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This paper presents a miniature energy harvesting device that makes use of stress-induced cyclic martensite variant reorientation in a Ni-Mn-Ga single crystal of 0.3x2x2 mm³ size. The stress- and magnetic field-induced reorientation is investigated for single crystalline Ni50.2Mn28.4Ga21.4 specimens of 0.3 mm thickness that are cut along the (100) direction and subjected to uniaxial compressive loading. A demonstrator is presented consisting of a FSMA specimen placed in the gap of a magnetic circuit to guide and enhance the field of biasing permanent magnets. The cyclic motion of a piezoelectric bimorph actuator is used to mechanically load the FSMA specimen. The corresponding change of magnetic flux induces an electrical voltage in a pick-up coil (N=2000 turns). The effects of biasing magnetic field, strain amplitude and strain velocity are investigated. An optimum magnetic field of 0.4 T exists, where the output voltage reaches 120 mV at a strain velocity of 0.006 ms-1.

A novel foil actuator of 15 × 3 mm 2 lateral dimensions is presented making use of the magnetic shape memory (MSM) effect. The actuation material is a Ni-Mn-Ga foil of 200 μm thickness that has been fabricated by cutting of a bulk Ni-Mn-Ga(100) single crystal consisting of 10 M martensite variants at room temperature. Stress-strain experiments on tensile test structures demonstrate that the stress needed for reorientation of martensite variants is about 1.2 MPa. The low twinning stress allows magnetic-field-induced variant switching, the basic mechanism of MSM actuation. A Ni-Mn-Ga foil actuator is fabricated by lithography and hybrid integration. The actuator shows a maximum magneto-strain of 4.9%, which is limited by the constraints of fixation and loading. Upon tensile loading at 1.5 MPa, linear actuation cycles are generated with an actuation stroke of 2.2%. The foil actuator is used as a benchmark system for modeling the coupled magneto-mechanical behavior of MSM actuation. We present finite element simulations based on a thermodynamic Gibbs free-energy model that qualitatively describes the observed tensile stress-dependence of magneto-strain.

Abstract:A single-crystal samples of near-stoichiometric Ni 2 MnGa alloys with large magnetic field induced strain (MSM effect) at room temperature have been investigated. It was found that there is a reversible reorientation of easy magnetization direction and magnetic domain structure under applied magnetic field. By means of x-ray Laue diffraction method and difractometric analysis it was ascertained that these phenomena are connected with the growth of martensitic twin variant with the short crystallographic axis (c-axis) directed along the applied magnetic field. Furthermore, by means magneto-optical investigations we have found the correlation between martensitic microstructure and magnetic domain structure. The field-induced reorientation of martensite was able to change the dimensional lengths of the samples more than 4.5 %. 1.IntroductionMagnetic field control of the shape of ferromagnetic alloys with martensitic structure was suggested recently [1,2]. Such materials are highly beneficial for actuators [3,4]. Field-induced strains, larger than 4 %, have been observed in the single-crystal Ni2MnGa alloys [5,6]. There are no detailed studies concerning the martensitic microstructure changes connected with MSM (magnetic shape memory) effect. The aim of this work was to fill up this gap. 2.Experimental proceduresThe alloy Ni4g.7Mn3o.iGa2i.3 was melted in an induction furnace in argon atmosphere. The composition of the alloy was measured by wave-length dispersive spectroscopy (WDS). After homogenization at 1000 C during 3 days and aging at 800 C 1 day the alloy was air cooled to room temperature. X-ray diffraction measurements revealed the Heusler type ordered structure (L2y) for the alloy in austenitic state. Martensitic transformation points M s , Mf, A s , A/, and Curie point T c were measured using low field ac magnetic susceptibility technique (M,=29° C, M f = 26° C, A s = 32° C, A f = 35° C, T c = 99° C). Samples for magnetic investigation with dimensions of 4x4x4 mm were cut using spark cutting machine and one single Ni4s.7Mn30.1Ga21.3 grain.All the M-H curves were obtained by using a vibrating sample magnetometer (VSM). The specimen stage was mounted between the pole pieces of a 1 T electromagnet and could be rotated for different field orientations. In all measurements the rotation of the samples was carried out at zero magnetic field. Strains induced by magnetic field were measured by strain gauges. The rather large changes in the sample shape and size gave us the possibility to confirm the strain gauge data by micrometric measurements. Temperatures were carefully controlled with a temperature control unit, i.e. a large volume alcohol circulator.Back-reflection Laue technique was used to determine the change in an orientation of crystals. X-ray tube with W-anode was used. A low voltage level V = 12 kV (/ = 30 mA) was applied in order to avoid the characteristic line effect and to decrease the fluorescent radiation from sample. The distance between the film Article published online by EDP Sciences a...

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