Ferromagnetic Shape Memory Alloys

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Authors: Volodymyr A. Chernenko
Abstract: The magnetically weakly anisotropic cubic Ni-Mn-Ga Heusler alloys exhibit martensitic transformation resulting in martensitic phases with elastically soft crystal lattices and strong magnetocrystalline anisotropies. The magnetic state of these martensites is coupled with a highly mobile twin structure through the ordinary magnetoelastic interactions giving rise to a giant magnetic-field-induced-strain effect. This effect is the key ingredient of a new scientific field. In the present article, the basic phenomena and concepts of this field, such as lattice instability, soft-mode behavior, electron concentration, ferromagnetic shape memory effect, magnetic-field-induced superelasticity, twinning strain-induced change of magnetization, and magnetoelastic mechanism of magnetostress are briefly reviewed.
Authors: Amitava Mitra, A.K. Panda
Abstract: Rapid solidification route by melt spinning has been adopted for preparation of a Ni52.5Mn24.5Ga23 (at %) ferromagnetic shape memory alloy in the form of ribbons. In the as-spun state, the ribbon revealed a predominant austenitic L21 structure in combination with martensitic feature as observed from x-ray diffraction studies. Transmission electron microscopic (TEM) evaluation showed these features in the form of martensitic plates. At low temperature, martensite to austenite transformation was exhibited by an increase in magnetization during heating cycle. The reverse effect was observed during cooling cycle. Annealing temperature and magnetising field was also found to effect this transformation.
Authors: Stefan Roth, Uwe Gaitzsch, Martin Pötschke, Ludwig Schultz
Abstract: Textured polycrystalline NiMnGa alloys were prepared by directional solidification. Alloys were chosen to have either the 7M or the 5M modulated martensitic structure after proper heat treatment. Mechanical training allowed to reduce the twin boundary pinning stress to below the magnetically induced stress. Thus, magnetic field induced changes in the mechanical behaviour could be demonstrated. The conditions of preparation and mechanical training will be discussed together with their influence on structure, microstructure, and the magneto-mechanical behaviour.
Authors: Volodymyr A. Chernenko, Ricardo López Antón, Stefano Besseghini, José M. Barandiarán, Makoto Ohtsuka, Andrea Gambardella, Peter Müllner
Abstract: A series of Ni51.4Mn28.3Ga20.3 films sputter-deposited on Si(100) wafer (with 500 nm thick buffer layer of SiNx) and annealed at 800 oC for 1h. are investigated with respect to their transformation behavior and magnetic properties. The film thickness, d, varies from 0.1 to 5.0 μm. Resistivity measurements reveal martensitic transformation above room temperature for all the films except for 0.1μm-thick film which is transforming at much lower temperature. The magnetic characteristics of martensitic films such as susceptibility and anisotropy field extracted from the inplane and out-of-plane magnetization curves show film thickness dependence likewise Curie temperature obtained from the resistivity curves. The surface topography and micromagnetic structure are studied by scanning probe microscopy. A stripe magnetic domain pattern featuring a large out-of-plane magnetization component is found in the films. The domain width, δ, depends on the film thickness, d, as δ ~ d .
Authors: Concepcio Seguí, Eduard Cesari, Jaume Pons
Abstract: Off-stoichiometric Ni2MnGa ferromagnetic shape memory alloys undergo a martensitic transformation (MT) from the L21 cubic phase to a martensitic crystal lattice consisting, in the majority of cases, of a periodic stacking sequence of nearly closed-packed planes with periodicity of 5, 7 or 2 planes, denoted as 10M (five layered tetragonal), 14M (seven layered orthorhombic) and 2M (non-modulated tetragonal). In addition to the parent to martensite transformation, Ni-Mn-Ga alloys tend to show stress or temperature induced intermartensitic transformations (IMTs) towards the most stable 2M phase, through the sequences 10M→14M→2M or 14M→2M depending on the first formed martensite. The IMTs reported in the literature show a variety of characteristics such as reversibility, completeness, hysteresis and temperature of occurrence, but, as a general trend, the role of internal stresses in favouring the occurrence of IMTs is recognised. Recently it has been shown that the L21 order degree favours the occurrence of the intermartensitic transformation from 14M to 2M martensite, stabilising the non modulated martensite through a decrease of its free energy with respect to the layered martensite. From this point of view, the occurrence of intermartensitic transformations in Ni-Mn-Ga alloys appears as a “chemical“ free energy effect. Aiming to go deeply into this aspect, in this work the occurrence of IMTs and their properties have been examined for an extensive set of off-stoichiometric Ni2MnGa. The results show the existence of a relationship between the IMTs temperatures and the alloys composition, as well as the dependence of all observed IMTs (i.e., 10M→14M, 14M→2M and their corresponding reverse transformations) on the L21 order degree.
Authors: R.K. Singh, R. Gopalan
Abstract: The effect of annealing temperature on martensite transformation temperature and magnetic properties has been investigated in a polycrystalline Ni53.69Mn26.06Ga20.25 alloy. The significant variation of the martensite transformation temperature and the magnetic properties is observed as a function of annealing temperature.

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