National Institute Of Materials Physics - Romania

Nanoscale thermometers with high sensitivity are needed in domains which study quantum and classical effects at cryogenic temperatures. Here, we present a micrometer sized and nanometer thick chromium selenide cryogenic temperature sensor capable of measuring a large domain of cryogenic temperatures down to tenths of K. Hexagonal Cr-Se flakes were obtained by a simple physical vapor transport method and investigated using scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray photoelectron spectroscopy measurements. The flakes were transferred onto Au contacts using a dry transfer method and resistivity measurements were performed in a temperature range from 7 K to 300 K. The collected data have been fitted by exponential functions. The excellent fit quality allowed for the further extrapolation of resistivity values down to tenths of K. It has been shown that the logarithmic sensitivity of the sensor computed over a large domain of cryogenic temperature is higher than the sensitivity of thermometers commonly used in industry and research. This study opens the way to produce Cr-Se sensors for classical and quantum cryogenic measurements.

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.

Fe oxide magnetic nanoparticles (MNPs) in general and cobalt ferrite nanoparticles in particular have immense potential for applications in catalysis, medicine, information and energy storage, etc. MNPs feature interesting physical and chemical properties, different to those of corresponding bulk materials. The magnetic anisotropy constant of almost spherical CoFe2O4 MNPs is much higher than that of magnetite (Fe3O4) MNPs of similar geometrical parameters due to the magnetocrystalline contribution. CoFe2O4 shows significant magnetization at saturation, high coercive field and Curie temperature, and good chemical and magnetic stability, being therefore preferable to the most usual Fe3O4 MNPs. A surfactant-assisted synthetic route was employed to synthesize Fe oxide and in particular cobalt ferrite MNPs over a wide pH range (3-13), endeavor which allowed analysis of transient and parasitic phase identified in acidic reaction conditions.

This paper describes for the first time two processing routes-the precursor method and the two-step wet chemical process-for the synthesis of magnetic cobalt ferrite using the Tamarindus indica fruit extract. These green approaches are eco-friendly, safe and efficient alternatives to classical chemical methods. The aqueous extract from tamarind fruit contains numerous metabolites (organic acids, aminoacids). All these bioactive components are able to chelate metal ions leading to the formation of the multimetallic complex (precursor of cobalt ferrite). The obtained precursor was characterized by Fourier transform infrared spectroscopy (FTIR), thermal analysis, X-ray diffraction analysis (XRD) and magnetic measurements. The structure, morphology and magnetic behavior of the cobalt ferrite samples prepared through both synthesis routes were investigated by various characterization techniques: FTIR, XRD, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), Mossbauer spectroscopy and magnetic measurements. XRD data confirmed that a cubic spinel structure was obtained for both ferrite powders with average crystallite size of 13 and 5 nm, respectively. The microstructure study by SEM revealed the formation of nanocrystallites assemblies using the precursor method and carbon-rich particles forming granulated micron-sized agglomerates, embedding ferrite nanocrystallites obtained through the two-step wet chemical process. Mossbauer spectroscopy results evidenced relaxation processes in the CoFe2O4 samples at room temperature, and the main characteristics of the involved sublattices were derived. The magnetic investigation revealed a typical magnetic behavior for a spinel, with CoFe2O4 nanoparticles ferrimagnetic at low temperature and superparamagnetic at room temperature.

The chemical precipitation was used to obtain carbon fibers (CF) with surface modified by magnetite particles (Fe3O4). Processing was carried out by employing up to three subsequent coating stages of ultrasonic treatments. After each sonication stage, the coating was 17, 33, and 47 wt. % of the total weight of the modified fibers. Raman spectroscopy indicates the presence in the coating of a mixture of iron II and III states. As-decorated fibers were used to fabricate composites with an epoxy resin (ED-20) matrix cured with PEPA. The quantity of the carbon fiber filler was of 1, 3, and 6 wt %. At room temperature, the saturation magnetization of the soft magnetic samples was 0.37, 0.83, and 1.72 emu/g for the indicated compositions. Carbon fiber reinforced polymer materials with extra functions such as magnetic in this case, are expected to be useful in applications from the power and energy industries.

Compact and defect-free high melting point oxide strengthened metallic matrix configurations are promising to resolve the hydrogen permeation and brittleness issues relevant to the fusion research community. Previous studies on oxide addition to metallic matrix demonstrated a mitigation in brittleness behavior, while deposition techniques and material configurations are still to be investigated. Thus, here, we report the structural, morphological, and mechanical characterization of metal-oxides thin layers co-deposited by radio frequency (RF)and direct current (DC) magnetron sputtering. A total of six configurations were deposited such as single thin layers of oxides (Al2O3, Er2O3) and co-deposition configurations as metal-oxides (W, Be)-(Al2O3, Er2O3). The study of films roughness by atomic force microscopy (AFM) method show that for Al2O3 metallic-oxides is increased to an extent that could favor gaseous trapping, while co-depositions with Be seem to promote an increased roughness and defects formation probability compared to Wco-depositions. Lower elastic modulus on metal-oxide co-depositions was observed, while the indentation hardness increased for Be and decreased for W matrix configurations. These outputs are highly relevant for choosing the proper compact and trap-free configuration that could be categorized as a permeation barrier for hydrogen and furtherly studied in laborious permeation yield campaigns.

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.

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.

In order to prove the usefulness of having a structurally disordered precursor to the formation of FePt L1(0) phase and to facilitate the co-existence of exchange coupled hard and soft magnetic phases with optimized magnetic properties in various conditions of annealing, a Fe-Pt-Zr-B melt spun alloy has been synthesized and detailed structural and magnetic investigations have been undertaken to probe its phase evolution during annealing. The dynamics of formation of the hard magnetic L1(0) phase during the gradual disorder-order phase transformation has been monitored by using a complex combination of X-ray diffraction methods and Fe-57 Mossbauer spectroscopy methods, over a wide range of annealing temperatures. Multiple phases co-existing in the annealed sample microstructures, observed in XRD, have been reconfirmed by the Mossbauer spectra analysis and, moreover, accurate quantitative data have been acquired in what concerns the relative abundance of each of the observed crystalline phases in every stage of annealing. It is shown that the formation of the hard magnetic phase, emerging from the chemically disordered precursor, is gradual and occurs via complex mechanisms, involving the presence of a disordered Fe-Zr-B-rich intergranular region which contributes to an increase in the abundance of the L1(0) phase for higher annealing temperatures. Magnetic measurements have confirmed the good performances of these alloys in terms of coercivity and remanence. These results contribute to the development of these alloys as the next generation of rare earth, free permanent magnets.

The discovery of multifunctional properties related to electro-activity of organic systems of biomolecules is important for a variety of applications, especially for devices in the realm of biocompatible sensors and/or bioactuators. A further step towards such applications is to prepare thin films with the required properties. Here, the investigation is focused on the characterization of films of guanine and cytosine nucleobases, prepared by thermal evaporation-an industrial accessible deposition technique. The cytosine films have an orthorhombic non-centrosymmetric structure and grow in two interconnected nanostructured fractal patterns, of nearly equal proportion. Piezoresponse force microscopy images acquired at room temperature on the cytosine films display large zones with antiparallel alignment of the vertical components of the polarization vector. Guanine films have a dense nano-grained morphology. Our studies reveal electrical polarization switching effects which can be related to ferroelectricity in the films of guanine molecules. Characteristic ferroelectric polarization-electric-field hysteresis loops showing large electrical polarization are observed at low temperatures up to 200 K. Above this temperature, the guanine films have a preponderant paraelectric phase containing residual or locally induced nano-scopic ferroelectric domains, as observed by piezoresponse force microscopy at room temperature.