Ga Single Crystals Research Articles

Acoustic energy absorption in Ni–Mn–Ga/polymer composites

Composites of Ni–Mn–Ga particles in a polyurethane (PU) matrix are known to show up to 70% energy absorption under quasi-static mechanical loading. Reported here are results showing that the large mechanical energy absorption in Ni–Mn–Ga/PU composites persists up to at least 1 kHz and exceeds that in each of these control samples of pure PU, as well as in Fe 2(Tb 1/3Dy 2/3) particles in epoxy, and single crystal Ni–Mn–Ga. The transmitted acoustic amplitude decreases linearly with increase in frequency, consistent with the theory, and at 1 kHz it is 0.7 V in the Ni–Mn–Ga single crystal, 0.65 V in pure PU, 0.45 in the Fe 2(Tb 1/3Dy 2/3)/PU composites and 0.25 V in Ni–Mn–Ga/PU composites.

Structure of the magnetic-field-induced modulated phase of an α-Fe2O3: Ga single crystal

The field and orientational dependences of magnetic linear birefringence in α-Fe2O3: Ga single crystals are investigated. The experimental results make it possible to refine and specify the structure and reveal the features of technical magnetization of the modulated magnetic phase of this weak ferromagnet. It is shown that the model of the modulated magnetic structure used in this study adequately describes the experimental situation.

Magnetostriction and Magnetic Heterogeneities in Iron-Gallium

Iron-gallium alloys Fe(1-x)Ga(x) exhibit an exceptional increase in magnetostriction with gallium content. We present small-angle neutron scattering investigations on a Fe(0.81)Ga(0.19) single crystal. We uncover heterogeneities with an average spacing of 15 nm and with magnetizations distinct from the matrix. The moments in and around the heterogeneities are observed to reorient with an applied magnetic field or mechanical strain. We discuss the possible roles played by nanoscale magnetic heterogeneities in the mechanism for magnetostriction in this material.

Special features of linear magnetic birefringence in an α-Fe2O3:Ga single crystal

Experimental investigations of the field dependence of the linear magnetic birefringence (LMB) in an α-Fe2O3:Ga single crystal are presented. It is established that during crystal magnetization in the basal plane near directions perpendicular to the С2 axes in the region of magnetic field saturation, the LMB changes nonmonotonically. The observed special feature of the field LMB dependence is due to the reorganization of the magnetic α-Fe2O3:Ga structure during magnetization.

Transformation twinning of Ni–Mn–Ga characterized with temperature-controlled atomic force microscopy

The magnetomechanical properties of ferromagnetic shape memory alloy Ni-Mn-Ga single crystals depend strongly on the twin microstructure, which can be modified through thermomagnetomechanical training. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) were used to characterize the evolution of twin microstructures during thermomechanical training of a Ni-Mn-Ga single crystal. Experiments were performed in the martensite phase at 25 degrees C and in the austenite phase at 55 degrees C. Two distinct twinning surface reliefs were observed at room temperature. At elevated temperature (55 degrees C), the surface relief of one twinning mode disappeared while the other relief remained unchanged. When cooled back to 25 degrees C, the twin surface relief recovered. The relief persisting at elevated temperature specifies the positions of twin boundaries that were present when the sample was polished prior to surface characterization. AFM and MFM following thermomechanical treatment provide a nondestructive method to identify the crystallographic orientation of each twin and of each twin boundary plane. Temperature dependent AFM and MFM experiments reveal the twinning history thereby establishing the technique as a unique predictive tool for revealing the path of the martensitic and reverse transformations of magnetic shape memory alloys.

Dc- and ac-magnetic field-induced strain effects in ferromagnetic shape memory composites of Ni–Mn–Ga single crystal and polyurethane polymer

Ferromagnetic shape memory composites of multilayer and sandwich types were fabricated by laminating Ni–Mn–Ga single-crystal plates with polyurethane (PU) polymer plates. The dc- and ac-magnetic field-induced strains (MFISs) in the composites were measured as functions of both magnetic field and mechanical load, and the results were compared with those of the single crystal. It was found that the load-free dc-MFISs were 5.6%, 1.5%, and 0.8%, while the load-free ac-MFISs were 0.3%, 0.8%, and 0.5% in the single crystal, multilayer composite, and sandwich composite, respectively. The relatively smaller load-free dc-MFISs and larger load-free ac-MFISs in the composites than the single crystal originated from the stress bias of the Ni–Mn–Ga plates by the PU plates in the composites. The dc-MFISs of all samples and the ac-MFISs of the composites decreased with the increase in mechanical load amplitude, while the ac-MFIS of the single crystal peaked at 1.6 MPa load.

Effects of surface roughness and training on the twinning stress of Ni–Mn–Ga single crystals

Spark eroding, which is commonly employed to cut samples out of magnetic shape-memory alloy single crystals, produces a rough surface layer. Directly after cutting, the single crystals exhibit a high twinning stress. After removal of a surface layer by electropolishing, the twinning stress reduces significantly and stress–strain curves become serrated. The reduction of twinning stress has previously been attributed to the removal of the defective surface layer. In this work, it is shown that different surface treatments in combination with repeated mechanical deformation experiments significantly reduce the twinning stress, regardless of whether or not electropolishing is used. The reduction of the twinning stress is due to softening that takes place as a mechanical training effect, which occurs with mechanical testing. In addition, the stress–strain curves of samples subjected to different surface treatments differed in so far as the curves of electropolished samples showed serrated flow, while the curves of unpolished samples and those of mechanically polished samples were smooth. Furthermore, the unpolished samples displayed significant hardening at higher strain. Following subsequent mechanical polishing, this hardening reduced to nearly zero, and the average twinning stress decreased another 30–50% to 1.6 MPa and below. For these samples, the twinning stress stayed at a very low level until twinning was complete.

Anisotropy of the electrical transport properties in a Ni2MnGa single crystal: Experiment and theory

Electrical transport properties in ferromagnetic shape memory Ni–Mn–Ga single crystal have been investigated both in experiment and theory by analyzing electrical resistivity along different crystallographic directions during heating. The experimental results show a clear first-order martensitic transformation and a large anisotropic resistivity (AR) of 23.7% at the tetragonal martensitic phase. The theoretical conductivity (σ=1/ρ), estimated using first-principles calculations combined with classical Boltzman transport theory, proves essential crystallographic anisotropic resistivity (AR=31%) in the martensitic phase and agrees well with experimental results. The AR in the martensitic phase is reveled to mainly originate from the splitting of the minority-spin Ni 3d and Ga 4p states near the Fermi level and hence reconstruction of the minority-spin Fermi surface upon martensitic transformation.

Twin microstructure dependent mechanical response in Ni–Mn–Ga single crystals

Ni–Mn–Ga single crystals with a twinning stress of about 0.1 MPa were studied. They showed a tendency to stay in a single variant state and to retain only one or very few twin boundaries during martensite reorientation induced by an external stress or magnetic field. This makes the crystals problematic for application in a magnetic actuator. To solve the issue, we introduced many parallel twin boundaries into the crystals by bending. However, this twin microstructure was not stable under cycling load. Additionally, it exhibited a twinning stress of 0.8 MPa—about ten times higher than a crystal with a single boundary.

Effect of Mn concentration on the phase transformation in Ni–Mn–Ga single crystal

Aim of this paper is to study the transformation temperature of Ni–Mn–Ga single crystals by using differential scanning calorimetry. It shows that martensitic temperature abruptly increase with increasing Mn concentration whereas Curie transition to decrease. The calculated value of electron-to-atom ratio ( e/ a) reveal that it increases with increased doping of Mn in place of Ga at fixed Ni content. The crystal structures at room temperature were studied using XRD pattern. Thermo-magnetic behavior of the Ni–Mn–Ga alloy shows the ferromagnetic nature and its behaviour is slightly losing due to antiferromagnetic ordering of excess Mn atoms. Microstructure from SEM image confirm that the alloys are martensitic plates. The martensitic plates contain fewer stripes like magnetic domains.

Ultrahigh anisotropic damping in ferromagnetic shape memory Ni–Mn–Ga single crystal

Damping behavior with an oscillating bending force applied to each of the three different crystallographic planes a– b, a– c, and c– a is measured in a tetragonal ferromagnetic shape memory Ni 50Mn 29Ga 21 single crystal at different combinations of frequency and temperature by a dynamic mechanical analysis method. The results show a frequency-independent anisotropic damping behavior in the martensitic phase with a loss factor of 0.03, a high loss factor of 0.15, and an ultrahigh loss factor of 0.26 for the a– b, a– c, and c– a crystallographic planes, respectively. The observed anisotropic damping behavior is found to originate from the reorientation of different numbers of martensitic twin variants upon bending. It is also adjustable by an applied magnetic field via the control of the martensitic twin-variant bending states.

Temperature Dependence of Twinning Pseudoelasticity in Fe<SUB>3</SUB>Ga Single Crystals

Temperature dependence of twinning pseudoelasticity in D0 3 -ordered Fe 3 Ga single crystals was examined. In the crystals annealed in the D0 3 single-phase region, 2.2 T -type pseudo-twins were frequently introduced during loading at and below room temperature. Since the pseudo-twins had a high energy compared with a perfect twin, the untwinning took place during unloading to decrease their energy resulting in the twinning pseudoelasticity. Moreover, the twinning pseudoelasticity became more dominant with decreasing temperature, in contrast to that based on dislocation motion. In particular, high strain recovery during unloading was observed even at — 180°C. Furthermore, the formation and annihilation stresses of the pseudo-twins were almost constant irrespective of deformation temperature at and below 100°C. This means that high driving force for the twinning pseudoelasticity was preserved even at low temperatures.

10.1007/s11449-008-2010-0

The spectral--kinetic characteristics of a ZnO:Ga single crystal upon excitation in the vacuum UV region have been studied. At a temperature of 8 K, the exciton luminescence line peaking at 3.356 eV has an extremely small half-width (7.2 meV) and a short decay time (360 ps). In the visible range, a wide luminescence band peaking at ~2.1 eV with a long luminescence time at 8 K and a decay time in the nanosecond range at 300 K is observed. The luminescence excitation spectra of ZnO:Ga have been measured in the range from 4 to 12.5 eV

Giant two-way shape memory effect in high-temperature Ni–Mn–Ga single crystal

A perfect two-way shape memory effect (TWSME) with reversible strain of about 9% has been found in the high-temperature Ni57.5Mn22.5Ga20.0 single crystal transforming into 2M non-modulated martensitic phase. Two thermal/mechanical treatment routes were utilized in the experimental procedures. The outstanding high-temperature TWSME is observed as a result of tensile stress-strain cycling along 〈100〉 axis involving huge superelastic strains due to stress-induced martensitic transformation. It is argued that the TWSME is related to the anisotropic internal stresses produced by a network of lattice defects. The defects are generated in the course of oriented growth/shrinkage of the dominating martensitic variant in the tensile sample.

Production and characterization of large single crystals made of ferromagnetic shape memory alloys Ni–Mn–Ga

Ferromagnetic shape memory alloys (MSMA) are well-known smart materials for actuation applications, due to their large shape change in applied magnetic fields. Off-stoichiometric Ni–Mn–Ga single crystals of MSMA exhibit a magnetic-field induced strain of about 10%. In the past decade a lot of effort has been put into understanding and optimizing the properties of Ni-Mn-Ga alloys in terms of the magnetic shape memory effect, starting with the production of single crystals and going all the way to heat treatment and magneto-mechanical training.Here we focus on and present the production and characterization of single crystal Ni–Mn–Ga alloys with off-stoichiometric composition. The molten material was cast into preheated ceramic shells and directionally solidified in a Bridgman-type process. Cylindrical single crystal bars of diameters up to 48 mm and a length of 110 mm were obtained by using suitable grain selectors. Only small misorientations between the single crystal c-axis and the bar axis were found in these bars. The crystals were characterized in terms of composition, segregation, crystal orientation and microstructural features by means of SEM/EDS, EBSD and light microscopy.

Nonlinear magneto-mechanical finite element analysis of Ni–Mn–Ga single crystals

This paper presents a three-dimensional (3D) nonlinear quasi-static isothermalfinite element formulation for single crystals of Ni–Mn–Ga ferromagnetic shapememory alloy (FSMA). This formulation is directly coupled (strong coupling)between the magnetic vector potential and mechanical displacement. As a result thisformulation is applicable to general structural configurations, where the mechanical andmagnetic behaviour of the entire structural and magnetic system can be computedsimultaneously. This finite element formulation has the potential for applications thatrequire accurate prediction of the magnetic field distribution and strain response ofNi–Mn–Ga single crystals. One of the major advantages of this finite elementformulation is the ability to predict the behaviour of the material for generalized orcomplicated shapes and geometries where analytical methods may not be adequate.The two-dimensional nonlinear finite element analysis computer software MAGSTRAN(magnetic structural analysis) is developed based on this formulation. Numerical results forselected examples are validated against experimental data available in the literature. It isdemonstrated that there is reasonable correlation between MAGSTRAN results andexperimental data available in the literature.

Short range ordering in Fe–Ge and Fe–Ga single crystals

We report on the nature of short range ordering (SRO) in slow cooled (100) and (111) Fe81.6Ga18.4 and Fe81Ge9 single crystals determined by x-ray θ-2θ scans. The SRO of the Ga atoms has at least some D03 character. In contrast Ge atoms only exhibit B2 chemical ordering symmetries in the short range order. It has been proposed that the D03 character in the SRO is important for the enhancement in magnetoelasticity in Fe-based alloys; however the presence of B2 character in Fe–Ge alloys suggest that the SRO is not of primary importance in increasing the magnetostriction.

Large magnetoelectric effect from mechanically mediated magnetic field-induced strain effect in Ni–Mn–Ga single crystal and piezoelectric effect in PVDF polymer

Abstract Magnetoelectric (ME) effect is observed in a bilayered composite fabricated by laminating one ferromagnetic shape memory Ni–Mn–Ga single-crystal plate with one piezoelectric PVDF polymer film. The reported ME effect originates from the mechanically mediated magnetic field-induced strain effect in the Ni–Mn–Ga plate and piezoelectric effect in the PVDF film. The induced ME voltage shows a good linear response to the applied ac magnetic field in the field range of 0–5 Oe. A large ME coefficient ( α E ) of 1.24 V/(cm Oe) is obtained at an optimal bias magnetic field of 5.1 kOe due to the reversible reorientation of martensitic twin variants in the Ni–Mn–Ga plate. Based on the combination of pseudo-piezomagnetism in the Ni–Mn–Ga plate and piezoelectricity in the PVDF film, the ME effect in the bilayered composite is predicted and found to agree with the experimental results.

Electron sound in metals

This paper is devoted to the investigation of electron sound -- oscillations of the electron distribution function coupled with elastic deformation and propagating with the Fermi velocity. The amplitude-phase relations characterizing the behavior of the electron sound in Ga single crystals are determined experimentally. A model problem of excitation of electron sound in a compensated metal with equivalent bands is solved for a finite sample with diffusive scattering of electrons at the interfaces. It was found that the displacement amplitude of the receiving interface is two orders of magnitude larger than the elastic amplitude of the wave due to electron pressure. It was established that the changes occurring in the amplitude and phase of the electron sound waves at a superconducting transition do not depend on the path traversed by the wave, i.e. they refer only to the behavior of the transformation coefficient.

Unusual pseudoelastic behaviour of Fe3Ga shape memory alloys

Pseudoelastic behaviour of Fe-23.8at%Ga single crystals compressed with different loading axes at room temperature was examined focusing on the activated deformation mode. In the crystals, {101}<111> slip and {211}<111> twin were mainly activated depending on the loading axis. Pseudoelastic behaviour of the crystals depended strongly on the deformation mode. If {101}<111> slip was operative, sole and paired 1/4<111> superpartial dislocations moved dragging antiphase boundaries (APB). During unloading, the APB pulled back the superpartials due to its tension resulting in pseudoelasticity. In contrast, twinning and untwinning of {211}<111> pseudo-twins also led to large strain recovery accompanying a serrated flow during loading and unloading. It is suggested the energy of the pseudo-twin interface was the driving force for the twinning pseudoelasticity.