Dr. Mihaela SOFRONIE

The Ni49+xMn32-2xGa19+x (x = 0; 2) Heusler ferromagnetic shape memory alloys were prepared using spark plasma sintering using raw flake-type powders obtained by soft grinding melt-spun ribbons. Samples were characterized using x-ray diffraction, electron microscopy, thermal analysis, and bending tests. Although the properties of ribbons and corresponding powders show similar properties' tendencies, they are opposite in the bulk sintered alloys when compared with precursor powders. Namely, Ni49Mn32Ga19 bulk shows a higher enthalpy (5.8 J g-1), an increased martensitic transformation (MT) temperature (by 9 K), and a reduced hysteresis span (5 K). Conversely, for the Ni51Mn28Ga21 sintered sample, a lower enthalpy (2 J g-1), a significant decrease (by 40 K) in the MT starting temperature, and a broadening of the hysteresis range (26 K) were observed. This difference is analyzed versus specific features of the microstructure. Moreover, the activation energy and the pre-exponential factor of the MT, extracted through kinetic analysis within two non-isothermal models, Kissinger and Friedman, complement and sustain these findings. Fractography details of the sintered samples are discussed in relation to the stress-strain curves from the bending tests. The Ni49Mn32Ga19 bulk sample exhibits a higher bending strength (260 MPa) and a lower strain (0.55%) than the Ni51Mn28Ga21 sample (177 MPa and 0.61%). The observed dependence of functional characteristics on preparation enables the possibility of property control required for various applications and suggests that the proposed route is promising in this regard for further investigations.

When a severe plastic deformation (SPD) process is performed at high temperatures, it becomes more versatile. Designed originally for the bulk nanoconstruction of hard-to-deform alloys, high-speed high-pressure torsion (HSHPT) is an SPD method used in this research for assembling multiple layers of shape memory nanocomposites. Three hard-to-deform magnetic alloys in the cast state were used. Soft magnetic shape memory alloys, NiFeGa and FePdMn, and a potentially hard magnetic alloy, CoZr, were assembled in various composites. Both grain refinement and strong layer bonding were achieved in ZrCo/FePdMn and ZrCo/NiFeGa composites in seconds. The very short SPD time is specific to HSHPT because of the intense friction that occurs under high pressures, which generates huge amounts of heat. After SPD, the temperature rises in bulk material like a pulse, being dissipated mostly through heat conduction. The SPD parameters were carefully controlled with an advanced automation system using a programmable logic controller. Nevertheless, the major drawbacks of high-pressure torsion were overcome, and large SPD discs were obtained. Various investigation techniques (optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and atomic force microscopy) show well-defined interfaces as well as a fine and ultrafine structure.

Fe, Co and Si powders were exposed to mechanochemical activation by high-energy ball milling for 0, 2, 4, 8 and 12 h. The samples were subsequently characterized by Mossbauer spectroscopy, X-ray powder diffraction (XRPD), magnetic measurements and optical diffuse reflectance spectroscopy. The room temperature Mossbauer measurements were consistent with the occurrence of FeCo2Si and Fe0.5Co0.5Si crystalline phases. The low temperature Mossbauer spectra confirmed the absence of superparamagnetism up to 44 K in the milled system. XRPD patterns supported the phase sequence derived from Mossbauer spectroscopy. The coercive field was found to increase with the ball milling time (BMT). Zero-field-cooling-field-cooling (ZFC-FC) measurements performed at 200 Oe in the temperature range 5-300 K evidenced the transition to the skyrmion phase of the Fe0.5Co0.5Si material below the critical temperature of 44 K. The optical absorption in the UV-Vis-NIR region of the spectrum was found to increase with BMT.

In our work, the kinetics of martensitic transformations and the influence of thermal treatments on martensitic transformations, as well as the related magnetic properties of the Ni49Mn32Ga19 ferromagnetic shape memory melt-spun ribbons, have been investigated. Thermal treatments at 673 K for 1, 4 and 8 h can be considered an instrument for fine-tuning the performance parameters of alloys. One-hour thermal treatments promote an improvement in the crystallinity of these otherwise highly textured ribbons, reducing internal defects and stress induced by the melt-spinning technique. Longer thermal treatments induce an important magnetization rise concomitantly with a slight and continuous increase in martensitic temperatures and transformation enthalpy. The activation energy, evaluated from differential scanning calorimeter experimental data with a Friedman model, significantly increases after thermal treatments as a result of the multi-phase coexistence and stabilization of the non-modulated martensitic phase, which increases the reverse martensitic transformation hindrance.

We present a simple two-dimensional model for a phase transition, then study its predictions, in particular the memory properties. The direct transformation is modeled by randomly placing small squares, "nuclei", on an initially empty surface. Then, the nuclei expand ("grow") up to finite final sizes which are randomly chosen in a given range, while keeping their square shape. An important issue is the "interaction" which forces some squares to remain at smaller sizes if the surrounding squares get in the way of their growth. Interestingly, this naturally leads to quasiequal total area covered by the squares of each size after a complete direct transformation. Next, it is shown that the system "remembers" incomplete ("arrested") reverse transformations taking place in reversed order of the squares sizes. The memory is "encrypted" in the distribution of the squares sizes after a next direct transformation and manifests as a significant imbalance between the areas covered by the "big" and "small" (relative to the arrest size) squares. We are able to also reproduce the so-called "hammer effect" and the memorizing of multiple arrest points. Our model is particularly relevant for the thermal memory effect in shape memory alloys, and we actually borrowed many features from existing thermodynamic models addressing this effect. However, here we eliminate the explicit thermodynamics and end up with a statistical geometry model, presumably easier to reproduce.

Yttrium iron garnet nanoparticles were exposed to mechanochemical activation by high-energy ball milling for 0, 2, 4, 8 and 12 h, with and without graphene nanoparticles. The samples were subsequently characterized by Mo center dot ssbauer spectroscopy, X-ray powder diffraction (XRPD), magnetic measurements and optical diffuse reflec-tance spectroscopy. Examination of the quadrupole doublet's abundance as function of ball milling time indi-cated that graphene slowed down the precipitation of the yttrium iron perovskite (yttrium orthoferrite) phase. The increased linewidth of the doublet showed that the carbon from graphene preferentially entered the lattice of the yttrium orthoferrite. The saturation magnetization decreased with decreasing particle size for prolonged milling due to the occurrence of the antiferromagnetic hematite phase. The enhanced absorption in the infrared region could be associated with the incorporation of carbon from graphene in the lattice of the yttrium ortho-ferrite. The results are interesting for sensing and microwave applications.

Shape memory alloys, especially ferromagnetic shape memory alloys, are interesting new materials for the manufacturing of stents. Iron-palladium alloys in particular can be used to manufacture self-expanding temporary stents due to their optimum rate of degradation, which is between that of magnesium and pure iron, two metals commonly used in temporary stent research. In order to avoid blood clotting upon the introduction of the stent, they are often coated with anticoagulants. In this study, sulfated pectin, a heparin mimetic, was synthesized in different ways and used as coating on multiple iron-palladium alloys. The static and dynamic prothrombin time (PT) and activated partial thromboplastin time (APTT) of the prepared materials were compared to samples uncoated or coated with polyethylene glycol. While no large differences were observed in the prothrombin time measurements, the activated partial thromboplastin time increased significantly with all alloys coated with sulfated pectin. Aside from that, sulfated pectin synthesized by different methods also caused slight changes in the activated partial thromboplastin time. These findings show that iron-palladium alloys can be coated with anticoagulants to improve their utility as material for temporary stents. Sulfated pectin was characterized by nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopy, and the coated alloys by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX).

This work reports the effect of the rapid solidification technique and thermal treatment on the martensitic transformation (MT), magnetic and magnetostrictive properties on the off-stoichiometric Ni49Mn31Ga20 and Ni51Mn28Ga21 ferromagnetic shape memory ribbons. The samples were investigated by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, magnetic and magnetostrictive measurements. The temperature dependence of the X-ray phases analysis shows the presence of martensite structures, both tetragonal and monoclinic, at room temperature and allowed to study their evolution through MT. The thermal treatment induces changes in the microstructure with implications in MT and Curie temperatures evolution. The competition between the magnetization orientation and twin boundary motion within martensitic variants under magnetic field evidenced in the magnetic-strain curves was discussed and correlated with the magnetic data.

The influence of the severe plastic deformation via high-speed high-pressure torsion (HSHPT) on the structural and magnetic properties of the Zr13Co87 alloys is investigated. Moderate applied deformation promotes the growth of the rhombohedral hard magnetic phase leading to the increase of the sample's hardness and magnetic coercivity. A higher degree of deformation affects the samples morphology leading to a critical value of the grain size under which the exchange coupling of the soft phase is less effective. Additionally, it produces a random alignment of the anisotropy axes, which are both detrimental to the hard magnetic properties.

The influence of processing on the martensitic transformation and related magnetic properties of the Ni55Fe18Nd2Ga25 ferromagnetic shape memory alloy, as bulk and ribbons prepared by the melt spinning method and subjected to different thermal treatments, is investigated. Structural, calorimetric, and magnetic characterizations are performed. Thermal treatment at 1173 K induces a decrease in both the Curie and the martensitic transformation temperatures, while a treatment at 673 K produces the structural ordering of the ribbons, hence an increase in T-C. A maximum value of the magnetic entropy variation of -5.41 J/kgK was recorded at 310 K for the as quenched ribbons. The evaluation of the magnetoresistive effect shows a remarkable value of -13.5% at 275 K on the bulk sample, which is much higher than in the ribbons.

Magnetic ceramic nanoparticles system xNd(2)O(3)-(1-x)alpha-Fe2O3 (x = 0.1, 0.3 and 0.5) was synthesized by mechanochemical activation starting from hematite and neodymium oxide precursors and characterized by X-ray diffraction (XRD) and Mossbauer spectroscopy. Rietveld refinement of XRD data evidenced the formation of neodymium orthoferrite NdFeO3 as an end-product with a particle size of about 22 nm, determined using the Scherrer method for x = 0.5. The Mossbauer spectra were typically analyzed considering 2 sextets, corresponding to hematite and neodymium orthoferrite and a doublet, representing superparamagnetic particles (SPM). The recoilless fractions were determined using our dual absorber method and were found consistent with a decrease in particle size as consequence of the ball milling process performed.

Durable biocompatible metal vascular implants are still one of the significant challenges of contemporary medicine. This work presents the preparation of ferromagnetic biomaterials with shape memory in metal strips based on FePd (30 at% Pd) that is either not doped or doped with Ga and Mn, coated with poly(benzofuran-co-arylacetic acid) or polyglutamic acid. The coating of the metal strips with polymers was achieved after the metal surface had been previously treated with open-air cold plasma. The final functionalization was performed to induce anti-thrombogenic/thrombolytic properties in the resulting materials. SEM-EDX microscopy and X-ray photoelectron microscopy (XPS) determined the morphology and composition of the metal strips covered with polymers. In vitro tests of standardized thromboplastin time (PTT) and prothrombin time (PT) were performed to evaluate the thrombogenicity of these biofunctionalized materials for future possible monitoring of the implant in patients.

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K-which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples.

The influence of the partial substitution of Fe by Si and thermal treatments on the structural, magnetic and magnetostrictive properties of the Fe67.5Pd30.5Si2 rapidly solidified ribbons has been investigated. A remarkable decrease in the martensite transformation temperature, with similar to 65 K lower than that of the Fe-Pd archetype alloy, is observed in the as-prepared ribbons. The thermal treatments shift the martensite transformation temperatures upward, with approximately 13 K for the higher thermal treatment. Also, these induce an improvement in the crystallinity in these ribbons with high texture and an increase in the crystallite size as a result of reducing the internal defects and stress. The thermodynamic considerations discussed in the frame of the Clapeyron-Clausius relation by using the calorimetric and thermomagnetic measurements (up to 7 T) reveal a weak influence of the magnetic fields on the martensitic transformation temperatures (similar to 0.5 K/T). The magnetostriction decrease with temperature under small magnetic fields was discussed, beside an unusual behaviour in the technically saturated domain. This behaviour is based on the coexistence of the ordinary and forced magnetostrictions, the last one increasing faster with the temperature decreasing.

The effects of severe plastic deformation (SPD) process via high-speed high-pressure torsion technique on martensitic transformation of Ni-Fe-Ga Heusler shape memory alloy are the subject of this work. The results show that moderate degrees of deformation lead to a decrease in the martensitic transformation temperatures, while the heat of reaction is enhanced only for the sample processed with the lowest degree of deformation. The results are explained by the interplay between the constituent tetragonal L10 and the cubic gamma crystal structures and the evolution of the samples morphology with the severity of deformation. The reduction in the samples granulation due to the progressive increase in the SPD is reflected by the magnetic properties of the samples with decreasing coercivity and Curie temperatures. At the highest applied degree of deformation, sample nanostructuring and a possible amorphization might explain the vanishing of MT.

The paper presents the research results in the field of TiNiCu shape memory alloys processed by powder metallurgy techniques. The powders mixtures used to synthesize Ti50Ni30Cu20 alloys were obtained by blended constitutive elemental powders or by mechanical alloying of powder mixtures for 10 and 20 hours, respectively. Finally, a subsequent annealing process was carried out, followed by a rapid cooling in ice water. All experiments were performed under 99.9% argon atmosphere. At room temperature, the obtained materials contained monoclinic and orthorhombic martensite type. The structural and thermal investigation of these materials were discussed in view of the potential development of actuators.

This work reports the effect of the Mn substitution, rapid solidification technique and heat treatments on the martensitic transformation, magnetic and magnetostrictive properties on the Fe70-xPd30Mnx (x = 1, 3) ferromagnetic shape memory ribbons. The samples were investigated by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, magnetic and magnetostrictive measurements. The thermal treatments induce significant changes in the microstructure and magnetocrystalline anisotropy of the martensitic phase, for Fe67Pd30Mn3 compared to Fe69Pd30Mn1. The competition between the magnetization orientation and twin boundary motion within martensitic variants under magnetic field evidenced in the magnetic-strain curves was discussed and correlated with the magnetic data.

Specific functional characteristics of the ferromagnetic shape memory alloy Fe67.5Pd30.5Ga2 prepared as ribbons by rapid quenching technique are reported. The shift of the martensitic transformation temperature induced by the applied magnetic field was determined from the thermomagnetic measurements at various fields up to 5T being proved to be in good agreement with the result obtained from calorimetric data via the Clapeyron - Claussius relation. Magnetostriction measurements reveal a pure spin rotation mechanism under low applied magnetic fields, allowed by the reduced magnetic anisotropy. In high magnetic fields and at temperatures close to martensitic transformation, the magnetostriction has a linear increase up to the maximum considered magnetic induction of 3T. This behavior has been discussed in connection with the forced magnetostiction aspects.

The work is dedicated to the investigation of optical, structural, magnetic and magneto-optical properties of an aluminophosphate glass doped with Dy3+ ions, for specific applications as Faraday rotators in the visible spectral domain. The vitreous material belongs to the 16Li(2)O-8Al(2)O(3)-6BaO center dot 60P(2)O(5)-10Dy(2)O(3) system. Optical homogeneity measured by a polariscopic method, as well as by polarimetry and interferometry revealed an optical quality glass. Time dependent electrical conductance measurements have shown a high chemical strength of the glass. Optical absorption of the doped glass in the visible domain evidenced the specific absorption lines of dysprosium ions, whereas structural investigations made by FTIR and Raman spectroscopy put in evidence the vitreous network forming role of phosphorous pentoxide. Magnetic and magneto-optical measurements demonstrated paramagnetic features of the doped glass, as well as a Verdet constant of about -0.05 min/Oe/cm at 600 nm wavelength.

Ce3+ and Tb3+-doped silico-phosphate films were obtained by using the sol-gel method, followed by the spin-coating deposition on silicon substrate. The homogeneity of the films was investigated by the conoscopy method. It was observed that the analysed films are isotropic but relatively inhomogeneous due to the specificity of the deposition technique. The morphology of the sol-gel films was investigated by Scanning Electron Microscopy and Atomic Force Microscopy. The elemental composition was determined by Energy Dispersive X-ray analysis. The magneto-optical investigations evidenced the capability of Ce and Tb-doped films of less than 2 mu m thickness to produce measurable Kerr rotations of 1 mdeg/T and 0.28 mdeg/T, respectively. (C) 2018 Elsevier B.V. All rights reserved.

Ferromagnetic shape memory alloys (FSMA) have attained strong interest over the last years and Fe-Pd alloys seems to be more suited for engineering and medical applications, due to their improved ductility and biocompatibility compared to the well-known Ni2MnGa alloy. The shape memory effect in disordered Fe-Pd (30 at.% Pd) is associated with FCC-FCT thermoelastic martensitic transformation. The melt-spinning technique enables to get ribbons with the FCC meta-stable structure preventing the precipitation of undesirable BCT irreversible phase and subsequent proper thermal treatments could improve the characteristic parameters of the martensitic transformation. The present work reports the effect of the rapid solidification (via melt-spinning technique) and the different thermal treatments on the microstructure, martensitic transformation and magnetic properties of the Fe69.4Pd30.6 ribbons. The samples were investigated by calorimetry, X-ray diffractometry, scanning electron microscopy and magnetometry. Two different structures induced by the distinct thermal treatments and responsible for the characteristic behavior of the martensitic transformation, were noticed and discussed in details. The high temperature treatment for short time stabilized the FCC phase and slightly decrease the martensitic transformation temperature, while the annealing at low temperature for longer time promotes the reduction of the amount of transforming FCC phase by its partial decomposition in the stable phases, causing the fall of the heat of transformation.

In this paper, we investigate the influence of Cu, Co, and Al substitutions on the transport properties, and in particular, the magnetoresistive effect in Ni-Fe-Ga ferromagnetic shape memory alloys (FSMAs) prepared as ribbons by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetometry, and magnetoresistive characterizations were performed. In the range of the martensitic transformation (MT), different FSMA compositions show a rich spectrum of different behaviors. For one of the compositions (Ni52Fe20Co2Ga23Al3), the magnetoresistance (MR) showed a local minimum or, on the contrary, a local maxima of reduced amplitude on cooling, in the range of the MT, depending on the performed thermal treatments. In the same composition, by replacing one Al atom with a Co one, no local extremes are seen, the alloy having a concomitant magneto-structural transition. When the magnetic field was varied, the MR showed a nonmonotonic variation in the martensite phase for some compounds, possibly due to the dynamics of the martensite variants realignment. From the studied compositions, the highest MR found on the MT of -9% for 5 T is for Ni50Fe20Ga27Cu3.

Samples of Ni57-xNdxFe18Ga25 with x=2 and 4 were prepared in ribbon form by rapid quenching via melt spinning route. The samples were analyzed by X-ray diffraction (XRD), magnetic measurements and Mossbauer spectroscopy, both in the as-quenched form and after thermal annealing at 900 degrees C for 2 min and 400 degrees C for 2 hours. For x=2 the Nd atoms are completely dissolved in the Ni-Fe-Ga matrix, while for x=4 the additional occurrence of the secondary 2:17 phase could be resolved. These findings were supported by the analysis of hyperfine magnetic field distributions obtained from the non-linear least-squares fitting of the Mossbauer spectra.

The influence of the heat treatments on the martensitic transformation, magnetic properties and thermo- and magnetic induced strain on Ni50Fe20Ga27Cu3 ferromagnetic shape memory alloy prepared as ribbons by melt spinning technique are investigated. The degree of atomic order as effect of different thermal treatments produces important changes in the magneto-crystalline anisotropy of the martensite phase. The anomalies evidenced in the thermo-and magnetic-strain curves are discussed and correlated with the thermo-magnetic data. The transformation-induced strains with and without magnetic field have been measured, the results setting out the influence of the pre-martensitic transformation. Copyright (C) 2016 The Authors. Published by Elsevier B.V.

Aluminophosphate glasses from the Li2O-BaO-Al2O3-P2O5 system with the addition of nonmagnetic and paramagnetic rare earth ions, were prepared using a wet nonconventional method to process the raw materials, followed by a melting-quenching procedure. The glasses obtained were characterized with respect to their magnetic and magneto-optical properties using superconducting quantum interference device magnetometry and spectroscopic ellipsometry. The assumption of a linear dependence of the Verdet constant on the magnetic susceptibility, with a proportionality constant dependent on the type of vitreous matrix and doping ion, is critically discussed. The diamagnetic and paramagnetic contributions to the Faraday rotation were separately analyzed and specific designs for optimal active and passive elements are proposed.

The effect of "in situ" thermal treatments (by DSC measurements) on the martensitic transformation in two representative Ni-Fe-Ga and Ni-Mn-Ga alloys has been studied and discussed by correlating the structural and magnetic properties. The alloys were prepared from high purity elements, by arc melting under argon protective atmosphere as bulk and also as melt-spun ribbons - an alternative preparation route that also allows to assess the influences of grains size and strain induced by this processing method. All samples presented reversible thermo-elastic transformations. The thermal treatments promote a reduction of the martensitic transformation temperatures in the Ni-Fe-Ga investigated samples, as opposed to the stoichiometric Ni2MnGa where the temperatures increase with increasing the annealing temperatures. Interestingly however, the off-stoichiometric Ni-Mn-Ga with increased Ni content recovers the behaviour with reduction of transformation temperatures by thermal treatments. The precipitation of the secondary FCC (gamma) phase is inherently found in Ni-Fe-Ga alloys with Ga <= 27% at, and also-although in lower amounts- in the off-stoichiometric Ni-Mn-Ga. The gamma phase is considered to contribute to the decrease of the MT temperatures (via valence electrons concentration depletion of the main matrix) and of the transformation heat as well as to the final structural degradation if the temperature of the thermal treatments is further increased. In addition, this phase, located mainly at the grain boundaries, is responsible for the improved ductility of Ni-Fe-Ga based alloys. Changes in the transformation heat due to thermal treatments are observed and discussed in both types of alloys, the maxima of the transformation heat being associated with the highest atomic order. Thermo-magnetic measurements show that Ni-Fe-Ga alloys have close magnetic and structural transitions temperatures, with promising applications for magnetic refrigeration. Copyright (C) 2016 The Authors. Published by Elsevier B.V.

A phenomenological 2D model, simulating the martensitic transformation, is built upon existing experimental observations that the size of the formed plates - in direct transformation - decreases as the temperature is lowered; then they transform back in reversed order. As such, if a reverse transformation is incomplete (arrested), the subsequent direct one will show anomalously a large number of big size plates - old plus newly formed - but consequentially a depletion of intermediate sizes, due to geometrical constraints, phenomenon that generates thermal memory. (C) 2015 Elsevier Ltd. All rights reserved.

Influence of Nd substitution for Ni on the magnetic properties and the martensitic transformation (MT) characteristics are investigated on Ni57-xNdxFe18Ga25 (x=0 divided by 4) ferromagnetic shape memory alloys (FSMAs) in bulk and also in ribbons prepared by melt spinning method and subjected to different thermal treatments. Increasing the Nd content induces a decrease of both the Curie and the MT temperatures. (C) 2015 The Authors. Published by Elsevier Ltd.

The present work addresses the shape memory and (ferro) magnetic properties of Fe52Ni29-xCo15+xTi4 (with x=0, 3 and 6) alloys. The analysed samples were prepared as ribbons by the melt spinning method and subjected to thermal treatments. X-ray diffraction, DSC, thermomagnetic measurements and Mossbauer spectroscopy were used for a complete structural and magnetic characterization. Both the preparation route and the different Co addition induce specific effects which are discussed in detail. The sample with x=0 sustains an irreversible transformation, while a partial reversible transformation and a relatively increased Curie temperature were observed for sample with x=3. However, further increasing the Co content to x= 6 leads to a loss of the martensitic transformation.

A rapid high sensitive method for determining the Faraday rotation of optical glasses is proposed. Starting from an experimental setup based on a Faraday rod coupled to a lock-in amplifier in the detection chain, two methodologies were developed for providing reliable results on samples presenting low and large Faraday rotations. The proposed methodologies were critically discussed and compared, via results obtained in transmission geometry, on a new series of aluminophosphate glasses with or without rare-earth doping ions. An example on how the method can be used for a rapid examination of the optical homogeneity of the sample with respect to magneto-optical effects is also provided. (C) 2015 AIP Publishing LLC.

The effect of thermal treatments on the martensitic transformation in three representative Ni-Fe-Ga alloys with or without Co substitutions has been studied by calorimetry, X-ray diffractometry, scanning electron microscopy and magnetometry. The alloys were prepared as ribbons, by the melt spinning technique. The thermal treatments promote a reduction of the martensitic transformation temperature in all investigated samples, with the most pronounced decrease for the alloys with lower Ga content. Three different mechanisms induced by specific thermal treatments and responsible for the characteristic behaviour of the martensitic transformation, with respect to temperature and heat of transition, were observed and discussed in details. (C) 2015 Elsevier B.V. All rights reserved.

Ribbons of Fe-Pd ferromagnetic shape memory alloys were prepared by rapid solidification via the melt-spinning technique. The effect of moderate and high temperature heat treatments on the martensitic transformation and the related changes of the magnetic behavior was analyzed by X-ray diffraction, differential scanning calorimetry and temperature, and field-dependent magnetometry. In addition, information about magnetic easy axis and magnetically field-induced strains (MFIS) were achieved from linear thermal expansion measurements performed under cooling at different applied fields. The observed low values of MFIS in the as-prepared ribbons are due, besides to the small size and random orientation of the grains, to the high atomic disorder inside the crystalline grains. The anisotropy field is enhanced by the reduced atomic disorder, as reflected by the increased MFIS values after thermal treatments.

The functionality of the ferromagnetic shape memory alloys is related to the martensitic and magnetic order-disorder transformations, both of which may be tailored by doping with other elements or by suitable thermal treatments, so that alloys with concomitant (or sequential but close) structural and magnetic phase transitions may be obtained. Concerning the magnetocaloric applications, it is assumed that the thin melt-spun ribbons assure a more efficient heat transfer. In the present work we investigate the influence of Co and Al substitutions on magnetocaloric effect characteristics of NiFeGa in bulk and also in ribbons prepared by melt spinning method and subjected to different thermal treatments. X-ray diffraction, differential scanning calorimetry, magnetocaloric and magnetoresistive characterizations have been performed. The results highlight the differences between the bulk and the ribbons (both as prepared and annealed) and the role of substitutions.

The martensitic transformation characteristics in two series of cobalt substituted Ni-Fe-Ga Ferromagnetic Shape Memory alloys have been studied by differential scanning calorimetry, X-ray diffraction, electrical resistivity and thermomagnetic measurements. Co substitution for Fe or Ni promotes an increase of the Curie temperatures but the changes of the martensitic transformation temperatures are in agreement with the valence electron concentration and unit cell volume variations. A large hysteresis observed between cooling and heating curves as feature of a first order phase transition was evidenced by r(T) and thermomagnetic measurements.

Phase transformation behavior of Ti50Ni30Cu20 shape memory alloys prepared by powder metallurgy is analyzed with respect to the duration of mechanical alloying. The processed blends were studied by differential scanning calorimetry and room temperature X-ray diffraction. The martensitic transformations evidenced by thermal scans are discussed in correlation with the relative phase content obtained from the refinement of the X-ray diffraction patterns. (c) 2011 Elsevier B.V. All rights reserved.

The influence of different crystallization conditions on the microstructure and magnetic hardening of RE-Fe-B amorphous ribbons with different Fe concentrations and Pr and Nd as rare earth elements was analyzed. The microalloying effects of Zr and Ti substitution on the evolution of crystallization process and the magnetic hardening was also discussed. The final aim of this experimental study was to obtain performing exchange spring magnets with as much as lower content of the expensive RE material.

Two series of cobalt substituted Ni-Fe-Ga ferromagnetic shape memory alloys with lower gallium content (< 27 at. %) were studied by differential scanning calorimetry, x-ray diffraction, and thermomagnetic measurements. Co substitution for Fe or Ni promotes an increase in the Curie temperatures and a variation in the martensitic transformation (MT) temperatures, in accord with the alloy valence electron concentration change. For alloys with MTs below room temperature, a field dependent thermomagnetic hysteresis was evidenced and discussed in connection with the effect of cobalt substitution, on the magnetic hardness of the martensite phase. A direct interpretation of the evolution of the thermomagnetic hysteresis versus the applied field was provided. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3429231]

The paper presents the research results in the field of TiNiCu shape memory alloys processed by powder metallurgy techniques. The powders mixtures used to synthesize Ti50Ni30Cu20 alloys were obtained by blended constitutive elemental powders or by mechanical alloying of powder mixtures for 10 and 20 hours, respectively. Finally, a subsequent annealing process was carried out, followed by a rapid cooling in ice water. All experiments were performed under 99.9% argon atmosphere. At room temperature, the obtained materials contained monoclinic and orthorhombic martensite type. The structural and thermal investigation of these materials were discussed in view of the potential development of actuators.

The influence of the severe plastic deformation via high-speed high-pressure torsion (HSHPT) on the structural and magnetic properties of the Zr13Co87 alloys is investigated. Moderate applied deformation promotes the growth of the rhombohedral hard magnetic phase leading to the increase of the sample's hardness and magnetic coercivity. A higher degree of deformation affects the samples morphology leading to a critical value of the grain size under which the exchange coupling of the soft phase is less effective. Additionally, it produces a random alignment of the anisotropy axes, which are both detrimental to the hard magnetic properties.

This work reports the effect of the rapid solidification technique and thermal treatment on the martensitic transformation (MT), magnetic and magnetostrictive properties on the off-stoichiometric Ni49Mn31Ga20 and Ni51Mn28Ga21 ferromagnetic shape memory ribbons. The samples were investigated by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, magnetic and magnetostrictive measurements. The temperature dependence of the X-ray phases analysis shows the presence of martensite structures, both tetragonal and monoclinic, at room temperature and allowed to study their evolution through MT. The thermal treatment induces changes in the microstructure with implications in MT and Curie temperatures evolution. The competition between the magnetization orientation and twin boundary motion within martensitic variants under magnetic field evidenced in the magnetic-strain curves was discussed and correlated with the magnetic data.

The effects of severe plastic deformation (SPD) process via high-speed high-pressure torsion technique on martensitic transformation of Ni-Fe-Ga Heusler shape memory alloy are the subject of this work. The results show that moderate degrees of deformation lead to a decrease in the martensitic transformation temperatures, while the heat of reaction is enhanced only for the sample processed with the lowest degree of deformation. The results are explained by the interplay between the constituent tetragonal L10 and the cubic gamma crystal structures and the evolution of the samples morphology with the severity of deformation. The reduction in the samples granulation due to the progressive increase in the SPD is reflected by the magnetic properties of the samples with decreasing coercivity and Curie temperatures. At the highest applied degree of deformation, sample nanostructuring and a possible amorphization might explain the vanishing of MT.