Detailed measurements of the critical current density j c of YBa 2 Cu 3 O 7Ϫ␦ films grown by pulsed laser deposition reveal the increase of j c as a function of film thickness. Both this thickness dependence and the field dependence of the critical current are consistently described using a generalization of the theory of strong pinning of Ovchinnikov and Ivlev ͓Phys. Rev. B 43, 8024 ͑1991͔͒. From the model, we deduce values of the defect density (10 21 m Ϫ3 ) and the elementary pinning force, which are in good agreement with the generally accepted values for Y 2 O 3 inclusions. In the absence of clear evidence that the critical current is determined by linear defects or modulations of the film thickness, our model provides an alternative explanation for the rather universal field dependence of the critical current density found in YBa 2 Cu 3 O 7Ϫ␦ films deposited by different methods.

We investigated the influence of UV (KrF, បϭ5.01 eV) pulsed laser irradiation on the atomic order and optical properties of amorphous YBaCuO films containing crystalline clusters of nanometer ͑up to 25 nm͒ size and characterized by high mobility of structural elements due to the lack of sharp interphase boundaries. The presence of crystalline clusters in amorphous medium leads to higher disorder of the latter, while electronic states in relatively narrow (Ϸ2 eV) Cu 3d 10 band become localized and practically do not participate in optical transitions. We found that UV radiation destroys the crystalline clusters, increases order in the amorphous medium and initializes the processes of (11l)ϩ(10l)⇒(00l) orientational transition. Increase of atomic order results in partial delocalization of electron states in the Cu 3d 10 band and the conduction band switches over from Cu 4s 1 to Cu 3d 10 .

The magnetic-domain induced vortex pinning is studied in the ferromagnet/superconductor bilayers (FSB's), in which the F layers are Co/Pt multilayers with perpendicular magnetic anisotropy, and the S layers are either niobium or high temperature superconductor YBa 2 Cu 3 O 7 (YBCO). The magnetization measurements reveal the enhancement of the flux pinning in both types of FSB's during the reversal of the magnetization of the F layer, but the details of the behavior depend on the type of the S layer. In the case of niobium FSB the maximum of pinning appears when the F layer is in the final stage of the magnetic reversal process, while the FSB with YBCO shows the maximum when the F layer is saturated. The possible origins of these differences are discussed. IntroductionThe vortex pinning in superconductors determines the critical current density and therefore has a direct impact on the possible applications of superconducting materials. A large effort is directed towards the development of methods of the enhancement of flux pinning. Recently, a novel method, with the use of ferromagnet/superconductor bilayers (FSB), has been suggested [1]. It is based on the idea that the magnetic stripe domains in the F layer with a perpendicular magnetic anisotropy pin the vortex core in the S layer. The relative ease of the realignment of the stripe domains offers the possibility of the adjustment of vortex pinning using small magnetic fields, provided that the S and F layers are sufficiently separated by the buffer layer which eliminates the proximity effect. To date, several attempts have been made to study various FSB's, and the evidence has been accumulated that some pinning enhancement indeed occurs [2][3][4][5]. However, only one study, of the FSB's with lead S layers (weak type II superconductor in thin film form) addressed directly the question of the origins of the pinning enhancement, and found it related not to stripe domains, but to the isolated magnetic domains nucleated during the reversal of magnetization of the F layer [4].To shed more light on the behavior of the magnetic domain induced pinning, we compare here two classes of FSB's, containing either conventional type II superconductor, niobium, or the high temperature superconductor, YBCO. The intrinsic pinning is much stronger in YBCO, and one may expect that this has some impact on the behavior of FSB's. The preliminary results on the behavior of the niobium FSB's have been described elsewhere [6].

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The peculiarities of magnetic field penetration in the form of fingering or dendritic instabilities are studied by magneto-optical technique in the niobium films of different thickness. It is observed that the reduction of Nb film thickness reduces the threshold field for instabilities, in agreement with the theoretical predictions. In addition, it is shown that the silver overlayer deposited on the top of Nb film leads also to the enhancement of instabilities, in marked contrast to the effect of gold or aluminum overlayers, which are known to suppress instabilities.