We studied the changes of martensite average temperature TM in a wide range of Heusler alloys derived from a Ni-Mn-Ga multifunctional compound prepared by arc melting. Based on prepared alloys and supplemented by the literature data, we demonstrated that criteria based on valence electron or non-bonding electron concentration per atom often failed in many different cases, in particular for isoelectronic compounds and Heusler alloys with Sb and Sn. Thus, we suggest an empirical criterion for estimating the temperature of martensitic transformation TM in Ni-Mn-based Heusler alloys. It is built on valence electron concentration per atomic volume. Suggested criterion well-describes the experiment and data available in literature. Although it can be used for predicting TM in complexly alloyed Ni-Mn-based Heusler alloys.

Magnetic shape memory effect provides up to 12% deformation in magnetic field however the application of the effect is limited due to low transformation temperature to low symmetry martensite phase, $\mathrm{T}_{\mathrm{m}}$, and by low ferromagnetic Curie point, $\mathrm{T}_{\mathrm{c}}$. Stoichiometric $\text{Ni}_{2} \text{MnGa}$ exhibits $\mathrm{T}_{\mathrm{M}} \approx 200 \mathrm{K}$ and $\mathrm{T}_{\mathrm{C}}=380 \mathrm{K}$. By changing the $\text{Ni} / \text{Mn} / \text{Ga}$ ratio the $\mathrm{T}_{\mathrm{M}}$ can be increased up to 350 K while $\mathrm{T}_{\mathrm{C}}$ stays constant or decreases. Here we pursued the path to modify transformation temperatures by systematic, combinatoric alloying by two transitional metals (up to $10 \text{at} \text{\%}$) simultaneously. It was found that alloying which resulted in an increase of martensite temperature usually decreased Curie point and vice versa. The highest $\mathrm{T}_{\mathrm{M}}$ was reached by Cu alloying, but this also resulted in lowest $\mathrm{T}_{\mathrm{C}}$. The highest $\mathrm{T}_{\mathrm{C}}$ was reached by combined Fe and Co alloying.

Epitaxially grown full Heusler alloy Rh2MnSb thin films were prepared for the first time using DC magnetron sputtering. The films were deposited on MgO [001] substrates at deposition temperatures of 600 degrees C, 700 degrees C, and 800 degrees C. We report on the structural, morphological, optical, magneto-optical, and magnetic properties of these films, which had a nominal thickness of 200 nm. The film grown at 600 degrees C was nearly stoichiometric and exhibited the almost perfect L21 ordering typical of Heusler alloys. The single-phase Rh2MnSb film possessed a tetragonal structure with lattice parameters close to the bulk material. X-ray photoelectron spectroscopy revealed the metallic character of the contamination-free film. The tetragonal films displayed discernible regular twinning, where most twin domains oriented with their c-axis perpendicular to the surface, attributed to substrate constraint. Twin formation was investigated using atomic force microscopy, transmission electron microscopy, and X-ray diffraction. Magnetic measurements of the film prepared at 600 degrees C revealed a Curie temperature (TC) of approximately 220-275 K and a saturation magnetization of around 55 emu/g at 10 K, both values close to the bulk material's. Magneto-optical Kerr effect measurements confirmed paramagnetic behavior at room temperature. These observed properties highlight the need for further studies of Rh2MnSb's thin films, focusing on compositional and structural control.

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.

Full Heusler Co2TiSn thin film were grown by DC magnetron sputtering on MgO(001) substrates at temperatures from ambient up to 800 degrees C. The epitaxial films with full Heusler structure and high purity were achieved at 700 degrees C with thickness ranging from similar to 20 nm to over 1000 nm enabling deeper study of the film growth and the effect of the thickness on film properties. The state of films was investigated by wide range of techniques including atomic force and electron microscopy, X-ray diffraction and photoelectron spectroscopy, magnetometry, and magnetooptical ellipsometry supported by ab initio calculation. Structural diffraction study and magnetooptical spectral analysis reveal that under suitable deposition conditions at 700 degrees C almost perfect L2(1) ordering can be attained with the [100] film direction along diagonal of cubic MgO substrate. Photoelectron spectroscopy indicated the purity of the deposited material and the fourfold symmetry of deposited films in agreement with XRD. The only downside of these films is a relatively high surface roughness as indicated by SEM and measured by AFM. Saturation magnetization at 10 K and Curie point at 370 K was comparable to bulk. The ordering as well as agreement between experimental and calculated Kerr spectra improve with increasing film thickness. In comparison the properties of Co2TiSn are similar to other alloys in the family such as Co2FeSi and Co2FeGa.

The impact of neutron irradiation on the properties of second-generation REBaCuO tapes was studied. The main aim of the present study was to identify the fast neutrons irradiation fluence threshold, φ th , at which the high-field critical current at low temperatures, I c (10 K, 8 T), starts degrading. Following up on our previous publication for low irradiation fluences, two new irradiation steps were added. Up to the last but one irradiation step, the T c in all samples followed a linear dependence on the fast neutrons irradiation fluence, T c ( φ ) ≈ T c (0) − τφ ( τ being a numerical constant in units of K/10 22 m −2 and φ in 10 22 m −2 ). In parallel, I c (10 K, 8 T) continuously grew with φ up to the threshold fluence, φ th ≈ 3× to 4.3 × 10 22 m −2 . The last gradual irradiation by φ ≈ 1.83 × 10 22 m −2 brought a severe degradation both in T c and I c (10 K, 8 T) in most samples, irrespective of their irradiation history. This effect was attributed to radiation overheating. Annealing of pristine tapes in dilute Ar confirmed this scenario. Two of the SuperPower tapes, doped by Zr, appeared to be exceptionally stable against both types of overheating. The tape doped with 7.5% Zr, T c ( φ ) followed the linear dependence up to the highest cumulative fluence φ = 6.16 × 10 22 m −2 , while the I c (10 K, 8 T) drop was several orders of magnitude lower than in the samples without Zr. TEM study found that the Zr-doped tapes survived the overheating during the last irradiation step in a crystalline form, while all other samples were amorphized.

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 (1 (1) over bar0) 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.

The key for the existence of magnetic induced reorientation is strong magnetocrystalline anisotropy, i.e., the coupling between ferroelastic and ferromagnetic ordering. To increase the transformation temperatures and thus functionality, various elemental alloying in Ni-Mn-Ga is tried. We analyzed more than twenty polycrystalline alloys alloyed by small amount (up to 5atom%) of transitional metals Co, Fe, Ni, and Cu for the value of magnetic anisotropy in search of general trends with alloying. In agreement with previous reports, we found that maximum anisotropy occurs at stoichiometric Ni2MnGa and any alloying decreases its value. The strongest decrease of the anisotropy is observed in the case where the alloyed elements substitute Ga.

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.

Low twinning stress (TS) is a prerequisite for magnetic shape memory functionality in ferromagnetic martensites. We compare Ni 50 Mn 28 Ga 22 (nominal at.%) single crystals from four different producers to reveal the effect of crystal quality on the TS. Near the reverse martensite transformation, the TS is generally low, about 1 MPa, regardless of mosaicity of up to 1.7° and chemical composition deviations of up to 2 at.% of Mn. Pure type I and type II twin boundaries occur in crystals with smooth chemical composition gradients. The corresponding temperature dependences of TS follow universal linear trends with the slopes of −0.045 MPa/K for type I twins and −0.001 MPa/K for type II twins, enabling a very low TS down to 2 K for the latter. An intermediate slope of −0.023 MPa/K appears for mixed type I/type II twin boundaries in a crystal with sharp local chemical deviations. We conclude that the crystal quality affects the magnitude of the TS indirectly, through its impact on the character of the twin boundaries. The effect is weak near the martensite transformation temperature and strong at low temperatures.

Fe75−xMn25Gax Heusler-like compounds were investigated in a wide range of Fe/Ga ratios while keeping the Mn content constant and equal 25 at% in order to elucidate the interplay between magnetic properties and composition. Materials were prepared by arc-melting from pure elements and subsequently annealed. Experimental investigations were focused on magnetization behavior in a wide temperature range from 4 to 1000 K and magnetic field up to 9 T. Optical and magneto-optical (MO) measurements were employed to shed more light on the magnetic state and electronic structure of investigated materials. Magnetization measurements indicated that in the vicinity of stoichiometry (Fe2MnGa) the compounds are ferro/ferrimagnetic, whereas the Fe-deficient compound is paramagnetic and at high Fe concentration the antiferromagnetic interaction prevails. Theoretical calculations of corresponding ordered and disordered stoichiometric compounds were carried out and compared to the experiment on the level of net magnetic moment as well as magneto-optical spectra. This comparison suggests that the Heusler crystal structure, L21, is not present even close to stoichiometry. Moreover, the comparison of density of states (DOS) for ordered and disordered structures allowed us to explain missing martensitic transformation (MT) in investigated materials.

In bulk high-temperature superconductors, Ag is usually used as an additive thus improving mechanical performance. In MgB(2)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 MgB(2)doped with 1 wt% MgB(4,)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 theF(n)(b) dependence (b=B/B-irr,B(irr)being the irreversibility field) was observed at aroundb= 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 MgB(2)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 MgB(2)grains control the main pinning mechanism, the small precipitates seem to determine details of the current flow through the grain boundaries.

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 microstructural, 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 γ-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 γ 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 γ phase, is thus crucial for the optimal behaviour of the alloys.

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.

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 degrees 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.

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.

Superconducting REBaCuO (RE=Gd, Y+Gd) tapes intended for wiring magnets for fusion reactors were investigated in response to neutron irradiation by fast neutron fluences up to 4 x 10(22) m(-2). The results indicate that the tapes are appropriate for the given purpose, in particular at temperatures below 25 K. There, due to neutron irradiation, the critical currents at low magnetic fields are gradually reduced, while in high magnetic fields they are first enhanced, then degraded. The position of the crossover differs from one tape type to another, in dependence on the initial pinning landscape. To find reasons for such behavior, SEM and TEM analyses started. Some preliminary SEM results are presented. The effect of irradiation on pinning landscape is discussed.

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.

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 <2 mT. Water quenching resulted in fine magnetic and APB patterns and enlarged the coercivity to 24 mT at room temperature, and this further increased with decreasing temperature up to 41 mT at 10 K. The analysis of magnetization approach to saturation indicated the antiferromagnetic character of APBs on which the magnetic domain walls were pinned. Despite the one to two order increase of coercivity, the twinning stress remained low, between 0.7 to 1.4 MPa, and about 6% MSM effect was observed. The best ratio between coercivity and twinning stress (17 mT / 0.7 MPa) was obtained for the sample quenched in air. Contrary to previous reports, 100 ppm B doping of Ni-Mn-Ga had no or weak effect on the magnetic coercivity. Instead, the major effect originated from the high density of antiphase boundaries.

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 N-2+H-2 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, T-c, (T-c above T-c of bulk Ni) and perpendicular magnetic anisotropy due to the formation of Ni particles into chains, normal to the substrate.