The nucleation of superconductivity in a hybrid film (HF) that consists of randomly distributed CoPt ferromagnetic nanoparticles (FNs) embedded in a 200 nm Nb layer was studied by combined magnetic and magnetoresistance measurements. The FNs exhibit both soft and hard magnetic behaviour. It is found that both magnetic phases contribute to the modulation of bulk and surface-like superconductivity by broadening their intermediate regime. More specifically, while the soft FNs (SFNs) promote bulk superconductivity, the hard FNs (HFNs) suppress the bulk superconducting state. In contrast, both SFNs and HFNs promote surface-like superconductivity since on the H-T phase diagram the respective boundary line presents abrupt upturns at their corresponding saturation fields. The dependence of the observed effects on the magnetic history gives clear evidence that they are mainly induced by the cooperative action of the FNs. Our experimental results are compared with current theoretical studies on relevant hybrid systems. Finally, the possible applications that such HFs could find in the near future are discussed.

Model combinatorial films (CFs) which host a pure superconductor adjacent to a ferromagnetic-superconducting hybrid film (HF) are manufactured for the study of the influence of ferromagnetic nanoparticles (FNs) on the nucleation of superconductivity. Careful resistance measurements were performed simultaneously on two different sites of the CFs. Enhancement of superconductivity and magnetic memory effects were observed only on the hybrid site of the CFs but were absent on their purely superconducting part. Our results give direct proof that the FNs modulate the superconducting order parameter in an efficient and controlled way giving us the possibility of miscellaneous practical applications.

We studied the crystal and magnetic structure of the La1-xCaxMnO3 compound for (0.11 <= x <= 0.175) using stoichiometric samples. For x < 0.13 the system's ground state is insulating canted antiferromagnetic. For 0.13 <= x <= 0.175 below the Jahn-Teller transition temperature (T-JT) the crystal structure undergoes a monoclinic distortion. The crystal structure can be described with P2(1)/c space group which permits two Mn sites. The unit-cell strain parameter s=2(a-c)/(a+c) increases for T < T-JT, taking the maximum value at the Curie point, and then decreases. Below T-M'M''approximate to 60 K s abruptly changes slope and finally approaches T=0 K with nearly zero slope. The change of s at T-M'M'' is connected to a characteristic feature in the magnetic measurements. As x increases towards the ferromagnetic metallic boundary, although s is reduced appreciably, the monoclinic structure is preserved. The monoclinic structure is discussed with relation to the orbital ordering, which can produce the ferromagnetic insulating ground state. We also studied samples that were prepared in air atmosphere. This category of samples shows ferromagnetic insulating behavior without following the particular variation of the s parameter. The crystal structure of these samples is related to the so-called O-* (c > a > b/root 2) structure.

In this paper we demonstrate the dependence of MgB2 as for its superconducting properties with respect to different thermal profiles used at the manufacturing stage of the compound. A colligation is made between the change of the superconducting capabilities and the structure of the final material. The samples were characterized, as far as the structure is concerned, with an X-ray diffractometer, a scanning electron microscope, and a differential thermal analyzer. As for the superconductivity aspect of the compound, the dependence of the magnetic ac susceptibility and resistivity with respect to temperature was determined. (c) 2004 Elsevier B.V. All rights reserved.

In this work we present magnetization data on hybrids consisting of multilayers (MLs) of manganites [La0.33Ca0.67MnO3/La0.60Ca0.40MnO3](15) in contact with a low-T-c Nb superconductor (SC). Although a pure SC should behave diamagnetically in respect to the external magnetic field in our ML-SC hybrids we observed that the magnetization of the SC follows that of the ML. Our intriguing experimental results show that the SC below its T-c(SC) becomes ferromagnetically coupled to the ML. As a result in the regime where diamagnetic behavior of the SC was expected its bulk magnetization switches only whenever the coercive field of the ML is exceeded. By employing specific experiments where the ML was selectively biased or not we demonstrate that the ML inflicts its magnetic properties on the whole hybrid. Possible explanations are discussed in connection to recent theoretical proposals and experimental findings that were obtained in relative hybrids.

We report on the microstructural evolution in Cu 15%Nb multifilamentary wires upon annealing and the corresponding effects on their magnetic properties. During annealing at temperatures higher than 800 degrees C thermal instability mechanisms take place in the microstructure of the Cu 15 %Nb composite, leading to the spheroidization of niobium filaments. Another important consequence of annealing is the recrystallization of Nb, that has a straightforward influence on the magnetization data. When compared to the as-drawn composite, in the annealed Cu15%Nb samples the bulk upper critical field H-c2(T) is reduced significantly, while the lower critical field H-c1 (T) is increased as a function of the annealing temperature and time. This indicates that we may employ this process as an efficient method to modify the basic superconducting parameters of Nb, namely the coherence length (T) and the penetration depth lambda(T).