We have investigated the growth of superconductive thin films of magnesium diboride (MgB2) by molecular-beam epitaxy. A Si(111) substrate with a seed layer of MgO was used for the growth of these films by varying parameters such as the growth temperature, Mg:B flux ratio and deposition rate as well as the background pressure. It was found that highly crystallized films could already form at 250 °C; however, only in a narrow window of growth parameters. The highest critical temperature of 35.2 K with a sharp transition (ΔTC of 0.5 K) was observed for films grown at 300 °C. Using a capping layer of MgO proved to be highly beneficial for the preservation and the smoothness of these films. Together with the fact that MgO proved to be a good seed layer for thin films of MgB2 makes it an ideal candidate for growing all epitaxial MgB2 Josephson junctions.

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MgO based magnetic tunnel junctions are prepared to investigate the dielectric breakdown of the tunnel barrier. The breakdown is directly visualized by transmission electron microscopy measurements. The broken tunnel junctions are prepared for the microscopy measurements by focussed ion beam out of the junctions characterized by transport investigations. Consequently, a direct comparison of transport behavior and structure of the intact and broken junctions is obtained. Compared to earlier findings in Alumina based junctions, the MgO barrier shows much more microscopic pinholes after breakdown. This can be explained within a simple model assuming a relationship between the current density at the breakdown and the rate of pinhole formation.PACS numbers: 68.37. Lp, 85.30.Mn, In 1975 the tunnel magnetoresistance effect (TMR) in ferromagnet/ insulator/ ferromagnet systems was discovered by Jullière [1]. In 1995 significant TMR was found at room temperature [2,3]. Since then, magnetic tunnel junctions (MTJ) became potential candidates for magnetic random access memory (MRAM) [4] and have already replaced the giant magnetoresistance [5,6] read heads in hard discs.The dielectric breakdown of the junctions, which is of major importance for their reliability, has been investigated for the formerly used Alumina based [7,8,9, 10] as well as the recently introduced MgO based MTJs [11] due to their possible use in electronic devices. These studies were generally done by analyzing the transport properties of the MTJs [7,8,9,11] and/ or indirect imaging of the proposed breakdown mechanism [10], generally single pinholes in the insulating barrier. Direct investigations of intact MTJs were also performed [12,13].Here, we try to directly image the broken barriers of the tunnel junctions. In order to do this, we prepared MTJs and performed standard transport measurements. Then, one half of the junctions was stressed by high voltages to induce the dielectric breakdown. Finally, slices were cut out of the broken and intact junctions by focussed ion beam (FIB) and investigated with transmission electron microscopy (TEM). This allows to directly compare structure and transport properties of the intact and broken samples and to compare the findings with those of alumina based MTJs. A simple model is presented to explain the behavior. The magnetic tunnel junctions are prepared in a magnetron sputter system with a base pressure of 1 × 10 −7 mbar. The layer stack is * Electronic address: athomas@physik.uni-bielefeld.de; URL: http: //www.spinelectronics.de (100) wafer. To activate the exchange biasing and for the crystallization of the MgO barrier, the layer stack is annealed after sputtering at 623 K for 60 minutes in a magnetic field of 6500 Oe. The stack is patterned by optical lithography and ion beam etching. The junction sizes are between 7 × 7 and 22.5 × 22.5 µm 2 . All measurements are done by conventional two probe technique.The transmission electron microscopy samples were prepared by FIB with a FEI NOVA NANOLAB 600, which allow...

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