Precursor effects and premartensitic transformation in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ni</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>MnGa

Zheludev, A.; Shapiro, S. M.; Wochner, P.; Tanner, L.E.

doi:10.1103/physrevb.54.15045

Cited by 205 publications

(262 citation statements)

References 34 publications

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“…Figure 4 shows the phonon-dispersion curve measured about the ͑2,2,0͒ Brillouin zone at three different temperatures. For comparison, the room-temperature measurements 16 of the isostructural compound Ni 2 MnGa are also presented. It is clear that the dispersion curve behaves in a normal nearly sinusoidal way without any anomalies and it is not temperature dependent.…”

Section: B Phonon-dispersion Relationsmentioning

confidence: 99%

“…14 In austenite Ni 2 MnGa, which has a L2 1 -Heusler-type structure, the structural instability is often linked with an anomalous phonon softening of the transversal acoustic TA 2 branch in ͓110͔ direction. [15][16][17][18] The softening occurs at a fractional wave vector of = 0.3− 0.4 depending upon the composition and becomes more prominent with decreasing temperature, approaching the martensitic transformation, and was frequently interpreted as precursor of the martensitic transformation. Similar anomalies can also be found in conventional shape-memory materials 19,20 and have been related to nesting parts of the Fermi surface in combination with strong electron-phonon coupling.…”

Section: Introductionmentioning

confidence: 99%

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Electronic structure and lattice dynamics of the magnetic shape-memory alloyCo2NiGa

et al. 2010

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In addition to the prototypical Ni-Mn-based Heusler alloys, the Co-Ni-Ga systems have recently been suggested as another prospective materials class for magnetic shape-memory applications. We provide a characterization of the dynamical properties of this material and their relation to the electronic structure within a combined experimental and theoretical approach. This relies on inelastic neutron scattering to obtain the phonon dispersion while first-principles calculations provide the link between dynamical properties and electronic structure. In contrast to Ni 2 MnGa, where the softening of the TA 2 phonon branch is related to Fermi-surface nesting, our results reveal that the respective anomalies are absent in Co-Ni-Ga, in the phonon dispersions as well as in the electronic structure. Keywords Materials Science and Engineering Disciplines Condensed Matter Physics | Materials Science and Engineering CommentsThis article is from Physical Review B 82 (2010) In addition to the prototypical Ni-Mn-based Heusler alloys, the Co-Ni-Ga systems have recently been suggested as another prospective materials class for magnetic shape-memory applications. We provide a characterization of the dynamical properties of this material and their relation to the electronic structure within a combined experimental and theoretical approach. This relies on inelastic neutron scattering to obtain the phonon dispersion while first-principles calculations provide the link between dynamical properties and electronic structure. In contrast to Ni 2 MnGa, where the softening of the TA 2 phonon branch is related to Fermisurface nesting, our results reveal that the respective anomalies are absent in Co-Ni-Ga, in the phonon dispersions as well as in the electronic structure.

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Section: B Phonon-dispersion Relationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Electronic structure and lattice dynamics of the magnetic shape-memory alloyCo2NiGa

et al. 2010

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“…Different behaviors have been reported in the literature. For some samples [9, 11,12] there exists evidence for a true phase transition at Tj of very weak first-order which is driven by a magnetoelastic coupling. The main proof of that is the fact that 7/ shifts with the external applied field [9] while no (significant) magnetic field dependence has been found for the TM temperature [13,15].…”

Section: Introductionmentioning

confidence: 99%

“…For instance in the sample studied by Stuhr et al [14], T m w 364K and T M « 284K with precursor signs for T > TM-, but no intermediate phase was found. In the sample studied by Zheludev et al [11,12] In this paper we present a microscopic theoretical model aimed to investigate the role of the magnetic coupling in Ni-Mn-Ga. The model has been solved by means of both Mean-Field (MF) and Monte Carlo (MC) techniques and we have obtained that both methods render the same qualitative agreement.…”

Section: Introductionmentioning

confidence: 99%

“…At the MT the structure of the system changes from a cubic phase (which can be viewed as a bec when neglecting the atomic order) to a low temperature tetragonal phase bet, which is modulated by a five-plane shuffle strain [7,8]. Particularly intriguing is that for nearly stoichiometric composition the full MT may be preceded by an intermediate phase transition (IT) at T/ in which apparently only the shuffle strain is activated, but not the tetragonal strain(the cubic symmetry is preserved) [9,10,11,12]. This is accompanied by a significant, although not complete, softening of the TA2 phonon branch at a wave vector fo = 0.33.…”

Section: Introductionmentioning

confidence: 99%

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Mean field and Monte Carlo simulation studies of premartensitic effects in Ni₂MnGa

2001

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We have extended the degenerate BEG model [l] to include magnetic degrees of freedom in order to study the premartensitic effects in Ni 2 MnGa. The model is solved by using mean-field theory and Monte Carlo techniques [2]. The numerical simulations reveal the crucial importance of fluctuations in pretransitional effects. Moreover, we find that a large variety of premartensitic effects may appear due to the magnetoelastic coupling. For large values of the coupling parameter a first-order transition line ending in a critical point appears. This critical point is responsible for the existence of large premartensitic fluctuations which manifest as broad peaks in the specific heat, not always associated with a true phase transition. The main conclusion is that premartensitic effects result from the interplay between the softness of the anomalous phonon driving the modulation and the magnetoelastic coupling strength. In particular, the premartensitic transition occurs when such coupling is strong enough to prevent a complete softening of the involved phonon mode. The implication of the results in relation to the available experimental data is discussed.

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On Order-Disorder (L21 ? B2?) Phase Transition in Ni2+xMn1?xGa Heusler Alloys

Ховайло

Takagi

Васильев

et al. 2001

phys. stat. sol. (a)

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Subject classification: 64.60.Cn; S1.2The ferromagnetic Ni 2þx Mn 1Àx Ga Heusler alloys are of considerable interest due to their potential applicability as magnetically driven shape memory materials [1]. For the stoichiometric composition of Ni 2 MnGa the melting temperature is 1382 K [2]. At cooling from the liquid phase these triple alloys usually solidify in the disordered A2 phase characterized by an arbitrary occupation of every site in the crystal lattice. In principle, the chemical ordering in the solid state of Heusler alloys is possible either through an intermediate partially ordered B2 0 phase or directly to the completely ordered body-centered cubic L2 1 phase [3]. In the B2 0 phase Ni atoms order while Mn and Ga atoms occupy their sites in the crystal lattice randomly. However, the neutron diffraction measurements as well as the differential thermal analysis [2] of the Ni-Mn-Ga system presented no clear evidence for the presence of the A2-B2 0 phase transition, meaning presumably that these alloys solidify in the partially ordered B2 0 phase. For the stoichiometric composition of Ni 2 MnGa the L2 1 ! B2 0 phase transition temperature is 1071 K.At further cooling, the Ni 2þx Mn 1Àx Ga Heusler alloys undergo a structural phase transition from the body-centered cubic phase to the body-centered tetragonal (c=a ¼ 0:94) martensitic phase, characterized by pronounced effects of shape memory and superelasticity [4][5][6]. For the stoichiometric Ni 2 MnGa composition the martensitic transition temperature is equal to 202 K. This transition is preceded by a premartensitic transition at 260 K, which is the formation of static displacement waves in the lattice with the wavevector [1/3, 1/3, 0] (Refs. [7,8]). The deviation from stoichiometry in Ni 2þx Mn 1Àx Ga alloys results in merging of the premartensitic and martensitic transitions, so that the tetragonal phase appears to be modulated by static displacement waves.While being in cubic L2 1 phase, Ni 2 MnGa exhibits a ferromagnetic phase transition with the Curie temperature T C ¼ 376 K. The change of composition in the Ni 2þx Mn 1Àx Ga system results in a decrease of Curie temperature and an increase of the martensitic transition temperature until they merge at x ¼ 0:18-0.20 [9]. While the low temperature phase transitions in the Ni 2þx Mn 1Àx Ga system were studied in many aspects, the high temperature L2 1 ! B2 0 phase transition needs further investigation. In the present work we studied this transition in Ni 2þx Mn 1Àx Ga (x ¼ 0:16-0.20) by means of differential scanning calorimetry (DSC) measurements.The Ni 2þx Mn 1Àx Ga polycrystalline ingots were prepared by a conventional arc-melting method under argon atmosphere. The ingots were annealed at 1100 K for nine days in quartz ampoules and quenched in ice water. Samples for the measurements were cut from the middle part of the ingots. The measurements were done by a NETZSCH-404 high temperature differential scanning calorimeter in the temperature range from 750 to 1200 K.The results of DSC measurements for...

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Precursor effects and premartensitic transformation inNi2MnGa

Cited by 205 publications

References 34 publications

Electronic structure and lattice dynamics of the magnetic shape-memory alloyCo2NiGa

Electronic structure and lattice dynamics of the magnetic shape-memory alloyCo2NiGa

Mean field and Monte Carlo simulation studies of premartensitic effects in Ni₂MnGa

On Order-Disorder (L21 ? B2?) Phase Transition in Ni2+xMn1?xGa Heusler Alloys

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Precursor effects and premartensitic transformation inNi2MnGa

Cited by 205 publications

References 34 publications

Electronic structure and lattice dynamics of the magnetic shape-memory alloyCo2NiGa

Electronic structure and lattice dynamics of the magnetic shape-memory alloyCo2NiGa

Mean field and Monte Carlo simulation studies of premartensitic effects in Ni2MnGa

On Order-Disorder (L21 ? B2?) Phase Transition in Ni2+xMn1?xGa Heusler Alloys

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Mean field and Monte Carlo simulation studies of premartensitic effects in Ni₂MnGa