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The magneto-optical (MO) Kerr effects for ZnO and ZnO:Ni-doped nanolaminate structures prepared using atomic layer deposition (ALD) have been investigated. The chemical composition and corresponding structural and morphological properties were studied using XRD and XPS and compared for both nanostructures. The 2D array gradient maps of microscale variations of the Kerr angle polarization rotation were acquired by means of MO Kerr microscopy. The obtained data revealed complex behavior and broad statistical dispersion and showed distinct qualitative and quantitative differences between the undoped ZnO and ZnO:Ni-doped nanolaminates. The detected magneto-optical response is extensively inhomogeneous in ZnO:Ni films, and a giant Kerr polarization rotation angle reaching up to ~2° was established. This marks the prospects for further development of magneto-optical effects in ALD ZnO modified by transition metal oxide nanostructures. [ABSTRACT FROM AUTHOR]

The versatile AC magnetic susceptibility technique offers a detailed insight into the complex electrodynamic phenomena in superconductors. In the present study, we outline the key effects related to the temperature, AC field amplitude and frequency variations of the fundamental and harmonic components for an investigation of the vortex dynamics in a flux-grown FeSe crystal. By means of higher harmonic (nonlinear) analysis, we have explored certain atypical, asymmetric features in the AC magnetic response. These effects were identified through the detection of an even (second) harmonic and an unusual temperature shift in the odd (third) harmonic, possibly due to the complex interactions related to the composite superconducting/magnetic morphology of the crystal. Using the high-frequency sensitivity of the third harmonic, the basic functional dependencies of the pinning activation energy, as the main mixed state parameter, were determined with the implementation of the Kim–Anderson Arrhenius relation in the framework of the collective creep theory. [ABSTRACT FROM AUTHOR]

The AC magnetic properties of a sample of Fe3O4 nanoparticles coated with oleic acid have been investigated with the help of AC susceptibility measurements. In particular, several DC magnetic fields have been superimposed on the AC field, and their effect on the magnetic response of the sample has been analysed. The results show the presence of a double peak structure in the imaginary component of the complex AC susceptibility measured as a function of the temperature. A preliminary evaluation of the Mydosh parameter for both peaks gives the information that each one of them is associated with a different state of interaction between nanoparticles. The two peaks evolve both in amplitude and position when the intensity of the DC field is changed. The field dependence of the peak position shows two different trends, and it is possible to study them in the framework of the currently existing theoretical models. In particular, a model of non-interacting magnetic nanoparticles has been used to describe the behaviour of the peak at lower temperatures, whereas the behaviour of the peak at higher temperatures has been analysed in the framework of a spin-glass-like model. The proposed analysis technique can be useful for the characterisation of magnetic nanoparticles used in several types of applications, such as biomedical and magnetic fluids. [ABSTRACT FROM AUTHOR]

A Fe(Se,Te) iron based crystal has been studied using dc magnetic measurements as a function of temperature (T) and magnetic field (H). Firstly, the field dependence of the critical current density Jc has been extracted from the superconducting hysteresis loops m(H) at different temperatures within the Bean critical state model in order to obtain information about the transport properties of the sample. The knowledge of the critical current density values as a function of the field for temperatures near Tc allow us to determine the values and the behavior of the irreversibility field Hirr as a function of the temperature. On the other hand, by means of zero field cooling m(T) measurements at different magnetic fields, the upper critical field Hc2 has been evaluated as a function of the temperature. Both irreversibility field and upper critical field have been fitted by the equation H(T) = H(0)(1 − T/T*)n allowing us to obtain the values of the related parameters which are in good agreement with those found in literature for the IBS systems. [ABSTRACT FROM AUTHOR]

Hydrotalcites (HTlcs) are a class of nanostructured layered materials that may be employed in a variety of applications, from green to bio technologies. In this paper, we report an investigation on HTlcs made of Mg and Fe, recently employed to improve the growth in vitro of osteoblasts within a keratin sponge. We carried out an analysis of powder materials and of HTlcs dispersed in keratin and spin-coated on a Si/SiO2 substrate at different temperatures. A magnetic study of the powders was carried out with a Quantum Design Physical Property Measurement System equipped with a Vibrating Sample Magnetometer. The data gathered prove that these HTlcs are fully paramagnetic, and keratin showed a very small magnetic response. Optical and Atomic Force Microscopy analyses of the thin films provide a detailed picture of clusters randomly dispersed in the films with various dimensions. The magnetic properties of these films were characterized using the Nano Magneto Optical Kerr Effect (NanoMOKE) down to 7.5 K. The data collected show that the local magnetic properties can be mapped with a micrometric resolution distinguishing HTlc regions from keratin ones. This approach opens new perspectives in the characterization of these composite materials. [ABSTRACT FROM AUTHOR]

We analyze the magnetic behavior of a CaKFe4As4 polycrystalline sample fabricated by a mechanochemically assisted synthesis route. By means of DC magnetization (M) measurements as a function of the temperature (T) and DC magnetic field (H) we study its critical parameters and pinning features. The critical temperature Tc has been evaluated by M(T) curves performed in Zero Field Cooling-Field Cooling conditions. These curves show the presence of a little magnetic background for temperatures above Tc, as also confirmed by the hysteresis loops M(H). Starting from the M(H) curves, the critical current density Jc of the sample has been calculated as a function of the field at different temperatures in the framework of the Bean critical state model. The Jc(H) values are in line with the ones reported in the literature for this typology of samples. By analyzing the temperature dependence of the critical current density Jc(T) at different magnetic fields, it has been found that the sample is characterized by a strong type pinning regime. This sample peculiarity can open perspectives for future improvement in the fabrication of this material. [ABSTRACT FROM AUTHOR]

The magnetization M of an Fe(Se, Te) single crystal has been measured as a function of temperature T and dc magnetic field H. The sample properties have been analyzed in the case of a magnetic field parallel to its largest face H||ab. From the M(T) measurement, the Tc of the sample and a magnetic background have been revealed. The superconducting hysteresis loops M(H) were between 2.5 K and 15 K showing a tilt due to the presence of a magnetic signal measured at T > Tc. From the M(H) curves, the critical current density Jc(H) has been extracted at different temperatures showing the presence of a second magnetization peak phenomenon. By extracting and fitting the Jc(T) curves at different fields, a pinning regime crossover has been identified and shown to be responsible for the origin of the second magnetization peak phenomenon. Then, the different kinds of pinning centers of the sample were investigated by means of Dew-Hughes analysis, showing that the pinning mechanism in the sample can be described in the framework of the collective pinning theory. Finally, the values of the pinning force density have been calculated at different temperatures and compared with the literature in order to understand if the sample is promising for high-current and high-power applications. [ABSTRACT FROM AUTHOR]

In the category of biopolymers, polylactide or polylactic acid (PLA) is one of the most promising candidates considered for future developments, as it is not only biodegradable under industrial composting conditions, but it is produced from renewable natural resources. The modification of PLA through the addition of nanofillers is considered as a modern approach to improve its main characteristic features (mechanical, thermal, barrier, etc.) and to obtain specific end-use properties. Iron oxide nanoparticles (NPs) of low dimension (10–20 nm) such as magnetite (Fe3O4), exhibit strong magnetization in magnetic field, are biocompatible and show low toxicity, and can be considered in the production of polymer nanocomposites requiring superparamagnetic properties. Accordingly, PLA was mixed by melt-compounding with 4–16 wt.% magnetite NPs. Surface treatment of NPs with a reactive polymethylhydrogensiloxane (MHX) was investigated to render the nanofiller water repellent, less sensitive to moisture and to reduce the catalytic effects at high temperature of iron (from magnetite) on PLA macromolecular chains. The characterization of nanocomposites was focused on the differences of the rheology and morphology, modification, and improvements in the thermal properties using surface treated NPs, while the superparamagnetic behavior was confirmed by VSM (vibrating sample magnetometer) measurements. The PLA−magnetite nanocomposites had strong magnetization properties at low magnetic field (values close to 70% of Mmax at H = 0.2 T), while the maximum magnetic signal (Mmax) was mainly determined by the loading of the nanofiller, without any significant differences linked to the surface treatment of MNPs. These bionanocomposites showing superparamagnetic properties, close to zero magnetic remanence, and coercivity, can be further produced at a larger scale by melt-compounding and can be designed for special end-use applications, going from biomedical to technical areas. [ABSTRACT FROM AUTHOR]

This paper reports the fabrication of novel hybrid and structured materials by using hemp fibers as organic support. Two in-situ methodologies were proposed and applied to modify the surface of hemp fibers in order to improve their physical properties and to confer a magnetic behavior: i) coprecipitation of Fe/Al layered double hydroxides through urea method (MHF@LDH Fe/Al) and ii) deposition of Fe 3 O 4 particles onto AlOOH modified hemp fibers in ammonia solution (MHF@AlOOH-FePs). The obtained hybrid materials have been widely characterized in terms of spectroscopic, morphological, thermal and barrier properties. Finally, a further investigation concerned the evaluation of surface charge and the estimation of magnetic properties showing a ferromagnetic behavior (Curie Temperature T c > 300 K) for both samples with a coercive field of H c ≈± 40 Oe for MHF@LDH Fe/Al and H c ≈ ± 10 Oe for MHF@AlOOH-FePs. [Display omitted] • Fabrication of novel hybrid materials based on hemp fibers and Fe/Al. • Two in-situ surface modification methodologies have been investigated. • Ferromagnetic behavior of Fe/Al-hemp based materials has been observed. [ABSTRACT FROM AUTHOR]

For high-field power applications of high-temperature superconductors, it became obvious in recent years that nano-engineered artificial pinning centers are needed for increasing the critical current and pinning potential. As opposed to the artificial pinning centers obtained by irradiation with various particles, which is a quite expensive approach, we have studied superconducting samples having self-assembled defects, created during the sample fabrication, that act as effective pinning centers. We introduced a simple, straight-forward method of estimating the frequency-dependent critical current density by using frequency-dependent AC susceptibility measurements, in fixed temperatures and DC magnetic fields, from the positions of the maxima in the dependence of the out-of-phase susceptibility on the amplitude of AC excitation magnetic field. The results are compatible with a model that stipulates a logarithmic dependence of the pinning potential on the probing current. A mathematical derivation allowed us to estimate from the experimental data the pinning potentials in various samples, and in various DC magnetic fields. The resulted values indicate large pinning potentials, leading to very small probability of magnetic flux escaping the pinning wells, hence, leading to very high critical currents in high magnetic fields. [ABSTRACT FROM AUTHOR]

In this work, we aim to demonstrate the potential of the flux flow instability (FFI) tool as a probe for the evaluation of the quasiparticle energy relaxation time τϵ in iron-based superconductors (IBS). The knowledge of this microscopic parameter, its temperature dependence and the magnetic field influence, turns particularly useful to implement IBS materials in photon detection applications, as well as to get information on the gap symmetry or its anisotropy. Here, we focus on Fe(Se,Te) thin films that both from structural and magnetic properties show the simpler behaviour, thus it can be a reference test for any more complex IBS. By current-voltage characterizations and resistance measurements, we investigate the FFI features in the presence of an external applied magnetic field as a function of the angular dependence between the crystal structure of the film and the orientation of the field. We describe the observed experimental characteristics of FFI within the intrinsic electronic mechanism of Larkin-Ovchinnikov model. In this way, we are able to give a quantitative estimate of τϵ in Fe(Se,Te) that can be compared with evaluation from other techniques such as pump and probe measurements. Thus, the angular measurements of FFI in high magnetic fields are a viable route to the possible mechanisms of quasiparticle relaxation and to the complementary knowledge on its anisotropy. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles embedded in polymer matrices have excellent potential for electromagnetic device applications, like electromagnetic interference suppression or strain sensing applications in the structural health monitoring domain. In this work, polymer nanocomposites of polypropylene loaded with varying concentrations of magnetite nanoparticles (from 2 to 40 wt%) have been synthesized by an ex situ process. The magnetite nanoparticles were produced using a simple co‐precipitation technique. The nanocomposites were characterized by differential scanning calorimetry, scanning electron microscopy, tensile and hardness testing, vibrating sample magnetometry. The mechanical properties were found to be dependent on the degree of dispersion and on the strong nucleating ability of magnetite nanoparticles, while a superparamagnetic behavior was reported. These composites have tailorable mechanical and magnetic properties highly dependent on the content of magnetic filler. POLYM. COMPOS., 39:E1742–E1750, 2018. © 2018 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

In a quench event determined by instability of the flux-flowregime, the quenching current I* can be much higher than the critical current Ic; thus, it sets the upper limit to the superconducting operational regime. In iron-based Fe(Se,Te) superconducting thin films, which can be relevant to fabricate conductors for high magnetic field applications, substantial differences between the Ic and I* behavior as a function of the applied magnetic field have recently been observed. Both current values are strongly influenced by flux pinning; thus, anisotropy effects are expected to play a role. Here, we report the first study of the angular dependence on themagnetic field orientation of the quenching current in Fe(Se,Te) thin films grown on a CaF2 substrate, which have already shown a weak anisotropic critical current dependence on the applied magnetic field orientation. The quenching current is found even less sensitive than the critical current with respect to angular variations. [ABSTRACT FROM AUTHOR]

The role of a layered structure in superconducting pinning properties is still at a debate. The effects of the vortex shape, which can assume for example a staircase form, could influence the interplay with extrinsic pinning coming from the specific defects of the material, thus inducing an effective magnetic field dependence. To enlighten this role, we analysed the angular dependence of flux pinning energy U(H,θ) as a function of magnetic field in FeSe0.5Te0.5 thin film by considering the field components along the ab-plane of the crystal structure and the c-axis direction. U(H,θ) has been evaluated from magneto-resistivity measurements acquired at different orientations between the applied field up to 16 T and FeSe0.5Te0.5 thin films grown on a CaF2 substrate. We observed that the U(H,θ) shows an anisotropic trend as a function of both the intensity and the direction of the applied field. Such a behaviour can be correlated to the presence of extended defects elongated in the ab-planes, thus mimicking a layered superconductor, as we observed in the microstructure of the compound. The comparison of FeSe0.5Te0.5 with other superconducting materials provides a more general understanding on the flux pinning energy in layered superconductors. [ABSTRACT FROM AUTHOR]