Influence of structural disorder on the optical and transport properties of Co_0.50Ti_0.50 alloy films

Co_0.50Ti_0.50 alloy films with a total thickness of about 100 nm were prepared by flash evaporation of the crushed alloy powders onto heated (730 K for the ordered state) and LN₂-cooled (150 K for the disordered state) substrates. Structural analysis of the films was performed by using transmission electron microscopy. The optical conductivity (OC) of the samples was measured at room temperature in a spectral range of 265-2500 nm (4.7-0.5 eV). The resistivity measurements were carried out by using the four-probe technique in a temperature range of 4.2 - 300 K. The experimental OC spectra for the Co_0.50Ti_0.50 alloys show the most significant changes in the infrared region upon the order-disorder transformation. The structural disorder in the Co_0.50Ti_0.50 alloy film leads to a change in the sign of the temperature coefficient of the resistivity from positive to negative. The observed changes in the optical properties and the temperature dependences of resistivity caused by the order-disorder structural transition are analyzed in the framework of the lattice symmetry and the electronic structure of the ordered CoTi compound.