Magnetic neutron diffraction study of the charge ordered chain compounds Rb11Mn8O16 and Cs3Mn2O4

The magnetic ordering patterns of Rb11Mn8O16 and Cs3Mn2O4 were determined by neutron powder diffraction with and without applied magnetic fields. The crystal structures of these compounds exhibit infinite chains of edge sharing MnO4 tetrahedra with periodically alternating Mn2 and Mn3 valence. Both Rb11Mn8O16 and Cs3Mn2O4 show collinear magnetic order with antiferromagnetic alignment of Mn moments along the chains below the N el temperatures TN 38 1 K and 13.5 5 K, respectively. In Cs3Mn2O4 the Mn2 and Mn3 moments could be separately refined. The full magnetic structure in zero magnetic field can be viewed as a set of ferrimagnetic chains whose net moments are coupled antiferromagnetically perpendicular to the chain direction. For this compound, we further observe a magnetic field induced transition into a high field phase with uniformly aligned ferrimagnetic moment

Theoretical study on the electronic and magnetic properties of double perovskite La 2−x Sr xMnCoO 6 (x = 0,1,2)

In this paper, the electronic and magnetic properties of double perovskite La2−xSrxMnCoO6 (x = 0,1,2) have been studied using the local-spin-density approximation + U method. For the three compositions investigated, the low symmetry P21/n structure yields consistently lower energy than that of the high symmetry \hbox{$Fm\bar{3}m$} structure. The strong electronic correlation and the orbital polarization of Co-d electrons play crucial roles. In agreement with experiments, we find that La2MnCoO6 is a ferromagnetic insulator with both Mn and Co ions in their high-spin states. The tilting of oxygen octahedrons is most significant in this case and is responsible for its insulating behavior; for LaSrMnCoO6, the ground state remains a ferromagnetic insulator with Mn and Co ions in their high-spin states. The optimized P21/n and \hbox{$Fm\bar{3}m$} crystal structures are nearly the same, and the P21/n structure is stabilized by the spontaneous layer-wise antiferro-orbital ordering of Co-d electrons. We also predict that Sr2MnCoO6 is a ferromagnetic metal, and its electronic structure can be viewed as a rigid band shifting from that of LaSrMnCoO6. Due to the strong covalency between transition metal and oxygen ions, the valences of Mn and Co ions differ considerably from those derived from purely ionic model. Also, doping induced holes mainly go to oxygen sites though the density of states near the Fermi energy has strong mixed character. This feature, together with the orbital ordering phenomenon, should be observable via the X-ray near-edge absorption spectroscopy and the polarized X-ray diffraction spectra

Quantum chemical study of Co 3+ spin states in LaCoO 3

Ab initio quantum-chemical cluster calculations are performed for the perovskite LaCoO 3. The main concern is to calculate the energy level ordering of different spin states of Co 3+ , which is an issue of great controversy for many years. The calculations performed for the trigonal lattice structure at T=5 K and 300 K, with the structural data taken from experiment, display that the low-spin (LS, S=0) ground state is separated from the first excited high-spin (HS, S=2) state by a gap >100 meV, while the intermediate-spin (IS, S=1) state is located at much higher energy ≈0.5 eV. We suggest that the local lattice relaxation around the Co 3+ ion excited to the HS state and the spin-orbit coupling reduce the spin gap to a value ~10 meV. Coupling of the IS state to the Jahn-Teller local lattice distortion is found to be rather strong and reduces its energy position to a value of 200 $\div$ 300 meV. Details of the quantum-chemical cluster calculation procedure and the obtained results are extensively discussed and compared with those reported earlier by other authors. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010