Papers by Author: Franca Albertini

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Authors: Lara Righi, Franca Albertini, Simone Fabbrici, Antonio Paoluzi
Abstract: Multifunctional ferromagnetic shape memory Heusler alloys are frequently characterized by structural modulation in martensitic phases. In particular, modulated martensitic phases, showing the higher magnetic field induced strain (MFIS) performance, are the most promising candidates for technological applications. Depending on the composition, as well as pressure and temperature conditions, this periodic structural distortion, consisting of shuffling of atomic layers along defined crystallographic directions, accompanies the martensitic transformation. Over the years, different Ni-Mn-Ga modulated martensitic structures have been observed and classified depending upon the periodicity of corresponding ideal nM superstructure (where n indicates the number of basic unit cells constituting the superlattices). On the other hand, it has been demonstrated that in most cases such structural modulation is incommensurate and the crystal structure has been fully determined by applying superspace formalism. The results, obtained by structure refinements on powder diffraction data, suggest a unified crystallographic description of the modulated martensitic structures, here presented, where every different “nM” periodicity can be straightforwardly represented.
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Authors: Franca Albertini, Z. Arnold, J. Kamarád, Fabrizio Leccabue, L. Pareti, Bernard Enrico Watts
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Authors: Lara Righi, Franca Albertini, Antonio Paoluzi, Simone Fabbrici, Elena Villa, Gianluca Calestani, Stefano Besseghini
Abstract: Magnetic 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.
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Authors: Franca Albertini, Massimo Solzi, Antonio Paoluzi, Lara Righi
Abstract: The giant magnetocaloric properties of NiMnGa alloys can be enhanced by suitable composition changes that make structural and magnetic transition temperatures to coincide. In this paper we report results on critical temperatures, magnetic anisotropy, and magnetocaloric effect in Ni- and Mn-rich alloys as a function of composition. A phenomenological phase diagram, useful for the identification by thermomagnetic analysis of magnetic and structural transitions in the vicinity of their coincidence, is proposed. Particular emphasis is given to the discussion of giant magnetocaloric effect of those alloys showing a first order magnetostructural transition, the method of its determination, and the potentialities for applications in the field of magnetic refrigeration.
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Authors: Franca Albertini, Simone Fabbrici, Antonio Paoluzi, Jiri Kamarad, Zdenek Arnold, Lara Righi, Massimo Solzi, Giacomo Porcari, Chiara Pernechele, David Serrate, Pedro Algarabel
Abstract: We review the composition dependence of the structural and magnetic properties of the Co-doped Ni–Mn–Ga Ferromagnetic Shape Memory Alloy around the Mn-rich composition Ni50Mn30Ga20. The presence of Co affects the critical temperatures and alters the exchange interactions of martensite and austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a “paramagnetic gap” between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation. The magnetic and structural properties display noticeable discontinuities across the martensitic transformation: remarkable values of the saturation magnetization jump at the transformation (DM), of the field dependence of the martensitic transformation temperature (dTM/dH), and of the crystalline volume change (DV/V) are reported, and are considerably enhanced by additional Indoping of the quaternary alloy. These properties give rise to a remarkable phenomenology which is of interest for multifunctional applications; magnetic superelasticity and high values of reversible strain are found.
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Authors: Bernard Enrico Watts, Daniele Regonini, Fabrizio Leccabue, Francesca Casoli, Franca Albertini, Gabriele Bocelli, David S. Schmool
Abstract: M-type lead hexaferrites (PbFe12O19) have been prepared using sol-gel methods. Lead and iron precursors were dissolved in butoxyethanol and stabilised with acetylacetone to produce a sol with good wetting and surface coating characteristics. Powder samples were investigated to optimise conditions for the growth of thin films. The «spin on» technique was used to provide homogeneous films. X-ray diffraction and thermomagnetic analysis vibrating sample magnetometry have been performed to assess phase distribution and transformation. The results show that the hexaferrite phase can be crystallised above 700 °C and that for the first indication that this phase is metastable in air below 650 °C.
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Authors: Franca Albertini, L. Morellon, P.A. Algarabel, M.R. Ibarra, L. Pareti, Gianluca Calestani
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Authors: Franca Albertini, Antonio Paoluzi, L. Pareti, L. Nasi, Giancarlo Salviati, Gianluca Calestani
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