 2-variant films. We show that the in-plane twinning angle is almost completely relaxed. However, the relaxation of shear strain by the out-of-plane twinning angle and the monoclinic distortion is only partial. Furthermore, the film thickness dependence of the domain width reveals that domain formation is a universal mechanism for shear strain relaxation. Finally, we show that the structural response to the transition from the paramagnetic to the ferromagnetic phase of La_0.7Sr_0.3MnO₃ at 345?K is smaller in 4-variant films compared to 2-variant films.     

High resolution transmission electron microscope observation of zero-strain deformation twinning mechanisms in Ag

We have observed a new deformation-twinning mechanism using the high resolution transmission electron microscope in polycrystalline Ag films, zero-strain twinning via nucleation, and the migration of a Σ3{112} incoherent twin boundary (ITB). This twinning mechanism produces a near zero macroscopic strain because the net Burgers vectors either equal zero or are equivalent to a Shockley partial dislocation. This observation provides new insight into the understanding of deformation twinning and confirms a previous hypothesis: detwinning could be accomplished via the nucleation and migration of Σ3{112} ITBs. The zero-strain twinning mechanism may be unique to low staking fault energy metals with implications for their deformation behavior.     

Experiments on the reflection of cold atoms from magnetic thin films

We report the results from a series of experiments in which ferromagnetic thin films were used as atom mirrors for laser-cooled rubidium atoms released from a magneto-optical trap. The thin films were made of cobalt and lanthanum calcium manganite (LCMO) with thicknesses between 20 and 300 nm. The magnetic domains in these thin films have a periodic structure where the spatial period is of the order of the thickness of the film, and the field decays exponentially above the film over a length scale comparable to the domain size. Thus, the neutral atoms reflect off these films from distances comparable to the thickness of the film, resulting in modification of the reflectivity due to the competition between the repulsive magnetic force and the attractive short-range forces such as van der Waals and Casimir forces. The smoothness of the atom mirror is also modified due to the proximity of the magnetic domains. The reflectivity is sensitive to the domain structure and size, which can be modified in LCMO by applying a modest external magnetic field. In this paper, we discuss the evaluation of the thin films as magnetic mirrors for atom optics, and the measurement of the van der Waals force with an accuracy of about 15%, using cobalt thin films. We also discuss some preliminary results on the temperature-dependent reflectivity for atoms near the ferromagnetic transition at 250 K in the LCMO film, and on the domain dynamics and relaxation.     

twins in Mo-doped TiSi 2 thin films with the C54 structure

The defect structures in orthorhombic C54 crystallites in thin films of Mo-doped TiSi 2 produced by co-sputtering have been investigated by transmission electron microscopy. Almost all C54 crystallites contain a twin boundary parallel to (101), dividing a crystallite into two regions, each of which also contains many thin twins with the habit plane parallel to (001), which is inclined by about 45° from (101). Both of the two regions divided by the twin boundary parallel to (101) tend to have facets on (001) as well as thin twins with the habit plane parallel to (001). As a result, C54 crystallites exhibit a characteristic shape just like an oak leaf. While twins with the (001) habit plane have been observed in C54 crystallites in both binary and Mo-doped TiSi 2 thin films, those with the (101) habit plane are present only in Mo-doped TiSi 2 thin films. The twinning elements for (101) twins are determined to be K 1 = (101), , K 2 = (001) and m2 = [100]. The origin of (101) twins in Mo-doped TiSi 2 is discussed in terms of the change in the c / a axial ratio upon alloying TiSi 2 with Mo.     

Magnetic nanostructures by adaptive twinning in strained epitaxial films

We exploit the intrinsic structural instability of the Fe(70)Pd(30) magnetic shape memory alloy to obtain functional epitaxial films exhibiting a self-organized nanostructure. We demonstrate that coherent epitaxial straining by 54% is possible. The combination of thin film experiments and large-scale first-principles calculations enables us to establish a lattice relaxation mechanism, which is not expected for stable materials. We identify a low twin boundary energy compared to a high elastic energy as key prerequisite for the adaptive nanotwinning. Our approach is versatile as it allows to control both, nanostructure and intrinsic properties for ferromagnetic, ferroelastic, and ferroelectric materials.     

Investigation of interface properties by nanoscale elastic modulus mapping

We present a method for investigating the spatial changes of elastic moduli in a nm-scale vicinity of interfaces. The method is demonstrated on twin walls in PbTiO(3) single crystals. It is revealed that the region near the twin wall is significantly softer than the two domains surrounding it. A comparison with finite element simulations relates this effect to an anelastic relaxation due to point defect accumulation around the twin wall. Local softening around the twin wall can affect the overall elastic modulus in thin films and nanostructured ferroelectric materials, in which the average distance between twin walls is smaller than the thickness of the softer region.     

Molecular dynamics simulation of the formation of twin boundaries during agglomeration of nanoparticles

The processes controlling early stages of agglomeration of nanoparticles have been investigated by the molecular dynamics method. It has been established that the formation of boundaries with twin misorientation is the main mechanism of structural relaxation during primary agglomeration of nanoparticles. It has been shown that an increase in the temperature leads to an increase in the number of twin boundaries and that their mutual arrangement depends on the misorientation of the nanoparticles. In the case where twin boundaries are noncoplanar, structure relaxation results in the formation of pentagonal twin boundaries. The role of twinning in the formation of interfaces upon compaction of nanoparticles has been discussed.     

Dislocation model for an incoherent nonthin twin

A condition for the equilibrium of a nonthin twin is obtained from a dislocation model using the approximation of a continuous distribution of twinning dislocations at twin boundaries. The model of a nonthin twin is shown to be applicable to the elastic and inelastic stages of twinning. The theory of a thin twin is found to be a particular case of the developed dislocation model of a nonthin twin.     

人工水晶-x面上巴西双晶的形成机制

本文以人工水晶-x面上的巴西双晶作为研究对象,在实验的基础上,利用X射线衍射形貌术和电子探针显微分析等手段,研究了双晶的形成机制,同时分析了Kern的双晶成核动力学理论的不足之处。作者提出了双晶界应变能势垒的概念,认为由于晶体的不完整性提高了晶体本身的能量状态,结果相对降低了应变能势垒的高度,使得双晶易于发生。本文结合晶体生长实验和晶体缺陷的测试,分析了双晶形成与籽晶取向、杂质、晶体缺陷的相互关系,着重指出缺陷对双晶形成的影响。本文还以凹入角生长机制讨论了双晶的发育。 关键词：     

Dynamical pattern formation for diblock copolymer films under stretch by moving walls

We propose a computational method that takes into account the dynamical influence of moving rigid walls over the pattern formation for thin films of diblock copolymers. The competition between the surface field energy and elastic stretching energy, and the effects of the molecular relaxation on pattern formation are studied. Finally, it is also observed that stretching the film enhances the ordering of patterns in it.     

Giant magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films

Complex experimental investigations of the structural, optical, and magneto-optical properties (magnetotransmission, magnetoreflection, and transversal Kerr effect, as well as the magnetoresistance, of La_0.7Ca_0.3MnO₃ epitaxial films indicate that magnetoreflection and magnetotransmission in manganite films can reach giant values and depend strongly on the magnetic and charge homogeneity of the films, their thickness, and spectral range under investigation. It has been shown that the optical enhancement of the magnetorefractive effect occurs in thin films as compared to manganite crystals. In the region of the minimum of the reflectance near the first phonon band, the resonance-like magnetorefractive effect has been observed, which is accompanied by change of the sign of the magnetoreflection. A model based on the theory of the magnetorefractive effect has been proposed to qualitatively explain this behavior.     

Mobility in thin polymer films ranging from local segmental motion, Rouse modes to whole chain motion: A coupling model consideration

Large increases of mobility of local segmental relaxation observed in polymer films as the film thickness is decreased, as evidenced by decreases of the glass temperature, are not found for relaxation mechanisms that have longer length scales including the Rouse relaxation modes and the diffusion of entire polymer chains. We show that the coupling model predictions, when extended to consider polymer thin films, are consistent with a large increase of the mobility of the local segmental motions and the lack of such a change for the Rouse modes and the diffusion of entire polymer chains. There are two effects that can reduce the coupling parameter of the local segmental relaxation in thin films. One is the chain orientation that is induced parallel to the surface when the film thickness h becomes smaller than the end-to-end distance of the chains and the other is a finite-size effect when h is no longer large compared to the cooperative length scale. Extremely thin ( ≈ 1.5 nm) films obtained by intercalating a polymer into layered silicates have thickness significantly less than the cooperative length scale near the bulk polymer glass transition temperature. As a result, the coupling parameter of the local segmental relaxation in such thin films is reduced almost to zero. With this plausible assumption, we show the coupling model can explain quantitatively the large decrease of the local segmental relaxation time found experimentally. Received 1 August 2001 and Received in final form 1 December 2001     

Editorial: Special issue on Properties of Thin Polymer Films

The behavior of polymers in thin films or close to interfaces is far from being understood. Many observations, encompassing both structural and dynamical behavior, indicate that the properties of polymers in thin films deviate from what we know from the bulk. Questions about the density (irreversible and reversible changes after annealing even above the bulk glass transition temperature), film stability and dewetting, glass transition temperature, diffusion coefficient and chain conformation and relaxation are intensively debated. In particular, it is not yet clear how the size of such chain-like molecules (their molecular weight) comes into play, especially if the thickness of the film is less than the radius of gyration of these macromolecules. In addition, due to the high surface-to-volume ratio the influence of surface and interfacial properties becomes important if not dominant. This interfacial sensitivity highlights the importance of the properties of the near surface region in polymer films; a topic whose importance is beginning to be recognized. This special issue presents experimental and theoretical works on a variety of questions related to polymers at interfaces and in thin films, ranging from space-averaged properties like adhesion to surface ordering or dynamic molecular (segmental) motion in confining geometries. Since the first reports of anomalous dynamics in thin polymer films almost a decade ago, the subject of dynamics in thin films has gained considerable momentum. Until very recently, this body of work focused almost solely on measurements of the thermodynamic signature of the kinetic glass transition. Such measurements are, at best, a very indirect probe of the microscopic dynamics and convolute the temperature dependence, time dependence and sometimes even the thermal history into a single measured value. The articles focusing on dynamics in thin films published in this special issue illustrate an important shift in this rapidly evolving field. There is now a strong focus on many different fronts. Measurements of dynamics are more varied, ranging from indirect studies such as adhesion to direct measurement of the segmental relaxation using dielectric spectroscopy. There is a concerted effort now to draw analogies to bulk systems in order to learn about what effects may contribute to the observation made in thin films. There is also a strong effort using numerical simulations to make quite direct comparisons to measured values in thin films. Finally, in a way that signals a new maturity to this field, a significant fraction of literature being currently published concerns ideas as to why the dynamics in thin films behave the way they do. In this special issue we have aimed at capturing a cross-section of problems of high current interest. While all contributions definitely provide highly valuable insight in the behavior of polymers in thin films, many questions are still unanswered and await further in-depth-going studies. We just want to note a few of these questions, also highlighted in several commentaries published in this issue: Can polymers in experimental studies on thin films ever be fully equilibrated? What is the relaxation behavior of macromolecules in confining geometries with interacting boundaries? How does the relaxation behavior depend on the length scale over which it is measured? What are the mechanical properties of thin polymer films? We tried to assemble different approaches and opinions from various viewpoints. We hope that such a complementary presentation is helpful and stimulates further discussions in order to dissolve some of the confusion in this area, leading eventually to a clear understanding of thin-film properties of polymers. The European Physical Journal E - Soft Matter will continue to provide a forum for the discussion of such questions and a place for the publication of future work on properties of polymers in thin films for all colleagues interested in these questions. Günter Reiter (Editor) James Forrest (Guest Editor)     

Molecular dynamics simulation of twinning in devitrite,Na2Ca3Si6O16

Reports of Type II twins are quite rare for most crystal structures. When they do occur, they are usually one of a number of possible twinning modes observed in a particular material. However, for the triclinic phase devitrite, Na₂Ca₃Si₆O₁₆, which nucleates from commercial soda?lime?silica float glass subjected to suitable heat treatments, the only reported twinning mode to date is a Type II twinning mode. In this study, this Type II twinning mode is first examined by molecular dynamics simulation to determine the lowest energy configuration of perfect twin boundaries for the twin mode. This is then compared with the lowest energy configurations of perfect twin boundaries found for six possible Type I twinning modes for devitrite for which the formal deformation twinning shear is less than 0.6. The most favourable twin plane configuration for the Type II twinning crystallography is shown to produce reasonably low twin boundary energies and sensible predictions for the optimum locations of the twin plane, K ₁, and the [1?0?0] rotation axis, η ₁, about which the 180° Type II twinning operation takes place. By comparison, all the Type I twinning modes were found to have very energetically unstable atomic configurations, and for each of these twinning modes, the lowest energy configurations found all led to high effective K ₁ twin boundary energies relative to perfect crystal. These results therefore provide a rationale for the experimental observation of the particular Type II twinning mode seen in devitrite.     

面心立方晶体高次孪晶化为一次旋转对称关系

根据孪生几何与旋转矩阵对应关系以及旋转操作归并定理,总可以将高次孪晶的多次孪生操作化为一次旋转操作。本文计算了面心立方晶体一至五次孪晶化为一次旋转操作的旋转轴[uvw]和旋转角φ,并给出适用于计算机自动分析高次孪晶电子衍射图的计算步骤。 关键词：     

High-quality in situ manganite thin films by pulsed laser deposition at low background pressures

We show that by decreasing the laser fluence it is possible to improve the oxidation process in manganite thin films under low background oxygen pressure, allowing the in situ use of conventional Reflection High Energy Electron Diffraction diagnostic. Films deposited at low fluence (corresponding to a deposition rate per pulse lower than 10^-2 unit cells per laser shot) show a two-dimensional growth mode and possess very good transport properties without the necessity of any further post-growth annealing treatment. A physical model, based on the plume-background interaction as a primary mechanism of film oxidation during growth, is proposed to explain the experimental findings.     

New insight on spin polarized current injection in high-T$\mathsf{_c}$ cuprate/manganite devices

Recently, numerous experiments have been reported on critical current reductions in thin films cuprates as being due to spin injection from ferromagnetic manganites. However, little theoretical justification for these very strong effects exists, and the necessary spin relaxation length is always larger than predictions. In the present work, we investigate the possibility that these effects are due to a different origin and we report on devices designed such that the temperature of the layer itself can be measured in situ. Our data show that similar reductions of the critical current are quantitatively correlated to heating in the manganite electrode due to dissipation of the polarization current.Received: 3 July 2003, Published online: 15 October 2003PACS: 72.25.Hg Electrical injection of spin polarized carriers - 74.76.Bz High-Tc films - 74.80.Dm Superconducting layer structures: superlattices, heterojunctions, and multilayers     

Optimization of Pr(0.9)Ca(0.1)MnO(3) thin films and observation of coexisting spin-glass and ferromagnetic phases at low temperature

Optimization of thin films of small bandwidth manganite, Pr(1-x)Ca(x)MnO3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750?°C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn3+ and Mn4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.     

Substrate and chain size dependence of near surface dynamics of glassy polymers

We use nanohole relaxation to study the surface relaxation of films of glassy isotactic poly (methyl methacrylate) (i-PMMA) films. These measurements allow us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2-3 nm of the free surface in a system often used as a model system for the effect of the substrate on thin film dynamics. The surface is observed to relax at temperatures up to 42 K below the bulk Tg value, even on systems where the thin film Tg is known to be greater than the bulk value. We are able to determine the range over which the substrate directly affects the free surface relaxation, and determine a surprisingly large (Mw independent) limiting thickness of approximately 180 nm where the free surface relaxation is not affected by the substrate. For thick films (h>200 nm) we find an unexpected linear Mw dependence of the near surface relaxation time.     

Characterization of twinning in electrodeposited Ni–Mn alloys

Twinning is ubiquitous in electroplated metals. Here, we identify and discuss unique aspects of twinning found in electrodeposited Ni–Mn alloys. Previous reports concluded that the twin boundaries effectively refine the grain size, which enhances mechanical strength. Quantitative measurements from transmission electron microscopy (TEM) images show that the relative boundary length in the as-plated microstructure primarily comprises twin interfaces. Detailed TEM characterization reveals a range of length scales associated with twinning beginning with colonies (～1000?nm) down to the width of individual twins, which is typically <50?nm. We also consider the connection between the crystallographic texture of the electrodeposit and the orientation of the twin planes with respect to the plating direction. The Ni–Mn alloy deposits in this work possess a {110}-fiber texture. While twinning can occur on {111} planes either perpendicular or oblique to the plating direction in {110}-oriented grains, plan-view TEM images show that twins form primarily on those planes parallel to the plating direction. Therefore, grains enclosed by twins and multiply twinned particles are produced. Another important consequence of a high twin density is the formation of large numbers of twin-related junctions. We measure an area density of twin junctions that is comparable to the density of dislocations in a heavily cold-worked metal.