Magnetic Shape Memory Heusler Alloys Research Articles

Nano Scaled Checkerboards: A Long Range Ordering in NiCoMnAl Magnetic Shape Memory Alloy Thin Films with Martensitic Intercalations

Magnetic shape memory Heusler alloys, such as NiCoMnAl, are considered as promising candidates for magnetocaloric cooling applications. Grown in thin film systems of adjacent layers with austenite and martensite crystal structures of almost equal thicknesses, a long-range ordering phenomenon in the shape of a 3D checkerboard pattern occurs in NiCoMnAl samples. The crystallographic origin of the pattern is proven by transmission electron microscopy (TEM) techniques. The darker fields of the arrangement consist of martensite nuclei superposed with austenite, while the purely austenite regions appear bright in TEM cross sections. The nucleation process is presumably triggered by inhomogeneous local elastic stray fields of primary martensitic nuclei in the austenite matrix and limited by the thicknesses of the martensite and austenite thin films.

Improved crystallographic compatibility and magnetocaloric reversibility in Pt substituted Ni2Mn1.4In0.6 magnetic shape memory Heusler alloy

We present here the improved crystallographic/geometric compatibility and magnetocaloric reversibility by measurement of magnetic entropy change using different protocols in 10% Pt substituted Ni2Mn1.4In0.6 magnetic shape memory alloy. The substitution of Pt reduces the thermal hysteresis about 50% to the Ni2Mn1.4In0.6. The origin of the reduced thermal hysteresis is investigated by the crystallographic compatibility of the austenite and martensite phases. The calculated middle eigenvalue of the transformation matrix turned out to be 0.9982, which is very close to 1 (deviation is only 0.18%) suggests for the crystallographic compatibility between the austenite and martensite phases in Ni1.9Pt0.1Mn1.4In0.6. A very small thermal hysteresis and crystallographic compatibility between two phases in this alloy system indicate a stress-free transition layer (i.e. perfect habit plane) between the austenite and martensite phase, which is expected to give reversible martensite phase transition and therefore reversible magnetocaloric effect (MCE) as well. The calculated value of the isothermal entropy change (ΔSiso) using the magnetization curve under three different measurement protocols (i.e. isothermal, loop, and isofield measurement protocol) is found to be nearly same indicating a reversible MCE in the present alloy system. Our work provides a path to design new magnetic shape memory Heusler alloys for magnetic refrigeration and also suggest that any of the above measurement protocol can be used for the calculation of ΔSiso for materials satisfying geometrical compatibility condition.

Effect of chemical and hydrostatic pressure on the coupled magnetostructural transition of Ni-Mn-In Heusler alloys

Ni-Mn-In magnetic shape-memory Heusler alloys exhibit generally a large thermal hysteresis at their first-order martensitic phase transition which hinder a technological application in magnetic refrigeration. By optimizing the Cu content in Ni$_2$Cu$_x$Mn$_{1.4-x}$In$_{0.6}$, we obtained a thermal hysteresis of the martensitic phase transition in Ni$_{2}$Cu$_{0.2}$Mn$_{1.2}$In$_{0.6}$ of only 6 K. We can explain this very small hysteresis by an almost perfect habit plane at the interface of martensite and austenite phases. Application of hydrostatic pressure does not reduce the hysteresis further, but shifts the martensitic transition close to room temperature. The isothermal entropy change does not depend on warming or cooling protocols and is pressure independent. Experiments in pulsed-magnetic fields on Ni$_{2}$Cu$_{0.2}$Mn$_{1.2}$In$_{0.6}$ find a reversible magnetocaloric effect with a maximum adiabatic temperature change of -13 K.

Improved magnetostructural and magnetocaloric reversibility in magnetic Ni-Mn-In shape-memory Heusler alloy by optimizing the geometric compatibility condition

We report an improved reversibility of magnetostriction and inverse magnetocaloric effect (MCE) for the magnetic shape-memory Heusler alloy Ni$_{1.8}$Mn$_{1.8}$In$_{0.4}$. We show that the magnetostriction and MCE crucially depends on the geometrical compatibility of the austenite and martensite phases. Detailed information on the compatibility of both phases has been obtained from the transformation matrix calculated from x-ray diffraction data. The uniqueness of the lattice parameters results in an improved reversibility of the magnetostriction and the MCE. In the thermal hysteresis region of the martensitic transformation, the maximum relative length change is 0.3% and the adiabatic temperature change $\Delta T_{ad}\approx -10$ K in pulsed magnetic fields. Our results reveal that the approach of geometric compatibility will allow one to design materials with reversible magnetostriction and reversible inverse MCE at a first-order magnetostructural phase transition in shape-memory Heusler alloys.

Effect of substitution on elastic stability, electronic structure and magnetic property of Ni–Mn based Heusler alloys: An ab initio comparison

First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson’s ratio and Young’s modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt’s and Reuss’s method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively.

Structural and magnetic dynamics in the magnetic shape-memory alloyNi2MnGa

Magnetic shape memory Heusler alloys are multiferroics stabilized by the correlations between electronic, magnetic and structural order. To study these correlations we use time resolved x-ray diffraction and magneto-optical Kerr effect experiments to measure the laser induced dynamics in a Heusler alloy Ni$_2$MnGa film and reveal a set of timescales intrinsic to the system. We observe a coherent phonon which we identify as the amplitudon of the modulated structure and an ultrafast phase transition leading to a quenching of the incommensurate modulation within 300~fs with a recovery time of a few ps. The thermally driven martensitic transition to the high temperature cubic phase proceeds via nucleation within a few ps and domain growth limited by the speed of sound. The demagnetization time is 320~fs, which is comparable to the quenching of the structural modulation.

A First‐Principles Investigation of the Compositional Dependent Properties of Magnetic Shape Memory Heusler Alloys

AbstractThe interplay of structural and magnetic properties of magnetic shape memory alloys is closely related to their composition. In this study the influence of the valence electron concentration on the tetragonal transformation in Ni2Mn1 + xZ1 − x (Z = Ga, In, Sn, Sb) and Co2Ni1 + xGa1 − x is investigated by means of ab initio calculations. While the type of magnetic interaction is different for the two series, the trends of the total energy changes under a tetragonal transformation are very similar. We find that tetragonal structures become energetically preferred with respect to the cubic one as the valence electron concentration e/a is increased regardless of the system under consideration. In particular, the energy difference between the austenite and martensite structures increases linearly with e/a, which is in part responsible for the linear increase of the matensite transformation temperature. The substitution of nickel by platinum increases even further the transformation temperature.

Designing shape-memory Heusler alloys from first-principles

The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z = Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14%.

Temperature and field induced strain in polycrystalline Ni 50Mn 35In 15− xSi x magnetic shape memory Heusler alloys

Magnetic shape memory properties of polycrystalline Ni 50Mn 35In 15− x Si x were investigated. A reversible strain of more than 0.4% was observed for x = 0 at a magnetic field H = 5 T that was found to be associated with a field induced reverse martensitic transformation. The strains were found to increase with the substitution of In by Si and strains larger than 1% were observed for x = 2 at H = 5 T. Both the positive and negative strain changes were observed in the vicinity of martensitic transition temperatures. The strain in Ni 50Mn 35In 15− x Si x was found to depend on silicon concentration, and on samples texture.

Large inverse magnetic entropy changes and magnetoresistance in the vicinity of a field-induced martensitic transformation in Ni50−xCoxMn32−yFeyGa18

Significantly large inverse magnetic entropy changes (ΔSM) and magnetoresistance (MR) were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys: Ni50−xCoxMn32−yFeyGa18. The crystal structures of alloys were tetragonal at 300 K and the phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion. The maximum peak values of the ΔSM and MR at H=5 T were determined as ≈(+)31 J Kg−1 K−1 and ≈−21%, respectively, for x=8 and y=2. The relatively small hysteretic loss and large refrigeration capacity observed in this system make these compounds promising materials for applications.

Ab initio investigation of twin boundary motion in the magnetic shape memory Heusler alloy Ni2MnGa

Magnetic shape memory (MSM) alloys, which transform martensitically below the Curie temperature in the ferromagnetic (FM) state, represent a new class of actuators. In Ni2MnGa, unusually large magnetic field-induced strains of about 10% have been observed. This effect is related to a high mobility of martensitic twin boundaries in connection with a large magneto-crystalline anisotropy. MSM materials exist in a variety of different martensitic structures depending on temperature and compositions. We investigate the energetics of L10 phase twin boundary motion quasi-statically with ab initio methods and relate the results to calculations of the magneto-crystalline anisotropy energy. Our results indicate that for the L10 structure the energy needed for a coherent shift of a twin boundary may be too large to be overcome solely by magnetic field-induced strains.

A first-principles study of the martensitic instabilities in magnetic shape memory alloys

AbstractAmong the magnetic shape memory Heusler alloys, Ni-Mn-Ga near stoichiometry displays the largest shape change in the martensitic 5M or 7M structure with a strain of the order of 10% in an external magnetic field of less than one Tesla. In addition, the alloys exhibit a sequence of intermediate martensites with the modulated structures usually appearing at c/a < 1 while the low-temperature nonmodulated tetragonal structures have c/a > 1. Typically, the martensitic phase changes are accompanied by a shift of a peak in the electronic density arising from the non-bonding Ni states, a reconstruction of the associated Fermi surface, and, in some cases, by pronounced phonon anomalies. These appear in the cubic high-temperature austenitic and premartensitic phases but also in the modulated phases. In addition, the modulated phases have highly mobile twins which can be rearranged under the action of an external magnetic field due to the high magnetic anisotropy, which builds up in martensite and which is at the origin of the magnetic shape memory effect. First-principles calculations confirm the overall scenario.

希少・有害元素代替のための3d多元系機能材料の開発研究

Research and development of 3d multinary alloys and compounds for functional materials in terms of substitution of materials containing rare and toxic elements have been carried out extensively. Fe2VAl Heusler alloy and its related alloys exhibit a large Seebeck coefficient, therefore, these alloys have a potential as a substitute of the Bi-Te system for room-temperature thermoelectric materials. Furthermore, Pb-Zn-Te electrostrictive and TbFe2-based magnetostrictive materials are probably replaced with Ni-based magnetic shape memory Heusler alloys, which exhibit a large magnetic-field induced strain. In addition, the control of a large negative thermal expansion of Mn3XN (X=Zn, Ga)anti-Perovskite compounds by partial substitution is useful way to overcome a usage of expensive W element and its compounds. Finally, we focus on magnetic refrigerants which are keenly desired in utilization of magnetic refrigeration. The existing candidates for magnetic refrigerants are limited to compounds containing elements such as As, Gd and Sb. On the other hand, La(FexSi1-x)13 compounds offer advantages on account of environmental safety, low-prices, and a large magnetocaloric effects. To utilize these 3d-based functional materials by taking advantage of the ability to control the electronic state in multinary system, the material designs based on electronic structures and supports of computer-aided phase diagrams are highly useful.

Ab initio modeling of martensitic transformations (MT) in magnetic shape memory alloys

The magnetic field-induced (MFI) changes of the electronic, magnetic and phonon properties of magnetic shape memory Heusler alloys Ni–Mn–Ga and Ni–Mn–In are discussed on the basis of first-principles calculations. Without an external magnetic field, the lattice instability to shear wave propagation in [1 1 0] direction shows up in the calculation as softening of the TA 2 ζ [1 1 0] ( 2 π / a ) mode at ζ ≈ 1 3 . The ab initio modeling of magnetic field effects shows that the phase (austenite or martensite) is favored which has larger magnetization. This is in agreement with experiments on Ni–Mn–In, for which the martensitic transformation (MT) can be shifted by 40 K to lower temperatures in an external field of 5 T.

Modelling the phase diagram of magnetic shape memory Heusler alloys

We have modelled the phase diagram of magnetic shape memory alloys of the Heusler type by using the phenomenological Ginzburg–Landau theory. When fixing the parameters by realistic values taken from experiment we are able to reproduce most details of, for example, the phase diagram of Ni2+xMn1−xGa in the (T, x) plane. We present the results of ab initio calculations of the electronic and phonon properties of several ferromagnetic Heusler alloys, which allow one to characterize the structural changes associated with the martensitic instability leading to the modulated and tetragonal phases. From the ab initio investigations emerges a complex pattern of the interplay of magic valence electron per atom numbers (Hume–Rothery rules for magnetic ternary alloys), Fermi surface nesting and phonon instability. As the main result, we find that the driving force for structural transformations is considerably enhanced by the extremely low lying optical modes of Ni in the Ni-based Heusler alloys, which interfere with the acoustical modes enhancing phonon softening of the TA2 mode. In contrast, the ferromagnetic Co-based Heusler alloys show no tendency for phonon softening.