Surface dependent structural phase transition in SrTiO₃ observed with spin relaxation of ⁸Li

We investigate the 105 K structural phase transition in SrTiO₃ using depth controlled measurements of the spin relaxation of ⁸Li. The measurements were performed in zero external magnetic field and rely on the local electric field gradient (EFG) at the crystalline implantation site of the ⁸Li (I = 2) to hold the nuclear polarization. The tetragonal distortion accompanying the phase transition modifies the EFG in some ⁸Li implantation sites, resulting in an observable loss of ⁸Li polarization. This loss of polarization begins at a temperature T^* = 150 K, indicating there is some loss of cubic symmetry well above the bulk transition. We find that the value of T^* is unaffected by the range of implantation depths available (10-150 nm); however, the temperature dependence of the polarization depends on the surface preparation of the SrTiO₃ sample.