The material class of rare earth nickelates with high Ni3+ oxidation state is generating continued interest due to the occurrence of a metal-insulator transition with charge order and the appearance of non-collinear magnetic phases within this insulating regime. The recent theoretical prediction for superconductivity in LaNiO3 thin films has also triggered intensive research efforts. LaNiO3 seems to be the only rare earth nickelate that stays metallic and paramagnetic down to lowest temperatures. So far, centimeter-sized impurity-free single crystal growth has not been reported for the rare earth nickelates material class since elevated oxygen pressures are required for their synthesis. Here, we report on the successful growth of centimeter-sized LaNiO3 single crystals by the floating zone technique at oxygen pressures of up to 150 bar. Our crystals are essentially free from Ni2+ impurities and exhibit metallic properties together with an unexpected but clear antiferromagnetic transition.

The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism, which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors.

Static and high-frequency magnetic properties have been studied for BaFe 12Ϫ2x Co x Zr x O 19 with x ϭ0-1.2. The x-ray diffraction and M-H curves of aligned samples show that Co-Zr substitution can modify the anisotropy of BaFe 12Ϫ2x Co x Zr x O 19. BaFe 12Ϫ2x Co x Zr x O 19 with xр0.6 has an easy c-axis anisotropy and the anisotropy fields rapidly decrease with Co-Zr substitutions. The sample of xϭ1.2 has an easy c-plane anisotropy with H a ϭ5 kOe. The saturation magnetizations of BaFe 12Ϫ2x Co x Zr x O 19 are almost constant at xр0.4 and decrease with higher substitutions. The coercivity decreases rapidly from 2400 Oe for the sample of xϭ0 to about 10 Oe for the sample of xϭ0.8. The natural resonance frequencies are observed at 4.5 and 5.0 GHz for the samples of x ϭ0.8 and xϭ1.2, respectively. Based on microwave measurement and theoretical estimations on reflectivity, BaFe 12Ϫ2x Co x Zr x O 19 may be a good candidate for electromagnetic compatibility and other practical applications at high frequency.

The static and high frequency magnetic properties have been studied for BaW ferrites BaZn2−xCoxFe16O27 with x=0, 0.5, 0.7, 1.0, 1.5, and 2.0. The results showed that Co ions are able to modify the anisotropy from c axis to c plane at x=0.6–0.7. For the BaW composites with c-plane anisotropy, as the Co concentration x increases, the resonance frequencies fR are shifted to higher frequency, from 2.5 GHz at x=0.7 to 12.0 GHz at x=1.5. The permeabilities μ0′ and μmax″ are 2.4–2.2 and ∼0.8, respectively, for composites with 35% (by volume) BaW ferrite powders at x=0.7 and 1.0. The parameters indicate that BaZn2−xCoxFe16O27 composites have suitable high frequency properties for use as electromagnetic materials with low reflectivity at microwave frequencies. In addition, for BaW composites with c-axis anisotropy, there exist both the domain wall and natural resonances. However, only natural resonance is observed for composites with c-plane anisotropy.

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2−xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2−xSrxCoO4.