IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 3261 Superconducting and Microstructural Properties of Two Types of Films Prepared by Pulsed Laser Deposition Yue Zhao, Mihail Ionescu, Marie Roussel, Alexey V. Pan, Josip Horvat, and Shi X. Dou Abstract—Significant differences in superconducting and microstructural properties between two types of films prepared by pulsed laser deposition were determined. A very high slope of 1.1 T/K was achieved in the in situ film. The curves of the in situ film also show a much weaker field de- pendence than that of the ex situ film. The magneto-optical (MO) images show that at 4 K the flux penetrates the in situ film through random paths, while for the ex situ film, the flux pene- tration pattern is mostly repeatable, indicating a defect-controlled flux penetration. Microstructural study (transmission electron microscopy and atomic force microscopy) revealed a relatively big grain size in the ex situ film. The correlation between the super- conducting properties, microstructure and preparation conditions is discussed with regard to the two types of films. Index Terms—Magneto-optic imaging, superconducting films, transmission electron microscopy. I. INTRODUCTION T HE two-gap phonon mediated superconductor has some very attractive properties and great interest has been focused on the preparation of thin films for both theoret- ical and application purposes. Shortly after the discovery of su- perconductivity in this material, thin films with bulk-like were achieved by ex situ annealing of a boron or Mg-B pre- cursor film in Mg vapor [1], [2]. Kang and co-workers optimized the ex situ annealing conditions by fine-tuning the annealing temperature and time, and a very high of at 5 K, 0 T with a weak field dependence was obtained [3]. Eom and coworkers found that a high oxygen level in the film prepared by a two-step ex situ annealing procedure signifi- cantly improved the in high fields [2]. Zeng et al. developed an in situ hybrid physical-chemical vapor deposition (HPCVD) method to grow high quality film epitaxially [4]. The value of their film reached at 4.2 K, 0 T, but it dropped rather quickly to as the field in- creased to 4 T. As-grown and in situ annealed films, prepared by pulsed laser deposition (PLD), molecular beam epitaxy (MBE) or magneto-sputtering employing much lower substrate temper- atures and shorter annealing times, generally show a small-grain Manuscript received October 5, 2004. This work was supported in part by Australian Research Council (ARC) under a Linkage Project (LP0219629) co- operating with Alphatech International and the Hyper Tech Research Inc. Y. Zhao, M. Roussel, A. V. Pan, J. Horvat, and S. X. Dou are with the Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2500, Australia (e-mail: yz70@uow.edu.au). M. Ionescu is with Australian Nuclear Science and Technology Organization (ANSTO), Australia (e-mail: mionescu@uow.edu.au). Digital Object Identifier 10.1109/TASC.2005.848847 feature and poor crystallization [5]–[9]. This category of films has suppressed , but the properties are quite good in high fields, which indicates strong pinning and intra- or inter-band scattering due to a high disorder level in the films. As can be seen, the properties of thin films vary significantly with different preparation processes, which are closely related to the variety of microstructures and impurity levels of those films. A study of this relationship could be relevant to the opti- mization of the film preparation. In this paper, we report a comparative study on in situ and ex situ annealed films prepared by pulsed laser deposi- tion. The relationship between the superconducting properties and the microstructure is investigated, and the influence of the preparation conditions on the two types of films is dis- cussed. II. EXPERIMENTAL In the preparation of the in situ annealed film, the precursor film was deposited on a 250 substrate from a stoichiometric target (84% density). A pulsed excimer laser beam with energy fluence of 300 mJ/pulse was focused to a 8 ellipse spot on the target. During the deposition process, the atmosphere was 120 mTorr higher pu- rity Argon. An 800 nm Mg cap layer was deposited on top of the precursor film to compensate the Mg loss due to annealing. The film was then heated to 685 in 12 min and kept at this temperature for 1 min in a 1 atm Ar atmosphere. For the ex situ annealed film, a boron precursor film was deposited from a boron target ( 40% density) onto an substrate in a vacuum. The precursor film was then an- nealed at 900 for 30 min in a sealed stainless steel tube with Mg pellets. The details of the preparation are described in [10]. The transport measurements were carried out on a PPMS-9T magnetometer system (Quantum Design), using a standard 4-probe method and a dc current density of 1 . The zero-field-cooled (ZFC) magnetization vs. temperature curves and magnetization hysteresis loops of the films were measured on an MPMS-5 T magnetometer. In each measurement the applied field was perpendicular to the film plane. The was calculated from the magnetization loops using the Bean model. The upper critical fields in different temperatures were obtained using 90% points in the resistivity-temperature curves measured in deferent fields. The MO imaging was carried out at the University of Oslo, employing a bismuth substituted yttrium iron garnet indicator 1051-8223/$20.00 © 2005 IEEE