Low temperature EPR investigation of Co~(2+) ion doped into rutile TiO_2 single crystal: Experiments and simulations

摘要

In this paper, we present the results of X-band EPR spectra of Co~(2+) ion doped rutile (TiO_2) which is one of the most promising memristor material. We obtained the angular variation of spectra in three mutually perpendicular planes at liquid helium (7-13 K) temperatures. Since the impurity ions have 1/2 effective spin and 7/2 nuclear spin, a relatively simple spin Hamiltonian containing only electronic Zeeman and hyperfine terms was utilized. Two different methods were used in theoretical analysis. Firstly, a linear regression analysis of spectra based on perturbation theory was studied. However, this approach is not sufficient for analyzing Co_(+2) spectra and leads to complex eigenvectors for G and A tensors due to large anisotropy of eigenvalues. Therefore, all spectra were analyzed again with exact diagonalization of spin Hamiltonian and the high accuracy eigenvalues and eigenvectors of G and A tensors were obtained by taking into account the effect of small sample misalignment from the exact crystallographic planes due to experimental conditions. Our results show that eigen-axes of g and A tensors are parallel to crystallographic directions. Hence, our EPR experiments proves that Co~(2+) ions substitute for Ti~(4+) ions in lattice. The obtained principal values of g tensor are g_x=2.110(6), g_y=5.890(2), g_z=3.725(7) and principal values of hyperfine tensor are A_x=42.4, A_y=152.7, A_z=26 (in 10~(-4)/cm).