1,720,964 research outputs found
Effects of film thickness and composition on the structure and martensitic transition of epitaxial off-stoichiometric Ni-Mn-Ga magnetic shape memory films
No full textEpitaxial Ni-Mn-Ga magnetic shape memory films with varied thickness and variable stoichiometry were prepared by magnetron sputtering on MgO(001) substrates and investigated by using x-ray diffraction (XRD), scanning tunneling microscopy (STM) and scanning electron microscopy (SEM). The results demonstrate that the structure of the films is either a non-modulated martensite or a seven-layer-modulated martensite. At small film thicknesses, we observe a preferential alignment of the {110} twin planes titled 45 degrees from the substrate surface, which gives rise to the surface corrugation, whereas at large thicknesses, twin planes align additionally along the perpendicular planes. The biaxial tensile stress due to the film-substrate lattice mismatch is shown to have an important role in the selection of the possible twinning planes. An exponential relation between the stress and the film thickness is identified. In contrast, the magnetic properties of the films are found to be independent of the stress. A resistance maximum is measured close to the Curie temperature, which can be attributed to the relaxation of the lattice distortion induced by magnetoelastic coupling.BMBF [13N10060, 13N10061, 13N10062
Origin of steps in magnetization loops of martensitic Ni-Mn-Ga films on MgO(001)
No full textWe study the temperature dependent magnetization properties of (010)-oriented Ni-Mn-Ga epitaxial films on MgO(001) substrates. In the martensitic phase, we observe pronounced abrupt slope changes in the magnetization loops for all studied samples. Our experimental findings are discussed in conjunction with the micromagnetic simulations, revealing that the characteristic magnetization behavior is governed solely by the magnetization switching within the specific martensitic variant pattern, and no reorientation of twin variants is involved in the process. Our study emphasizes the important role of the magnetostatic interactions in the magnetization behavior of magnetic shape memory alloy thin films.publishe
Structural and electronic properties of Ni-Mn-Ga magnetic shape memory alloys
No full textIn recent years there has been an increased demand for the development of smart and functional materials for integration into actuators, sensors and energy harvesting devices. Magnetic shape memory alloys represent a new type of multifunctional materials, which display a strong coupling between structural and magnetic degrees of freedom, accompanied by a strong magnetocrystalline anisotropy. Thus, exposed to sufficiently high magnetic fields, they show a macroscopic shape change. The effect arises from the reversible rearrangement of the martensitic microstructure. To date, Ni-Mn-Ga based alloys have demonstrated many remarkable properties, like 10% magnetic field induced strain and operating frequencies up to the lower kHz range, thus being of particular technological interest. In order to perform their rational design, the microscopic origin of their functional properties must be understood.Since the discovery of the magnetic shape memory effect in Ni-Mn-Ga a strong effort has been first of all invested into improvement of the performance of bulk materials. However, the fundamental microscopic properties of these multifunctional materials were not intensively investigated from the experimental point of view. This thesis is devoted to the implementation of surface sensitive techniques for the investigation of structural and electronic properties of Ni-Mn-Ga magnetic shape memory alloys. There is also an emphasis on the fabrication of freestanding Ni-Mn-Ga thin film microstructures, relevant for device applications.A fabrication process was developed to prepare freestanding single crystalline Ni-Mn-Ga film microstructures. This process is based on electron beam lithography, focused ion beam milling and wet chemical etching. A thermally induced change of the martensitic twin variant configuration was observed.Temperature dependent scanning tunneling microscopy was used to investigate the surface structure of Ni-Mn-Ga alloys in both austenitic and martensitic phases. Film samples epitaxially grown on MgO(001) substrates and single crystal samples were studied. The microscopic surface structure was addressed from the micrometer scale down to the atomic level. Measurements revealed a well-ordered surface exhibiting a Mn-Ga termination. In the martensitic phase a distinct surface corrugation feature originating from the modulated nature of martensite was observed. Topography measurements demonstrate that martensitic Ni-Mn-Ga exhibits two distinctly different martensitic phases with respect to the nature of the modulation. On the one hand, sinusoidal modulation with incommensurate five-fold period was found for stoichiometric alloys. The periodic lattice distortion is accompanied by a charge density wave. On the other hand, a nearly seven-fold nanotwinned configuration with random stacking faults was identified for alloys with off-stoichiometric compositions. The structural modulation appears to be similar to the hierarchical microstructure predicted by the concept of adaptive martensite.Electronic properties of the stoichiometric Ni2MnGa compound have been studied by means of angle-resolved photoemission spectroscopy. Fermi surface topology has been examined in the austenitic state. A nesting feature, which has been previously identified in theoretical calculations, has been observed. The nesting vector matches the modulation period of the martensitic structure. Spectra, obtained in the martensitic state, provide a convincing proof of the a pseudogap formation for the nested parts of the Fermi surface. These results are in line with the observation of a charge density modulation by STM and support the assumption of a Peierls like instability. Temperature dependent ultraviolet photoemission spectra demonstrate a similar transfer of spectral weight near the Fermi level across the martensitic transformation for off-stoichiometric and stoichiometric alloys.It can be consistently accounted for the seemingly conflicting observations of two different Ni-Mn-Ga martensitic microstructures if a strong electron-lattice coupling scenario is considered.publishe
Microstructure and atomic configuration of the (001)-oriented surface of epitaxial Ni-Mn-Ga thin films
No full textThe (001)-oriented surface of epitaxial off-stoichiometric Ni-Mn-Ga ferromagnetic shape memory alloys was studied in both austenitic and martensitic phases. Scanning tunneling microscopy (STM) imaging of the austenitic surface reveals a well-ordered and reconstruction-free surface exhibiting predominantly Mn-Ga termination. We found that only one of the two atomic species (Ga or Mn) is visible in STM, which is attributed to a pronounced geometric corrugation of the surface layer. After a transformation of the sample from the initial austenitic phase to the martensitic phase upon a high-temperature annealing step, a thorough investigation of the martensitic surface was conducted. On a larger scale, pronounced corrugation lines arise from the macroscopically twinned surface. A second corrugation feature is found on a distinctly smaller scale and is shown to originate from the modulated nature of the martensitic film structure. The irregularly spaced corrugation lines support the model of adaptive martensites.Bundmesministerium fur Bildung und Forschung (BMBF) [13N10061, 13N10062
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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