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Abstract:
Surface morphology, element distribution and superconducting properties
of MgB2 filins prepared by ex situ annealing of multilayer B/Mg
precursors in a flow of Mg vapour have been studied. The composition of
the films and distribution of the elements have been analysed by both
Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy
(XPS). The films were found to be composed of a metallic Mg and an
Mg-deficient Mg1-xB2 phase. Al and O have also been detected across the
whole film, suggesting a reaction of the substrate with the adjacent Mg
layer of the precursor. The films are nano-crystal line or even
amorphous, which is ascribed to precipitation of impurities at grain
boundaries. The critical current density of the MgB2 films extracted
from both magneto-optical and magnetization measurements is J(c)
similar to 1 X 10(7) A cm(-2) at 10 K and self-field, and J(c) similar
to 3 x 10(4) A cm(-2) at 2 T and 5 K. The upper critical field is H-c2
= 6.5 T at 5 K. Additionally, an unknown phase is found in the
Mg-Al-2-O-3 System after annealing of Mg films on sapphire in
conditions similar to the synthesis process.
