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Influence of the transition width on the magnetocaloric effect across the magnetostructural transition of Heusler alloys
Get PDFWe report a complete structural and magneto-thermodynamic characterization of four samples of the Heusler alloy Ni-Co-Mn-Ga-In, characterized by similar compositions, critical temperatures and high inverse magnetocaloric effect across their metamagnetic transformation, but different transition widths. The object of this study is precisely the sharpness of the martensitic transformation, which plays a key role in the effective use of materials and which has its origin in both intrinsic and extrinsic effects. The influence of the transition width on the magnetocaloric properties has been evaluated by exploiting a phenomenological model of the transformation built through geometrical considerations on the entropy vs. temperature curves. A clear result is that a large temperature span of the transformation is unfavourable to the magnetocaloric performance of a material, reducing both isothermal entropy change and adiabatic temperature change obtainable in a given magnetic field and increasing the value of the maximum field needed to fully induce the transformation. The model, which is based on standard magnetometric and conventional calorimetric measurements, turns out to be a convenient tool for the determination of the optimum values of transformation temperature span in a trade-off between sheer performance and amplitude of the operating range of a material
Ferromagnetic shape memory Ni-Mn-Ga alloys: a new synergy between structure and properties
No full textThe family of Ni-Mn based Heusler alloys provides an extended playground of physical properties. The interplay between a reversible martensitic transformation (MT) and magnetically ordered states gives rise to a series of functional properties that can be exploited for developing innovative devices [1] which originate from the possibility to dramatically change the materials properties by an applied external stimulus, such as magnetic field, stress of pressure. One of the most interesting property rely on the occurrence of large strains activated by external magnetic field. This phenomenon which takes the name of Magnetic Shape Memory (MSM) is mainly related to the particular crystalline structure assumed by the martesntic phase characterizing such family of this alloy. The martensitic transformation induced by the temperature change (corresponding to TM) consists of a deformation of the lattice of the austenitic phase having a structure type L21 with cubic symmetry. In Ni-Mn-Ga alloys the austenite is associated to a ferromagnetic state which is retained during the martensitic transformation. Upon the application of a magnetic field the martensite is magnetized by sliding the twinning boundaries typically featuring the product phase. The macroscopic effect of this process is represented by giant strains and this special synergy between the crystal structure of martensite and magnetic properties is at the basis of several international studies focused on the possible technological applications especially in the field of micromechanics. The cooperative aspect between crystal structure and magnetism is also manifested in the giant magnetocaloric effect. This property has triggered attention to the possible applications in the refrigeration industry (green technology). Basically, the observed magnetocaloric effect is closely related to the large difference in entropy between the martensitic and austenitic phase. In this paper we present the main physical properties of the Ni-Mn-Ga Heusler alloys potentially suitable for the design of a new generation of multifunctional smart devices
Incommensurate and Commensurate Structural Modulation in Martensitic Phases of FSMA
No full textMagnetic and structural properties in multifunctional FSMA (Ferromagnetic Shape Memory Alloys) belonging to Heusler family are frequently related to the occurrence of structural modulation in martensitic phases. The highest MFIS (Magnetic Field Induced Strain) effect has been observed in Ni-Mn-Ga alloys showing martensitic modulated structures. Depending on the composition, pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along specific crystallographic directions, accompanies the martensitic transformation. Over the years, different modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding superstructure (nM with n=3, 5, 6, 7, 12 etc). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure can be solved by applying superspace approach. The crystallographic representation of different modulated structures, obtained by structure refinement on powder diffraction data, suggests a unified description where every different “nM” periodicity can be straightforwardly represented. It will be presented an overview illustrating structural features of several displacive modulated martensitic lattices. For a specific Ni-Mn-Ga composition, the evolution of structural modulation upon temperature change will be illustrated
Structural properties and multiferroic phase diagram of K0.6Fe0.6(II)Fe0.4(III)F3 fluoride with TTB structure
No full textEvidence for in-plane tetragonal c-axis in Mn<sub>x</sub>Ga<sub>1-x</sub> thin films using transmission electron microscopy
No full textTetragonal MnxGa1-x (x = 0.70, 0.75) thin films grown on SrTiO3 substrates exhibit perpendicular magnetic anisotropy with coercive fields between 1 and 2 T. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) reveal that 40 nm samples grown at 300-350 degrees C lead to films with the tetragonal c-axis oriented primarily perpendicular to the film plane but with some fraction of the sample exhibiting the c-axis in the film plane. This structure results in an undesirable secondary magnetic component in the out of plane magnetization. Growth at 300 degrees C with a reduced thickness or Mn concentration significantly decreases the tetragonal c-axis in the film plane. (C) 2015 Elsevier Ltd. All rights reserved
On the broadening of the martensitic transition in Heusler alloys: from microscopic features to magnetocaloric properties
No full textWe report on the sharpness of the magnetostructural transition in a Ni-Co-Mn-Ga-In Heusler alloy, which has a key role in defining the magnetocaloric effect exploitable in efficient and environmentally friendly magnetic refrigerators. Magnetic and calorimetric analysis together with optical microscopy in phase-imaging mode are compared to correlate the transition broadening with microscopic features of the sample. The three techniques give the same temperature behavior of the martensitic transition. Microscopy analysis of different sample areas and during cycles in temperature reveals the spatial homogeneity of the transition width and its reproducibility in subsequent transformations
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
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