First-principles study of the electronic and magnetic structures of CaCu3Ti4O12

The high-dielectric-constant perovskite-related oxide CaCu3Ti4O12 (CCTO) is calculated to be an antiferromagnetic (AFM) semiconductor using density-functional theory within the generalized gradient approximation. The calculated indirect band gap of 0.51 eV is much better than earlier results of about 0.19 eV within the local spin-density approximation. A metastable state is found to be ferromagnetic (FM) and semiconducting, in terms of spin-polarized and fixed-spin-moment methods, indicating semiconducting property and an AFM-FM transition in CCTO. The calculated magnetic moment on CuO4 complex is 0.84μB, in good agreement with experimental results. Based upon the calculated AFM-FM splitting, the sum of the exchange couplings J1+2J3 is estimated to be 14 meV, comparable to the observed characterized magnetic excitations. Besides the Ti path suggested first by Lacroix [C. Lacroix, J. Phys. C 13 (1980) 5125], a possible oxygen path is proposed in the superexchange interaction between Cu spins.