The spin state transition in LaCoO3_{3}; revising a revision

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-$L_{2,3}$ edge we reveal that the spin state transition in LaCoO$_{3}$ can be well described by a low-spin ground state and a triply-degenerate high-spin first excited state. From the temperature dependence of the spectral lineshapes we find that LaCoO$_{3}$ at finite temperatures is an inhomogeneous mixed-spin-state system. Crucial is that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low-/intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance and inelastic neutron data

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Observation and theoretical description of the pure Fano-effect in the valence-band photo-emission of ferromagnets

The pure Fano-effect in angle-integrated valence-band photo-emission of ferromagnets has been observed for the first time. A contribution of the intrinsic spin polarization to the spin polarization of the photo-electrons has been avoided by an appropriate choice of the experimental parameters. The theoretical description of the resulting spectra reveals a complete analogy to the Fano-effect observed before for paramagnetic transition metals. While the theoretical photo-current and spin difference spectra are found in good quantitative agreement with experiment in the case of Fe and Co only a qualitative agreement could be achieved in the case of Ni by calculations on the basis of plain local spin density approximation (LSDA). Agreement with experimental data could be improved in this case in a very substantial way by a treatment of correlation effects on the basis of dynamical mean field theory (DMFT).Comment: 11 pages, 3 figures accepted by PR

Ising magnetism and ferroelectricity in Ca3_3CoMnO6_6

The origin of both the Ising chain magnetism and ferroelectricity in Ca$_3$CoMnO$_6$ is studied by $ab$ $initio$ electronic structure calculations and x-ray absorption spectroscopy. We find that Ca$_3$CoMnO$_6$ has the alternate trigonal prismatic Co$^{2+}$ and octahedral Mn$^{4+}$ sites in the spin chain. Both the Co$^{2+}$ and Mn$^{4+}$ are in the high spin state. In addition, the Co$^{2+}$ has a huge orbital moment of 1.7 $\mu_B$ which is responsible for the significant Ising magnetism. The centrosymmetric crystal structure known so far is calculated to be unstable with respect to exchange striction in the experimentally observed $\uparrow\uparrow\downarrow\downarrow$ antiferromagnetic structure for the Ising chain. The calculated inequivalence of the Co-Mn distances accounts for the ferroelectricity.Comment: 4 pages, 3 figures, PRL in press (changes made upon referees comments

Valence, spin, and orbital state of the Co ions in the one-dimensional Ca3Co2O6: an x-ray absorption and magnetic circular dichroism study

We have investigated the valence, spin, and orbital state of the Co ions in the one-dimensional cobaltate Ca3Co2O6 using x-ray absorption and x-ray magnetic circular dichroism at the Co-L2,3 edges. The Co ions at both the octahedral Co_oct and trigonal Co_trig sites are found to be in a 3+ state. From the analysis of the dichroism we established a low-spin state for the Co_oct and a high-spin state with an anomalously large orbital moment of 1.7 muB at the Co3+ trig ions. This large orbital moment along the c-axis chain and the unusually large magnetocrystalline anisotropy can be traced back to the double occupancy of the d2 orbital in trigonal crystal field.Comment: 5 pages, 4 figure

Orbital order in La0.5Sr1.5MnO4: beyond a common local Jahn-Teller picture

The standard way to find the orbital occupation of Jahn-Teller (JT) ions is to use structural data, with the assumption of a one-to-one correspondence between the orbital occupation and the associated JT distortion, e.g. in O6 octahedron. We show, however, that this approach in principle does not work for layered systems. Specifically, using the layered manganite La0.5Sr1.5MnO4 as an example, we found from our x-ray absorption measurements and theoretical calculations, that the type of orbital ordering strongly contradicts the standard local distortion approach for the Mn3+O6 octahedra, and that the generally ignored long-range crystal field effect and anisotropic hopping integrals are actually crucial to determine the orbital occupation. Our findings may open a pathway to control of the orbital state in multilayer systems and thus of their physical properties.Comment: 4+ pages, 4 figure