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Castel Guelfo di Bologna, Italy

Manosa L.,University of Barcelona | Stern-Taulats E.,University of Barcelona | Planes A.,University of Barcelona | Lloveras P.,Polytechnic University of Catalonia | And 7 more authors.
Physica Status Solidi (B) Basic Research | Year: 2014

We report on calorimetric measurements under hydrostatic pressure in a series of composition related metamagnetic shape memory alloys. We show that metamagnetic shape memory alloys exhibit a barocaloric effect whose magnitude compares well to the magnetocaloric effect exibited by this kind of alloys. While in metamagnetic alloys the magnetocaloric effect is inverse, the barocaloric effect has been found to be conventional. The values obtained for the pressure-induced entropy changes at moderate pressures are in the range of those reported for giant caloric materials. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Kamarad J.,ASCR Institute of Physics Prague | Albertini F.,CNR Institute of Materials for Electronics and Magnetism | Arnold Z.,ASCR Institute of Physics Prague | Fabbrici S.,CNR Institute of Materials for Electronics and Magnetism | And 2 more authors.
Acta Materialia | Year: 2014

To show and to describe a contrast between the magnetic and structural properties of the stoichiometric Ni2MnGa alloy and the off-stoichiometric and the Co-doped Ni-Mn-Ga alloys, we report the effect of high hydrostatic pressure (up to 1.2 GPa) on magnetization and the critical temperatures of structural and magnetic transitions in the off-stoichiometric Ni50-xMn25+x +yGa25-y and the Co-doped Ni50-xCoxMn25+ yGa25-y Heusler alloys. A huge decrease in their saturation magnetization MM(5 K) under pressure was observed with an extreme value of pressure parameter d ln MM(5 K)/dP = -56 × 10-3 GPa-1 in the case of the Ni 39Co11Mn34Ga16 alloy. The Curie temperature TCA increases under pressure in all the studied alloys. Almost identical values of dTCA/dP = +4.7 ÷ +5.9 K GPa-1 point to a normalized steepness of a dependence of exchange interactions on interatomic distances. The values of paramagnetic effective moments, meff, are identical in both structural phases of the alloys and they lie in a narrow interval from 4.6 μB to 5.7 μB. Furthermore, the Co-doped alloys show a very strong pressure dependence on the structural critical temperature TM- A, with values of dTM-A/dP up to +34 K GPa-1, the highest reported values for the Ni-Mn-based Heusler alloys. The results are discussed in light of the fundamental magnetic interactions in the Heusler alloys and from the point of view of a further exploration of their multifunctional applications. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Emre B.,University of Barcelona | Emre B.,Ankara University | Yuce S.,University of Barcelona | Yuce S.,Ondokuz Mayis University | And 6 more authors.
Journal of Applied Physics | Year: 2013

Calorimetry under magnetic field has been used to study the inverse magnetocaloric effect in Ni-Co-Mn-Ga-In magnetic shape memory alloys. It is shown that the energy dissipated during a complete transformation loop only represents a small fraction (5% to 7%) of the latent heat of the martensitic transition. It is found that the entropy values obtained from isofield temperature scans agree well with those obtained from isothermal magnetic field scans. The reproducibility of the magnetocaloric effect has been studied from isothermal measurements. Reproducible entropy values under field cycling have been found within a temperature interval bounded by the start temperature of the forward transition at zero field and the start temperature of the reverse transition under applied field. Large reversible entropy changes around 11 J/kg K have been found for fields up to 6 T. © 2013 AIP Publishing LLC. Source

Porcari G.,University of Parma | Cugini F.,University of Parma | Fabbrici S.,MIST E R Laboratory | Fabbrici S.,CNR Institute of Materials for Electronics and Magnetism | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The study of two aspects of the magnetocaloric effect (MCE), namely, the matching between isothermal entropy change and direct adiabatic temperature change, is not straightforward since huge differences between these two quantities have often been reported. Here we put in relation the direct and indirect measurements on the first order magnetostructural martensitic transformation occurring in Ni-Co-Mn-Ga alloys. In order to complete the characterization of the MCE and to find an explanation of these mismatches, differential scanning calorimeter measurements have been performed at different applied magnetic fields. © 2012 American Physical Society. Source

Ranzieri P.,CNR Institute of Materials for Electronics and Magnetism | Fabbrici S.,CNR Institute of Materials for Electronics and Magnetism | Fabbrici S.,MIST E R Laboratory | Nasi L.,CNR Institute of Materials for Electronics and Magnetism | And 6 more authors.
Acta Materialia | Year: 2013

Thin films of Ni-Mn-Ga alloy ranging in thickness from 10 to 100 nm have been epitaxially grown on MgO(1 0 0) substrate. Temperature-dependent X-ray diffraction measurements combined with room-temperature atomic force microscopy and transmission electron microscopy highlight the structural features of the martensitic structure from the atomic level to the microscopic scale, in particular the relationship between crystallographic orientations and twin formation. Depending on the film thickness, different crystallographic and microstructural behaviours have been observed: for thinner Ni-Mn-Ga films (10 and 20 nm), the L2 1 austenitic cubic phase is present throughout the temperature range being constrained to the substrate. When the thickness of the film exceeds the critical value of 40 nm, the austenite-to-martensite phase transition is allowed. The martensitic phase is present with the unique axis of the pseudo-orthorhombic 7M modulated martensitic structure perpendicular to the film plane. A second critical thickness has been identified at 100 nm where the unique axis has been found both perpendicular and parallel to the film plane. Magnetic force microscopy reveals the out-of-plane magnetic domain structure for thick films. For the film thickness below 40 nm, no magnetic contrast is observed, indicating an in-plane orientation of the magnetization. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

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