Elsevier

Thin Solid Films

Volume 520, Issue 23, 30 September 2012, Pages 6985-6988
Thin Solid Films

Microstructure and superconducting properties of MgB2 films prepared by solid state reaction of multilayer precursors of the elements

https://doi.org/10.1016/j.tsf.2012.07.089Get rights and content

Abstract

Surface morphology and superconducting properties of MgB2 superconducting thin films prepared by ex-situ annealing of multilayer Mg/B precursors in Mg vapor are studied.

Depending on the precursor structure different physical and microstructural properties of the superconductor evolve. Structure and composition of the films are analyzed by scanning electron microscopy and wavelength dispersive x-ray spectroscopy. It is found that certain precursor structures can lead to high quality superconducting films, however, in specific precursor structures mechanical stress leads to the formation of wrinkles strongly affecting the superconducting homogeneity of the films. A correlation between microstructure and superconducting properties, such as pinning or critical current density, can be provided via magneto-optical Faraday microscopy.

Highlights

► Correlation of microstructure and properties of MgB2 films ► Electron and ion based analysis of microstructure ► Superconducting properties by magneto-optical imaging ► Enhanced magnetic field stability by a particular microstructure

Introduction

Thin magnesium diboride films are highly attractive for superconducting applications since they exhibit the highest critical temperature of all simple metallic compounds. Nearly anisotropic superconducting properties, robustness against the presence of grain boundaries [1] and high ductility allowing good mechanical deformation are advantages that cannot be provided by any material with higher superconducting transition temperatures such as the cuprates or the pnictides [2]. However, the preparation of MgB2 thin films with reasonable properties is a task not easy to cope with.

The preparation out of the elements is difficult because of the extremely different vapor pressures of magnesium and boron. A stoichiometric driven process often lacks magnesium escaping during the process, which in the end leads to boron-rich phases such as MgB4. A comprehensive review on possible solutions for adequate preparation paths is given by Naito and Ueda [3]. In this paper we follow the route of depositing a Mg/B multilayer precursor which is then post annealed in Mg vapor at high temperatures. This process leads to MgB2 films with transition temperatures between Tc = 30 K and Tc = 35 K exhibiting high critical currents of up to jc = 1011 A/m2 at T = 10 K [4], [5]. However, it turned out that the process is extremely sensitive to both precursor structure and substrate properties.

In this paper we present an analysis of superconducting MgB2 films that have been prepared from different precursor multilayer sequences. We show that changing the substrate near layer from magnesium to boron leads to a completely different microstructure of the finally produced superconducting film. In one case homogeneous films are obtained exhibiting consistent superconducting properties. In the other case, a strong distortion of the superconducting layer takes place which leads to highly inhomogeneous films.

Section snippets

Sample production process

MgB2 films have been prepared by Mg/B – multilayer electron beam evaporation on r – cut Al2O3 substrates and a subsequent ex-situ annealing process [4], [5], [6]. The precursor contained a first layer of 30 nm Mg followed by four times the combination (B 30 nm/Mg 60 nm). In order to investigate the role of the multilayer sequence we used an incomplete initial magnesium layer, which is only covering the inner part of the substrate. This allows the comparative analysis of two precursor scenarios on

Microstructure analysis

To get a deeper insight into the microstructural properties of different regions of the film we characterized the samples via scanning electron microscopy (SEM) which is installed within a Nova NanoLab 600 DualBeam. The SEM images were taken with scattered electrons with an accelerating voltage of 5 kV and a beam current of 0.4 nA. The electron beam was at an angle of 52° to the vertical, thus it is possible to view the surface and a vertical cut within the same setup. The desired area for the

Superconducting properties

It is interesting to correlate the microstructure to the superconducting properties of the film in the different areas. For this purpose a method has to be applied which is able to provide local information on the different regions. We perform a characterization in terms of the magneto-optical Faraday effect. For this method a magneto-optically active iron garnet film is placed on top of the sample acting as the field sensing element. The iron garnet film is illuminated with polarized light. A

Conclusion

Magnesium diboride thin films were prepared by a post annealing process of layers of the elements. The layer sequence of the precursor is of importance. With the bottom layer being either magnesium or boron either homogeneous or inhomogeneous MgB2 films emerge. The microstructure and superconducting properties of these two different structures are characterized. SEM pictures reveal that in the inhomogeneous area the MgB2 film breaks open and undulates. Magneto-optical images reveal enhanced

Acknowledgments

The authors are thankful to U. Eigenthaler for FIB implementation and to Y. Link for evaporation of the precursor.

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