The susceptibility - Knight-shift relation in La2NiO4.17 and K2NiF4 by 61Ni NMR
The NiO4 plaquettes in La2NiO4.17, a cousin of the hole-doped high-temperature superconductor La{2-x}SrxCuO4, have been studied by 61Ni-NMR in 14 T in a single crystal enriched in 61Ni. Doped and undoped plaquettes are discriminated by the shift of the NMR resonance, leading to a small line splitting, which hardly depends on temperature or susceptibility. The smallness of the effect is additional evidence for the location of the holes as deduced by Schussler-Langenheine et al., Phys. Rev. Lett. 95, 156402 (2005). The increase in linewidth with decreasing temperature shows a local field redistribution, consistent with the formation of charge density waves or stripes. For comparison, we studied in particular the grandmother of all planar antiferromagnets K2NiF in the paramagnetic state using natural abundant 61Ni. The hyperfine fields in both 2-dimensional compounds appear to be remarkably small, which is well explained by super(transferred) hyperfine interaction. In K2NiF4 the temperature dependence of the susceptibility and Knight shift cannot be brought onto a simple scaling curve. This unique feature is ascribed to a different sensitivity for correlations of these two parameters.