Kötzler et al. Reply: The Comment [1] picks up two features of our Letter [2] and proposes alternative interpretations of both in terms of results for the 2D XY model [3]. In this Reply, we argue that our key conclusion, i.e., the loss of superconducting (SC) phase coherence via a thickness-induced vortex-loop (VL) unbinding at T ? d below the virtual 3D bulk transition T c [see Fig. 1(a)], is not weakened by the given reasons. The other point of the Comment addresses the onset of SC phase fluctuations observed in the dynamical conductance (see Fig. 1) at temperatures T ! > T c . Here we agree that 2D vortexantivortex (v=a) fluctuations in partially coupled CuO 2 planes are operative; however, we provide evidence against 2D XY, i.e., Kosterlitz-Thouless(KT) -type ordering effects, as emphasized in the Comment.We follow the order of the Comment and first discuss the onset phenomena. There we assumed a dynamical scaling function S ! to fit the larger, inductive component of the conductance, i.e., !G 00 !. As shown by Fig. 1 of Ref.[1] for d 100 nm, the Kronig-Kramers transform of !G 00 underestimates the loss component !G 0 . We observe this enhancement of the !G 0 peak on rather homogeneous (from AFM images) films , such as d 50 nm ( 1) shown in Fig. 1, as well as on more inhomogeneous ones ( 0:4). Upon decreasing homogeneity, the normalized !G 0 peaks broaden and decrease to below 1=. We, therefore, suspect that the film disorder affects the dynamical shape. We should emphasize, however, that our main conclusion was drawn from the leading quantities, i.e., the peak temperatures T ! . They clearly reveal an Arrhenius-type behavior of the phase relaxation time T, the barriers of which could be associated with the nucleation of vortex cores. There and also in the experimental literature discussed in Ref.[2], no indication is seen for cooperative KT, i.e., v=a binding features, as inferred in the Comment. We cannot exclude that such effects occur closer to T c of our films; however, there is no evidence at the present frequencies.Within the KT scenario, phase coherence is expected below the virtual ordering temperature T KT d < T c , and Medvedyeva et al.[1] take our ''universal'' inverse kinetic conductance L ÿ1 k Td 35 nH ÿ1 to validate their interpretation. However, since they do not provide any argument for the huge, i.e., 500%, difference to the universal KT value of L ÿ1 k T KT d 7 nH ÿ1 , this interpretation is yet unjustified. Moreover, we have obtained from the universal value the c-axis coherence lengths for all our films, c T ? 0:4d. This constitutes our strong evidence for the thickness-induced vortex-loop unbinding, since d varied by more than 1 order of magnitude and c T measures the mean VL (loop) diameter. Note that the ratio c =d has been extracted from data, rather than introduced as an additional condition, as claimed in the Comment. Moreover, the number c T ? =d 0:4 is fully consistent with the values 0.3 and 1.0 obtained from data

The magnetoresistance (MR) of 10 nm to 200 nm thin polycrystalline Co-films, deposited on glass and insulating Si(100), is studied in fields up to 120 kOe, aligned along the three principal directions with respect to the current: longitudinal, transverse (in-plane), and polar (out-of-plane). At technical saturation, the anisotropic MR (AMR) in polar fields turns out to be up to twice as large as in transverse fields, which resembles the yet unexplained geometrical size-effect (GSE), previously reported for Ni-and Permalloy films. Upon increasing temperature, the polar and transverse AMR's are reduced by phonon-mediated sd-scattering, but their ratio, i.e. the GSE remains unchanged. Basing on Potters's theory [Phys.Rev.B 10, 4626(1974)], we associate the GSE with an anisotropic effect of the spin-orbit interaction on the sd-scattering of the minority spins due to a film texture. Below magnetic saturation, the magnitudes and signs of all three MR's depend significantly on the domain structures depicted by magnetic force microscopy. Based on hysteresis loops and taking into account the GSE within an effective medium approach, the three MR's are explained by the different magnetization processes in the domain states. These reveal the importance of in-plane uniaxial anisotropy and out-of-plane texture for the thinnest and thickest films, respectively.

A systematic study is presented on the superconductivity (sc) parameters of the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz cavities for the linear collider project TESLA. Cylindrical Nb samples have been subjected to the same surface treatments that are applied to the TESLA cavities: buffered chemical polishing (BCP), electrolytic polishing (EP), low-temperature bakeout (LTB). The magnetization curves and the complex magnetic susceptibility have been measured over a wide range of temperatures and dc magnetic fields, and also for different frequencies of the applied ac magnetic field. The bulk superconductivity parameters such as the critical temperature T c = 9.26 K and the upper critical field B c2 (0) = 410 mT are found to be in good agreement with previous data. Evidence for surface superconductivity at fields above B c2 is found in all samples. The critical surface field exceeds the Ginzburg-Landau field B c3 = 1.695B c2 by about 10% in BCP-treated samples and increases even further if EP or LTB are applied. From the field dependence of the susceptibility and a power-law analysis of the complex ac conductivity and resistivity the existence of two different phases of surface superconductivity can be established which resemble the Meissner and Abrikosov phases in the bulk: (1) "coherent surface superconductivity", allowing sc shielding currents flowing around the entire cylindrical sample, for external fields B in the range B c2 < B < B coh c3 , and (2) "incoherent surface superconductivity" with disconnected sc domains for B coh c3 < B < B c3 . The "coherent" critical surface field separating the two phases is found to be B coh c3 = 0.81 B c3 for all samples. The exponents in the power law analysis are different for BCP and EP samples, pointing to different surface topologies.

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Based on the high-temperature organometallic route (Sun et al. Science 287, 1989(2000), we have synthesized powders containing CoPt3 single crystals with mean diameters of 3.3(2) nm and 6.0(2) nm and small log-normal widths σ=0.15(1). In the entire temperature range from 5 K to 400 K, the zero-field cooled susceptibility χ(T ) displays significant deviations from ideal superparamagnetism. Approaching the Curie temperature of 450(10) K, the deviations arise from the (mean-field) type reduction of the ferromagnetic moments, while below the blocking temperature T b , χ(T ) is suppressed by the presence of energy barriers, the distributions of which scale with the particle volumes obtained from transmission electron microscopy (TEM). This indication for volume anisotropy is supported by scaling analyses of the shape of the magnetic absorption χ ′′ (T, ω) which reveal distribution functions for the barriers being also consistent with the volume distributions observed by TEM. Above 200 K, the magnetization isotherms M(H,T) display Langevin behavior providing 2.5(1) µB per CoPt3 in agreement with reports on bulk and thin film CoPt3. The non-Langevin shape of the magnetization curves at lower temperatures is for the first time interpreted as anisotropic superparamagnetism by taking into account an anisotropy energy of the nanoparticles EA(T ). Using the magnitude and temperature variation of EA(T ), the mean energy barriers and 'unphysical' small switching times of the particles obtained from the analyses of χ ′′ (T, ω) are explained. Below T b hysteresis loops appear and are quantitatively described by a blocking model, which also ignores particle interactions, but takes the size distributions from TEM and the conventional field dependence of EA into account.