Study On The Spin State Of Co Perovskites

Since the discovery of the colossal magnetoresistance in the perovskites, there have been extensive experimental studies on crystal structures, electron-magnetic properties, etc. to find the origin of the magnetoresistance. Up to now, there are two main viewpoints on the mechanisms:one is the magnetic scattering origin based on the double-exchange model; another is the role of the Jahn-Teller distortion effect. The central focus is to explain the increase of the resistance and even the metal-to-insulator transition around the Curie temperature Tc. In this thesis, we concentrate our study on the magnetic and electronic structures of the material to reveal the relationship between the magnetoresistance and magnetic properties of the system excluding the crystal distortion。In all perovskites, magnetoresistance main exist in the manganite and cobaltite, and it is found that there is a difference between these systems: in Co perovskite no Jahn-Teller effect exists,but there are different spin-states. This will be the key difference to the origin of the magnetoresistance in the Co perovskite materials.We apply the multiband Hubbard model to describe the realistic perovskite, that is, take the degeneracy of d and p orbitals, and the on-site Coulomb and exchange interactions. Within the unrestricted Hartree–Fock approximation, we linearize the Hamiltonian. Meanwhile, we use the real space recursion method to calculate the electronic structures of the different magnetic states. We obtain the possible magnetic structure and its properties of the system.First, we study the spin-state transition in perovskite CeCoO₃ in detail. Because of the competition between the crystal-field strength and the Hund's coupling, there are low-spin state ( t_2g⁶e_g⁰ , LS, low-spin state), the intermediate-spin state ( t_2g⁵e_g¹ , IS, intermediate-spin state), and the high-spin state ( t_2g⁴e_g² , HS, high-spin state) from the configuration of the Co³⁺. In the same manner, Co⁴⁺ions also has the low-spin state( t_2g⁵e_g⁰ , LS), intermediate-spin state( t_2g⁴e_g¹ , IS), and high-spin state ( t_2g³e_g², HS), but Co²⁺ has only the low-spin state ( t_2g⁶e_g¹ , LS) and high-spin state ( t_2g⁵e_g² , HS). According to the experiments, there may also exist the neighboring ordered states among the different spin-states, we have studied various magnetically ordered states of an enlarged double cell. We find that the ground state of the CeCoO₃ is the high-spin antiferromagnetically ordered state for given prameters.Next, we investigate the spin-state transition in the Ce-doped SrCoO₃. From the configuration of the Co⁴⁺ions, we obtain the new three possible states: the low-spin state(LS, t_2g⁵e_g² x), the intermediate-spin state(IS, t_2g^4+xe_g^1+x)and the high-spin state(HS, t_2g^3+2xe_g²)as well as their combinations on the doubled cell are taken as the initial configurations. It is found that with doping, the number of the iterant e_g electrons decreases while the t_2g electrons remain localized. Our results suggest that the metals here are all half-metals in the whole doping and become insulator in the last. When doping concentration crosses x=0.2, the IS state is replaced by the LS -IS -FM state. As doping increases further, IS Co ions are replaced by the HS ions with the same moment direction for 0.38≤x<0.79, the LS–HS FM state is the ground state. When 0.79≤x<0.9, the IS-CAFM state becomes the ground state of the system. The insulator-metal transition occurs in the region, 0.93, especially the enhancing Co-d character at the top of the valence band with the doping concentration, and O-p character at the bottom of the valence band.