National Institute Of Materials Physics - Romania

We present the magnetic and transport properties of superconducting composites fabricated by admixing carbon-encapsulated Fe nanospheres and MgB2 powder. The addition of nanoparticles is expected to enhance the critical current density by carbon-doping the MgB2 matrix and by providing artificial pinning sites. Three samples with estimated amounts of 0.35, 0.6, and 1.0 wt. % metallic Fe were prepared using the spark plasma sintering technique. The average size of these nanoparticles is comparable to the superconducting coherence length of MgB2 at approximately 5 nm. We found that the additions do not significantly alter the critical temperature which is very high, close to that of the pure MgB2 samples. We have also observed improved current densities, as high as 1100 kA/cm(2) for the samples with 0.35 wt. % metallic Fe at 5 K and 1 T. A core-shell model for explaining the transport data is presented. The field and temperature dependence of the reduced pinning force is described in terms of pinning on grain boundaries and/or on point defects. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3672820]

We report on the characterization of metallurgical phases and their magnetism at the interfaces of nanoscale MgB2/Fe layered structures. MgB2/Fe-57 multilayers with varying layer thicknesses were prepared by vacuum deposition and investigated, before and after annealing by electrical resistance measurements, x-ray diffraction and Fe-57 conversion-electron Mossbauer spectroscopy (CEMS) down to 5 K. Interfacial Fe-B phases, such as Fe2B, were identified by CEMS. A superparamagnetic-to-ferromagnetic transition is observed with increasing Fe-57 film thickness. Ultrahigh vacuum annealing at 500 degrees C of the multilayers leads to strong diffusion of Fe atoms into the boundary regions of the MgB2 layers. MgB2 in the as-grown multilayers is non-superconducting. Structural disorder and the effect of Fe interdiffusion contribute to the suppression of superconductivity in the MgB2 films of all the as-grown multilayers and the thinner annealed multilayers. However, an annealed MgB2/Fe-57/MgB2 trilayer with thicker (500 angstrom) MgB2 layers is observed to be superconducting with an onset temperature of 25 K. At 5 K, the annealed trilayer can be conceived as being strongly chemically modulated, consisting of two partially Fe-doped superconducting MgB2 layers separated by an interdiffused weakly magnetic Fe-B interlayer, which is characterized by a low hyperfine magnetic field B-hf of similar to 11 T. This chemically modulated layer structure of the trilayer after annealing was verified by Rutherford backscattering.

We have used scanning Hall probe microscopy to investigate vortex structures and vortex dynamics in Bi2Sr2CaCu2O8+delta thin films in very low perpendicular magnetic fields. After nominally zero field cooling in the Earth's field we find that the vortices appear to be in a stable glassy state in our highly disordered samples. After applying a cancellation field of a few Oersted at low temperature, however, the system enters a new regime at very low magnetic induction when the only image contrast is due to vortices that are intermittently trapped on strong pinning centres. This state shares many of the signatures of the re-entrant vortex liquid phase that has been theoretically predicted in these highly anisotropic materials at very low vortex densities. Analysing the trapping times for vortices in the fluctuating state we estimate that the pinning potential of typical strong pinning centres is about 900 K under our experimental conditions. To our knowledge, this is the first direct experimental evidence for the existence of a dynamic liquid-like vortex state in this highly anisotropic material at very low magnetic induction.

Multiband superconductors can have several types of domains that are inhibited in conventional single-band superconductors. These domains are phase domains and chiral domains and their domain wall are an interband phase difference soliton. In a superconductor with an odd number of electronic bands (five or more) and with positive interband Josephson interactions, we find other types of domains with different interband phase differences. We call these domains configuration domains because pseudo-order parameters for each band are dispersed in the complex plain and several configurations, which have several local minima. Fractional vortices serve as hubs for phase difference solitons (configuration domain walls). The divergence of the number of configurations with local minima would pose a serious problem for the stability of superconductivity. (C) 2011 Elsevier B.V. All rights reserved.

The first field for magnetic flux penetration H in Bi2Sr2CaCu2O8+delta (Bi-2212) single crystals near the critical temperature T-c was investigated from the local magnetic hysteresis loops registered for different magnetic field H sweeping rates by using a scanning Hall probe microscope (SHPM) with similar to 1 mu m effective spatial resolution. Evidences for a significant role of the surface barrier were obtained: the asymmetric shape of the magnetization loops and an anomalous change in the slope of H-p(T) close to T-c. (C) 2011 Elsevier B.V. All rights reserved.

A novel nanocomposite FePt-based exchange-coupled magnet has been synthesized and structurally and magnetically characterized. We report for the first time the direct formation of the L1(0) FePt phase without the need for post-synthesis annealing procedures in Fe-Pt-based melt-spun ribbons, obtained by a conventional melt spinning method. The structure and magnetic properties are investigated and the occurrence of the L1(0) ordered phase in the as-cast state of Fe-Pt-Ag-B melt-spun ribbons is confirmed by XRD and magnetic measurements. A microstructure consisting of fine, uniformly dispersed, 22-24 nm FePt grains dispersed within a soft magnetic matrix is observed by scanning transmission electron microscopy imaging. Coercive fields as high as 727 kAm(-1), saturation magnetization of about 1.2 T and energy product around 87 kJm(-3) are determined from 270 K hysteresis loops of the as-cast ribbons, making one of the best FePt-based nanocomposite magnet ribbons even without further annealing treatments.

We show that the optical infrared magneto-refractive effect can be used to probe magneto-resistance effects in absolute terms. A spin valve sample with synthetic anti-ferromagnet has been studied using non-contact infrared reflection spectroscopy measurements. Direct experimental comparison shows excellent agreement between electrical and optical measurements in both shape of the magneto-resistance curves and absolute values. This opens the possibility of developing fast and efficient tests of magneto-resistive samples and sensors using an in situ, non-contact, non-destructive optical approach, without the need to determine a calibration factor between the electrical and optical measurements. [doi:10.1063/1.3631778]

The relaxation of the irreversible magnetic moment m(t) in YBa(2)Cu(3)O(7) (YBCO) films was investigated as a function of temperature T and the external magnetic field H along the c axis applied in zero-field cooling conditions, for the determination of vortex creep parameters. The data analysis was performed using the T and current density dependence of the normalized vortex creep activation energy, or by the fit of the m(t) data with the well known interpolation formula in the framework of the general vortex creep equation. It was found that (i) even for specimens with strong static pinning the characteristic pinning energy remains small in the low-T range, where the vortex creep appearing in standard magnetization measurements is elastic, (ii) the observed strong increase of the time scale for creep and the decrease of the creep exponent with increasing H beyond the theoretical predictions can be attributed to the crossover elastic-creep-plastic-creep generated by the macroscopic currents induced in the specimen during magnetization measurements, and (iii) the creep parameters extracted with the interpolation formula are highly affected by the m(t) data registered at short t, which may lead to unphysical creep parameter values.

In the so-called 'step-shape' angular spin distribution model for layered systems, the non-collinear directions of the atomic magnetic moments are confined to the film plane and form a homogeneous fan spanning inside an (in-plane) angular interval Delta phi centered at an angle phi(0). A general approach for deriving the two parameters phi(0) and Delta phi via Fe-57 Mossbauer spectroscopy measurements is discussed. The analysis extends our previously reported treatment, which assumed that the angular aperture Delta phi develops symmetrically versus a fixed direction phi(0) (e.g., the in-plane easy axis of magnetization) oriented either along or perpendicular to the in-plane projection of the Mossbauer gamma-ray direction. The proposed approach is also applicable for those cases when not only the spin aperture Delta phi is changing but also the aperture center phi(0) is rotating under the influence of different external parameters, such as applied field, temperature, stress, etc. The method is suitable for applications to nanoscale layered heterostructures with in-plane uniaxial or unidirectional magnetic anisotropy. The method is applied to experimental data obtained on a 2-nm thick defected Fe layer with in-plane magnetic texture. (C) 2011 Elsevier B.V. All rights reserved.

The results of the electric and magnetic measurements performed on PbZr0.2Ti0.8O3-BiFeO3 symmetric structures, deposited on Pt/Si wafers, were compared for different number of layers in order to analyse the effect of interfaces over the macroscopic properties. It was found that the shape and magnitude of the capacitance-voltage characteristic, as well as the shape and parameters of the ferroelectric and magnetic hysteresis, depend on the number of interfaces in the intended multilayer structure. A temperature induced gradual transition from a magnetically disordered spin glass like phase of low temperature to an uncompensated antiferromagnetic phase at room temperature takes place in the BiFeO3 films, under low applied magnetic fields. A partial ferromagnetic like order can be obtained at low temperatures by increasing the field. The observed changes in the electric and magnetic behaviour of the systems were related to an increased degree of disorder for electric dipoles and magnetic moments, due to the increased number of layers and crystallization treatments. (C) 2011 Elsevier B.V. All rights reserved.