'Colossal magnetoresistance' in perovskite manganites such as La0.7Ca0.3MnO3 (LCMO), is caused by the interplay of ferro-paramagnetic, metal-insulator and structural phase transitions. Moreover, different electronic phases can coexist on a very fine scale resulting in percolative electron transport. Here we report on (LCMO)1-x:(MgO)x (0 < x < or = 0.8) epitaxial nano-composite films in which the structure and magnetotransport properties of the manganite nanoclusters can be tuned by the tensile stress originating from the MgO second phase. With increasing x, the lattice of LCMO was found to expand, yielding a bulk tensile strain. The largest colossal magnetoresistance of 10(5)% was observed at the percolation threshold in the conductivity at xc 0.3, which is coupled to a structural phase transition from orthorhombic (0 < x < or 0.1) to rhombohedral R3c structure (0.33 < or = x < or = 0.8). An increase of the Curie temperature for the Rc phase was observed. These results may provide a general method for controlling the magnetotransport properties of manganite-based composite films by appropriate choice of the second phase.

We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state established due to the proximity effect in superconducting Nb/Cu 41 Ni 59 bilayers. Using a special wedge-type deposition technique, series of 20-35 samples could be fabricated by magnetron sputtering during one run. The layer thickness of only a few nanometers, the composition of the alloy, and the quality of interfaces were controlled by Rutherford backscattering spectrometry, high resolution transmission electron microscopy, and Auger spectroscopy. The magnetic properties of the ferromagnetic alloy layer were characterized with superconducting quantum interference device (SQUID) magnetometry. These studies yield precise information about the thickness, and demonstrate the homogeneity of the alloy composition and magnetic properties along the sample series. The dependencies of the critical temperature on the Nb and Cu 41 Ni 59 layer thickness, T c (d S ) and T c (d F ), were investigated for constant thickness d F of the magnetic alloy layer and d S of the superconducting layer, respectively. All types of non-monotonic behaviors of T c versus d F predicted by the theory could be realized experimentally: from reentrant superconducting behavior with a broad extinction region to a slight suppression of superconductivity with a shallow minimum. Even a double extinction of superconductivity was observed, giving evidence for the multiple reentrant behavior predicted by theory. All critical temperature curves were fitted with suitable sets of parameters. Then, T c (d F ) diagrams of a hypothetical F/S/F spin-switch core structure were calculated using these parameters. Finally, superconducting spin-switch fabrication issues are discussed in detail in view of the achieved results.

We report on the first observation of a pronounced re-entrant superconductivity phenomenon in superconductor/ferromagnetic layered systems. The results were obtained using a superconductor/ferromagnetic-alloy bilayer of Nb/Cu1−xNix. The superconducting transition temperature Tc drops sharply with increasing thickness dCuNi of the ferromagnetic layer, until complete suppression of superconductivity is observed at dCuNi ≈4 nm. Increasing the Cu1−xNix layer thickness further, superconductivity reappears at dCuNi≈13 nm. Our experiments give evidence for the pairing function oscillations associated with a realization of the quasi-one dimensional Fulde-FerrellLarkin-Ovchinnikov (FFLO) like state in the ferromagnetic layer.The coexistence of superconductivity (S) and ferromagnetism (F) in a homogeneous material, described by Fulde-Ferrell and Larkin-Ovchinnikov (FFLO) [1,2], is restricted to an extremely narrow range of parameters [3]. So far no indisputable experimental evidence for the FFLO state exists.In general, superconductivity and ferromagnetism do not coexist, since superconductivity requires the conduction electrons to form Cooper pairs with antiparallel spins, whereas ferromagnetism forces the electrons to align their spins parallel. This antagonism can be overcome if superconducting and ferromagnetic regions are spatially separated, as for example, in artificially layered superconductor/ferromagnet (S/F) nanostructures (see, e.g. [4], for an early review). The two long-range ordered states influence each other via the penetration of electrons through their common interface. Superconductivity in such a proximity system can survive, even if the exchange splitting energy E ex ∼ k B θ Curie in the ferromagnetic layer is orders of magnitude larger than the superconducting order parameter ∆ ∼ k B T c , with T c the superconducting transition temperature. Cooper pairs entering from the superconducting into the ferromagnetic region experience conditions drastically different from those in a non-magnetic metal. This is due to the fact that spin-up and spin-down partners in a Cooper pair occupy different exchange-split spin-subbands of the conduction band in the ferromagnet. Thus, the spin-up and spin-down wave-vectors of electrons in a pair, which have opposite directions, cannot longer be of equal magnitude and the Cooper pair acquires a finite pairing momentum [5]. This results in a pairing function that does not simply decay as in a non-magnetic metal, but in addition oscillates on a characteristic length scale. This length scale is the magnetic coherence length ξ F , which will be specified below.Various unusual phenomena follow from the oscillation of the pairing wave function in ferromagnets (see, e.g. the recent reviews [6,7,8] and references therein). A prominent example is the oscillatory S/F proximity effect. It can be qualitatively described using the analogy with the interference of reflected light in a Fabry-Pérot interferometer at normal incidence. As the conditions change periodically between construc...

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Nanocrystalline europium doped yttria was synthesized using a chemical vapor technique. The powder was characterized by x-ray diffraction, transmission electron microscopy, and ultraviolet spectroscopy. For the first time it was possible to obtain single phase Y2O3:Eu nanoparticles crystallized in the cubic structure with an average particle size of only 10 nm. The reflection, excitation, and emission spectra were studied. The nanoparticles show blue shifted absorption bands with respect to coarse grained material.