Author
K. Ullakko
Other affiliations: Massachusetts Institute of Technology
Bio: K. Ullakko is an academic researcher from Helsinki University of Technology. The author has contributed to research in topics: Magnetic shape-memory alloy & Magnetization. The author has an hindex of 23, co-authored 31 publications receiving 6745 citations. Previous affiliations of K. Ullakko include Massachusetts Institute of Technology.
Papers
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TL;DR: In this paper, the superelastic motion of twin boundaries in the martensitic phase of Ni2MnGa with magnetic fields of 8 kOe applied at 265 K was found to be associated with the staining.
Abstract: Strains of nearly 0.2% have been induced along [001] in unstressed crystals of Ni2MnGa with magnetic fields of 8 kOe applied at 265 K. These stains are associated with the superelastic motion of twin boundaries in the martensitic phase that is stable below about 274 K.
2,442 citations
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TL;DR: In this paper, a magnetic-field-induced strain of about 9.5% was observed at ambient temperature in a magnetic field of less than 1 T in NiMnGa orthorhombic seven-layered martensitic phase.
Abstract: Giant magnetic-field-induced strain of about 9.5% was observed at ambient temperature in a magnetic field of less than 1 T in NiMnGa orthorhombic seven-layered martensitic phase. The strain proved to be caused by magnetic-field-controlled twin boundary motion. According to an analysis of x-ray diffraction data, the crystal structure of this phase is nearly orthorhombic, having lattice parameters a=0.619 nm, b=0.580 nm, and c=0.553 nm (in cubic parent phase coordinates) at ambient temperature. Seven-layer shuffling-type modulation along the (110)[110]p system was recorded. The results of mechanical tests and magnetic anisotropy property measurements are also reported.
1,494 citations
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01 Sep 2000TL;DR: In this article, a room temperature extensional strain of 5.1% was observed in martensitic Ni/sub 48/Mn/sub 31/Ga/sub 21/ alloy in the magnetic field of 480 kA/m.
Abstract: A room temperature extensional strain of 5.1% was observed in martensitic Ni/sub 48/Mn/sub 31/Ga/sub 21/ alloy in the magnetic field of 480 kA/m. The magnitude of field-induced strain decreases with increasing external compressive stress applied in the direction of expansion. The compressive stress of about 3 MPa prevents the development of the substantial field-induced strain. Magnetization curves obtained by VSM exhibit an abrupt magnetization change and a transient hysteresis in the first quadrant. Large reversible field-induced strain and the abrupt magnetization change are due to the rearrangement or redistribution of martensitic twin variants by the applied magnetic field. It was confirmed by optical observation of movement and nucleation of martensitic twin boundaries.
336 citations
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TL;DR: In this article, the crystal structure of ferromagnetic near-stoichiometric Ni2MnGa alloys with different compositions has been studied at ambient temperature and the range of the average number of valence electrons per atom was determined for phases 5M, 7M, and nonmodulated martensite.
Abstract: The crystal structure of ferromagnetic near-stoichiometric Ni2MnGa alloys with different compositions has been studied at ambient temperature. The studied alloys, with five-layered (5M) and seven-layered (7M) martensitic phases, exhibit the martensitic transformation temperature (TM) up to 353 K. Alloys with these crystal structures are the best candidates for magnetic-field-induced strain applications. The range of the average number of valence electrons per atom (e/a) was determined for phases 5M, 7M, and nonmodulated martensite. Furthermore, a correlation between the martensitic crystal structure, TM and e/a has been established. The lattice parameters ratio (c/a) as a function of e/a or TM has been obtained at ambient temperature for all martensitic phases. That the paramagnetic-ferromagnetic transition influences the structural phase transformation in the Ni–Mn–Ga system has been confirmed.
299 citations
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TL;DR: In this article, the authors presented some new experimental results and the quantitative model describing large magneto-strain effect and main mechanical and magnetic properties observed in several ferromagnetic shape-memory alloys.
282 citations
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TL;DR: Shape memory alloys (SMAs) are a class of shape memory materials (SMMs) which have the ability to "memorise" or retain their previous form when subjected to certain stimulus such as thermomechanical or magnetic variations.
2,818 citations
01 Sep 1955
TL;DR: In this paper, the authors restrict their attention to the ferrites and a few other closely related materials, which are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present.
Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.
2,659 citations
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IBM1
TL;DR: Heusler compounds as discussed by the authors are a remarkable class of intermetallic materials with 1:1:1 or 2:1-1 composition comprising more than 1500 members, and their properties can easily be predicted by the valence electron count.
1,675 citations
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TL;DR: The magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy is reported, attributing this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase in the Ni45Co5Mn36.7In13.3 single crystal.
Abstract: Large magnetic-field-induced strains1 have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field1,2,3. These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy4. We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni45Co5Mn36.7In13.3 single crystal.
1,581 citations
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TL;DR: In this paper, a magnetic-field-induced strain of about 9.5% was observed at ambient temperature in a magnetic field of less than 1 T in NiMnGa orthorhombic seven-layered martensitic phase.
Abstract: Giant magnetic-field-induced strain of about 9.5% was observed at ambient temperature in a magnetic field of less than 1 T in NiMnGa orthorhombic seven-layered martensitic phase. The strain proved to be caused by magnetic-field-controlled twin boundary motion. According to an analysis of x-ray diffraction data, the crystal structure of this phase is nearly orthorhombic, having lattice parameters a=0.619 nm, b=0.580 nm, and c=0.553 nm (in cubic parent phase coordinates) at ambient temperature. Seven-layer shuffling-type modulation along the (110)[110]p system was recorded. The results of mechanical tests and magnetic anisotropy property measurements are also reported.
1,494 citations