Eng. Aurel LECA
Technological Development Engineer III
Laboratory of Magnetism and Superconductivity
aurel[DOT]leca[AT]infim[DOT]roPublications
1. Effect of P2O5 Content on Luminescence of Reduced Graphene-Oxide-Doped ZnO-P2O5 Nano-Structured Films Prepared via the Sol-Gel Method
Authors:
Vasiliu, IC; Filip, AV; Chilibon, I; Elisa, M; Bartha, C; Kuncser, V; Leca, A; Boroica, L; Sava, BA; Trusca, R; Eftimie, M; Nicoara, A
Published: SEP 2023, MATERIALS, 16, 6156, DOI: 10.3390/ma16186156
A convenient and low-cost sol-gel approach for the one-step synthesis of ZnO-P2O5-rGO nanostructures with tuned bandgap and fluorescence was investigated. The obtained hybrid nanostructures exploit the properties of zinc oxide, graphene oxide and phosphorous oxide as promising candidates for a wide range of optoelectronic applications. A predominant amorphous structure, ZnO-P2O5-rGO, containing ZnO nanorods was evidenced by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The estimated size of the ZnO nanorods in nanostructures with P2O5 was noticed to decrease when the P2O5/ZnO ratio was increased. The presence of ZnO, P2O5 and rGO was confirmed by Fourier-transform infrared spectroscopy (FTIR) and Raman investigation. P2O5 was noticed to tune the bandgap and the fluorescence emissions of the nanostructured films, as estimated by UV-Vis-NIR and fluorescence spectroscopy, respectively. The electrical measurements performed at room temperature showed that the main influence on the film's resistivity does not come from the 1% rGO doping but from the P2O5/ZnO ratio. It was found that a 10/90 molar ratio of P2O5/ZnO decreases the resistivity almost seven-fold compared with rGO-doped ZnO films.
2. Magnetic Properties of Nanosized Fe and FeCo Systems on Trenched Mo Templates
Authors:
Stanciu, AE; Schinteie, G; Kuncser, AC; Locovei, C; Trupina, L; Iacob, N; Leca, A; Borca, B; Kuncser, V
Published: SEP 2022, COATINGS, 12, 1366, DOI: 10.3390/coatings12091366
The manipulation of magnetic anisotropy represents the fundamental prerequisite for the application of magnetic materials. Here we present the vectorial magnetic properties of nanostructured systems and thin films of Fe and FeCo prepared on linearly trenched Mo templates with thermally controlled periodicity. The magnetic properties of the nanosystems are engineered by tuning the shape, size, thickness, and composition parameters of the thin films. Thus, we control coercivity, magnetization, orientation of the easy axis of magnetization, and the long-range magnetic order of the system in the function of the temperature. We distinguish magnetic components that emerge from the complex morpho-structural features of the undulating Fe or FeCo nanostructured films on trenched Mo templates: (i) assembly of magnetic nanowires and (ii) assembly of magnetic islands/clusters. Uniaxial anisotropy at room temperature was proven, characterized, and explained in the case of all systems. Our work contributes to the understanding of magnetic properties necessary for possible further applications of linear systems and undulated thin films.
3. Thermal Stability, Blocking Regime and Superparamagnetic Behavior in Mn-Al-C Melt Spun Ribbons
Authors:
Crisan, AD; Leca, A; Dan, I; Crisan, O
Published: NOV 2021, NANOMATERIALS, 11, DOI: 10.3390/nano11112898
Alloys possessing nominal compositions Mn53Al45C2 and Mn52Al46C2 were prepared by the melt spinning method and were subjected to complex structural, morphological and magnetic investigations. As these alloys can exhibit tetragonal L1(0)-type and tau phase, they have good potential as rare earth (RE)-free magnets. It is, therefore, important to monitor the epsilon-tau phase transformation and the stability and the magnetic features of the tetragonal phase in an entire temperature interval. By using synchrotron X-ray diffraction, it has been proven that the epsilon-tau phase transformation occurs gradually, with the tau phase becoming predominant only after 450 & DEG;C. Moreover, this phase has been proven to be quite stable without any grain growth even at the highest temperature investigated at 800 & DEG;C. Low temperature behavior was thoroughly investigated by using a complex combination of major and minor hysteresis loops combined with the zero field cooled-field cooled magnetization protocols (ZFC-FC). Two different regimes, blocking and superparamagnetic, were documented. A spin reorientation transition was proven to occur at 55 K while a maximum magnetization observed in ZFC-FC curves proved that at about 75 K, a transition from ferro to superparamagnetic state occurs. The existence of a blocking regime below 55 K that is characteristic to nanogranular systems with superparamagnetic behavior has shown further development towards obtaining RE-free magnets.
4. New superconductor/ferromagnet heterostructure formed by YBa2Cu3O7-x and CaRuO3
Authors:
Ivan, I; Pasuk, I; Crisan, A; Sandu, V; Onea, M; Leca, A; Cosar, C; Burdusel, M
Published: NOV 2021, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 34, DOI: 10.1088/1361-6668/ac2622
Almost all proposed configurations and practical achievements based on superconductor/ferromagnet (S/F) heterostrucutres focus on s-wave superconductors. However, several attempts targeted also high temperature superconductors, most of them using manganite ferromagnets LaXMnO3 (X: Ca or Sr) and Y1Ba2Cu3O7-x (YBCO). Here we propose a new ferromagnetic material that can be used with YBCO for the fabrication of S/F hybrid structures. We show that a ferromagnetic order can be induced in a thin layer (similar to 130 nm thickness) of CaRuO3 grown by pulsed laser deposition on epitaxial YBCO film. Detailed magnetic and structural investigations show that the observations of the weak ferromagnetism are consistent with the magnetic order induced by in-plane tensile strain of about 1.7% and the easy-magnetization axis forms an angle of similar to 180o with the layer plane. The value of the Curie temperature T (Curie) estimated using the Curie-Weiss law was 340 K. An unusual temperature dependence of the magnetic moment around the superconducting transition was observed in both field-cooled and zero-field-cooled configurations which is attributed to the paramagnetic Meissner effect.
5. Influence of Thickness on the Magnetic and Magnetotransport Properties of Epitaxial La0.7Sr0.3MnO3 Films Deposited on STO (001)
Authors:
Greculeasa, SG; Stanciu, AE; Leca, A; Kuncser, A; Hrib, L; Chirila, C; Pasuk, I; Kuncser, V
Published: DEC 2021, NANOMATERIALS, 11, DOI: 10.3390/nano11123389
Epitaxial La0.7Sr0.3MnO3 films with different thicknesses (9-90 nm) were deposited on SrTiO3 (0 0 1) substrates by pulsed laser deposition. The films have been investigated with respect to morpho-structural, magnetic, and magneto-transport properties, which have been proven to be thickness dependent. Magnetic contributions with different switching mechanisms were evidenced, depending on the perovskite film thickness. The Curie temperature increases with the film thickness. In addition, colossal magnetoresistance effects of up to 29% above room temperature were evidenced and discussed in respect to the magnetic behavior and film thickness.
6. Magnetism and epsilon-tau Phase Transformation in MnAl-Based Nanocomposite Magnets
Authors:
Crisan, AD; Leca, A; Bartha, C; Dan, I; Crisan, O
Published: APR 2021, NANOMATERIALS, 11, DOI: 10.3390/nano11040896
Melt spun ribbons of Mn53Al45C2 and Mn52Al46C2 have been synthesized by rapid quenching of the melt with the purpose of monitoring the epsilon-tau phase transformation to show technologically feasible ways to increase magnetic parameters and to illustrate the viability of these alloys as the next generation of rare earth (RE)-free magnets. By differential scanning calorimetry (DSC), activation energies and temperatures of onset of the epsilon-tau phase transformation were obtained. Structural analysis was performed using X-ray diffraction (XRD) and the resulting XRD patterns were quantitatively assessed using full profile Rietveld-type analysis. Appropriate annealing was performed in order to enable the epsilon-tau phase transformation. While hcp epsilon-phase was found to be predominant in the as-cast samples, after appropriate annealing, the tetragonal tau-phase, the one that furnishes the relevant magnetic response, was found to be predominant with an abundance of about 90%. The data suggested a mechanism of hcp epsilon-phase decomposition controlled by the segregation towards the interfacial regions, having the rate of transformation governed by antiphase boundary diffusion processes. Magnetic measurements of annealed sample Mn53Al45C2, consisting of predominant tetragonal tau-phase, showed high values of magnetization and increased coercivity, consistent with an energy product of about 10 MGOe, similar with previously reported magnetization measurements, providing further insight into the realization of future class of RE-free low-cost permanent magnets.
7. Tuning structural and magnetic properties of Fe oxide nanoparticles by specific hydrogenation treatments
Authors:
Greculeasa, SG; Palade, P; Schinteie, G; Leca, A; Dumitrache, F; Lungu, I; Prodan, G; Kuncser, A; Kuncser, V
Published: OCT 14 2020, SCIENTIFIC REPORTS, 10, DOI: 10.1038/s41598-020-74188-5
Structural and magnetic properties of Fe oxide nanoparticles prepared by laser pyrolysis and annealed in high pressure hydrogen atmosphere were investigated. The annealing treatments were performed at 200 degrees C (sample A200C) and 300 degrees C (sample A300C). The as prepared sample, A, consists of nanoparticles with similar to 4 nm mean particle size and contains C (similar to 11 at.%), Fe and O. The Fe/O ratio is between gamma-Fe2O3 and Fe3O4 stoichiometric ratios. A change in the oxidation state, crystallinity and particle size is evidenced for the nanoparticles in sample A200C. The Fe oxide nanoparticles are completely reduced in sample A300C to alpha-Fe single phase. The blocking temperature increases from 106 K in A to 110 K in A200C and above room temperature in A300C, where strong inter-particle interactions are evidenced. Magnetic parameters, of interest for applications, have been considerably varied by the specific hydrogenation treatments, in direct connection to the induced specific changes of particle size, crystallinity and phase composition. For the A and A200C samples, a field cooling dependent unidirectional anisotropy was observed especially at low temperatures, supporting the presence of nanoparticles with core-shell-like structures. Surprisingly high M-S values, almost 50% higher than for bulk metallic Fe, were evidenced in sample A300C.
8. Magnetic Phase Coexistence and Hard-Soft Exchange Coupling in FePt Nanocomposite Magnets
Authors:
Crisan, O; Dan, I; Palade, P; Crisan, AD; Leca, A; Pantelica, A
Published: AUG 2020, NANOMATERIALS, 10, DOI: 10.3390/nano10081618
With the aim of demonstrating phase coexistence of two magnetic phases in an intermediate annealing regime and obtaining highly coercive FePt nanocomposite magnets, two alloys of slightly off-equiatomic composition of a binary Fe-Pt system were prepared by dynamic rotation switching and ball milling. The alloys, with a composition Fe(53)Pt(47)and Fe55Pt45, were subsequently annealed at 400 degrees C and 550 degrees C and structurally and magnetically characterized by means of X-ray diffraction,Fe-57 Mossbauer spectrometry and Superconducting Quantum Interference Device (SQUID) magnetometry measurements. Gradual disorder-order phase transformation and temperature-dependent evolution of the phase structure were monitored using X-ray diffraction of synchrotron radiation. It was shown that for annealing temperatures as low as 400 degrees C, a predominant, highly ordered L1(0)phase is formed in both alloys, coexisting with a cubic L1(2)soft magnetic FePt phase. The coexistence of the two phases is evidenced through all the investigating techniques that we employed. SQUID magnetometry hysteresis loops of samples annealed at 400 degrees C exhibit inflection points that witness the coexistence of the soft and hard magnetic phases and high values of coercivity and remanence are obtained. For the samples annealed at 500 degrees C, the hysteresis loops are continuous, without inflection points, witnessing complete exchange coupling of the hard and soft magnetic phases and further enhancement of the coercive field. Maximum energy products comparable with values of current permanent magnets are found for both samples for annealing temperatures as low as 500 degrees C. These findings demonstrate an interesting method to obtain rare earth-free permanent nanocomposite magnets with hard-soft exchange-coupled magnetic phases.
9. Mn-Induced Thermal Stability of L1(0)Phase in Fept Magnetic Nanoscale Ribbons
Authors:
Crisan, AD; Leca, A; Pantelica, D; Dan, I; Crisan, O
Published: JUL 2020, NANOMATERIALS, 10, DOI: 10.3390/nano10071278
Magnetic nanoscale materials exhibiting the L1(0)tetragonal phase such as FePt or ternary alloys derived from FePt show most promising magnetic properties as a novel class of rare earth free permanent magnets with high operating temperature. A granular alloy derived from binary FePt with low Pt content and the addition of Mn with the nominal composition Fe(57)Mn(8)Pt(35)has been synthesized in the shape of melt-spun ribbons and subsequently annealed at 600 degrees C and 700 degrees C for promoting the formation of single phase, L1(0)tetragonal, hard magnetic phase. Proton-induced X-ray emission spectroscopy PIXE has been utilized for checking the compositional effect of Mn addition. Structural properties were analyzed using X-ray diffraction and diffractograms were analyzed using full profile Rietveld-type analysis with MAUD (Materials Analysis Using Diffraction) software. By using temperature-dependent synchrotron X-ray diffraction, the disorder-order phase transformation and the stability of the hard magnetic L1(0)phase were monitored over a large temperature range (50-800 degrees C). A large interval of structural stability of the L1(0)phase was observed and this stability was interpreted in terms of higher ordering of the L1(0)phase promoted by the Mn addition. It was moreover found that both crystal growth and unit cell expansion are inhibited, up to the highest temperature investigated (800 degrees C), proving thus that the Mn addition stabilizes the formed L1(0)structure further. Magnetic hysteresis loops confirmed structural data, revealing a strong coercive field for a sample wherein single phase, hard, magnetic tetragonal L1(0)exists. These findings open good perspectives for use as nanocomposite, rare earth free magnets, working in extreme operation conditions.
10. Temperature dependence and defect related structure, photoluminescence, (ferro)magnetism and ammonia sensitivity of un-doped nanocrystalline ZnO
Authors:
Mihalache, V; Secu, M; Negrila, C; Bercu, V; Mercioniu, I; Leca, A
Published: DEC 2020, MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS, 262, DOI: 10.1016/j.mseb.2020.114748
ZnO nanostructures with intrinsic and extended defects were prepared by rapid decomposition of zinc-propio-nate and annealing at 400 degrees C-970 degrees C. The correlation between the structure/morphology, type of native defects (photoluminescence (PL), EPR) and ferromagnetism was investigated, together with ammonia adsorption capacity. All the samples show room temperature ferromagnetism (RTFM). Crystallite size increases while the unit cell volume, c-axis constant, microstrain and saturation magnetization relax with increasing temperature; morphology varies from aggregated nanoparticles to frameworks of well-welded crystals. 400 degrees C-800 degrees C annealed samples show a broad visible and/or a prominent violet-blue PL emission and, two narrow g = 2.0065 and g = 1.9632 EPR signals. 800 degrees C-970 degrees C annealed samples exhibit very intense green-yellow photoluminescence. The intrinsic defects in conjunction with a deformed lattice and/or pinned by grain-boundaries appear responsible for RTFM and Curie temperature exceeding 700 degrees C. Tuning the morphology, PL intensity and ferromagnetic signal by choice of annealing temperature can find applications in (gas) sensing, photonic/optoelectronic and spintronic devices.
11. Towards a Correlation between Structural, Magnetic, and Luminescence Properties of CeF3:Tb(3+)Nanocrystals
Authors:
Bartha, C; Secu, C; Matei, E; Negrila, C; Leca, A; Secu, M
Published: JUL 2020, MATERIALS, 13, DOI: 10.3390/ma13132980
In this study, we report on the structural, magnetic, and optical properties of Tb3+-doped CeF(3)nanocrystals prepared via a polyol-assisted route, followed by calcination. X-ray diffraction analysis and electron microscopy investigations have shown the formation of a dominant Ce(0.75)F(3)nanocrystalline phase (of about 99%), with a relatively uniform distribution of nanocrystals about 15 nm in size. Magnetization curves showed typical paramagnetic properties related to the presence of Ce(3+)and Tb(3+)ions. The magnetic susceptibility showed a weak inflexion at about 150 K, assigned to the cerium ions' crystal field splitting. Under UV light excitation of the Ce(3+)ions, we observed Tb(3+)green luminescence with a quantum yield of about 20%.
12. Structure and magnetic properties of highly coercive L1(0) nanocomposite FeMnPt thin films
Authors:
Crisan, O; Vasiliu, F; Crisan, AD; Mercioniu, I; Schinteie, G; Leca, A
Published: JUN 2019, MATERIALS CHARACTERIZATION, 152, 252, DOI: 10.1016/j.matchar.2019.04.028
Among the rare-earth-free systems that are currently investigated in search for novel permanent magnet solutions for various applications, with special emphasis on the magnets required to operate in extreme conditions, the FePt binary system, where the tetragonal hard magnetic L1(0) phase can be formed by suitable microstructure processing via annealing, has been extensively studied. A variation of this system, the ternary FeMnPt system, has been also recently shown to exhibit good permanent magnet behavior due to the suitable formation of the L1(0) phase. In addition to be likely to form the L1(0) phase as its parent binary system, the ternary FeMnPt benefits from the reduced costs due to the reduced amount of Pt and may exhibit particular magnetic structure due to the influence of the antiferromagnetic Mn. In the present work, we have employed a mixed sputtering technique, based on the use of both elemental and compound target for developing L1(0) FeMnPt thin films with specific structural features that triggers better magnetic performances in terms of coercivity and maximum energy products. The as-obtained films have been thermally annealed and characterized by means of transmission electron microscopy, X-ray diffraction, Mossbauer spectroscopy, magneto-optic Kerr effect (MORE) and SQUID magnetometry. The aim is to correlate the Mn induced microstructural and lattice changes with the magnetic properties and to optimize the microstructure for an early formation of the ordered L1(0) phase and increased coercivity compared to the as-prepared, structurally disordered, face centred cubic initial state of the films.
13. 3D Superparamagnetic Scaffolds for Bone Mineralization under Static Magnetic Field Stimulation
Authors:
Paun, IA; Calin, BS; Mustaciosu, CC; Mihailescu, M; Moldovan, A; Crisan, O; Leca, A; Luculescu, CR
Published: SEP 2019, MATERIALS, 12, DOI: 10.3390/ma12172834
We reported on three-dimensional (3D) superparamagnetic scaffolds that enhanced the mineralization of magnetic nanoparticle-free osteoblast cells. The scaffolds were fabricated with submicronic resolution by laser direct writing via two photons polymerization of Ormocore/magnetic nanoparticles (MNPs) composites and possessed complex and reproducible architectures. MNPs with a diameter of 4.9 +/- 1.5 nm and saturation magnetization of 30 emu/g were added to Ormocore, in concentrations of 0, 2 and 4 mg/mL. The homogenous distribution and the concentration of the MNPs from the unpolymerized Ormocore/MNPs composite were preserved after the photopolymerization process. The MNPs in the scaffolds retained their superparamagnetic behavior. The specific magnetizations of the scaffolds with 2 and 4 mg/mL MNPs concentrations were of 14 emu/g and 17 emu/g, respectively. The MNPs reduced the shrinkage of the structures from 80.2 +/- 5.3% for scaffolds without MNPs to 20.7 +/- 4.7% for scaffolds with 4 mg/mL MNPs. Osteoblast cells seeded on scaffolds exposed to static magnetic field of 1.3 T deformed the regular architecture of the scaffolds and evoked faster mineralization in comparison to unstimulated samples. Scaffolds deformation and extracellular matrix mineralization under static magnetic field (SMF) exposure increased with increasing MNPs concentration. The results are discussed in the frame of gradient magnetic fields of similar to 3 x 10(-4) T/m generated by MNPs over the cells bodies.
14. Local configurations and atomic intermixing in as-quenched and annealed Fe1-xCrx and Fe1-xMox ribbons
Authors:
Stanciu, AE; Greculeasa, SG; Bartha, C; Schinteie, G; Palade, P; Kuncser, A; Leca, A; Filoti, G; Birsan, A; Crisan, O; Kuncser, V
Published: 2018, PHILOSOPHICAL MAGAZINE, 98, 1067, DOI: 10.1080/14786435.2018.1425556
Local atomic configuration, phase composition and atomic intermixing in Fe-rich Fe1-xCrx and Fe1-xMox ribbons (x = 0.05, 0.10, 0.15), of potential interest for high-temperature applications and nuclear devices, are investigated in this study in relation to specific processing and annealing routes. The Fe-based thin ribbons have been prepared by induction melting, followed by melt spinning and further annealed in He at temperatures up to 1250 degrees C. The complex structural, compositional and atomic configuration characterisation has been performed by means of X-ray diffraction (XRD), transmission Mossbauer spectroscopy and differential scanning calorimetry (TG-DSC). The XRD analysis indicates the formation of the desired solid solutions with body-centred cubic (bcc) structure in the as-quenched state. The Mossbauer spectroscopy results have been analysed in terms of the two-shell model. The distribution of Cr/Mo atoms in the first two coordination spheres is not homogeneous, especially after annealing, as supported by the short-range order parameters. In addition, high-temperature annealing treatments give rise to oxidation of Fe (to haematite, maghemite and magnetite) at the surface of the ribbons. Fe1-xCrx alloys are structurally more stable than the Mo counterpart under annealing at 700 degrees C. Annealing at 1250 degrees C in He enhances drastically the Cr clustering around Fe nuclei.
15. Structural, Magnetic, and Mossbauer Investigation of Ordered Iron Nitride with Martensitic Structure Obtained from Amorphous Hematite Synthesized via the Microwave Route
Authors:
Palade, P; Plapcianu, C; Mercioniu, I; Comanescu, C; Schinteie, G; Leca, A; Vidu, R
Published: MAR 22 2017, INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH, 56, 2966, DOI: 10.1021/acs.iecr.6b04574
Amorphous hematite synthesized by a simple and fast microwave route was used to obtain Fe16N2 fine particles by reducing in 5% H-2/Ar gas flow, followed by long time nitridation in ammonia gas flow at temperatures below 200 degrees C. Depending on nitridation temperature, various amounts of metallic iron were present along with the alpha" -Fe16N2 main phase. A small amount of iron oxide was observed by Mossbauer spectroscopy, but it was undetected by X-ray diffraction due to its high degree of amorphization. Increased amounts of Fe3N and Fe4N phases were observed at a nitridation temperature above 150 degrees C, which had a detrimental effect on the magnetic properties. Structural information and phase composition were extracted from Rietveld refinement of the XRD data. Values of the magnetization at saturation measured at 40 kOe and 25 degrees C of 222 emu/g for alpha"-Fe16N2 and 192 emu/g for metallic iron were obtained via magnetic measurements, Rietveld, and Mossbauer analysis.
16. Single Wall Carbon Nanotubes Based Cryogenic Temperature Sensor Platforms
Authors:
Monea, BF; Ionete, EI; Spiridon, SI; Leca, A; Stanciu, A; Petre, E; Vaseashta, A
Published: SEP 2017, SENSORS, 17, DOI: 10.3390/s17092071
We present an investigation consisting of single walled carbon nanotubes (SWCNTs) based cryogenic temperature sensors, capable of measuring temperatures in the range of 2-77 K. Carbon nanotubes (CNTs) due to their extremely small size, superior thermal and electrical properties have suggested that it is possible to create devices that will meet necessary requirements for miniaturization and better performance, by comparison to temperature sensors currently available on the market. Starting from SWCNTs, as starting material, a resistive structure was designed. Employing dropcast method, the carbon nanotubes were deposited over pairs of gold electrodes and in between the structure electrodes from a solution. The procedure was followed by an alignment process between the electrodes using a dielectrophoretic method. Two sensor structures were tested in cryogenic field down to 2 K, and the resistance was measured using a standard four-point method. The measurement results suggest that, at temperatures below 20 K, the temperature coefficient of resistance average for sensor 1 is 1.473%/K and for sensor 2 is 0.365%/K. From the experimental data, it can be concluded that the dependence of electrical resistance versus temperature can be approximated by an exponential equation and, correspondingly, a set of coefficients are calculated. It is further concluded that the proposed approach described here offers several advantages, which can be employed in the fabrication of a microsensors for cryogenic applications.
17. Tuning magneto-transport properties of Fe-Au granular thin films by cluster organization
Authors:
Stanciu, AE; Kuncser, A; Schinteie, G; Palade, P; Leca, A; Greculeasa, SG; Catrina, A; Kuncser, V
Published: JUL 12 2017, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 50, DOI: 10.1088/1361-6463/aa7405
A comparative study of morpho-structural, magnetic and magneto-transport properties of two Fe-Au granular films with different concentrations of Fe nanoclusters of almost similar size is reported. Different organizations of the Fe clusters, i.e. in lamellar-like or random-like configuration, were obtained by varying the amount of Fe in the Fe-Au films. The specific magnetic behaviour was investigated with respect to local structure and morpho-structural aspects by combining magneto-optic Kerr effect and superconducting quantum interference device magnetometry, Fe-57 conversion electron Mossbauer spectroscopy and a wide range of electron microscopy techniques. A strong in-plane magnetic texture with uniaxial anisotropy was observed in the case of the lamellar-like organization of the clusters (specific to the Fe-Au film with higher Fe concentration) whereas a superparamagnetic behaviour was evidenced in the case of random distribution of the clusters (specific to the Fe-Au film with lower Fe concentration), despite the similar average size of the clusters in the two samples. Specific magnetoresistance effects were investigated with respect to both the involved magnetic configurations and magnetic interactions of the Fe clusters.
18. 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.
19. Structure, transition temperature, and magnetoresistance of titanium-doped lanthanum barium manganite epilayers onto STO 001 substrates
Authors:
Galca, AC; Oumezzine, M; Leca, A; Chirila, CF; Kuncser, V; Kuncser, A; Ghica, C; Pasuk, I; Oumezzine, M
Published: OCT 30 2017, APPLIED PHYSICS LETTERS, 111, DOI: 10.1063/1.4998011
We have developed a thin film structure with a maximum magnetoresistance effect (MRE) at room temperature, which is one of the operating requirements for many applications. It is shown that La0.67Ba0.33Ti0.02Mn0.98O3 epilayers obtained by pulsed laser deposition onto (001) SrTiO3 single crystal substrates exhibit the highest MRE, Delta R/R(H) approximate to 150% or Delta R/R(0) approximate to 60% under 5 T, at 300 K, a temperature near to the corresponding Curie temperature (T-C). Both doping with a tiny amount of titanium and induced stress due to lattice mismatch between the thin film and the substrate contribute to a decrease in T-C as compared to the pristine compound and therefore to the decrease in the temperature where the highest MRE is recorded. Published by AIP Publishing.
20. Physical Mechanisms of Exchange Coupling Effects in Nanoparticulate Diluted Magnetic Oxides Obtained by Laser Pyrolysis
Authors:
Kuncser, VE; Schinteie, GA; Kuncser, AC; Leca, A; Scarisoreanu, M; Morjan, I; Filoti, G
Published: APR 27 2017, JOURNAL OF PHYSICAL CHEMISTRY C, 121, 9069, DOI: 10.1021/acs.jpcc.7b01500
TiO2 nanoparticles, undoped and doped with Fe, have been prepared by laser pyrolysis and further investigated with respect to morphological, structural and magnetic aspects by transmission electron microscopy, diffractometry, Mossbauer spectroscopy, and magnetometry. The obtained nanoparticles, consisting of mainly anatase phase, agglomerate in clusters of tenths of units and present a large size distribution in the range from 5 to 40 nm. The anatase to rutile weight ratio (about 9) and the morphology of particles is similar in all analyzed samples (doped by up to 12 0,0 at. % Fe). Only Fe3+ ions in high spin-configuration were observed mainly at the, surface of TiO2 nanoparticles, either distributed or forming fine clusters of Fe oxide.;:Both a paramagnetic phase and a superparamagnetic one with blocking temperature lower than 50 K are superposed over a long-range ferromagnetic phase specific to diluted magnetic oxide systems. The influence of doping Fe ions on the magnetic behavior of each phase is discussed in detail. Evidences for interface exchange couplings (with unidirectional anisotropy in specific conditions) between the long-range ferromagnetic phase and the fine clusters (antiferromagnetic in nature), which become frozen below temperature of 50, K, are provided. The specificity of the processing route and the physical mechanisms responsible observed relevant magnetic features, which can be tailored for suitable applications, are discussed.
21. STRUCTURAL AND MAGNETIC PROPERTIES OF Sr2FeMoO6 OBTAINED AT LOW TEMPERATURES
Authors:
Bartha, C; Plapcianu, C; Crisan, A; Enculescu, M; Leca, A
Published: JUL-SEP 2016, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 11, 780, DOI:
The double-perovskite Sr2FeMoO6 has been obtained by solid state method at low temperature (1060 degrees C) and a very short time of synthesis (up to 4h). Both, X-ray diffraction and scanning electron microscopy (SEM) confirmed the formation of Sr2FeMoO6 oxide with grain sizes around 160 mm, and a small amount of SrMoO4 as an impurity. Mossbauer spectroscopy revealed a mixed site population with Fe and Mo ions generating a structure type with population inversion. This structure has a critical influence on the magnetic properties, as confirmed by the magnetization and TC values, i.e 3.56 mu(B)/f.u and 415 K, respectively. The Sr2FeMoO6 behavior was interpreted in terms of ferrimagnetic couplings generated by the various distributions of local interactions between Fe and Mo neighbors while comparing the ideal structure should show antiferromagnetic coupling between the two sublattices.
22. Structural, magnetic and magnetocaloric effects in epitaxial La0.67Ba0.33Ti0.02Mn0.98O3 ferromagnetic thin films grown on 001-oriented SrTiO3 substrates
Authors:
Oumezzine, M; Galca, AC; Pasuk, I; Chirila, CF; Leca, A; Kuncser, V; Tanase, LC; Kuncser, A; Ghica, C; Oumezzine, M
Published: 2016, DALTON TRANSACTIONS, 45, 15040, DOI: 10.1039/c6dt01914e
Epitaxial La0.67Ba0.33Ti0.02Mn0.98O3 (denoted as LBTMO hereafter) thin films of approximately 95 nm thickness were deposited by a pulsed laser deposition technique onto SrTiO3 (STO) (001) substrates. High-resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM) investigations revealed that the films are epilayers with a four-fold symmetry around the [001] direction. Cross-sectional TEM and the presence of Pendellosung fringes in the XRD profiles demonstrate smooth interfaces. The STO substrate induces an in-plane compressive strain, which leads to a slight tetragonality of the film structure. The epilayers exhibit paramagnetic-to-ferromagnetic phase transitions at the Curie temperature T-C (286 K), close to room temperature. The magnetization easy axis lies in the film plane along the [100] direction of the (001) substrate. The magnetic entropy change (Delta S-M) associated with the second-order magnetic phase transition was determined via magnetization measurements in the temperature range between 210 and 350 K under different magnetic fields. The relative cooling power (RCP) of this film is about 220 J kg(-1), somewhat lower than that of bulk Gd (410 J kg(-1)) for a 50 kOe field change, making the LBTMO ferromagnetic thin films a promising candidate for micro/nanomagnetic refrigeration around room temperature. The proposed universal curve provides a simple method for extrapolating Delta S-M in a wide range of fields and temperatures, thus confirming the order of the magnetic transition in this system. The magnetic entropy (Delta S-M)(max) around T-C is proportional to (mu H-0/T-C)(2/3) in agreement with the mean-field theory, indicating the existence of long-range ferromagnetic interactions in epitaxial LBTMO thin films.
23. Direct low temperature hydrothermal synthesis of YFeO3 microcrystals
Authors:
Racu, AV; Ursu, DH; Kuliukova, OV; Logofatu, C; Leca, A; Miclau, M
Published: FEB 1 2015, MATERIALS LETTERS, 140, 110, DOI: 10.1016/j.matlet.2014.10.129
In this work, we present a new, fast and high yield synthesis method for obtaining microcrystalline yttrium orthoferrite YFeO3, based on hydrothermal technique. The advantage of the method consists in the direct crystallization of the material without the necessity of the calcination. The quality of the so-obtained compound is checked by X-ray diffraction and XPS measurements. Unusual behavior of the zero field cooled (ZFC) - field cooled (FC) magnetization curves (ZFC > FC) was also observed in YFeO3 and a possible explanation could be related to the effect of the synthesis methods on the magnetic properties of yttrium orthoferrite. (C) 2014 Elsevier B.V. All rights reserved.
24. Insulin loaded iron magnetic nanoparticle-graphene oxide composites: synthesis, characterization and application for in vivo delivery of insulin
Authors:
Turcheniuk, K; Khanal, M; Motorina, A; Subramanian, P; Barras, A; Zaitsev, V; Kuncser, V; Leca, A; Martoriati, A; Cailliau, K; Bodart, JF; Boukherroub, R; Szunerits, S
Published: 2014, RSC ADVANCES, 4, 875, DOI: 10.1039/c3ra46307a
One of the focal subjects in insulin delivery is the development of insulin formulations that protect the native insulin from degradation under acidic pH in the stomach. In this work we show, for the first time, that a graphene oxide (GO) based matrix can ensure the stability of insulin at low pH. GO and GO modified with 2-nitrodopamine coated magnetic particle (GO-MPdop) matrices loaded with insulin were prepared and the pH triggered release of the insulin was studied. The loading of insulin on the GO nanomaterials proved to be extremely high at pH < 5.4 with a loading capacity of 100 +/- 3% on GO and 88 +/- 3% on GO-MPdop. The insulin-containing GO matrices were stable at acidic pH, while insulin was released when exposed to basic solutions (pH = 9.2). Using Xenopus laevis oocytes as a model we showed that the meiotic resumption rate of GO and GO-MPdop remained unaltered when pre-treated in acidic conditions, while pre-incubated insulin (without GO nanomaterials) has lost almost entirely its maturation effect. These results suggest that GO based nanomatrices are promising systems for the protection of insulin.
25. Magnetic configurations of Ni-Cu alloy nanowires obtained by the template method
Authors:
Matei, E; Enculescu, I; Toimil-Molares, ME; Leca, A; Ghica, C; Kuncser, V
Published: AUG 2013, JOURNAL OF NANOPARTICLE RESEARCH, 15, DOI: 10.1007/s11051-013-1863-3
High aspect ratio nanowires of Ni-Cu alloys have been synthesized by potentiostatic electrochemical deposition in etched ion-track membranes. The nickel-to-copper ratio in the nanowires was controlled via the deposition potential and electrochemical bath composition. We present a detailed study of nanowire properties including morphology, composition, and magnetic behavior. We report the magnetic configurations measured as function of the nanowire composition and discuss domain formation, anisotropy aspects, and local easy axis distributions.
26. Superparamagnetic magnetite-divinylbenzene-maleic anhydride copolymer nanocomposites obtained by dispersion polymerization
Authors:
Donescu, D; Raditoiu, V; Spataru, CI; Somoghi, R; Ghiurea, M; Radovici, C; Fierascu, RC; Schinteie, G; Leca, A; Kuncser, V
Published: OCT 2012, EUROPEAN POLYMER JOURNAL, 48, 1716, DOI: 10.1016/j.eurpolymj.2012.07.012
Magnetite alternating copolymers divinylbenzene-maleic anhydride (DVB-MA) composites were prepared by dispersion polymerization. Because magnetite is used as a complex with oleic acid (Fe(3)O(4)OLA), the final hybrids show good dispersion of inorganic nanofillers in the polymer matrix. The obtained composites were analyzed by infrared absorption spectrometry, diffuse reflectance in visible light, thermogravimetry, X-ray fluorescence, X-ray diffraction, dynamic light scattering, scanning electron microscopy and vibrating sample magnetometry. The obtained results indicate the successful preparation of magnetite nanoparticles with an average size of about 23 nm dispersed in micrometer size copolymer spherical particles, which relative content can be controlled via the processing parameters. A relationship between the relative content of magnetite nanoparticles and the size of the polymer particles, with direct influence on the diffuse reflectance in the visible domain, was observed. A superparamagnetic behavior was evidenced at room temperature with a blocking temperature lower than as expected from the bulk anisotropy constant and the average size of the magnetite nanoparticles. Both the unexpected low blocking temperature and the observed low specific magnetizations were explained by a defected and poor crystalline structure of the magnetite nanoparticles, giving rise to spin disorder and diminished crystalline anisotropy constant. (C) 2012 Elsevier Ltd. All rights reserved.
27. Bovine Serum Albumin 3D Structure Determination by THz Spectroscopy and Molecular Modeling
Authors:
Mernea, M; Leca, A; Dascalu, T; Mihailescu, D
Published: 2011, TERAHERTZ AND MID INFRARED RADIATION: GENERATION, DETECTION AND APPLICATIONS, +, DOI: 10.1007/978-94-007-0769-6_14
The low frequency collective movements of proteins associated with biologically relevant conformational transitions are situated in the terahertz (THz) spectral region. THz spectroscopy experiments have revealed that each molecular species has a unique absorption pattern in THz domain. We used time-domain THz spectroscopy (TDS) to study the conformation and flexibility of bovine serum albumin (BSA), a protein whose 3D structure is unknown. We performed THz spectroscopy experiments on lyophilized BSA. Theoretical spectra were obtained by normal modes analysis performed on BSA structures generated by homology modeling and molecular dynamics simulations. The agreement between experimental and theoretical data allowed us to validate the model of BSA and also to gain insight into BSA vibrations in THz domain.
28. Transmission THz time domain system for biomolecules spectroscopy
Authors:
Dinca, MP; Leca, A; Apostol, D; Mernea, M; Calborean, O; Mihailescu, D; Dascalu, T
Published: JAN 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 114, DOI:
A THz Time Domain Spectroscopy experimental set-up was built for studying the biomolecules absorption spectra in the THz range and obtaining information on their collective motions. Details on the set-up calibration and performances are presented, together with the used noise and artefacts reduction data processing techniques. Preliminary results on the absorbance spectrum of the solvated Bovine Serum Albumin (BSA) for different water concentration are given.
29. Q-switched Nd lasers pumped directly into the F-4(3/2) emitting level
Authors:
Pavel, N; Dascalu, T; Salamu, G; Sandu, O; Leca, A; Lupei, V
Published: DEC 15 2009, OPTICS COMMUNICATIONS, 282, 4754, DOI: 10.1016/j.optcom.2009.08.067
The influence of the direct pumping into the F-4(3/2) emitting level on the output characteristics of continuous-wave (CW) pumped, passively or actively (acoustooptic, AO) Q-switched Nd lasers is discussed. In case of passive Q-switching by Cr4+: YAG saturable absorber (SA) crystal, the change of pumping wavelength from 0.81 mu m into the highly-absorbing F-4(5/2) level to 0.88 mu m into the F-4(3/2) level of Nd does not modify the energy of the Q-switch pulse, but increases the pulse repetition rate and the laser average power for the same absorbed pump power. This is demonstrated with 0.81 and 0.88 gm CW laser diode-pumped Nd:YAG and Nd-vanadate lasers with average output power in the watt-level range at 1.06 mu m. The effect is explained by the control of passive Q-switching by the intracavity photon flux that is influenced by the pump wavelength and by the initial transmission of the SA crystal. On the other hand, it is discussed and experimentally proved that due to the possibility to control externally the frequency of switching, in case of the AO Q-switched Nd laser the change of the pump wavelength from 0.81 to 0.88 pm increases the pulse energy for a fixed frequency, leading to a corresponding increase of the average laser power. (C) 2009 Elsevier B.V. All rights reserved.
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