Study of the sputtered MgB₂ film on Al₂O₃ by X-ray absorption spectroscopy

In our laboratory we successfully produced thin films on Al ₂O₃ by sputtering a target, which is made of a mixture of MgB₂ (commercial powder) and Mg, and went post-annealing process under different conditions. However, the films were not axial oriented and their superconductivity was not as great as MgB₂ powders. We study the B and Mg K-edge absorption spectra and find followings: 1. The sputtered film (before ex situ annealing treatment) contains small amount of B ₂O₃ due to residual oxygen in the vacuum chamber, and unknown Mg compound. No signal of MgO is observed. 2. Under annealing temperature at 900°C, B₂O₃ is reduced to B by reacting with Mg and MgO is formed. No superconductivity was found. 3. Under annealing temperature at 600° ∼ 700°C, the T_c increases from 11.5 K to 25.5 K. The chemical shift of the pre-edge of the Mg K-absorption edge is greater as compared with that of MgB₂ powder (T_c = 39 K). The Mg K-absorption spectrum indicates that the sample contains some amount of MgO.