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Lanthanum nickelate La2NiO4 is an insulator oxide with a 2D-antiferromagnetic character. When doped with holes, either by substituting part of the La3+ by divalent ions or by adding an excess of oxygen, or both, these compounds present a very rich phase diagram, with structural and electronic phases depending on the hole concentration (nH). Neutron diffraction studies showed the presence of a complex order of charge and spins in different compositions [1,2], indicating that this order may be a universal behaviour in these doped-nickelate compounds. This behaviour is also observed in cuprate supraconductors and manganite oxides. We prepared, by floating zone method, a cylindrical single crystal of La2NiO(4+d) of 50 mm height and around 5 mm diameter. Combining thermogravimetric analysis and EDX (Energy Dispersive X-ray Spectroscopy), we determined the composition to be La2Ni0.95O4.12. The incommensurate charge and spin ordering in the (a,b) plane have been studied by neutron diffraction on a piece of around 0.4cm3 cut from the as-grown crystal. The data indicate a singular behavior versus temperature different to the one expected in similar systems. Electron spin resonance (ESR) and magnetic susceptibility have been used to study the magnetic properties and to try to understand the anomalies found in the neutron diffraction study. ESR, being a local magnetic probe, may give additional information about the complex order of charge and spins presented by these compounds. Optical conductivity as a function of temperature and transmission electron microscopy (TEM) were also used to complete this study. [1].J.M. Tranquada et al. Phys. Rev. B 55, R6113(1997) [2] R. Kajimoto et al. Phys. Rev. B 67, 014511 (2003) |
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