NIMP| Felicia TOLEA, Author at NIMP

Positions

CS II

Publications

1. Formation of skyrmion phase in the Fe-Co-Si system by mechanochemical activation

Authors: Sorescu, M; Tolea, F; Sofronie, M; Kuncser, V; Craig, AJ; Aitken, JA

Published: SEP 1 2024, PHYSICA B-CONDENSED MATTER, 688, 416153, DOI: 10.1016/j.physb.2024.416153

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 Mo<spacing diaeresis>ssbauer spectroscopy, X-ray powder diffraction (XRPD), magnetic measurements and optical diffuse reflectance spectroscopy. The room temperature Mo<spacing diaeresis>ssbauer measurements were consistent with the occurrence of FeCo2Si and Fe0.5Co0.5Si crystalline phases. The low temperature Mo<spacing diaeresis>ssbauer spectra confirmed the absence of superparamagnetism up to 44 K in the milled system. XRPD patterns supported the phase sequence derived from Mo<spacing diaeresis>ssbauer 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.

2. Magnetic Shape Memory Nanocomposites Assembled with High Speed High Pressure Torsion

Authors: Gurau, C; Tolea, F; Cimpoesu, N; Sofronie, M; Ceoromila, AC; Stefanescu, C; Gurau, G

Published: MAR 2024, NANOMATERIALS, 14, 405, DOI: 10.3390/nano14050405

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.

3. Thermodynamic and Kinetic Simulations Used for the Study of the Influence of Precipitates on Thermophysical Properties in NiTiCu Alloys Obtained by Spark Plasma Sintering

Authors: Cirstea, CD; Povoden-Karadeniz, E; Cirstea, V; Tolea, F; Kozeschnik, E

Published: MAR 2024, NANOMATERIALS, 14, 461, DOI: 10.3390/nano14050461

The thermodynamic and kinetic simulations based on the re-assessment of the thermodynamic and kinetic database of the Ni-Ti-Cu system were employed to predict the phenomena of mechanical alloying, spark plasma sintering and thermal properties of the intriguing Ni-Ti-Cu system. Thermodynamic calculations are presented for the stable and unstable phases of NiTiCu materials and support a correlation with the evolving microstructure during the technological process. Also, the thermal conductivity, the thermal diffusivity and the specific heat of spark plasma sintered and aged Cu-alloyed NiTi-based shape memory alloys (NiTiCu) with two compositions, Ni45Ti50Cu5 and Ni40Ti50Cu10, are evaluated and the influence of mechanical alloying and precipitates on thermal properties is discussed. Measurements of these thermal properties were carried out from 25 degrees C up to 175 degrees C using the laser flash method, as well as differential scanning calorimetry. The thermal hysteresis of the 20 mm diameter samples was between 8.8 and 24.5 degrees C. The observed T0 temperatures from DSC experimental transformation features are in reasonable accordance with the thermodynamic predictions. The determined k values are between 20.04 and 26.87 W/m K and in agreement with the literature results. Moreover, this paper can provide some suggestions for the preparation of NiTiCu shape memory alloys and their applications.

4. Kinetics and the Effect of Thermal Treatments on the Martensitic Transformation and Magnetic Properties in Ni49Mn32Ga19 Ferromagnetic Shape Memory Ribbons

Authors: Tolea, F; Popescu, B; Bartha, C; Enculescu, M; Tolea, M; Sofronie, M

Published: JAN 2023, MAGNETOCHEMISTRY, 9, 7, DOI: 10.3390/magnetochemistry9010007

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.

5. Kinetics and the Effect of Thermal Treatments on the Martensitic Transformation and Magnetic Properties in Ni₄₉Mn₃₂Ga₁₉ Ferromagnetic Shape Memory Ribbons

Authors: Tolea, F; Popescu, B; Bartha, C; Enculescu, M; Tolea, M; Sofronie, M

Published: JAN 2023, MAGNETOCHEMISTRY, 9, 7, DOI: 10.3390/magnetochemistry9010007

6. Effects of mechanochemical activation on the structural, magnetic and optical properties of yttrium iron garnet-graphene nanoparticles

Authors: Glasser, S; Tolea, F; Craig, AJ; Sofronie, M; Aitken, JA; Sorescu, M

Published: FEB 1 2023, PHYSICA B-CONDENSED MATTER, 650, 414501, DOI: 10.1016/j.physb.2022.414501

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.

7. MICROSTRUCTURE, MAGNETIC AND MAGNETOSTRICTIVE BEHAVIOUR IN RAPIDLY QUENCHED OFF-STOICHIOMETRIC Ni-Mn-Ga FERROMAGNETIC SHAPE MEMORY ALLOYS

Authors: Sofronie, M; Tolea, F; Enculescu, M; Pasuk, I; Popescu, B

Published: 2022, ROMANIAN REPORTS IN PHYSICS, 74, 503, DOI:

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.

8. Unidirectional Magnetic Anisotropy in Molybdenum Dioxide-Hematite Mixed-Oxide Nanostructures

Authors: Tolea, F; Sorescu, M; Diamandescu, L; Iacob, N; Tolea, M; Kuncser, V

Published: MAR 2022, NANOMATERIALS, 12, 938, DOI: 10.3390/nano12060938

MoO2-Fe2O3 nanoparticle systems were successfully synthesized by mechanochemical activation of MoO2 and alpha-Fe2O3 equimolar mixtures throughout 0-12 h of ball-milling. The role of the long-range ferromagnetism of MoO2 on a fraction of more defect hematite nanoparticles supporting a defect antiferromagnetic phase down to the lowest temperatures was investigated in this work. The structure and the size evolution of the nanoparticles were investigated by X-ray diffraction, whereas the magnetic properties were investigated by SQUID magnetometry. The local electronic structure and the specific phase evolution in the analyzed system versus the milling time were investigated by temperature-dependent Mossbauer spectroscopy. The substantially shifted magnetic hysteresis loops were interpreted in terms of the unidirectional anisotropy induced by pinning the long-range ferromagnetic order of the local net magnetic moments in the defect antiferromagnetic phase, as mediated by the diluted magnetic oxide phase of MoO2, to those less defect hematite nanoparticles supporting Morin transition. The specific evolutions of the exchange bias and of the coercive field versus temperature in the samples were interpreted in the frame of the specific phase evolution pointed out by Mossbauer spectroscopy. Depending on the milling time, a different fraction of defect hematite nanoparticles is formed. Less nanoparticles supporting the Morin transition are formed for samples exposed to a longer milling time, with a direct influence on the induced unidirectional anisotropy and related effects.

9. SOME ASPECTS REGARDING PRECIPITATES IN THE NiTi SHAPE MEMORY MATERIALS

Authors: Cirstea, CD; Tolea, F; Patroi, D; Cirstea, V; Tsakiris, V

Published: 2022, ROMANIAN JOURNAL OF PHYSICS, 67, 903, DOI:

NiTi cylindrical materials were obtained by spark plasma sintering at 850??C or 900??C from high purity Ni and Ti powders. These mixtures were previously processed by mechanical alloying for 8 and 15 hours. In order to increase the alloy homogeneity, the NiTi materials were subjected to post aging treatment in protective atmosphere at constant temperature 400??C and cooling in water with ice. All samples were characterized by optical microscopy, differential calorimetry (DSC), X-ray measurements (XRD) and micro/nanoindentation measurements. The results demonstrate that the NiTi materials obtained by this preparation route have good mechanical properties and can serve as a quantitative reference for the microstructure design of shape memory materials for various applications, such as the biomedical ones.

10. Magnetic and Magnetostrictive Properties of Ni50Mn20Ga27Cu3 Rapidly Quenched Ribbons

Authors: Sofronie, M; Tolea, M; Popescu, B; Enculescu, M; Tolea, F

Published: SEP 2021, MATERIALS, 14, DOI: 10.3390/ma14185126

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.

11. The Effect of the In-Situ Heat Treatment on the Martensitic Transformation and Specific Properties of the Fe-Mn-Si-Cr Shape Memory Alloys Processed by HSHPT Severe Plastic Deformation

Authors: Gurau, C; Gurau, G; Tolea, F; Popescu, B; Banu, M; Bujoreanu, LG

Published: AUG 2021, MATERIALS, 14, DOI: 10.3390/ma14164621

This work focuses on the temperature evolution of the martensitic phase epsilon (hexagonal close packed) induced by the severe plastic deformation via High Speed High Pressure Torsion method in Fe57Mn27Si11Cr5 (at %) alloy. The iron rich alloy crystalline structure, magnetic and transport properties were investigated on samples subjected to room temperature High Speed High Pressure Torsion incorporating 1.86 degree of deformation and also hot-compression. Thermo-resistivity as well as thermomagnetic measurements indicate an antiferromagnetic behavior with the Neel temperature (T-N) around 244 K, directly related to the austenitic gamma-phase. The sudden increase of the resistivity on cooling below the Neel temperature can be explained by an increased phonon-electron interaction. In-situ magnetic and electric transport measurements up to 900 K are equivalent to thermal treatments and lead to the appearance of the bcc-ferrite-like type phase, to the detriment of the epsilon(hcp) martensite and the gamma (fcc) austenite phases.

12. Long- and short-range order in the Ni52Co2Fe20Ga26 ferromagnetic Heusler alloy

Authors: Macovei, D; Tolea, F

Published: AUG 2021, JOURNAL OF APPLIED CRYSTALLOGRAPHY, 54, 1216, DOI: 10.1107/S1600576721006415

The crystalline structure and Fe local environment in a Co-doped Ni-Fe-Ga Heusler alloy, prepared by the melt-spinning technique, were investigated by X-ray diffraction (XRD) and EXAFS at room and low temperatures. The characteristic temperatures of the austenite-martensite phase transitions were determined by differential scanning calorimetry via cooling and heating cycles of the alloy ribbons. As shown by room-temperature XRD, the austenitic phase of the alloy has the chemically ordered L2(1) Heusler structure. This was confirmed by EXAFS, although this technique was not able to conclusively distinguish between the L2(1) and B2 structures of the austenite for the analyzed alloy. The low-temperature martensitic phase and its structural evolution towards austenite with increasing temperature were studied by high-energy X-ray diffraction, which evinced the martensite modulation. However, the Fe environment could be fitted by EXAFS with the tetragonal L1(0) structure of the non-modulated martensite. This proves that the martensite modulation has structural effects on a long-range scale, without significant changes in the short-range order around the atoms. The changes in the local structure around iron on martensitic transformation were correlated with changes in the electronic structure, described by XANES spectroscopy at the Fe K edge.

13. Multifunctional GaFeO3 Obtained via Mechanochemical Activation Followed by Calcination of Equimolar Nano-System Ga2O3-Fe2O3

Authors: Diamandescu, L; Tolea, F; Feder, M; Vasiliu, F; Mercioniu, I; Enculescu, M; Popescu, T; Popescu, B

Published: JAN 2021, NANOMATERIALS, 11, DOI: 10.3390/nano11010057

The equimolar oxide mixture beta-Ga2O3-alpha-Fe2O3 was subjected to high-energy ball milling (HEBM) with the aim to obtain the nanoscaled GaFeO3 ortho-ferrite. X-ray diffraction, Fe-57 Mossbauer spectroscopy, and transmission electron microscopy were used to evidence the phase structure and evolution of the equimolar nano-system beta-Ga2O3-alpha-Fe2O3 under mechanochemical activation, either as-prepared or followed by subsequent calcination. The mechanical activation was performed for 2 h to 12 h in normal atmosphere. After 12 h of HEBM, only nanoscaled (similar to 20 nm) gallium-doped alpha-Fe2O3 was obtained. The GaFeO3 structure was obtained as single phase, merely after calcination at 950 degrees C for a couple of hours, of the sample being subjected to HEBM for 12 h. This temperature is 450 degrees C lower than used in the conventional solid phase reaction to obtain gallium orthoferrite. The optical and magnetic properties of representative nanoscaled samples, revealing their multifunctional character, were presented.

14. Martensitic Transformation and Magnetic Properties of Ni57Fe18Ga25 Shape Memory Alloy Subjected to Severe Plastic Deformation

Authors: Popescu, B; Gurau, C; Gurau, G; Tolea, M; Sofronie, M; Tolea, F

Published: OCT 2021, TRANSACTIONS OF THE INDIAN INSTITUTE OF METALS, 74, 2498, DOI: 10.1007/s12666-021-02293-8

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.

15. EFFECT OF Mn SUBSTITUTION ON THE STRUCTURAL, MAGNETIC AND MAGNETOSTRICTIVE PROPERTIES OF Fe-Pd FERROMAGNETIC SHAPE MEMORY RIBBONS

Authors: Sofronie, M; Enculescu, M; Crisan, AD; Tolea, F

Published: 2020, ROMANIAN REPORTS IN PHYSICS, 72, DOI:

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.

16. Magnetic and magnetostrictive properties of the ternary Fe67.5Pd30.5Ga2 ferromagnetic shape memory ribbons

Authors: Sofronie, M; Tolea, F; Tolea, M; Popescu, B; Valeanu, M

Published: JUL 2020, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 142, DOI: 10.1016/j.jpcs.2020.109446

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.

17. Structural Change in Ni-Fe-Ga Magnetic Shape Memory Alloys after Severe Plastic Deformation

Authors: Gurau, G; Gurau, C; Tolea, F; Sampath, V

Published: JUN 2 2019, MATERIALS, 12, DOI: 10.3390/ma12121939

Severe plastic deformation (SPD) is widely considered to be the most efficient process in obtaining ultrafine-grained bulk materials. The aim of this study is to examine the effects of the SPD process on Ni-Fe-Ga ferromagnetic shape memory alloys (FSMA). High-speed high-pressure torsion (HSHPT) was applied in the as-cast state. The exerted key parameters of deformation are described. Microstructural changes, including morphology that were the result of processing, were investigated by optical and scanning electron microscopy. Energy-dispersive X-ray spectroscopy was used to study the two-phase microstructure of the alloys. The influence of deformation on microstructural features, such as martensitic plates, intragranular gamma phase precipitates, and grain boundaries' dependence of the extent of deformation is disclosed by transmission electron microscopy. Moreover, the work brings to light the influence of deformation on the characteristics of martensitic transformation (MT). Vickers hardness measurements were carried out on disks obtained by SPD so as to correlate the hardness with the microstructure. The method represents a feasible alternative to obtain ultrafine-grained bulk Ni-Fe-Ga alloys.

18. STRUCTURAL EVOLUTION OF THE NiTi/NiFeGa SMART HYBRID MATERIAL DURING SEVERE PLASTIC DEFORMATION

Authors: Gurau, C; Gurau, G; Tolea, F; Sampath, V

Published: JUL-SEP 2019, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 14, 546, DOI:

High speed high pressure torsion (HSHPT) a patented new approach is proposed to fabricate nanocomposites. The goal of this work is to investigate the NiTi/NiFeGa bilayer hybrid material with nano- and submicrocrystalline structure under the influence of HSHPT. Apart from the grain refinement, the effectiveness of the joint are revealed scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The mechanical properties of the composite layers in bulk and after severe plastic deformation are investigated. Bi-layered composite disks consisting of NiTi shape memory alloy and NiFeGa - Heusler type alloy, exhibit thermoelastic structural martensitic transformation. Submicrocrystalline structure is formed in the both layers of the hybrid material. It is also ascertained significant hardening of each layer of the hybrid as a result of HSHPT. The results highlight market differences between the bulk and the hybrid and the role of severe plastic deformation on martensitic transformation.

19. Structural, Magnetic and Hyperfine Properties of Molybdenum Dioxide-Hematite Mixed Oxide Nanostructures

Authors: Trotta, R; Tolea, F; Valeanu, M; Diamandescu, L; Grabias, A; Sorescu, M

Published: 2018, MRS ADVANCES, 3, 2892, DOI: 10.1557/adv.2018.420

MoO2-Fe2O3 nanoparticle system was successfully synthesized by mechanochemical activation of MoO2 and alpha-Fe2O3 equimolar mixtures for 0-12 hours of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale. X-ray powder diffraction (XRD), Mossbauer spectroscopy and magnetic measurements were used to study the phase evolution of MoO2-Fe2O3 nanoparticle system under the mechanochemical activation process. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextet and a doublet upon duration of the milling process with molybdenum dioxide. Recoilless fraction was determined using our dual absorber method and was found to decrease with increasing ball milling time. Magnetic measurements recorded at 5 and 300 K in an applied magnetic field of 50,000 Oe showed the magnetic properties in the antiferromagnetic and canted ferromagnetic states. The Morin transformation was evidenced by zero-field cooling-field cooling (ZFC-FC) measurements in 200 Oe and the transformation characteristic temperatures were shifted to lower values.

20. Magneto-optical properties of Ce3+ and Tb3+-doped silico-phosphate sol-gel thin films

Authors: Stefan, CR; Elisa, M; Vasiliu, IC; Sava, BA; Boroica, L; Sofronie, M; Tolea, F; Enculescu, M; Kuncser, V; Beldiceanu, A; Volceanov, A; Eftimie, M

Published: AUG 1 2018, APPLIED SURFACE SCIENCE, 448, 480, DOI: 10.1016/j.apsusc.2018.04.067

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.

21. (Fe, Nd) codoped ZnO micro- and nanostructures with multifunctional characteristics like photocatalytic activity, optical and ferromagnetic properties

Authors: Diamandescu, L; Cernea, M; Tolea, F; Secu, EC; Trusca, R; Secu, M; Enculescu, M

Published: DEC 1 2018, CERAMICS INTERNATIONAL, 44, 21975, DOI: 10.1016/j.ceramint.2018.08.310

Zn1-x-yFexNdyO, x = 0.02 and y = 0.00-0.05 were successfully synthesized via a hydrothermal method at 200 degrees C. XRD and Mossbauer spectroscopy indicated that (Fe, Nd) codoped ZnO, calcined at 700 degrees C, 3 h in air, contains hexagonal ZnO phase as main phase and traces of ZnFe2O4; no phase containing neodymium was observed. In this study, a strong powder morphology dependence on Nd concentration and calcination temperature was evidenced by SEM. The photocatalytic properties were found to depend on the sample morphology and Nd content. Thermoluminescence signal is weak compared to the undoped ZnO and the curves shape was almost independent on the doping level and dependent on the calcination temperature and was related to the structural defects induced by the synthesis and calcination. Photoluminescence spectra showed that the green-yellow band due to the oxygen related defects, decreases by Fe and Nd-codoping. Thermo-magnetization curves measured in low applied fields and the magnetic hysteresis loops recorded in the temperature range 5-380 K, indicate two superimposed magnetic regimes, one predominant above 100 K the other below 100 K. The observed magnetic order is not directly related to the dopant Fe or Nd ions, but to different type of defects.

22. Martensitic transformation and related properties of Fe69.4Pd30.6 ferromagnetic shape memory ribbons

Authors: Tolea, F; Sofronie, M

Published: NOV-DEC 2018, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 20, 706, DOI:

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.

23. Thermal memory fading by heating to a lower temperature: Experimental data on polycrystalline NiFeGa ribbons and 2D statistical model predictions

Authors: Tolea, F; Tolea, M; Valeanu, M

Published: MAY 2017, SOLID STATE COMMUNICATIONS, 257, 41, DOI: 10.1016/j.ssc.2017.04.003

Shape memory alloys are known to memorise one -or several-temperatures at which the martensite-austenite transformation was stopped before completion in the past, the memory manifesting as specific dips in subsequent calorimetric scans. Previous studies have shown that this memory can be erased by heating to higher temperatures than the ones previously recorded. In this paper, we study a distinct memory fading effect which takes place by heating to a lower temperature. This effect is reported in NiFeGa as polycrystalline ribbons, the alloy being initially studied as bulk for which the thermal memory effect was not found. If, after an initial incomplete heating up to T-1 one performs a second incomplete heating up to T-2 < T-1, a new calorimetric dip appears at T-2, as expected, while less expected was that the dip corresponding to T-1 reduces in amplitude or even vanishes ( if the arrest at T-2 is repeated). The memory fading effect is more clear for small differences T-1-T-2 and less obvious or absent for large ones. The second part of the paper employs a statistical 2D model, which associates the memorized temperatures with a depletion of certain martensite plates sizes, and also supports the memory fading effect.

24. Specific Changes in the Magnetoresistance of Ni-Fe-Ga Heusler Alloys Induced by Cu, Co, and Al Substitutions

Authors: Tolea, F; Tolea, M; Sofronie, M; Popescu, B; Crisan, A; Leca, A; Valeanu, M

Published: APR 2017, IEEE TRANSACTIONS ON MAGNETICS, 53, DOI: 10.1109/TMAG.2016.2628386

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.

25. Structure, Magnetic Properties and Hyperfine Parameters of Nd-Substituted Ni-Fe-Ga Heusler Alloys

Authors: Sorescu, M; Tolea, F; Valeanu, M; Sofronie, M

Published: 2017, MRS ADVANCES, 2, 1346, DOI: 10.1557/adv.2017.70

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.

26. Tuning the transmission phase by the dot size

Authors: Tolea, F; Tolea, M

Published: JAN 2017, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 85, 173, DOI: 10.1016/j.physe.2016.08.034

The phase shift of the electron's wave function after a tunneling event (i.e. the transmission phase) was at first measured for its fundamental or applicative relevance for quantum circuitry, but later the phase study self-motivated due to a number of unexpected results. One such result was the reduced increment of the phase on some resonances - with only fractions of pi - in the few-electrons "mesoscopic" regime. In this paper we address such a regime for a rectangular quantum dot and compute the total phase increase on the first four resonances by means of accurate configuration-interaction method and a generalized Friedel sum rule as proposed by Rontani (2006) [17]. Our findings confirm that the electronic correlations reduce the on-resonance phase growth which is also found to decrease quasi-linearly with the dot size, the decrease being more pronounced as the number of electrons on the dot is raised. Sudden jumps (of small amplitude) of the phase are found to accompany ground states spin transitions. (C) 2016 Elsevier B.V. All rights reserved.

27. Characterization of TiNi shape memory alloys obtained by spark plasma sintering process

Authors: Cirstea, CD; Tolea, F; Leonat, L; Lungu, M; Cucos, A; Cirstea, V; Tsakiris, V

Published: SEP-OCT 2016, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 18, 862, DOI:

This paper presents the research results on the thermal behavior of the 48.5% at Ti-Ni alloys, obtained by spark plasma sintering in vacuum under a pressure of 50 MPa, at temperatures of 850 degrees C and 900 degrees C and holding time of 5 minutes, and subsequently aged in argon for up to 30 minutes at 450 degrees C. Differential scanning calorimetry, dynamic mechanical and dilatometry analysis were used to characterize the transformation properties of the TiNi samples. All the results of differential scanning calorimetry and dynamic mechanical analysis confirmed the hypothesis that the stress-free thermally induced martensitic transformation exists in the investigated TiNi alloys. Annealing at lower temperatures (450 degrees C) in Ni-rich TiNi alloys led to precipitation process which facilitates the formation of the R-phase due to internal stress contributed by precipitates.

28. Magnetoelastic properties in polycrystalline ferromagnetic shape memory Heusler alloys

Authors: Sofronie, M; Tolea, F; Crisan, AD; Popescu, B; Valeanu, M

Published: 2016, 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), 2, 1537, DOI: 10.1016/j.prostr.2016.06.194

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.

29. The relationship between magnetism and magneto-optical effects in rare earth doped aluminophosphate glasses

Authors: Valeanu, M; Sofronie, M; Galca, AC; Tolea, F; Elisa, M; Sava, B; Boroica, L; Kuncser, V

Published: FEB 25 2016, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 49, DOI: 10.1088/0022-3727/49/7/075001

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.

30. Effect of thermal treatments in Ni-Fe-Ga with Co substitutions and Ni-Mn-Ga melt spun ribbons

Authors: Tolea, F; Sofronie, M; Crisan, AD; Popescu, B; Tolea, M; Valeanu, M

Published: 2016, 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), 2, 1480, DOI: 10.1016/j.prostr.2016.06.187

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.

31. On the role of Fe ions on magnetic properties of doped TiO2 nanoparticles

Authors: Tolea, F; Grecu, MN; Kuncser, V; Constantinescu, SG; Ghica, D

Published: APR 6 2015, APPLIED PHYSICS LETTERS, 106, DOI: 10.1063/1.4917037

The role of iron doping on magnetic properties of hydrothermal anatase TiO2:Fe-57 (0-1 at. %) nanoparticles is investigated by combining superconducting quantum interference device magnetometry with Mossbauer and electron paramagnetic resonance techniques. The results on both as-prepared and thermally treated samples in reduced air atmosphere reveal complexity of magnetic interactions, in connection to certain iron ion electron configurations and defects (oxygen vacancies, F-center, and Ti3+ ions). The distribution of iron ions is predominantly at nanoparticle surface layers. Formation of weak ferromagnetic domains up to 380 K is mainly related to defects, supporting the bound magnetic polaron model. (C) 2015 AIP Publishing LLC.

32. Distribution of plates' sizes tell the thermal history in a simulated martensitic-like phase transition

Authors: Tolea, F; Tolea, M; Sofronie, M; Valeanu, M

Published: JUL 2015, SOLID STATE COMMUNICATIONS, 213, 41, DOI: 10.1016/j.ssc.2015.04.016

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.

33. Magnetic and Martensitic Transformations in the bulk and melt spun ribbons of Ni57-xNdxFe18Ga25 Ferromagnetic Shape Memory Alloys

Authors: Tolea, F; Crisan, AD; Sofronie, M; Tolea, M; Valeanu, M

Published: 2015, MATERIALS TODAY-PROCEEDINGS, 2, 878, DOI: 10.1016/j.matpr.2015.07.421

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.

34. Hearing shapes of few electrons quantum drums: A configuration-interaction study

Authors: Tolea, F; Tolea, M

Published: FEB 1 2015, PHYSICA B-CONDENSED MATTER, 458, 91, DOI: 10.1016/j.physb.2014.11.013

The - highly remarkable - existence of non-congruent yet vibrationally isospectral shapes has been first proved theoretically and then also tested experimentally - by using electromagnetic waves in cavities, vibrating smectic films or electrons in nanostructures. In this context, we address the question whether isospectrality holds if two or more electrons interact electrostatically, using the accurate configuration-interaction method, in a discrete representation of the Bilby and Hawk shapes. Isospectral pairs offer an unique possibility to test how identical sets of single-particle energies may combine differently in the few-electrons eigenmodes, due to different wave functions spatial distributions. Our results point towards the break down of isospectrality in the presence of interactions. Thus one should be able to "hear" the shapes of few electrons quantum drums. Interestingly however, for the analyzed two and three electrons cases, there exists an interaction strength (which can be tuned by changing the size of the shapes), for which the ground states energies of Bilby and Hawk coincide, but not the excited states as well Wigner localization is studied and shown to occur at about the same size for both Bilby and Hawk shapes. Next, an exercise is proposed to use the two-electrons charge density of the Bilby and Hawk ground states in the phase extraction scheme as proposed by Moon et al. (2008). Results show that out-of-phase regions appear if the linear size of the shapes exceeds the Bohr radius as occupation of higher Slater determinants becomes significant. (C) 2014 Elsevier B.V. All rights reserved.

35. SHAPE MEMORY PROPERTIES OF FeNiCoTi RIBBONS EVIDENCED BY MAGNETIC MEASUREMENTS

Authors: Tolea, F; Sofronie, M; Tolea, M; Kuncser, V; Valeanu, M

Published: APR-JUN 2015, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 10, 575, DOI:

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.

36. Ruthenium oxide-hematite magnetic ceramic nanostructures

Authors: Stroh, C; Tolea, F; Valeanu, M; Diamandescu, L; Xu, TH; Sorescu, M

Published: DEC 2015, CERAMICS INTERNATIONAL, 41, 14375, DOI: 10.1016/j.ceramint.2015.07.070

xRuO(2)-(1-x)alpha-Fe2O3 (x=0.1, 0.3,0.5 and 0.7) nanoparticle systems were successfully synthesized by mechanochemical activation of RuO2 and alpha-Fe2O3 mixtures for 0-12 h of ball milling. Mossbauer spectroscopy, X-ray diffraction (XRD), magnetic measurements, simultaneous differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were employed to investigate the phase evolution of xRuO(2)- (1-x)alpha-Fe2O3 nanoparticle systems under the mechanochemical activation process. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for alpha-Fe2O3 (hematite) to four sextets and a doublet upon duration of the milling process with ruthenium oxide. Rietveld refinement of the XRD patterns yielded the values of lattice parameters as function of composition and milling times. The presence of Ru-substituted hematite and Fe-doped ruthenium oxide was evidenced and correlated with differences between ionic radii of Fe3+ and Ru4+. Magnetic measurements recorded at 5 and 300 K in applied magnetic fields of 50,000 and 100,000 Oe showed that the estimated saturation magnetization of the milled samples increased with ball milling time while preserving a multidomain magnetic structure. The Morin transformation was evidenced by zero-field cooling-field cooling (ZFC-FC) measurements in 200 Oe and 1 T, for samples milled for 0 and 8 h of mechanochemical activation. These results correlate well with the DSC-TGA measurements, which support the formation of mixedoxide solid solutions in the system under investigation. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

37. Nanostructured iridium oxide-hematite magnetic ceramic semiconductors

Authors: Limongelli, J; Tolea, F; Valeanu, M; Diamandescu, L; Xu, TH; Sorescu, M

Published: JAN 2015, CERAMICS INTERNATIONAL, 41, 343, DOI: 10.1016/j.ceramint.2014.08.076

xIrO(2)-(1-x)alpha-Fe2O3(x=0.1, 0.3 and 0.5) nanoparticle systems were successfully synthesized by mechanochemical activation of IrO2 and alpha-Fe2O3 mixtures for 0-12 h of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale, which is of crucial importance for catalysis, sensing and electrochemical applications. X-ray powder diffraction (XRD), Mossbauer spectroscopy, magnetic measurements and simultaneous differential scanning calorimetry (DSC) and thermal gravirnetric analysis (TGA) were used to study the phase evolution of xIrO(2)-(1-x)alpha-Fe2O3 nanoparticle systems under the mechanochemical activation process. Rietveld refinement of the XRD patterns yielded the values of the particle size and lattice parameters as function of composition and milling times and indicated the presence of Ir-substituted hematite and Fe-doped iridium oxide for large x values and long milling times. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextets and a doublet upon duration of the milling process with iridium oxide. Magnetic measurements recorded at 5 K in an applied magnetic field of 40,000 Oe showed that the saturation magnetization of the milled samples increased with ball milling time while preserving a multidomain magnetic structure. The unmilled sample at 5 K showed a spinflop type metarnagnetic transition around 30,000 Oe. The Morin transformation was evidenced by zero-field cooling field cooling (ZFC-FC) measurements in 200 Oe and 1 T and the transformation characteristic temperatures were shifted to lower values. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

38. Effect of thermal treatments on the structural and magnetic transitions in melt-spun Ni-Fe-Ga-(Co) ribbons

Authors: Tolea, F; Sofronie, M; Crisan, AD; Enculescu, M; Kuncser, V; Valeanu, M

Published: NOV 25 2015, JOURNAL OF ALLOYS AND COMPOUNDS, 650, 670, DOI: 10.1016/j.jallcom.2015.07.296

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.

39. Magneto-Structural Properties and Magnetic Behavior of Fe-Pd Ribbons

Authors: Sofronie, MI; Tolea, F; Kuncser, V; Valeanu, MC; Filoti, G

Published: JAN 2015, IEEE TRANSACTIONS ON MAGNETICS, 51, DOI: 10.1109/TMAG.2014.2359226

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.

40. Magnetocaloric effect in Ni-Fe-Ga Heusler alloys with Co and Al substitutions

Authors: Tolea, F; Sofronie, M; Crisan, AD; Tolea, M; Valeanu, M

Published: 2015, ESOMAT 2015 - 10TH EUROPEAN SYMPOSIUM ON MARTENSITIC TRANSFORMATIONS, 33, DOI: 10.1051/matecconf/20153305008

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.

41. Structural and magnetic properties of Li2O-Fe2O3 ceramic nanostructures

Authors: Sorescu, M; Bushunow, V; Diamandescu, L; Tolea, F; Valeanu, M; Xu, TH

Published: MAY 2014, CERAMICS INTERNATIONAL, 40, 5395, DOI: 10.1016/j.ceramint.2013.10.120

xLi(2)O-(1 x)alpha-Fe2O3 (x=0.1, 0.3, 0.5, and 0.7) nanoparticle systems were successfully synthesized by mechanochemical activation of Li2O and alpha-Fe2O3 mixtures for 0-12 h of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale, which is of crucial importance for catalysis, sensing and electrochemical applications. X-ray powder diffraction (XRD), Mossbauer spectroscopy and magnetic measurements were used to study the phase evolution of xLi(2)O-(1 x)alpha-Fe2O3 nanoparticle systems under the mechanochemical activation process. Rietveld refinement of the XRD patterns yielded the values of the particle size as function of composition and milling times and indicated the presence of Li-substituted hematite and tetra lithium iron oxide LiFeO2, along with the formation of multiple phases for large x values and long milling times. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextets and a doublet upon duration of the milling process with lithium oxide. Magnetic measurements recorded at 5 K to room temperature (RT) in an applied magnetic field of 50,000 Oe showed that the magnetization of the milled samples is larger at low temperatures than at RT and increases with decreasing particle size. Zero field cooling measurements made possible the determination of the blocking temperatures of the specimens as function of ball milling time and evidenced the occurrence of superparamagnetism in the studied samples. This result correlates well with the observed presence of a quadrupole-split doublet in the Mossbauer spectra. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

42. Influence of mechanical milling on the physical properties of SmCo5/Fe65Co35 type hard/soft magnetic nanocomposite

Authors: Dorolti, E; Trifu, AV; Isnard, O; Chicinas, I; Tolea, F; Valeanu, M; Pop, V

Published: MAY 25 2013, JOURNAL OF ALLOYS AND COMPOUNDS, 560, 194, DOI: 10.1016/j.jallcom.2013.01.140

The influence of milling type on the structural and magnetic behaviour of mechanically milled SmCo5/Fe65Co35 has been studied. By adjusting the mill speed parameters, (Omega/omega), three types of milling have been performed: shock mode process (SMP), friction mode process (FMP) and a combined shock followed by a friction mode process. It was found that in SMP conditions the remanent magnetization and composite coercivity increase with increasing milling time. The best exchange coupling, with a coercivity of 1.05 T, was obtained for samples milled for 8 h in SMP and annealed for 2 h at 550 degrees C. On the other hand, the low energy of FMP does not succeed in producing a microstructure adapted to favour the interphase exchange interactions between the soft and hard magnetic phases. A combined milling process does not change significantly the microstructure obtained previously after 8 h of SMP. This indicates that the microstructure of the composite is formed primarily during SMP while FMP, less energetic, does not drastically affect the crystallite size and crystallite shape of SMP samples. X-ray diffraction patterns were used to follow the evolution of the structure and microstructure. The diffusion of Fe and Co from Fe65Co35 to SmCo5 phase leads to the formation of Sm-2(Co,Fe)(17)/Fe-Co composite. The magnetic behaviour was checked from hysteresis curves and dM/dH vs. H plots. (C) 2013 Elsevier B.V. All rights reserved.

43. Phase extraction in disordered isospectral shapes

Authors: Tolea, M; Ostahie, B; Nita, M; Tolea, F; Aldea, A

Published: MAR 14 2012, PHYSICAL REVIEW E, 85, DOI: 10.1103/PhysRevE.85.036604

The phase of the electronic wave function is not directly measurable but, quite remarkably, it becomes accessible in pairs of isospectral shapes, as recently proposed in the experiment by Moon et al. [Science 319, 782 (2008)]. The method is based on a special property, called transplantation, which relates the eigenfunctions of the isospectral pairs, and allows us to extract the phase distributions, if the amplitude distributions are known. We numerically simulate such a phase extraction procedure in the presence of disorder, which is introduced both as Anderson disorder and as roughness at edges. With disorder, the transplantation can no longer lead to a perfect fit of the wave functions, however we show that a phase can still be extracted-defined as the phase that minimizes the misfit. Interestingly, this extracted phase coincides with (or differs negligibly from) the phase of the disorder-free system, up to a certain disorder amplitude, and a misfit of the wave functions as high as similar to 5%, proving a robustness of the phase extraction method against disorder. However, if the disorder is increased further, the extracted phase shows a puzzle structure, no longer correlated with the phase of the disorder-free system. A discrete model is used, which is the natural approach for disorder analysis. We provide a proof that discretization preserves isospectrality and the transplantation can be adapted to the discrete systems.

44. Characterization of martensitic transformation in some Ni-Fe-Co-Ga ferromagnetic shape memory alloys

Authors: Tolea, F; Sofronie, M; Ghica, C; Valeanu, M

Published: MAY 2011, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 5, 566, DOI:

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.

45. THE EFFECT OF HYDROGENATION ON THE MAGNETIC PROPERTIES OF LAYERED SPIN VALVE STRUCTURES

Authors: Kuncser, V; Tolea, F; Schinteie, G; Jepu, I; Palade, P

Published: JAN-MAR 2011, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 6, 206, DOI:

Multilayer spin valve structures of type AF/F/Cu/F and AF/F/MgO/F (AF=antiferromagnetic layer and F=ferromagnetic layer) have been prepared by sputtering in radiofrequence. The samples were investigated at room temperature by magneto-optic Kerr effect and conversion electron Mossbauer spectroscopy, before and after thermal hydrogenation. Additional magnetic peculiarities were observed at low temperatures via SQUID magnetometry. The effect of the thermal hydrogenation on the magnetic properties of the systems was discussed with respect to the changes in the local spin configuration, observed by Mssbauer spectroscopy.

46. LASER INDUCED CRYSTALLIZATION IN SPUTTERED FeSiB THIN FILMS

Authors: Tolea, F; Stoica, M; Matei, E; Cristescu, CP

Published: 2010, UNIVERSITY POLITEHNICA OF BUCHAREST SCIENTIFIC BULLETIN-SERIES A-APPLIED MATHEMATICS AND PHYSICS, 72, 256, DOI:

Thin films with nominal composition Fe(78)Si(9)B(13) generated by radio-frequency (rf) sputtering on Si substrates with (100) orientation were irradiated with an eyeliner laser with 130mJ/cm(2) fluence. Information on the induced structural changes were obtained by X-ray diffraction and scanning electron microscopy, while the magnetization curves were determined by the magneto-optical Kerr effect. The effects of the irradiation consist in the growing of crystallites,appearence of melted and ultra-fast frozen zones, surface oxidation and changes in the magnetic properties of the thin films.

47. Magnetic Nanocomposites for Permanent Magnets

Authors: Tolea, F; Sofronie, M; Birsan, A; Schinteie, G; Kuncser, V; Valeanu, M

Published: 2010, TRENDS IN NANOPHYSICS: THEORY, EXPERIMENT AND TECHNOLOGY, 296, DOI: 10.1007/978-3-642-12070-1_12

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.

48. Martensitic transformation and accompanying magnetic changes in Ni-Fe-Ga-Co alloys

Authors: Sofronie, M; Tolea, F; Kuncser, V; Valeanu, M

Published: JUN 1 2010, JOURNAL OF APPLIED PHYSICS, 107, DOI: 10.1063/1.3429231

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]

49. Shape memory and associated properties in Fe-Mn-Si-based ribbons produced by melt-spinning

Authors: Valeanu, M; Filoti, G; Kuncser, V; Tolea, F; Popescu, B; Galatanu, A; Schinteie, G; Jianu, AD; Mitelea, I; Schinle, D; Craciunescu, CM

Published: JUL 2008, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 320, E167, DOI: 10.1016/j.jmmm.2008.02.105

Four Fe-Mn-Si alloys, Fe(62)Mn(32)Si(6), Fe(62)Mn(20)Si(5)Cr(8)Ni(5), Fe(62)Mn(16)Si(5)Cr(12)Ni(5) and Fe(65)Mn(9)Si(7)Cr(10)Ni(9), were obtained by the melt-spinning method. The samples were structurally, magnetic and shape memory effect (SME) investigated, both ''as quenched'' and thermally treated. The Mn-rich compositions show different phase, magnetic behavior and SME in comparison with Mn-poor compositions. The thermal treatments generate transformation between the two existing majority phases (alpha and gamma), related magnetization and SME behavior. The features are derived from the corroboration of structural, magnetic interaction and magnitude of SME data. (c) 2008 Elsevier B.V. All rights reserved.

50. The colloidal route of the sol-gel process - an alternative to produce Fe3O4-SiO2 nanocomposites

Authors: Raileanu, M; Crisan, M; Ballo, A; Cosma, C; Petre, J; Stan, C; Predoi, D; Valsangiacom, CM; Tolea, F; Secu, C

Published: MAY 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1402, DOI:

The iron oxide-based nanomaterials have a great importance because of their impact on a wide number of industries. They have many properties and applications in domains as: environmental protection, biomedical, catalysis, information displays and electronics. In order to avoid the tendency of nanoparticles to aggregate, they are often included in sol-gel derived silica matrices, thus being also ensured a homogeneous dispersion of the ultra-fine metal oxide particles in the host matrix. The reasons of the attempt are both an economic and a non-pollutant one, tacking into account the fact that it is well known that aqueous silica sol is cheaper and less toxic than TEOS. The Fe3O4 was introduced in the reaction mixture as aqueous suspension. A final iron content related to SiO2 of 3% wt. was chosen for the prepared nanocomposite. Thermal analysis, IR spectroscopy, XRD and TEM methods have been used for the structural characterisation,. Some experiments of water depollution (from As) have been proceeded using the prepared Fe3O4-SiO2 nanocomposite.

51. Structural (ME, XRD, NGR, ESR) investigations on some sulphides from costesti, valea lui stan and jidostita gold ores (Southern Carpathians, Romania)

Authors: Udubasa, SS; Constantinescu, S; Grecu, MN; Popescu-Pogrion, N; Udubasa, G; Popescu, GC; Tolea, F; Popescu, IV

Published: 2007, ROMANIAN JOURNAL OF PHYSICS, 52, 104, DOI:

An extensive investigation of the gold ores in the South Carpathians (Costesti, Valea lui Stan si Jidostita) has performed, using different chemical and physics experimental techniques. The used structural techniques have been the electron microscopy, X-ray diffraction, Mossbauer effect and electron spin resonance. The investigation of the representative samples by the structural techniques relieved new minerals in the gold ores, the presence of the "coral like" aggregates of nanometric gold solid bubbles.

52. Criteria for the occurrence of negative resonant magnetoresistance in magnetic tunnel junctions

Authors: Tolea, M; Aldea, A; Tolea, F

Published: SEP 2006, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243, R86, DOI: 10.1002/pssb.200642347

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.

53. Mossbauer spectroscopy and magnetic measurements on Fe-Ni-Co-Ti shape memory alloys

Authors: Tolea, F; Schinteie, G; Popescu, B

Published: AUG 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1506, DOI:

Fe-Ni-Co-Ti shape memory alloys were prepared by various techniques. A comparison between samples prepared by classical metallurgy and by melt spinning technique is performed in respect to the microscopic mechanisms responding for the shape memory effects. X-ray diffraction, thermomagnetic measurements and Mossbauer spectroscopy were applied for a complete structural and magnetic characterization. Only samples supporting an aging treatment gave evidence for shape memory effects and correlated structural transformations.

54. Effects of the Severe Plastic Deformation on the Magnetic Properties of Zr13Co87 Ribbons

Authors: Popescu, B; Palade, P; Sofronie, M; Kuncser, A; Gurau, C; Gurau, G; Tolea, F

Published: , METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE, DOI: 10.1007/s11661-021-06507-y

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.

55. MICROSTRUCTURE, MAGNETIC AND MAGNETOSTRICTIVE BEHAVIOUR IN RAPIDLY QUENCHED OFF-STOICHIOMETRIC Ni-Mn-Ga FERROMAGNETIC SHAPE MEMORY ALLOYS