Large magnetic-field-induced strains in Ni-Mn-Ga alloys in rotating magnetic field

Abstract

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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Influence of external stress on the reversibility of magnetic-field-controlled shape memory effect in Ni-Mn-Ga

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Simultaneous magnetooptic observation and thermomagnetic analysis of phase transitions in shape-memory Ni–Mn–Ga alloys

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