Magnetic Excitations in Disordered Striped Antiferromagnetic Insulators

The compounds of the La2-xSrxCoO4 series are isostructural to the 214 family of cuprate superconductors, but nevertheless remain insulating over a wide range of doping. Recently, it was observed that the x=1/4, 1/3, and 4/10 compounds display a peculiar type of magnetism, with short-ranged static antiferromagnetic order and an hour-glass-like magnetic excitation spectrum, which can be directly associated with the presence of stripe order. Here we argue that inhomogeneities, coming from deviations of perfectly ordered stripes, are of central importance and, by calculating the magnetic excitation spectrum, we show that a scenario of disordered charge stripes (formed by Co2+ and Co3+ ions) is consistent with the experimental data.