Search

Heusler Ni-Mn-Ga alloys with martensitic transformation being multiferroic materials are touted for magneto- and multi-caloric effect (MCE) and moreover, they exhibit magnetic shape memory (MSM) phenomena. MCE benefits from high saturation magnetization and also large variability and sensitivity of transformation temperatures on alloying. In contrast, MSM phenomena are limited by low temperatures of both ferromagnetic and ferroelastic transformations. In an attempt to increase the transformation temperatures and to understand the physical fundamentals beyond transformation variations and magnetic ordering, we experimentally investigate the magnetic and transformation properties in Ni2MnGa alloyed with transitional elements spanning from Cr to Cu. These elements are consecutively substituted on Ni, Mn, and Ga sites, to establish the effect of positional substitution and elemental evolution. The elemental dependence of saturation magnetization, magnetocrystalline anisotropy, Curie temperature, and martensitic transformation temperatures are compared with (non-)stoichiometric Ni-Mn-Ga Heusler compounds. The observed behavior is complex and there are no clear trends of transformation temperatures as a function of the number of valence (non-bonding) electrons per atom, however, the saturation magnetization at 10K and Curie temperature seems to follow the Pauli-Slater curve. Obtained results can serve as useful rectification for the theoretical prediction of new Ni-Mn-Ga-X Heusler alloys.

Publisher Copyright: © 2022 Elsevier Inc. Using common and high resolution transmission electron microscopy (TEM), we study the magnetic and crystal phase structures as well as their evolution in Ni50Mn25Ga20Fe5 magnetic shape memory material with high Curie point. The particular alloying by Fe and changing thickness of the TEM foil enable us to observe all martensitic phases known in Ni-Mn-Ga Heusler alloy system and their respective transitions simultaneously. Starting from cubic austenite at about 10 nm foil thickness, the structure evolves via peculiar interleaved stripes of austenite and five-layered modulated 10M martensite to pure 10M phase with a low density of stacking faults at 40 nm thickness. With further increasing thickness, the 10M phase transforms gradually to seven-layered modulated 14M martensite with an increased density of stacking faults. Finally, the non-modulated tetragonal NM phase appears within the 14M phase by detwinning of nanotwins forming the modulated phases. High resolution TEM further confirms that nanotwinning and stacking faults are inherent structure features tightly connected with the lattice modulation and intermartensite transformations. Overall, the evolution of average lattice shows the same general trend, an increasing simple shear with (11¯0) shuffling plane, across the whole phase sequence. Additionally, we found large local variation of lattice parameters in all phases, which is is ascribed to strong lattice softening in the vicinity of the martensitic transformation and high density of stacking faults in 14M martensite lattice.

Here we report a systematic research on effects of Fe and Cu upon properties relevant for the magnetic shape memory effect of Ni–Mn–Ga ferromagnetic shape memory alloys. Fe and Cu were identified as elements with potential synergism to increase the martensite transformation temperature of Ni–Mn–Ga magnetic shape memory (MSM) alloys. Eighteen Ni–Mn–Ga–Fe–Cu alloys with different systematic trends in substituting the ternary elements with Cu and Fe have been investigated. We found a method to describe the effectiveness of Ni, Mn, and Cu upon raising the martensitic transformation temperature, lowering the saturation magnetization, and varying the Curie temperature. We find the martensite transformation temperature most influenced by the Ni content, followed by Mn, with a smaller effect of Cu. The saturation magnetization decreases with similar coefficients for Mn and Cu alloying. The Curie temperature monotonously decreases with Mn, but not Cu. The 10M martensite structure is stable for the composition Ni46.5Mn25+XGa25−X−YFe3.5CuY with X and Y range of 0–5.7, and 0.8–3.0. Used in combination with the total e/a, the elemental e/a-ratio gives some insight into the complex behavior of quinary MSM alloys and is a useful method of analyzing MSM alloys for improved functional properties.

Systematic doping by transition elements Fe, Co and Ni on each site of Ni2MnGa alloy reveal that in bulk material the increase in martensitic transformation temperature is usually accompanied by the decrease in ferromagnetic Curie temperature, and vice versa. The highest martensitic transformation temperature (571 K) was found for Ni50.0Mn25.4(Ga20.3Ni4.3) with the result of a reduction in Curie temperature by 55 K. The highest Curie point (444 K) was found in alloy (Ni44.9Co5.1)Mn25.1Ga24.9; however, the transition temperature was reduced to 77 K. The dependence of transition temperature is better scaled with the Ne/a parameter (number of non-bonding electrons per atom) compared to usual e/a (valence electrons per atom). Ne/a dependence predicts a disappearance of martensitic transformation in (Ni45.3Fe5.3)Mn23.8Ga25.6, in agreement with our experiment. Although Curie temperature usually slightly decreases while the martensitic transition increases, there is no significant correlation of Curie temperature with e/a or Ne/a parameters. The doping effect of the same element is different for each compositional site. The cascade substitution is discussed and related to the experimental data.

Ferromagnetic shape-memory CoNiGa alloys have attracted much scientific interest due to their potential alternative use as high-temperature shape-memory alloys, bearing a high prospect for actuation and damping applications at elevated temperatures. Yet, polycrystalline CoNiGa, due to strong orientation dependence of transformation strains, suffers from intergranular fracture. Here, two multi-grain CoNiGa samples were prepared by a novel hot extrusion process that can promote favourable grain-boundary orientation distribution and improve the material's mechanical behaviour. The samples were investigated by multiple methods and their micro structural, magnetic, and mechanical properties are reported. It is found that a post-extrusion solutionising heat treatment leads to the formation of a two-phase oligocrystalline homogeneous microstructure consisting of an austenitic parent B2 phase and gamma-CoNiGa precipitates. Reconstruction of the full 3D grain morphology revealed large, nearly spherical grains with no low-angle grain boundaries throughout the entire sample volume. The presence of gamma precipitation affects the transformation behaviour of the samples, by lowering the martensitic transformation temperature, while, in conjunction with the oligocrystalline microstructure, it improves the ductility. Controlling the composition of the B2 matrix, as well as the phase fraction of the gamma phase, is thus crucial for the optimal behaviour of the alloys. (c) 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

We investigate experimentally the universality of the temperature dependences of twinning stress of type Ⅰ and type Ⅱ twins in Ni50Mn28Ga22 five-layered modulated martensite. Single crystals from different producers are compared to distinguish the universal behaviour and the effects of crystal quality. The twinning stress is both measured directly and calculated from magnetisation loops. In the vicinity of martensite transformation, the twinning stress is about the same for both types of twins. For type Ⅰ twins, it increases linearly with decreasing temperature with the slope of -0.045 MPa/K. For type Ⅱ twins, the twinning stress is almost temperature independent, but it increases rapidly near the intermartensite transformation temperature. In the crystal, which does not undergo an intermartensitic transformation and exhibits type Ⅱ twins, the magnetically induced reorientation of martensite is observed down to 2 K. Chemical inhomogeneity and mosaicity of the crystal results in mixed type twin boundaries with temperature dependences deviating from the universal behaviour of type Ⅰ and type Ⅱ twins.

Critical currents Ic of several commercial REBaCuO superconducting tapes, prepared by various producers, were tested inductively (vibrating sample magnetometer in magnetic fields up to 9 T). Measurements were performed before and after neutron irradiation. Irradiation was conducted in the LVR-15 fission reactor at the Research Centre qež with doses of 1 × 10 22 m -2 and 2.1 × 10 22 m -2 . After irradiation, we observed a slight decrease in Ic around the self-field in all tapes. At higher magnetic fields, neutron irradiation caused a large variation in Ic, with some tapes showing a strong enhancement, some tapes showing a moderate effect, whilst no effect or even an Ic degradation appeared in the remainder. The irradiation effect depended on the pinning medium, measurement temperature, and magnetic field.

The microstructures and transformation behaviour of self-accommodated and mechanically stabilized martensite of Cu69.4Ni3.4Al27.2 (at.%) single crystal were investigated by optical microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) and magnetometry. XRD and TEM analyses showed the presence of both 2H and 18R phases in self-accommodated martensite. The mechanical compression of martensite (≈ 100 MPa) increased markedly the transformation temperature to austenite, i.e. resulting in significant mechanical stabilization of martensite. The 18R phase disappeared after the compression. This reveals the important role of 18R phase in transformation behaviour of Cu–Ni–Al alloy. Furthermore, we demonstrate that thermo-magnetic measurement is suitable method to analyse the martensitic transformation even for diamagnetic material.

The RE-123 tapes represent the most advanced product in the field of high temperature superconductors, developed for transport of high electric currents. Although this product has already proceeded to the stage of industrial production, the variety in techniques employed by different manufacturers make the final products by far not identical. We studied electromagnetic properties of a series of 21 tapes from main global wire manufacturers. The tapes were tested by magnetic induction technique (vibrating sample magnetometer, VSM) and by a transport current measurements. The aim was to find the best candidates for superconducting coils for fusion reactors. Engineering currents were deduced from magnetic hysteresis loops measured by VSM at 10 K, 50 K, and 77 K. The data for 77 K were compared with the results of direct transport experiments done at the same temperature. Current transport experiments also provided information on the current anisotropy. The VSM tests were repeated after neutron irradiation of the tapes by neutron fluence of 2.12 × 1018 cm−2. A significant enhancement of engineering currents was observed in magnetic fields above 0.5 T in all investigated tapes and temperatures. The enhancement, similarly as the engineering current, was roughly exponential function of temperature.

Light-weight ferromagnetic Co-Ni-Al:Ti composites were consolidated by spark plasma sintering. Powders of ball-milled Co-Ni-Al and gas atomized α-Ti were mixed together in three different ratios and spark plasma sintered at 950°C, using two sintering times, i.e. dwell times at the maximal temperatures: 1 and 5min. The composites were successfully compacted and new intermetallic phases were formed at the Co-Ni-Al/Ti interfaces. The composites exhibit favourable flexural strength, excellent high-temperature stability, and relatively high saturation magnetization. On the other hand, the newly emerged intermetallic phases are non-ferromagnetic, which slightly deteriorates the magnetic properties. Keywords: Metal–metal composites, Spark plasma sintering, Light metals, Ferromagnetic alloys, Mechanical properties

Ferromagnetic resonance in an array of Co nanowires electrolytically deposited into nanoporous alumina template is investigated at four microwave frequencies in the range from 9.3 to 69.7 GHz. The array consists of highly textured hcp Co with hexagonal axes perpendicular to the nanowires. The spectra measured at higher frequencies can be decomposed into four wide resonances peaks. Different mechanisms, which can lead to the multi-peak resonance, are discussed.

The combination of enlarged magnetic coercivity and magnetic shape memory (MSM) functionality is essential for novel magnetomechanical effects in MSM alloys. We found that increasing the density of thermal antiphase boundaries (APBs) provides a method to increase the magnetic coercivity without deteriorating the MSM functionality. APB density was controlled by different heat treatments in Ni-Mn-Ga(-B) MSM single crystals with five-layered modulated martensite structure. Slow cooling ~1 K/min of Ni-Mn-Ga through the B2'-L21 transition resulted in a low density (

The combination of enlarged magnetic coercivity and magnetic shape memory (MSM) functionality is essential for novel magnetomechanical effects in MSM alloys. We found that increasing the density of thermal antiphase boundaries (APBs) provides a method to increase the magnetic coercivity without deteriorating the MSM functionality. APB density was controlled by different heat treatments in Ni–Mn–Ga (–B) MSM single crystals with five-layered modulated martensite structure. Slow cooling ∼1 K/min of Ni–Mn–Ga through the B2′–L21 transition resulted in a low density ( 1/micrometer) of APBs observed by magnetic force microscopy and low coercivity

We have demonstrated a method for preparing nickel nanoparticles in TiO2 matrix films, 500–900 nm thick, using magnetron sputtering followed by a heat treatment at temperatures 723–773 K for 1 h in various gas atmospheres. The X-ray diffraction indicated that the size of Ni crystallites was 4–6 nm depending on the heat treatment conditions. Magnetic force and scanning electron microscopy studies show clustering the nanoparticles to size of order hundreds of nm arranged in a shape of nanopillars separated by TiO2. The above mentioned clustering results in a magnetic anisotropy as indicated from the loops of the magnetic moment – magnetic field curves in the perpendicular and parallel directions. Some of the prepared nanocomposite samples exhibit different magnetic behavior within the perpendicular and parallel directions for |μ0H|>0.6 T, presumably due to the presence of antiferromagnetic oxides.

Epitaxial growth of ferromagnetic Rh2Mn5Bi4 thin films from the family of X2Mn5Bi4 (X = Cu, Ni, Pd and Rh) compounds was pursued employing magnetron sputtering on MgO(001) substrates. Composition, structure and microstructure of prepared films were studied using a wide range of diffraction, microscopy and spectroscopy techniques, while Superconducting quantum interference device and Magneto-Optical Kerr Effect were used for magnetic and magneto-optical characterizations. The huge difference between Rh and Bi in their physicochemical properties - melting points, vapor pressures and deposition yields - manifested in a substantial influence of growth temperature on resulting properties of prepared films. Rh2Mn5Bi4 alloy with characteristic microstructure and favorable ferromagnetic and magneto-optical response was obtained after growth at 400 °C. The arrangement of Mn atoms within the structure should give rise to the magnetic moment in a similar fashion as in Heusler alloys. Curie temperature of these films was found to be 270 K. The films appear to be promising for magnetic or spintronic applications.

In bulk high-temperature superconductors, Ag is usually used as an additive thus improving mechanical performance. In MgB2 doped with Ag, the Ag reacts with Mg, forming Mg–Ag phases acting as a vortex pinning medium. In this work, we analyze the electromagnetic and pinning properties of bulk MgB2 doped with 1 wt% MgB4, 4 wt% Ag, and 1 wt% Dy2O3, prepared at the Shibaura Institute of Technology (SIT), Tokyo. In three compounds of MgxB2 + 4 wt% Ag with x = 1, 1.075, and 1.1, the effect of Mg excess was studied. The magnetic moment was measured by a vibrating sample magnetometer (VSM). Pinning was studied in terms of a pinning diagram, i.e. the field dependence of the normalized pinning force density, F n = F/F max. In all studied samples, the peak of the F n (b) dependence (b= B/B irr, B irr being the irreversibility field) was observed at around b= 0.2, indicating a prevailing flux pinning at grain boundaries. A slight shift of the peak with decreasing temperature indicated a defect size distribution in the pinning landscape. Transmission electron microscopy (TEM) showed a granular structure of all samples composed of MgB2 grains of about 230 nm (average size), with ensembles of small grains (22 nm in average) of Ag3Mg, AgMg, Ag, Dy2O3, and MgB4. While the large MgB2 grains control the main pinning mechanism, the small precipitates seem to determine details of the current flow through the grain boundaries.

We investigate the origin of magnetocrystalline anisotropy (MCA) in non-modulated martensite of Ni-Mn-Ga-Co-Cu exhibiting magnetic-field-induced strain up to 12%. Experiments as well as theoretical calculations using density functional theory show that Co and Cu doping or deviation from Ni2MnGa stoichiometry decreases the MCA. As follows from the calculations, the decrease of MCA is much stronger for Cu in Ga sublattice in comparison to Cu in Mn sublattice. The decreasing effect of Co on the MCA is only indirect caused by deficiency in Ni, which is the main element governing the MCA. For further insight, we calculated MCA and magnetic moment as a function of lattice tetragonality c/a. The MCA reaches a maximum at the same c/a where Ni magnetic moment is maximum. However, the tetragonality of equilibrium does not coincide with these maxima. Consequently, in contrast to common expectation, decreasing tetragonality from equilibrium can increase the MCA.

Nickel nanoparticles in TiO2 matrix films, 500 nm thick, were prepared through co-sputtering from NiTi and Ti targets and post-annealing at 773 K for 1 h in the N2+H2 atmosphere. The microstructure of the samples was observed using a transmission electron microscopy. TiO2 crystallites forming TiO2 matrix were oriented with a preferential orientation whereas the crystallographic orientation of Ni particles was random. Magnetic moment at a constant magnetic field at various temperatures from 300 K to 1000 K was measured. The prepared samples exhibited high Curie temperature, Tc, (Tc above Tc of bulk Ni) and perpendicular magnetic anisotropy due to the formation of Ni particles into chains, normal to the substrate.

Electromagnetic performance of superconducting REBaCuO tapes from several manufacturers was investigated in response to neutron irradiation. The aim was to test the feasibility of using these tapes for wiring high-T c superconducting magnets for fusion reactors. Magnetic induction was employed to determine critical currents. In non-irradiated SuNAM, SuperOx, and SuperPower tapes without additional pinning centers, the normalized pinning force density as a function of the normalized magnetic field, h, exhibited the commonly observed single peak at about h = 0.2 in the whole temperature range 10 K–80 K. In the non-irradiated AP (advanced pinning) SuperPower tapes this dependence developed an extraordinary shape below 70 K. This feature changed with increasing neutron fluence. The effect of neutron irradiation on critical currents was found to be rather complex. In all samples, neutron irradiation by a total fluence of 8.03 × 1022 m−2 caused a critical current degradation at low magnetic fields in the temperature range 10 K–77 K. At high magnetic fields, the effect varied depending on the tape's initial pinning structure, temperature, and neutron fluence. At 77 K, the current decreased and the irreversibility field was reduced in all the samples. At intermediate temperatures the critical current nearly did not change, while at temperatures below 30 K the critical current increased.

Magnetic properties of Ni 46 Mn 24 Ga 22 Co 4 Cu 4, (Ni 45 Mn 23 Ga 22 Co 5 Cu 5 , Ni 46 Mn 22.5 Ga 23 Co 5 Cu 3.5 ) tetragonal martensites with c / a ≈ 1.14 exhibiting near 10% magnetic field-induced strain (MFIS) were investigated between 10 and 500 K and up to 9 T field. The (001) easy plane magnetocrystalline anisotropy with K 1 (300 K)= − 1.95 ∙ 10 5 J ∙ m − 3 ( K 2 K 1 |, K 3 K 1 |, M S (300 K) = 63 Am 2 /kg) was about 30% lower than in the non-doped Ni 50.5 Mn 30.4 Ga 19.1 reference, which can be related to a reduced c / a ratio in the doped material. In spite of different type of magnetocrystalline anisotropy compared to 10M martensite, the mechanism of MFIS is the same, i.e. with the typical first-quadrant magnetization hysteresis and {101} twin boundary motion.

Magnetic induction and current transport techniques were employed to test the electromagnetic performance of 22 samples of superconducting REBaCuO tapes of several manufacturers with the aim to select the best candidates for wiring new generation superconducting magnets for fusion reactors. An extraordinary shape of pinning force density as a function of magnetic field was observed in 'advanced pinning, AP' SuperPower tapes. The induced currents enabled us to classify the tapes and show the best ones, at low temperatures and high magnetic fields on the one hand and at high temperatures and low magnetic fields on the other hand. Transport current experiments confirmed the results. Moreover, they provided information on current anisotropy, which appeared to be specific for each tape type. The present results formed a basis for the ongoing study of neutron irradiation impact on the tapes' properties. The first experiment showed that neutron irradiation by the fluence of 2.1 E22 m−2 either enhances, reduces, or has no effect on the tape's properties, according to the tape pinning structure.