National Institute Of Materials Physics - Romania

In the quest for novel rare earth (RE)-free magnetic materials, which also exhibit other additional properties such as good corrosion resistance and potential to operate at higher temperatures, an alloy deriving from the binary FePt system, with Mo and B addition, has been synthesized for the first time, using the out-of-equilibrium method of rapid solidification form the melt. The alloy with the composition Fe49Pt26Mo2B23 has been subjected to thermal analysis through differential scanning calorimetry in order to detect the structural disorder - order phase transformation as well as to study the crystallization processes. For the stabilization of the formed hard magnetic phase, the sample has been annealed at 600 degrees C and further structurally and magnetically characterized by means of X-ray diffraction, transmission electron microscopy, Fe-57 Mossbauer spectrometry as well as magnetometry experiments. It has been proven that after annealing at 600 degrees C the tetragonal hard magnetic L1(0) phase emerges via crystallization from a disordered cubic precursor and becomes the predominant phase in terms of relative abundance. Moreover, it has been revealed by quantitative analysis via Mossbauer spectroscopy that the annealed sample exhibits a complex phase structure, where the L1(0) hard magnetic phase is accompanied by few other soft magnetic phases, in minority abundance: the cubic A1, orthorhombic Fe2B and residual intergranular region. The magnetic parameters have been derived from 300 K hysteresis loops. It was shown that, contrary to the as-cast sample which behaves as a typical soft magnet, the annealed sample presents strong coercivity and high remanent magnetization, accompanied by a large saturation magnetization. These findings offers good insight into the potential developing of novel class of RE-free permanent magnets, based on Fe-Pt-Mo-B, where the magnetic performance emerges from the co-existence of hard and soft magnetic phases in controlled and tunable proportions, capable of finding good applicability in fields requiring good catalytic properties and strong corrosion resistance.

In 1946 and 1949, Charles Kittel proposed a simple model for the size of ferromagnetic domains that is still widely used nowadays [C. Kittel, Phys. Rev. 70 (1946) 965-971 and Rev. Mod. Phys. 21 (1949) 541-583], including for other ferroic systems, such as ferroelectrics and multiferroics. Kittel's theory is revisited in this work, with a more detailed demonstration and emphasizing the main assumptions utilized, by using SI units instead of CGS units, as in the original Kittel's works. The validity limits of the Kittel's scaling law where the domain width varies with the square root of the sample thickness towards low thicknesses is derived, with the possibility of evolution towards large domains for ultralow thicknesses. Further, Kittel's model is extended to the case where the sample has a non-vanishing net magnetization and it is shown how magnetization curves at zero temperature can be obtained. This is discussed by supposing constant width of a pair of neighboring domains with opposed magnetization, or by allowing this width to vary as function on the net magnetization of the sample. Though this latter assumption seems to be more reasonable from the point of view of the evolution towards a single domain state at saturation, it seems that the model able to yield most accurate vales of the coercive field is the domain with fixed width of the pair of domains, which justifies the assumption of domain wall pinning". The introduction of the demagnetization factor associated with the finite size of the film yields a maximum thickness up to which the films present hysteresis curves. The validity of this theory for ferrelectric domains is also briefly discussed.

In the quest for novel rare earth (RE)-free magnetic materials, which also exhibit other additional properties such as good corrosion resistance and potential to operate at higher temperatures, an alloy deriving from the binary FePt system, with Mo and B addition, has been synthesized for the first time, using the out-of-equilibrium method of rapid solidification form the melt. The alloy with the composition Fe49Pt26Mo2B23 has been subjected to thermal analysis through differential scanning calorimetry in order to detect the structural disorder - order phase transformation as well as to study the crystallization processes. For the stabilization of the formed hard magnetic phase, the sample has been annealed at 600 degrees C and further structurally and magnetically characterized by means of X-ray diffraction, transmission electron microscopy, Fe-57 Mossbauer spectrometry as well as magnetometry experiments. It has been proven that after annealing at 600 degrees C the tetragonal hard magnetic L1(0) phase emerges via crystallization from a disordered cubic precursor and becomes the predominant phase in terms of relative abundance. Moreover, it has been revealed by quantitative analysis via Mossbauer spectroscopy that the annealed sample exhibits a complex phase structure, where the L1(0) hard magnetic phase is accompanied by few other soft magnetic phases, in minority abundance: the cubic A1, orthorhombic Fe2B and residual intergranular region. The magnetic parameters have been derived from 300 K hysteresis loops. It was shown that, contrary to the as-cast sample which behaves as a typical soft magnet, the annealed sample presents strong coercivity and high remanent magnetization, accompanied by a large saturation magnetization. These findings offers good insight into the potential developing of novel class of RE-free permanent magnets, based on Fe-Pt-Mo-B, where the magnetic performance emerges from the co-existence of hard and soft magnetic phases in controlled and tunable proportions, capable of finding good applicability in fields requiring good catalytic properties and strong corrosion resistance.

Cu2NiSnS4 (CNTS) absorber layers are elaborated by electrodeposition at various applied potentials followed by sulfurization treatment at 450 degrees C under sulfur atmosphere. The microstructural investigations revealed the presence of Cu4SnS4 secondary phases which can be reduced using an applied potential of -1.15 V vs. Ag/AgCl. Using the corresponding cathodic potential for Ni2+, the competing detrimental hydrogen evolution regresses the morphology and composition. The film with the highest Ni concentration has a band gap of 1.44 eV as inferred from diffuse reflectance data. The Randles cell model is probed by electrochemical impedance spectroscopy.

In fusion reactors, such as ITER or DEMO, the plasma used to generate nuclear reactions will reach temperatures that are an order of magnitude higher than in the Sun's core. Although the plasma is not supposed to be in contact with the reactor walls, a large amount of heat generated by electromagnetic radiation, electrons and ions being expelled from the plasma will reach the plasma-facing surface of the reactor. Especially for the divertor part, high heat fluxes of up to 20 MW/m(2) are expected even in normal operating conditions. An improvement in the plasma-facing material (which is, in the case of ITER, pure Tungsten, W) is desired at least in terms of both a higher recrystallization temperature and a lower brittle-to-ductile transition temperature. In the present work, we discuss three microengineering routes based on inclusions of nanometric dispersions, which are proposed to improve the W properties, and present the microstructural and thermophysical properties of the resulting W-based composites with such dispersions. The materials' behavior after 6 MeV electron irradiation tests is also presented, and their further development is discussed.

In this paper, we present a theoretical perspective regarding the interaction of graphene with circularly polarized light and magnetic field, from the topological insulators point of view. We analyze how these two external fields affect the spectral, topological and transport properties of graphene and correlate the findings in order to explain in a fundamental and unified way the emerging topological phase transitions. In this respect, in the first step we introduce a model for interaction and charge transport. Then, based on the derived theory, we present numerical results aimed to explain the underlying processes which give graphene topological properties. The central point is represented by the time-reversal symmetry breaking which generates chiral edge states, namely electronic states localized at the edges of the system, having opposite velocity directions. We find that the light frequency, intensity and polarization state drastically influence the formation of the chiral edge states and their number. We correlate this effect with quantum Hall transport, analyzing the resulting transversal (Hall) resistance plateaus and their values. Moreover, if a supplementary magnetic field driving is applied, there emerge intricate topological phase transitions, characterized by introducing or removing specific Hall resistance plateaus.

High-energy physics detectors with internal charge multiplication, like Low Gain Avalanche Detectors (LGADs), that will be used for fast timing in the High Luminosity LHC experiments, have to exhibit a significant radiation tolerance. In this context, the impact of radiation on the highly boron-doped gain layer is of particular interest, since due to the so-called Acceptor Removal Effect (ARE) a radiation-induced deactivation of active boron dopants takes place, that is causing a progressive loss in the gain with increasing irradiation level. In this paper we present defect-spectroscopy measurements (Deep-Level Transient Spectroscopy and Thermally Stimulated Current technique) on neutron, proton and electron irradiated p-type silicon pad diodes of different resistivity as well as LGADs neutron irradiated at fluences up to 1 x 1015 neq/cm2. We show that compared to silicon pad diodes the determination of LGAD defect introduction rates is less straightforward as they are strongly influenced by the impact of the gain layer. The measured gain layer capacitance has a strong frequency and temperature dependence which makes DLTS measurements challenging to perform with results difficult to interpret. With the TSC technique the defects formed in the LGADs are nicely observed and can be compared to the defects formed in the silicon pad diodes. However, the exact assignment of defects to the gain layer or bulk region remains challenging and the charge amplification effect of the LGADs impacts the exact determination of defect concentrations. We also demonstrate that, depending on the TSC measurement conditions, defect induced internal electric fields are built up in the irradiated LGADs which impact the signal current.

Nanocomposite films based on macrocyclic compounds (zinc phthalocyanine (ZnPc) and 5,10,15,20-tetra(4-pyridyl) 21H,23H-porphyrin (TPyP)) and metal oxide nanoparticles (ZnO or CuO) were deposited by matrix-assisted pulsed laser evaporation (MAPLE). 1,4-dioxane was used as a solvent in the preparation of MAPLE targets that favor the deposition of films with a low roughness, which is a key feature for their integration in structures for optoelectronic applications. The influence of the addition of ZnO nanoparticles (similar to 20 nm in size) or CuO nanoparticles (similar to 5 nm in size) in the ZnPc:TPyP mixture and the impact of the added metal oxide amount on the properties of the obtained composite films were evaluated in comparison to a reference layer based only on an organic blend. Thus, in the case of nanocomposite films, the vibrational fingerprints of both organic compounds were identified in the infrared spectra, their specific strong absorption bands were observed in the UV-Vis spectra, and a quenching of the TPyP emission band was visible in the photoluminescence spectra. The morphological analysis evidenced agglomerated particles on the composite film surface, but their presence has no significant impact on the roughness of the MAPLE deposited layers. The current density-voltage (J-V) characteristics of the structures based on the nanocomposite films deposited by MAPLE revealed the critical role played by the layer composition and component ratio, an improvement in the electrical parameters values being achieved only for the films with a certain type and optimum amount of metal oxide nanoparticles.

Whereas solar photovoltaic cells are promising for proper power production, their wide deployment is hampered by production costs, material availability and toxicity. One of these materials is CuO, which has great chemical stability as well as interesting physical properties, including a direct band gap, a high absorption coefficient, and p-type conductivity. These properties indicate CuO as an exciting semiconducting material to use an absorber layer in thin films solar cells. For this specific reason, this paper focuses on the synthesis of pristine and Ni-incorporated CuO thin films by spray pyrolysis process. Several techniques such as; X-ray diffraction (XRD), Raman spectroscopy, Scanning Electron Microscopy (SEM), Energy dispersive X-ray (EDX) and UV-Visible spectroscopy have been employed to characterize the synthesized samples. The XRD analysis indicated the formation of polycrystalline CuO thin films and the average crystallite size was decreased from 35 to 20 nm for the samples with x = 0-8 at%, respectively. Furthermore, Raman spectroscopy also confirms the single-phase formation of CuO films. The SEM images demonstrated that the incorporation of Ni in the CuO matrix improved the films surface. The EDX analysis and the elemental mapping belay the homogeneous distribution of the elements. Moreover, UV-Visible spectroscopy has shown a significant expansion in optical energy band gap from 1.45 to 1.53 eV with an increase of Ni concentration. On the other hand, to investigate the impact of Ni-incorporated CuO on nanostructure Ni:CuO/ZnO-NRs heterojunction solar cell performance, SCAPS-1D software is used. It has been found that, 8% Ni:CuO layer has been revealed as better for nanostructured solar cells. The results of this contribution will provide some vital guidelines for fabricating higher-efficiency solar cells.

Rhombohedral phase HfxZr1-xO2 (HZO, x from 0 to 1) films are promising for achieving robust ferroelectric polarization without the need for an initial wake-up pre-cycling, as is normally the case for the more commonly studied orthorhombic phase. However, a large spontaneous polarization observed in rhombohedral films is not fully understood, and there are also large discrepancies between experimental and theoretical predictions. In this work, in rhombohedral ZrO2 thin films, we show that oxygen vacancies are not only a key factor for stabilizing the phase, but they are also a source of ferroelectric polarization in the films. This is shown experimentally through the investigation of the structural properties, chemical composition and the ferroelectric properties of the films before and after an annealing at moderate temperature (400 degrees C) in an oxygen environment to reduce the V-O concentration compared. The experimental work is supported by density functional theory (DFT) calculations which show that the rhombohedral phase is the most stable one in highly oxygen defective ZrO2 films. The DFT calculations also show that V-O contribute to the ferroelectric polarization. Our findings reveal the importance of V-O for stabilizing rhombohedral ZrO2 thin films with superior ferroelectric properties.