National Institute Of Materials Physics - Romania

The effect of the demagnetizing factor, regarding the determination of the de-pairing current density Jdep, has been studied in the case of a Fe(Se,Te) crystal, using DC magnetic measurements as a function of a magnetic field (H) at different temperatures (T). First, the lower critical field Hc1(T) values were obtained, and the demagnetization effects acting on them were investigated after calculating the demagnetizing factor. The temperature behaviors of both the original Hc1 values and the ones obtained after considering the demagnetization effects (Hc1demag) were analyzed, and the temperature dependence of the London penetration depth lambda L(T) was obtained in both cases. In particular, the lambda L(T) curves were fitted with a power law dependence, indicating the presence of low-energy quasiparticle excitations. Furthermore, by plotting lambda L-2 as a function of T, we found that our sample behaves as a multigap superconductor, which is similar to other Fe-11 family iron-based compounds. After that, the coherence length xi values were extracted, starting with the Hc2(T) curve. The knowledge of lambda L and xi allowed us to determine the Jdep values and to observe how they are influenced by the demagnetizing factor.

Rare-earth-free permanent magnets with the L1(0) phase are actively researched for their potential as a future class of magnetic materials, capable of operating at higher temperatures and in challenging corrosion environments such as renewable energy applications. Among these classes, MnGa shows potential, being cost effective and having interesting magnetic properties. A MnGa magnetic alloy, with composition Mn73.6Ga26.4 in atomic percent, was produced via the out-of-equilibrium method, and its structural and magnetic properties were assessed using X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and extended magnetic characterization. We show that the MnGa alloy submitted to thermal annealing in optimal conditions exhibits a two-phase microstructure, where small nanocrystals of tetragonal L1(0)/D0(22) magnetic phase are embedded within a D0(19) MnGa matrix of a non-collinear antiferromagnetic nature. These co-existing, magnetically different phases produce an optimal set of promising magnetic properties, larger than the values reported in the literature for single-phase MnGa alloys and thin films. Such large values are explained by the exchange coupling between competing non-collinear magnetic sublattices of the D0(19) MnGa with the net moment of the small magnetic nanocrystals of tetragonal symmetry.

Technological progress has led to the development of analytical tools that promise a huge socio-economic impact on our daily lives and an improved quality of life for all. The use of plant extract synthesized nanoparticles in the development and fabrication of optical or electrochemical (bio)sensors presents major advantages. Besides their low-cost fabrication and scalability, these nanoparticles may have a dual role, serving as a transducer component and as a recognition element, the latter requiring their functionalization with specific components. Different approaches, such as surface modification techniques to facilitate precise biomolecule attachment, thereby augmenting recognition capabilities, or fine tuning functional groups on nanoparticle surfaces are preferred for ensuring stable biomolecule conjugation while preserving bioactivity. Size optimization, maximizing surface area, and tailored nanoparticle shapes increase the potential for robust interactions and enhance the transduction. This article specifically aims to illustrate the adaptability and effectiveness of these biosensing platforms in identifying precise biological targets along with their far-reaching implications across various domains, spanning healthcare diagnostics, environmental monitoring, and diverse bioanalytical fields. By exploring these applications, the article highlights the significance of prioritizing the use of natural resources for nanoparticle synthesis. This emphasis aligns with the worldwide goal of envisioning sustainable and customized biosensing solutions, emphasizing heightened sensitivity and selectivity.

Cu2ZnSnSe4 thin films have been synthesized by employing two magnetron-sputtering depositions, interlaced with two sequential post-deposition heat treatments in low vacuum, Sn+Se and Se-rich atmospheres at 550 degrees C. By employing successive structural analysis methods, namely Grazing Incidence X-Ray Diffraction (GIXRD) and Raman Spectroscopy, secondary phases such as ZnSe coexisting with the main kesterite phase have been identified. SEM peered into the surface morphology of the samples, detecting structural defects and grain profiles, while EDS experiments showed off-stoichiometric elemental composition. The optical bandgaps in our samples were calculated by a widely used extrapolation method from recorded transmission spectra, holding values from 1.42 to 2.01 eV. Understanding the processes behind the appearance of secondary phases and occurring structural defects accompanied by finding ways to mitigate their impact on the solar cells' properties is the prime goal of the research beforehand.

Nano-logic magnetic structures are of great interest for spintronic applications. While the methods used for developing arrays of magnetic L1(0)-phase dots are, in most cases, based on deposition followed by annealing at high temperatures, usually around 700 degrees C, we demonstrate here a technique where a much lower annealing temperature (i.e., 400 degrees C) is needed in order to promote fully the disorder-order phase transformation and achievement of highly coercive L1(0)-phase dots. In order to develop building blocks based on arrays of L1(0)-phase FePt dots for further spintronic applications, an engraving technique using electron beam lithography is employed. This paper describes the fabrication, as well as the morphological and magnetic characterization, of regularly placed FePt dots of various shapes, as pre-requisites for integration into nano-logic devices. As a proof of concept, regular arrays of FePt circular dots were devised and their structural characterization, using X-ray diffraction (XRD) and transmission electron microscopy (TEM), was performed. It has been shown that annealing at only 400 degrees C for 30 min proved the occurrence of the tetragonal L1(0) phase. Moreover, structural characterization showed that the disorder-order phase transformation was complete with only the L1(0) phase detected in high resolution TEM. The magnetic characterization provided more insight into the potential of such arrays of magnetic devices with convenient values of magnetic coercivity, remanent and saturation magnetization. These findings show good potential for developing regular arrays of uniformly shaped magnetic entities with encouraging magnetic performances in view of various applications.

Among various "families" of iron-based superconductors, the quite recently discovered AeAFe(4)As(4) (where Ae is an alkali-earth metal and A is an alkali metal) has high critical current density, a very high upper critical field, and a low anisotropy, and has recently received much interest for the possibility of high magnetic field applications at the liquid hydrogen temperature. We have performed DC magnetization relaxation and frequency-dependent AC susceptibility measurements on high-quality single crystals of CaKFe4As4 with the aim of determining the pinning potential U*. The temperature dependence of U* displays a clear crossover between elastic creep and plastic creep. At temperatures around 27-28 K, U* has a very high value, up to 1200 K, resulting in an infinitesimally small probability of thermally activated flux jumps. From the dependence of the normalized pinning potential on irreversible magnetization, we have determined the creep exponents in the two creep regimes, which are in complete agreement with theoretical models. The estimation of the pinning potential from multifrequency AC susceptibility measurements was possible only near the critical temperature due to equipment limitations, and the resulting value is very close to the one that resulted from the magnetization relaxation data. Magnetic hysteresis loops revealed a second magnetization peak and very high values of the critical current density.

Hydrothermally formed mesoporous SnO2 was used as a support for nickel chemical deposition and, after subsequent thermal treatment, a high specific surface area (36 m(2) g(-1)) Ni/SnO2 material was obtained. XPS analysis has shown that in the Sn 3d region the spectrum is similar to that of pristine SnO2, whereas Ni species are present on the surface as NiO, Ni2O3 and Ni(OH)(2). Mixing Ni/SnO2 with a small amount of Black Pearls (BP) leads to a significant enhancement of the resulting Ni/SnO2-BP composite activity for nitrite anodic oxidation, presumably due to the higher surface area (115 m(2) g(-1)), to better electrical conductivity and to a certain contribution of the BP to an increase in surface density of the active sites. Ni/SnO2-BP also outperforms pristine BP (in terms of Tafel slopes and electron-transfer rates), most likely due to the fact that the Ni(ii)/Ni(iii) couple can act as an electrocatalyst for nitrite oxidation. A voltammetric method is proposed for the determination of nitrite, over a concentration range of three orders of magnitude (0.05 to 20 mM), with good reproducibility, high stability and excellent sensitivity. The high upper limit of the dynamic range of the analytically useful response might provide a basis for the reliable quantification of nitrite in wastewater.

B doped as well as B and N co-doped highly reduced graphene oxide and multiwall carbon nanotubes/Ni oxide and Ni hydroxide nanohybrid layers were synthesized using a one-step laser technique. NiO nanoparticles, graphene oxide platelets, and multiwall carbon nanotubes were used as starting materials for the preparation of the composite layers. H3BO3 was used as B precursor and urea as well as NH3 as N precursors for B and N co -doping of the composite layers. The influence of the nature and concentration of the B and N precursors on the physico-chemical properties of the nanohybrid layers, chemical composition and chemical bonding states, crystalline structure, as well as charge transfer properties was systematically investigated. The relation between the physico-chemical properties and functional characteristics of the nanohybrid layers, photocatalytic removal of water contaminants under simulated sun irradiation conditions was studied, identifying the optimum con-centrations of the dopant precursor materials. The mechanism of the photodegradation process was explored, based of the band structure of the composites. The main intermediates, reactive oxygen species implied in the photocatalytic degradation processes of organic molecules, was investigated through the addition of scavengers to the organic dye test solutions. The highest photocatalytic activities were achieved for B and N co-doped samples synthesised using H3BO3 as B and urea as N precursors.

One of the new materials for next-generation thin film solar cells is Cu2ZnSnSe4 (CZTSe). However, achieving a single-phase CZTSe compound remains a challenge. This study describes the development of Cu2ZnSnSe4 thin films through the sequential deposition of stacked films of non-stoichiometric Cu2SnSe3 (CTSe) and ZnSe by magnetron sputtering. The structural, morphological, and electrical properties as well as the surface chemistry of the films were investigated and compared depending on the growth sequence of the thin films. By using Raman spectroscopy and grazing incidence X-ray diffraction, the tetragonal CZTSe structure was confirmed. Scanning electron microscopy and energy-dispersive spectroscopy measurements of the morphological and compositional properties indicated large grains and dense surfaces with an elemental composition close to the desired stoichiometry in SLG\SnSe2\Cu\ZnSe and SLG\SnSe2\Cu2Se\ZnSe stacks. To ascertain the surface chemistry and unique characteristics of the produced films, additional X-ray photoemission spectroscopy experiments were carried out. The optimal band gap values for the absorber layers were found using conventional spectroscopy, and they ranged from 0.88 to 1.47 eV. According to the electrical measurements, all the films were p-type and have high carrier concentrations between 1016 and 1020 cm-3. Our findings demonstrate that employing a sequential deposition approach and annealing in different atmospheres can yield CZTSe absorber layers with desirable properties, overcoming the challenge of non-stoichiometric CTSe precursors.

Dielectric spectroscopy is a well-known method to characterize different liquid or solid (nano) materials such as liquid crystals, polymers, composites, ceramics etc. More the material structure is complex, more the molecular dynamics are put in evidence with difficulty. In many such cases, the analysis of the spectra with the classic Havriliak-Negami (H-N) functions is hard to apply or even fails. Therefore, we propose a simple numerical approach (with two different procedures) for the deconvolution of complex spectra in dielectric spectroscopy (DS). The final goal is to obtain the frequency of maximum loss (fmax) depending on the temperatures, Arrhenius diagram, and the activation energy/energies. The developed procedures have as their starting point the "logarithmic derivative of the permittivity". To our knowledge, these procedures have not yet been presented in the literature. The proposed first numerical processing allows a better separation of the different relaxation processes, a decrease in the contribution of the electrical conductivity and a better localization of the frequencies where the dielectric loss has maximum values. The second of the procedures is applied in certain particular situations, and the other procedure is more general. Where the former can be applied, it is equivalent to using the classical H-N functions in terms of obtaining the fmax, but it is simpler to apply. The first procedure uses an "artificial" and simpler H-N function, in which the fewer fitting parameters have no physical meaning, but allow a good localization of the frequency of maximum dielectric loss. The proposed procedure was applied on experimental data of dielectric spectroscopy obtained on pristine materials (nematic mixture E7 and ZnO) and composites E7-ZnO, obtained by a "green technology". The complicated spectra of the dielectric permittivity presented by these samples represent the suitable test to highlight the advantages but also the limitations of our approach. Comparing the values of the activation energy obtained for the two procedures is the criterion for verifying their correctness. The volume of processed experimental data is large, for this reason we present only the results obtained based on numerical approaches. The differences between the results obtained with the two procedures are small. The procedures can be considered as alternatives to the application of fitting with H-N functions. One procedure replaces H-N by eliminating the use of functions with complex variables and the method of least squares and has the advantage that it can apply non-specialized software like Origin. The other allows a better analysis of spectra being an improved-modified H-N approach. Proposed procedures are