High quality CoPt(3) nanocrystals were synthesized via simultaneous reduction of platinum acetylacetonate and thermodecomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid and hexadecylamine as stabilizing agents. The high flexibility and reproducibility of the synthesis allows us to consider CoPt(3) nanocrystals as a model system for the hot organometallic synthesis of metal nanoparticles. Different experimental conditions (reaction temperature, concentration of stabilizing agents, ratio between cobalt and platinum precursors, etc.) have been investigated to reveal the processes governing the formation of the metal alloy nanocrystals. It was found that CoPt(3) nanocrystals nucleate and grow up to their final size at an early stage of the synthesis with no Ostwald ripening observed upon further heating. In this case, the nanocrystal size can be controlled only via proper balance between the rates for nucleation and for growth from the molecular precursors. Thus, the size of CoPt(3) nanocrystals can be precisely tuned from approximately 3 nm up to approximately 18 nm in a predictable and reproducible way. The mechanism of homogeneous nucleation, evolution of the nanocrystal ensemble in the absence of Ostwald ripening, nanocrystal faceting, and size-dependent magnetic properties are investigated and discussed on the example of CoPt(3) magnetic alloy nanocrystals. The developed approach was found to be applicable to other systems, e.g., FePt and CoPd(2) magnetic alloy nanocrystals.

The flux noise spectrum and complex impedance for a 500 Å thick YBCO film are measured and compared with predictions for two-dimensional vortex fluctuations. It is verified that the complex impedance and the flux noise spectra are proportional to each other, that the logarithm of the flux noise spectra for different temperatures has a common tangent with slope ഠ21, and that the amplitude of the noise decreases as d 23 , where d is the height above the film at which the magnetic flux is measured. A crossover from normal to anomalous vortex diffusion is indicated by the measurements and is discussed in terms of a two-dimensional decoupling. PACS numbers: 74.40. + k, 74.25.Nf, 74.76.Bz Spontaneously created two-dimensional (2D) vortices drive the phase transition of Kosterlitz-Thouless (KT) type between the superconducting and the normal state for thin film superconductors [1]. This means that the physics of the vortices is responsible for the features in a region around the transition [1]. More recently it has been found that high-T c superconductors also show behavior characteristic of 2D vortex fluctuations [1][2][3][4][5]. This is expected for films thin enough to be in the "quasi" 2D regime, while the phase transition for thicker films is related to the 3D bulk transition. However, also for such thick films 2D vortex fluctuations have been observed just above the transition [4,5], which has been ascribed to a decoupling of the superconducting planes associated with the CuO 2 layers [6][7][8]. The evidence in case of BSCCO, which has a fairly broad resistive transition, is better established than for YBCO which has a very narrow transition [4,5]. One may argue that for BSCCO, since it has a much larger anisotropy than YBCO, the interlayer coupling should be much weaker, and as a consequence the quasi 2D character much stronger [1]. From this perspective the possibility and the significance of 2D vortex fluctuations remains to be clarified for YBCO. In the present Letter, we address this question by focusing on the dynamical features of the vortices reflected in the flux noise spectrum and the complex impedance for a 500 Å thick YBCO film; a strategy which was also used in Ref.[3] for a quasi-2D BSCCO film. To resolve the narrow resistive region for our YBCO sample, we use a high resolution temperature control in the experimental setup.The sample is a square shaped (5 3 5 mm 2 ) 500 Å thick YBCO-123 film grown epitaxially on a LaAlO 3 substrate by pulsed laser deposition. According to x-ray u-2u and f scans the film is highly c-axis oriented and only [100] YBCOk͓100͔ LaAlO 3 in-plane orientation is observed. The experimental setup is a SQUID based system designed for complex impedance and flux noise measurements. The sample space is magnetically shielded by m-metal and niobium cans, resulting in a residual dc field of approximately 1 mOe. The pickup coil having a diameter of 1.2 mm is a first order gradiometer, each section containing 2 layers (8 1 7 turns) of 0.05 mm NbTi wire. The amplitude of the field in t...

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We have performed flux noise and AC-susceptibility measurements on two 400 nm thick MgB 2 films. Both measurement techniques give information about the vortex dynamics in the sample, and hence the superconducting transition, and can be linked to each other through the fluctuation-dissipation-theorem. The transition widths for the two films are 0.3 and 0.8 K, respectively, and the transitions show a multi step-like behavior in the AC-susceptibility measurements. The same phenomenon is observed in the flux noise measurements through a change in the frequency dependence of the spectral density at each step in the transition. The results are discussed and interpreted in terms of vortices carrying an arbitrary fraction of a flux quantum as well as in terms of different macroscopic regions in the films having slightly different compositions, and hence, different critical temperatures.