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Magnetic Domain Structure and Magnetically-Induced Reorientation in Ni-Mn-Ga Magnetic Shape Memory Alloy

100%
Heczko O., Bradshaw V.
Acta Physica Polonica A
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2017
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vol. 131
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issue 4
1063-1065
EN
Magnetization process during magnetically induced reorientation and related magnetic domains of cuboid Ni_{50}Mn_{28.5}Ga_{21.5} single crystal with {100} faces was investigated. Magnetic domains were visualized using magneto-optical indicator. The domains pattern is determined by strong uniaxial magnetic anisotropy of Ni-Mn-Ga martensite. Thanks to magnetically induced reorientation the domains arrangements for all three crystal orientations could be obtained and we showed that the size of domains scales with square root of the thickness and penetrates through whole crystal. Visualization of magnetic domains on all faces of cuboid provides the 3D model of magnetic domains.
2
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Equivalence of Mechanical and Magnetic Force in Magnetic Shape Memory Effect

71%
Kopecký V., Perevertov O., Straka L., Ševčík M., Heczko O.
|
EN
High mobility of twin boundary is crucial for magnetic shape memory effect. The twin boundary can be moved by applied magnetic field or mechanical stress. In Ni-Mn-Ga 10M martensite there are two different, field movable, a-c twin boundaries type I and II due to monoclinic lattice. For single twin boundary of both types we experimentally evaluated the equivalence of magnetic and mechanical force and the validity of generally used energy model using direct stress-strain and magnetization measurements. For type II, highly mobile twin boundary, the equivalence seems to be valid and model broadly agrees with measurement. However, for type I the calculated magnetic stress is much larger than mechanical stress needed for twin boundary motion.
3
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Effect of Compressive Load on Magnetic Shape Memory Effect in Ni-Mn-Ga Single Crystal

61%
Richterová K., Drahokoupil J., Kopecký V., Dragounová K., Landa M., Heczko O.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
704-708
EN
We investigated the effect of large compressive stress on magnetic shape memory effect in modulated 10M martensite of Ni-Mn-Ga Heusler alloy. The single crystalline sample deformed approximately elastically up to highest load 540 MPa. Structural and microstructural changes of a single crystal were analysed by X-ray diffraction 2D scan mapping in 2θ and ω. Although the crystal structure, lattice cell parameters, and twinned microstructure (a/b and modulation twinning) exhibited only small changes after the loading, the volume of the sample undergoing the magnetically induced reorientation decreased sharply while magnetic field necessary for the reorientation gradually increased. The extrapolation suggested that no magnetically induced reorientation might occur after compression of about 1 GPa.
4
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Martensitic Transformation in Co-Based Ferromagnetic Shape Memory Alloy

52%
Kopeček J., Yokaichiya F., Laufek F., Jarošová M., Jurek K., Drahokoupil J., Sedláková-Ignácová S., Molnár P., Heczko O.
Acta Physica Polonica A
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2012
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vol. 122
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issue 3
475-477
EN
The Co_{38}Ni_{33}Al_{29} alloy in both powder and bulk state was investigated in the presented study using neutron diffraction on E9 high resolution powder diffractometer at HZB (BER II). The reverse martensitic phase transformation from the tetragonal martensitic phase into the cubic austenitic phase was observed with the phase coexistence within the temperatures from 183 K to 133 K. The fcc cobalt solid solution particles distributed in the transforming matrix remained in the same state through the whole temperature range. The obtained results agree with magnetization measurements on the same annealed sample. The powder data are compared with experiments on bulk sample, although there was a texture present. The obtained results provide further information about the phase transition process in this important class of ferromagnetic shape memory alloys.
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