Hall effects of c-axis-oriented Mg_1−xAl_xB₂ thin film

We synthesized c-axis-oriented Mg_1-xAl_xB₂ thin film on an Al buffered layer of c-cut Al₂O₃ substrate via hybrid physical–chemical vapor deposition (HPCVD) and measured the longitudinal and Hall resistivities in the ab-plane. X-ray diffraction (XRD) showed the shift of the (000l) MgB₂ peaks to higher 2θ values, indicating that Al was substituted for Mg. The 185-nm Mg_1-xAl_xB₂ showed a critical temperature (T_c) of 26.8 K with a broadened transition width of approximately 11 K. In the normal state, the Hall coefficient was positive and decreased as the temperature increased. At T=100K, Hall coefficient (R_H) is 18.97×10^-11 m³/C, from which the hole charge carrier density (n_hole) was determined to be 3.29×10²² holes/cm³. The suppression of n_hole compared to pure MgB₂ of approximately 2×10²³ holes/cm³ supported the hypothesis that Al³⁺ substitutes to Mg²⁺. In the superconducting state, universal Hall scaling behavior with a constant β of 1.7±0.2 was observed due to the appearance of Al impurities.