Effect of temperature and compositional changes on the phonon properties of Ni-Mn-Ga shape memory alloys

Ener, Semih; Neuhaus, J.; Petry, W.; Mole, Richard A.; Hradil, K.; Siewert, Mario; Gruner, Markus E.; Entel, P.; Titov, Ivan; Acet, Mehmet

doi:10.1103/physrevb.86.144305

Cited by 24 publications

(27 citation statements)

References 47 publications

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“…The softening is strongest for Ni–Mn–Ga and depends on composition (i.e., the valence electron number

e / a

); compare, for example, the phonon dip anomaly for samples with slightly different compositions,

{Ni}_{52}

{Mn}_{23}

{Ga}_{25}

and

{Ni}_{50}

{Mn}_{29.5}

{Ga}_{20.0}

. Shift of the dip anomaly in Ni–Mn–Ga alloys to lower

false| q false|

has recently been discussed again for Mn‐rich Ni–Mn–Ga alloys (). It has been argued that in the ‘low’

e / a

crystals (around

e / a = 7.5

corresponding to stoichiometric

{Ni}_{2}

MnGa) the condensation of phonons leads to modulated phases because of magnetoelastic coupling with a first‐order transition to the intermediate martensitic phase.…”

Section: Phase Diagram and Strain‐glass Phase Of Ti50 Ni50−x Fexmentioning

confidence: 88%

Interacting magnetic cluster‐spin glasses and strain glasses in Ni–Mn based Heusler structured intermetallics

Entel

Gruner

Comtesse

et al. 2014

Physica Status Solidi (b)

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Magnetic Ni–Mn based Heusler intermetallics show complex magnetic behavior in connection with martensitic transformations (see, for instance, the phase diagram of Ni–Co–Mn–Sn on the right‐hand side). The cubic austenitic phase at high temperature shows long‐range ferromagnetic order which can considerably be weakened by the appearance of competing antiferromagnetic interactions which are induced by Mn excess and chemical disorder. With decreasing temperature a martensitic/magnetostructural transformation takes place from cubic to non‐modulated/modulated tetragonal/monoclinic or orthorhombic structure, where long‐range ferromagnetic order can no longer be maintained, leading to superparamagnetic behavior. At still lower temperatures superparamagnetism changes to superspin glass because of strong competition of ferromagnetic and antiferromagnetic interactions and chemical disorder. In addition, disorder and local structural distortions can lead to strain glass in austenite, as observed for some non‐magnetic martensitic systems. The magnetic intermetallics are of technological importance in view of their functional properties involving magnetic shape‐memory and exchange‐bias effects as well as magnetocaloric effects. The ‘ferroic cooling’ is of particular relevance since it avoids the use of ozone‐depleting and greenhouse chemicals compared with conventional fluid‐compression technology. Experimental phase diagram of Ni50−x Cox Mn39 Sn11 for 0≤x≤10. Here, TC is the Curie temperature of austenite; at TM the system transforms to paramagnetic martensite and at TS to superparamagnetic martensite (SPM) and then to superspin‐glass martensite (SSG) at TP. The possible strain‐glass phases (labeled by question marks) are predicted because of kinetic arrest phenomena and local distortions associated with the magnetostructural transition and ergodicity breaking by field‐cooling/zero‐field‐cooling experiments.

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“…The softening is strongest for Ni–Mn–Ga and depends on composition (i.e., the valence electron number

e / a

); compare, for example, the phonon dip anomaly for samples with slightly different compositions,

{Ni}_{52}

{Mn}_{23}

{Ga}_{25}

and

{Ni}_{50}

{Mn}_{29.5}

{Ga}_{20.0}

. Shift of the dip anomaly in Ni–Mn–Ga alloys to lower

false| q false|

has recently been discussed again for Mn‐rich Ni–Mn–Ga alloys (). It has been argued that in the ‘low’

e / a

crystals (around

e / a = 7.5

corresponding to stoichiometric

{Ni}_{2}

MnGa) the condensation of phonons leads to modulated phases because of magnetoelastic coupling with a first‐order transition to the intermediate martensitic phase.…”

Section: Phase Diagram and Strain‐glass Phase Of Ti50 Ni50−x Fexmentioning

confidence: 88%

Interacting magnetic cluster‐spin glasses and strain glasses in Ni–Mn based Heusler structured intermetallics

Entel

Gruner

Comtesse

et al. 2014

Physica Status Solidi (b)

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“…Both properties may change at a magnetostructural transition. This is expressed quantitatively in terms of the vibrational density of states (VDOS) g ( ϵ ), which can be measured by inelastic neutron scattering or nuclear resonant inelastic X‐ray scattering . VDOS and phonon dispersion relations have been calculated from first principles achieving excellent agreement with experimental data .…”

Section: Disentangling the Microscopic Contributions To The Entropy Cmentioning

confidence: 89%

“…This is expressed quantitatively in terms of the vibrational density of states (VDOS) g ( ϵ ), which can be measured by inelastic neutron scattering or nuclear resonant inelastic X‐ray scattering . VDOS and phonon dispersion relations have been calculated from first principles achieving excellent agreement with experimental data . The expression for S lat resembles S el , where the expression in the parentheses takes into account the Bosonic character of the phonons through the Bose–Einstein distribution function n ( ϵ , T )=(exp( ϵ / k B T )−1) −1 for the occupation numbers, which becomes large for high T or small phonon energies ϵ :

true S_{normall normala normalt} = 3 k_{B} \int_{0}^{\infty} g ((), ϵ) [(1 + n (() ϵ, T)) l n (1 + n (() ϵ, T)) - n (ϵ, T) l n (n (() ϵ, T))] normald ϵ

…”

Section: Disentangling the Microscopic Contributions To The Entropy Cmentioning

confidence: 94%

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

et al. 2018

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Magnetic refrigeration relies on a substantial entropy change in a magnetocaloric material when a magnetic field is applied. Such entropy changes are present at first‐order magnetostructural transitions around a specific temperature at which the applied magnetic field induces a magnetostructural phase transition and causes a conventional or inverse magnetocaloric effect (MCE). First‐order magnetostructural transitions show large effects, but involve transitional hysteresis, which is a loss source that hinders the reversibility of the adiabatic temperature change ΔTad. However, reversibility is required for the efficient operation of the heat pump. Thus, it is the mastering of that hysteresis that is the key challenge to advance magnetocaloric materials. We review the origin of the large MCE and of the hysteresis in the most promising first‐order magnetocaloric materials such as Ni–Mn‐based Heusler alloys, FeRh, La(FeSi)13‐based compounds, Mn3GaC antiperovskites, and Fe2P compounds. We discuss the microscopic contributions of the entropy change, the magnetic interactions, the effect of hysteresis on the reversible MCE, and the size‐ and time‐dependence of the MCE at magnetostructural transitions.

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“…Ab initio theory has become a sucessful and predictive tool for the description of magnetic shape memory alloys on the atomic scale—in particular for the prototype system Ni 2 MnGa. Regarding the close agreement to experiment with respect to lattice structure and magnetism , electronic structure , phonons , and thermodynamics , it appears natural to address the question about the origin of the complex phase sequence in MSM alloys in the framework of atomistic density functional theory calculations. In this work, we will put special emphasis on the interpretation of the intermediate MSM phases as adaptive microstructures.…”

Section: Introductionmentioning

confidence: 99%

Magnetoelastic coupling and the formation of adaptive martensite in magnetic shape memory alloys

Gruner

Fähler

Entel

2014

Physica Status Solidi (b)

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Reviewing the results of recent first‐principles calculations, we work out a close analogy between the two paradigmatic classes of magnetic shape memory materials, the ordered Ni2 MnGa Heusler compound and the disordered Fe70 Pd30 alloy. Despite fundamental differences between both systems, we can demonstrate that in both cases the very low formation energy for tetragonal twins on the smallest length scale opens an alternative transformation path into an adaptive hierarchical microstructure which is important for the functional behavior. The low energy of the (101) twin boundary corresponds to a shear instability which is associated to the soft transversal acoustic phonon in both systems. In turn, changes of the energy landscape upon magnetic disorder are responsible for the stability of austenite. This points out the strong influence of magnetoelastic coupling on the transformation process. Nanotwinned adaptive microstructures in Ni2 MnGa and Fe68 Pd32 magnetic shape memory alloys obtained from first‐principles calculations.

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Effect of temperature and compositional changes on the phonon properties of Ni-Mn-Ga shape memory alloys

Cited by 24 publications

References 47 publications

Interacting magnetic cluster‐spin glasses and strain glasses in Ni–Mn based Heusler structured intermetallics

Interacting magnetic cluster‐spin glasses and strain glasses in Ni–Mn based Heusler structured intermetallics

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

Magnetoelastic coupling and the formation of adaptive martensite in magnetic shape memory alloys

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