Effect of Nb concentration on the magnetotransport properties and the spin-orbit coupling strength in Nb-doped SrTiO₃ epitaxial thin films

Several oxides have attracted much interest in spintronics due to unusual properties originating from the correlated orbital and spin degrees of freedom. One missing part in oxide spintronics is a good spin channel featured by strong spin-orbit coupling (SOC) which enables an efficient control of the electron’s spin. Motivated by a recent finding of the strong SOC in a heavily-doped Sr(Nb_0.2Ti_0.8)O₃ epitaxial film, we have systematically studied the dependence of the SOC strength on Nb concentration (n_Nb=2 ~ 20 at. %) by investigating the magnetotransport properties. We have found that the magnetoresistance (MR) is well described by the weak antilocalization (WAL) and the SOC strength increases with n_Nb. In addition, we have observed an anomalous behavior near 10 K in the SOC strength independent of n_Nb, which might be associated with the temperature dependent Landé g-factor. Furthermore, MR is found to be anisotropic for the intermediate doping regime (n_Nb=5 and 10 at. %) while being isotropic for the low and high doping regime (n_Nb =2, 15, and 20 at. %). The calculation of band structure shows this behavior is associated with the splitting of Nb-d bands by the crystal field and the change of the Fermi level with n_Nb.