Premartensitic Phase Research Articles

Elasticity and magnetism of Ni2 MnGa premartensitic tweed

Magnetic, magneto‐elastic and elastic measurements were used to characterize the thermal evolution of the premartensite phase of Ni2 MnGa. The premartensitic transition is shown to correspond to a sharp maximum of magnetostriction and a pronounced minimum of the (110) shear stiffness (the elastic coefficient); no additional softening of the coefficient prior to the martensitic transformation was observed. In contrast, a significant softening of the (100) stiffness (the elastic coefficient) was observed in the vicinity of the martensitic transition, while this coefficient is fully unaffected by the premartensitic transition. A simple two‐dimensional model of the tweed is presented to explain this mutual independence between the and shears and the effect of tweed formation on both of them.

The incommensurate modulations of stoichiometric Ni2MnGa

We use X-ray diffraction to confirm that the 5M phase of the stoichiometric Heusler alloy Ni2MnGa has an incommensurate modulation with wavevector q=[0.4280.4280]. We find the crystal to be macroscopically twinned with a structure well described by the theory of martensitic phase transitions, though theoretical calculations of the X-ray diffraction rule out the existence of an adaptive phase. The premartensite phase is found to be slightly incommensurate with q=[0.3410.3410].

(3 + 1)D superspace description of the incommensurate modulation in the premartensite phase of Ni2MnGa: a high resolution synchrotron x-ray powder diffraction study

Le Bail and Rietveld analysis of high resolution synchrotron x-ray powder diffraction (SXRPD) data shows unambiguous signatures of the failure of the commensurate 3M modulation model. Using (3 + 1) dimensional superspace group formalism, we have not only confirmed the incommensurate modulation in the premartensite phase with a modulation wavevector of q = 0.337 61(5)c* but also determined the superspace group (Immm(00γ)s00), atomic positions and amplitude of modulations for the incommensurate premartensite phase of Ni2MnGa for the first time. Our results may have important implications in the understanding of the martensitic transition and hence the magnetic field induced strains.

Coherent phonon dynamics at the martensitic phase transition of Ni2MnGa

We use time-resolved optical reflectivity to study the laser stimulated dynamics in the magnetic shape memory alloy Ni2MnGa. We observe two coherent optical phonons, at 1.2 THz in the martensite phase and at 0.7 THz in the pre-martensite phase, which we interpret as a zone-folded acoustic phonon and a heavily damped amplitudon, respectively. In the martensite phase the martensitic phase transition can be induced by a fs laser pulse on a timescale of a few ps.

Martensitic transition during Ni growth on Fe(001): evidence of a precursor phase

We report evidence that the body-centered cubic (bcc)–face-centered cubic (fcc) transition that occurs during Ni film growth on a Fe(001) substrate is preceded by a pre-martensitic phase, as demonstrated by low-energy electron diffraction. The corresponding film superstructure is characterized by a displacement of Ni atoms along the main 〈100〉 crystallographic axes of iron, without any rotation of the unit cell with respect to the (001) plane, in contrast with the martensitic transition that shows four fcc Ni domains with the Ni〈211〉 crystallographic directions aligned with the Fe〈110〉 axes. In addition, the martensitic transition is detected not at 6 ML, as previously believed, but above 20 ML if the Ni sample is rigorously kept at room temperature. The surface morphology of the bcc–fcc transition is characterized by the development of Ni mounds oriented along the 〈110〉 directions, as shown by scanning tunneling microscopy.

Phase diagram of the ferromagnetic shape memory alloys Ni2MnGa1−xCox

Permeability, magnetization, and differential scanning calorimetry measurements are carried out on the ferromagnetic shape memory alloys Ni${}_{2}$MnGa${}_{1\ensuremath{-}x}$Co${}_{x}$ (0 \ensuremath{\le}$\phantom{\rule{0.28em}{0ex}}x$ \ensuremath{\le} 0.20). On the basis of the experimental results, the phase diagram in the temperature-concentration plane is determined. The determined phase diagram is spanned by a paramagnetic austenite (Para-A) phase, paramagnetic martensite phase, ferromagnetic austenite phase, ferromagnetic martensite (Ferro-M) phase, and a premartensite phase. The measurements show that a magnetostructural transition between the Para-A and Ferro-M phases does not appear in this alloy system. The phase diagram is discussed using phenomenological Landau theory.

Coexistence of charge-density wave and ferromagnetism in Ni2MnGa

We demonstrate the existence of a charge-density-wave (CDW) associated with an incommensurate periodic lattice distortion on Ni${}_{2}$MnGa surface in the ferromagnetic state. Our temperature-dependent photoemission spectra provide compelling evidence of a pseudogap at the Fermi level for ${T}_{\mathrm{CDW}}$ $\ensuremath{\leqslant}$ 270 K that appears at the onset of the premartensite phase and persists in the martensite phase. While the width of the pseudogap is about 25 meV, a spectral weight transfer is observed over a much wider energy range that is associated with the CDW.

Ni2MnGa(100) ferromagnetic shape memory alloy: A surface study

Ni2MnGa(100) single crystal studied using low energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) exhibits interesting modification of the surface properties that are mainly influenced by surface composition as well as intrinsic effects. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. In contrast, a commensurate modulation is observed for Mn-excess Ni–Mn–Ga surface. A pre-martensite phase is identified at the surface. Both the surface martensitic and pre-martensitic transition temperatures decrease as the Mn content increases. The UPS spectra in the austenite phase exhibit systematic change in shape as a function of surface composition that can be related to changes in the hybridization between Ni and Mn 3d states. The spectra in the martensite phase exhibit interesting modifications near the Fermi level, which has been compared to density of states calculated for a modulated structure by ab-initio density functional theory. Intrinsic surface properties dissimilar from the bulk are enhanced hysteresis width of the martensite transition and increased pre-martensitic transition temperature.

Magnetic Phase Diagram of the Ferromagnetic Shape Memory Alloys Ni<sub>2</sub>MnGa<sub>1-x</sub>Cu<sub>x</sub>

X-ray powder diffraction, permeability, magnetization and differential scanning calorimetry measurements were carried out on the magnetic shape memory alloys Ni2MnGa1−xCux (0 ≤ x ≤ 0.25). On the basis of the experimental results, the phase diagram in the temperature– concentration plane was determined for this alloy system. The determined phase diagram is spanned by the paramagnetic austenite phase (Para-A), paramagnetic martensite phase (Para-M), ferromagnetic austenite phase (Ferro-A), ferromagnetic martensite phase (Ferro-M) and the premartensite phase. It was found that the magnetostructural transition between the phases Para-A and Ferro-M can occur in the concentration region 0.12 < x ≤ 0.14 and that Ni2MnGa1−xCux has the characteristics of the phase diagram similar to those of the phase diagrams of Ni2+xMn1−xGa and Ni2Mn1−xCuxGa. In order to understand the phase diagram, the phenomenological free energy as a function of the martensitic distortion and magnetization was constructed and analyzed.

Martensitic transition, ferromagnetic transition, and their interplay in the shape memory alloysNi2Mn1−xCuxGa

Magnetization and initial permeability measurements were done on the ferromagnetic shape memory alloys ${\text{Ni}}_{2}{\text{Mn}}_{1\ensuremath{-}x}{\text{Cu}}_{x}\text{Ga}$ $(0\ensuremath{\le}x\ensuremath{\le}0.4)$. On the basis of the results, the phase diagram in the temperature-concentration plane was determined for this alloy system. The determined phase diagram is spanned by the paramagnetic austenite phase (I), paramagnetic martensite phase (II), ferromagnetic austenite phase (III), ferromagnetic martensite phase (IV), and the premartensite phase (V). It was found that the magnetostructural transitions between the phases I and IV can occur in the concentration region $0.23\ensuremath{\le}x\ensuremath{\le}0.30$ and that ${\text{Ni}}_{2}{\text{Mn}}_{1\ensuremath{-}x}{\text{Cu}}_{x}\text{Ga}$ has the characteristics of the phase diagram closely similar to those of the phase diagram of ${\text{Ni}}_{2+x}{\text{Mn}}_{1\ensuremath{-}x}\text{Ga}$. In order to understand the phase diagram, the phenomenological free energy as a function of the martensitic distortion and the magnetization was constructed and analyzed, where couplings between these order parameters in existing theories were improved. Satisfactory agreements between the theory and experiments were obtained except for the appearance of the premartensite phase. The analyses show that the biquadratic coupling term, together with a higher order coupling term, of the martensitic distortion to the magnetization, plays an important role in the interplay between the martensite phase and the ferromagnetic phase.

Strain-induced dimensionality crossover and associated pseudoelasticity in the premartensitic phase of Ni2MnGa

The in situ high-energy x-ray diffraction was used for revealing an atomic mechanism on the two-step pseudoelastic behavior found in the premartensitic phase of Ni2MnGa magnetic shape memory alloy. The applied stress first suppresses the three-dimensional modulated structure of the premartensitic phase to a two-dimensional modulated one, which is accompanied by a change in the modulation wave vector and accommodates a large lattice strain reaching ∼1%. With further increasing stress, the two-dimensional modulated premartensite transforms to the five-layered modulated martensite. The observation of the stress-induced dimensionality crossover of atomic modulation has broad impacts in understanding not only the mechanical properties of advanced shape memory alloys but also the physical properties of condensed matter with heterogeneous structures.

Reflection and refraction of acoustic waves at the insulator-ferromagnetic heusler alloy boundary

Reflection and refraction of longitudinal and transverse acoustic waves on a flat boundary between an insulator and a ferromagnetic crystal of a Heusler alloy Ni2 + x + y Mn1 − x Ga1 − y have been considered in the region of premartensitic and martensitic phase transitions. The possibility is shown of using temperature to effectively controlling the angles of wave refraction and the coefficients of transformation of wave modes by changing strong acoustic crystalline anisotropy induced with approaching the point of the phase transition. The conditions of the appearance of critical angles and the angles of total internal reflection, as well as the effect of temperature on them, have been considered. The possibility has been analyzed of emission, in the vicinity of the phase transition, of a wave that propagates along the boundary into the bulk of the material. Based on the experimental data obtained by Trivisonno for the temperature dependences of the sound velocity in the Ni2 + x + y Mn1 − x Ga1 − y crystal, the conclusions of the theory developed are illustrated by numerical calculations for a concrete structure, namely, Ni2MnGa-quartz.

Phase diagram ofTi50−xNi50+x: Crossover from martensite to strain glass

We systematically investigated the variation in transition behavior and physical properties over a wide excess Ni (acting as defect) concentration range $(x=0--2.5)$ in ${\text{Ti}}_{50\ensuremath{-}x}{\text{Ni}}_{50+x}$ alloys. This enables the establishment of an updated quantitative phase diagram for this important system. The phase diagram shows not only the well-known parent phase and martensite phase but also a premartensitic state and a strain glass state. Our experiments were able to determine quantitatively the borders of these states, the latter two having been unclear so far. The new phase diagram shows that a crossover from martensite to strain glass occurs at $x=1.3$, and the appearance of a ``premartensitic phase'' below a critical temperature ${T}_{\text{nd}}$ for defect-containing compositions $(x>0)$. We propose that point defects (excess Ni here) play two roles in a ferroelastic/martensitic system: (i) changing the thermodynamic driving force for the formation of long-range strain order (martensite) and (ii) creating random local stress that favors a premartensitic nanostructure and strain glass. Our work enables a simple explanation for several long-standing puzzles, such as the appearance of premartensitic nanostructure, the vanishing of transition latent heat with increasing Ni content and the anomalous negative temperature coefficient of electrical resistivity in Ni-rich Ti-Ni alloys.

The modeling of phase diagrams and premartensitic effects in Heusler Ni–Mn–Ga alloy by Monte Carlo method

In this work the coupled magnetostructural and premartensitic phase transitions in Heusler Ni–Mn–Ga alloy have been modeled by Monte Carlo method using Ising and Blume–Emery–Griffiths models. The phase diagrams in coordinates (Temperature–Magnetoelastic constant) were obtained. For the first time it is shown that the magnetoelastic interaction account in austenitic and martensitic phases allows to describe the coupled magnetostructural phase transition in Ni-Mn-Ga alloys. The area of the premartensitic phase transition in the temperature–magnetoelastic constant diagram has been found.

Angle-Dependent magneto-resistance near the pre-martensitic phase of Ni2MnGa

Angle-dependent magneto-resistance measurements in the vicinity of the pre-martensite phase (TPM∼219 K) in Ni2MnGa are reported. Magneto-resistance measurements detect a change in slope, from positive to negative near TPM that can be suppressed with a modest magnetic field. Mirroring this change in slope, published ARPES data [Opeil et al., Phys. Rev. Lett. 100, 165703 (2008).] show a significant depletion of states (pseudo-gap) forming at 0.3 eV below the Fermi energy.

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.

Reflection and refraction of acoustic waves at the boundary between the ferromagnetic Heusler alloy and liquid

Reflection and refraction of quasi-longitudinal and quasi-transverse acoustic waves at a flat interface between a liquid and a ferromagnetic crystal of the Heusler alloy Ni2 + x + y Mn1 − x Ga1 − y (in the region of its premartensitic and martensitic phase transitions) is considered. The directions of propagation and polarization and the amplitude of reflected quasi-longitudinal and quasi-transverse and of transmitted longitudinal waves in the (110) plane of the crystal are determined. In a wide region of phase transformations of this alloy, the possibility is shown of using temperature to efficiently control the angles of reflection and refraction of wave modes and the coefficients of their conversion due to a colossal acoustic anisotropy of the crystal. Beginning from a certain critical angle of the quasi-transverse wave, the quasi-longitudinal wave arising upon reflection acquires the nature of the accompanying surface vibration and at a large proximity to phase transition can be emitted into the bulk of the crystal. On the basis of the available experimental data for Ni2MnGa crystals, numerical estimations of the above acoustic effects have been carried out.

The structural and magnetic properties of Ni 2Mn 1−xB xGa Heusler alloys

The influence of the substitution of manganese by boron on the crystal structure and magnetic properties of Ni 2Mn 1− x B x Ga Heusler alloys with 0⩽ x⩽0.5 has been investigated using X-ray diffraction, thermal expansion, resistivity, and magnetization measurements. The samples with concentrations x<0.25 were found to be of single phase and belonged to the cubic L2 1 crystal structure at room temperature. Crystal cell parameters of the alloys decreased from 5.830 to 5.825 Å with increasing boron concentration ( x) from 0 to 0.25. The alloys were ferromagnetically ordered at 5 K and the saturation magnetization decreased with increasing boron concentration. The ferromagnetic ordering and structural transition temperatures for 0⩽ x⩽0.3 have been observed and the phase ( x− T) diagram of the Ni 2Mn 1− x B x Ga system was constructed. The phase ( x− T) diagram indicates that the ground state of Ni 2Mn 1− x B x Ga alloys belongs to ferromagnetic martensitic, premartensitic, and austenitic phases in x⩽0.12, 0.12< x⩽0.18, and 0.18< x⩽0.3, respectively. The relative influence of cell parameters and electron concentrations on the phase diagram is discussed.

Factors Influencing the Suitability of Thermal Methods for Stress Analysis of NiTi Shape Memory Alloys

The material assumptions made to facilitate Thermoelastic Stress Analysis (TSA) are linear elasticity, material homogeneity and isotropy, and mechanical properties that are independent of temperature. The unusual shape memory and superelastic properties of near equiatomic NiTi alloys complicate the application of any experimental stress analysis technique, and in the case of TSA, make these assumptions invalid. This paper describes a detailed analysis conducted to characterise the material properties of NiTi shape memory alloys and to identify loading conditions suitable for quantitative stress analysis using TSA. The mechanical behaviour of the material in three distinct regions is considered and the suitability of each region for TSA is discussed. It is shown that the thermoelastic response is dependent on the mean stress when tested at room temperature in the pre-martensitic phase, due the presence of an intermediate R-phase. Theoretical calculations are used to confirm that this effect is related to the high temperature dependence of the material’s Young’s modulus.

Magneto-Transport and Magnetic Properties of Ni-Mn-Ga

We report a detailed investigation of the magneto-transport and magnetic properties of Mn excess Ni-Mn-Ga using the resistivity and magnetization measurements. Magnetoresistance (MR) has been measured in the ferromagnetic state for different compositions in the austenitic, premartensitic and martensitic phases. With Mn doping in Ni2-yMn1+yGa, a decrease in magnetization and MR has been found, since the doped Mn atoms in Ni position are in the antiferromagnetic configuration with the Mn atoms in Mn position. MR for the parent stoichiometric composition Ni2MnGa varies almost linearly with field in the austenitic and pre-martensitic phases, and shows a cusp-like shape in the martensitic phase. This has been explained by the changes in twin and domain structures in the martensitic phase. Hysteresis in the heating and cooling cycles is a characteristic of the first order nature of the martensitic phase transition.