Heusler Alloy Ni2MnGa Research Articles

Deformation of hierarchically twinned martensite

Shape-memory alloys deform via the reorganization of a hierarchically twinned microstructure. Twin boundaries themselves present obstacles for twin boundary motion. In spite of a high density of obstacles, twinning stresses of Ni–Mn–Ga Heusler alloys are very low. Neither atomistic nor dislocation-based models account for such low yield stresses. Twinning mechanisms are studied here on a mesoscopic length scale making use of the disclination theory. In a first approach, a strictly periodic twin pattern containing periodic disclination walls with optimally screened stress fields is considered. Strict periodicity implies that the twin microstructure reorganizes homogeneously. In a second approach, a discontinuity of the fraction of secondary twins is introduced and modeled as a disclination dipole. The stress required for nucleation of this discontinuity is larger than the stress required for homogeneous reorganization. However, once the dipole is formed, it can move under a much smaller stress in agreement with experimental findings.

Magnetic and Magneto-Transport Properties of Nickel-Rich Ni–Mn–Ga Heusler Alloys

Electrical resistivity, ρ of nickel-rich Ni2+xMn1-xGa has been measured up to a magnetic field of 8 T and within a temperature range from 4.2 to 300 K. At lower temperatures (4.2–70 K), thermal variation of ρ(T) follows a combination of -√T dependent term coming from electron–electron interaction and BT2 term due to s–d interaction. At higher temperature region (up to ∼280 K), ρ(T) is interpreted in terms of electron–phonon and s–d interaction. A substantial jump in resistivity has been observed while the martensitic transformation takes place. A high negative magnetoresistance (∼8%) has been observed at room temperature due to a change of 8 T magnetic field for the sample Ni2.208Mn0.737Ga, where martensite–austenite structural and ferro–para magnetic transition temperatures almost coincide with each other. Magnetic properties of those samples have also been studied to understand their anomalous change in magnetic entropy and magnetoresistance.

Large negative magnetoresistance in nickel-rich Ni–Mn–Ga Heusler alloys

Electrical resistivity ρ and magnetoresistance of ferromagnetic Heusler alloys Ni2+xMn1−xGa have been measured up to a magnetic field of 8 T and within a temperature range from 4.2 to 300 K. A substantial jump in resistivity has been observed while the martensitic transformation takes place. A large negative magnetoresistance (∼8%) was obtained at room temperature due to a change of 8 T magnetic field for the sample Ni2.208Mn0.737Ga, where martensite-austenite structural and ferro-para magnetic transition temperatures almost coincide with each other.

Martensitic transformation and microstructure of polycrystalline Ni–Mn–Ga and C-doped Ni–Mn–Ga Heusler alloys

The martensitic transformations of Ni–21·7Mn–23·8Ga (at.-%) (NiMnGa) and Ni–19·4Mn–22·7Ga–1·6C (at.-%) (NiMnGaC) alloys were investigated by the measurement of resistivity. Two kinds of martensitic transformations occur in NiMnGa alloy. The first martensitic transformation is thermoelastic, which exhibits a steep increasing in resistivity. The second transformation exhibits a larger thermal hysteresis compared with the first transformation. NiMnGaC alloy only shows a single martensitic transformation and the C addition increases the first martensitic transformation temperatures. The first martensitic phase of NiMnGa alloy is of five layered structure while the martensitic phase of NiMnGaC alloy is of non-modulated structure. Combined with the observation of optical microscopy and TEM, NiMnGa alloy exhibits much wider martensite twins than NiMnGaC alloy does.

Effect of pressure on the magnetocaloric properties of nickel-rich Ni–Mn–Ga Heusler alloys

Nickel-rich Ni–Mn–Ga Heusler alloys were prepared by arc melting and subsequent homogenization by annealing. A large magnetic entropy change was observed around 291 K in the alloy where martensite-austenite structural and ferro-para magnetic transitions almost coincide with each other. The effect of hydrostatic pressure of up to 8 kbar on magneto-structural transitions, magnetocaloric effect, and magnetic hysteresis was studied. The martensitic transition temperature as well as the Curie temperature TC was found to increase, whereas the magnetic entropy change |ΔSM| decreases slightly due to the application of hydrostatic pressure. The large hysteresis observed in M versus H curve at the ambient pressure almost vanishes due to the application of 8 kbar pressure.

Magnetic entropy change of V substituted Ni–Mn–Ga Heusler alloy

The magnetization behaviors have been analyzed for Ni54Mn21−xVxGa25 (x=0,2,4) alloys which were prepared by conventional arc melting method in argon atmosphere. The Curie temperature TC was found to be 325, 300, and 265 K and the austenitic transition temperature TA on heating was found to be 315, 217, and 124 K for x=0, 2, and 4, respectively. The magnetic characteristics were performed with a Quantum Design superconducting quantum interference device magnetometer in the field of up to 20 kOe. A large magnetic entropy change ΔSM, which is calculated from H versus M curves associated with the ferromagnetic-paramagnetic transitions, has been observed. The maximum ΔSM for an applied field of 2.0 T is 2.49, 1.92, and 1.81 J/kg K for x=0, 2, and 4, respectively.

First-principles calculation of elastic and thermodynamic properties of Ni2MnGa Heusler alloy

The equilibrium lattice parameter, heat capacity, thermal expansion coefficient and bulk modulus of Ni2MnGa Heusler alloy are successfully obtained using the first-principles plane-wave pseudopotential (PW-PP) method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus B and temperature T up to 800 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure and decreases with increasing temperature. The pressure dependence of heat capacity Cv and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperature of Ni2MnGa is determined from the non-equilibrium Gibbs function. Our calculated results are in excellent agreement with the experimental data.

Martensite-austenite and Ferromagnetic-paramagnetic Transformations in Ni2-xAxMn1-yByGa1-zCz (A, B=Co; C=Al) Heusler Alloys

Ni2MnGa Heusler alloy is known as a potential smart material. At room temperature it has a L21 type structure, undergoing a martensitic transition (TM) at low temperatures. Some authors have classified Ni-Mn-Ga Heusler alloys on the values of martensitic transformation and Curie temperatures: first group formed by alloys with TM below room temperature and TC, second group with TM around room temperature and third group with TM]TC. The partial substitution of Ni with Mn leads to an increase of the transition temperature and a decrease of the Curie temperature. The Ni2-xMn1+xGa alloys have a complex tetragonal structure at room temperature. The substitution of Ga with Al can change the crystalline and magnetic structure of Heusler alloys: Ni2MnAl is antiferromagnetic for a B2 (CsCl type, with Mn and Al randomly distributed in the center of the cube) structure and ferromagnetic for a L21 (BiF3 type with Mn and Al ordered distributed in the cube center) structure. We intend to put in evidence a dependence between the applied magnetic field on a side, and the transition temperature, on other side, for the Ni2-xAxMn1-yByGa1-zCz (A, B=Co; C=Al) Heusler alloys.

Reflection of elastic waves in a crystal of Heusler alloy Ni2MnGa in the phase transition range

The reflection of longitudinal and transverse acoustic waves from the free surface of the ferromagnetic shape memory alloy Ni2MnGa that is located in the ranges of the premartensite and martensite phase transformations is considered. The propagation directions and amplitudes of the waves reflected in the (001) plane of the crystal are determined. They acquire the character of substantially quasi-longitudinal and quasi-transverse vibrations rather than being pure modes. The angles of wave reflection and conversion are shown to be effectively controlled by temperature and a magnetic field due to the colossal acoustic anisotropy of the crystal over the wide range of its phase transitions. Beginning from a certain critical angle of incidence of a quasi-transverse wave, the quasi-longitudinal wave having appeared upon reflection becomes an accompanying surface vibration, and it can be emitted into the bulk of the crystal when the phase transition point is approached. Two angles of full conversion of an incident quasi-longitudinal wave into a quasi-transverse wave are established, and their temperature dependences are found. Trivisonno’s experimental data for the ultrasound velocity and absorption in an Ni2MnGa crystal are used to numerically estimate these acoustic effects.

The effect of Co-doping on martensitic transformation temperatures in Ni–Mn–Ga Heusleralloys

The effect of Co-doping in non-stoichiometric Ni–Mn–Ga alloys on martensitictransformation temperatures is investigated. Experimental results showthat the effect of Co addition on martensitic transformation temperaturesis remarkable, but the Heusler structure of the alloys remains unchanged. ForNi48Mn27Ga25−xCox (x = 1,2, 3, 4) alloys, the martensitic transformation temperatures increase monotonically withincreasing Co substitution for Ga. The effect can be attributed to the variation ofintrinsic factors, i.e. the decrease in the unit-cell volumes and the increase in theelectronic concentrations and density. The martensitic transformation temperatures ofNi48Mn26−xCoxGa26 (x = 1,2, 3, 5) alloys do not change much with the variation of Co substitution and intrinsicfactors have no effect on the exceptional behaviour, which implies that many factors shouldbe considered simultaneously when designing practical alloys.

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 Charge Compton Profile Study of Ni<sub>2</sub>MnGa: Theory and Experiment

We present the first ever theoretical and experimental charge Compton profiles of Ni2MnGa Heusler alloy. The measurements have been made using magnetic Compton spectrometer at SPring8, Japan. The Compton profiles and energy bands have been computed using Hartree-Fock, density functional theory with local density and generalized gradient approximations. It is seen that the Hartree-Fock based Compton profile is relatively in better agreement with the experimental profiles. In addition, we also report the energy bands, density of states and valence charge densities using full potential linearized augmented plane-wave method.

Large anisotropy of spin polarization in Heusler alloy Ni2MnGa induced by martensitic transformation

Spin polarization both in the cubic austenitic and tetragonal martensitic phases of the Ni2MnGa alloy has been investigated using first-principles calculations combined with classical Bloch–Boltzmann transport theory. It is shown that the degree of spin polarization, while decreasing from 42% in the ⟨001⟩ directions of the austenitic phase to 30% in the [100] direction of the martensitic phase, rises to 75% in the [001] direction of the martensitic phase, resulting from a preferential reconstruction of the spin-down Fermi surfaces upon martensitic transformation. With this finding, various recent intriguing electrical measurements upon Ni2MnGa across the martensitic transformation can find an explanation. This also opens a way of searching for giant magnetoresistance materials.

Phase composition and the mechanical properties of a Heusler alloy Ni2 MnGa alloyed by iron and tungsten

Two series of samples of a Heusler alloy Ni2MnGa have been studied. The samples alloyed with iron (to 1.6 at %) and with tungsten (0.5 at %) are single-phase (L21). Alloying leads to a strengthening of the samples; during the compression tests, values of σ0.2 and σu have been found to grow. The deformation of the samples during compression increases from 2 to 5.5% in the alloy with 1.6 at % Fe and to 6.0% in the alloy with 0.5 at % W. At the concentration of 0.5 at %, the alloying with tungsten strengthens the alloy to a larger degree than alloying with iron. At the concentration 0.8 at %, no complete dissolution of tungsten in Ni2MnGa occurs. Pileups of particles of the second phase are observed, which represents a solid solution on the basis of tungsten. Temperatures of the magnetic and martensitic transformation have been determined.

Solid state reaction at the interface between Heusler alloys and Al cap accelerated by elevated temperature and rough surface

We have investigated the solid state reaction at the interface of Co2Cr0.6Fe0.4Al, Co2FeSi, and Ni2MnGa Heusler alloy films and Al cap layers using x-ray absorption spectroscopy in transmission and total electron yield mode. At elevated temperatures and at rough surfaces the deposited Al severely reacts with the surface of a Heusler alloy indicated by changes of the absorption spectra. Microspectroscopy using photoemission electron microscopy reveals that the reaction proceeds inhomogeneously with reaction nuclei separated on a micron length scale.

Commensurate and incommensurate “5M” modulated crystal structures in Ni–Mn–Ga martensitic phases

It is well known that the composition of ferromagnetic shape memory Ni–Mn–Ga Heusler alloys determines both temperature of martensitic transformations and the structure type of the product phase. In the present work we focused our attention on the structural study of the so-called “5M” modulated structure. In particular, the structure of Ni 1.95Mn 1.19Ga 0.86 martensitic phase is analysed by powder X-ray diffraction (PXRD) and compared with that of the stoichiometric Ni 2MnGa martensite. The study of the diffraction data reveals the occurrence of commensurate (C) structural modulation in Ni 1.95Mn 1.19Ga 0.86; this contrasts with Ni 2MnGa, where an incommensurate (IC) structural modulation was evident. The two phases also differ in the symmetry of the fundamental martensitic lattice. In fact, the incommensurate modulation is related to an orthorhombic basic structure, while the commensurate variant presents a monoclinic symmetry. The commensurate modulated structure has been investigated by using the superspace approach already adopted to solve the structure of Ni 2MnGa martensite. The structure has been determined by Rietveld refinement of PXRD data.

The phase diagram of Ni–Mn–Ga alloys with account of crystal lattice modulation and external magnetic field

The phase diagram of Ni–Mn–Ga Heusler alloys is investigated theoretically with the help of Ginzburg–Landau theory including magnetic, elastic and crystal lattice modulation order parameters as well as an external magnetic field. The influence of the parameters of the Ginzburg–Landau functional on the phase diagrams is discussed in detail. It is shown that the volume and anisotropic magnetostriction have significant influence on the phase diagrams of Ni–Mn–Ga alloys. For an appropriated combination of phenomenological parameters, the sequence of the phase transitions is in good agreement with the experimental ones. The condiditons for the existence of a critical point in the H– T diagram are discussed.

The way composition affects martensitic transformation temperatures of Ni–Mn–Ga Heusler alloys

AbstractA systematic substitution of Ge, Si, C and Co for Ga in the non‐stoichiometric Ni–Mn–Ga alloys was performed. The relationship between the compositions of different elements including Ni, Mn, Ga, Ge, Si, C, Co, In and martensitic transformation temperature (Ms) was studied in detail for the present alloys together with data collected from a variety of sources. It is found that Ms is a sensitive parameter to the composition. The size factor and electron concentration are usually thought to be the way the composition influences Ms in the Ni–Mn–Ga alloys. Here, analyzing by linear regression, the electron density maybe a most appropriate parameter to describe the way the composition influences Ms when compared with size factor and electron concentration. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic susceptibility anomaly associated with premartensitic transition in Heusler alloy

Magnetic susceptibility anomaly during the premartensitic transformation in Heusler alloy Ni 2MnGa is an important physical property and has been investigated in the present work by using Green's function technique based on the magnon–TA phonon interaction model. The results of the numerical calculation shows that this kind of anomalous effect is wavevector- and magnetic-field dependent.

Phase transitions and magnetic entropy change in Mn-rich Ni2MnGa alloys

Thermomagnetic and magnetocaloric (MC) properties of Ni2MnGa Heusler alloys are found to be very sensitive to small composition variations. It is shown that the temperature behavior of ac susceptibility mimics that of MC properties of the samples. The coincidence of magnetic and structural transformations is found for a Mn-rich composition. As in the case of Ni-rich alloys, the magnetocaloric effect is enhanced at the transitions co-occurrence. The increase of Mn content, with respect to the stoichiometric value, rapidly decreases anisotropy field and saturation magnetization.