We describe neutron-scattering experiments performed to investigate the magnetic order and dynamics of half-doped ${\text{La}}_{1.5}{\text{Sr}}_{0.5}{\text{CoO}}_4$. This layered perovskite exhibits a near-ideal checkerboard pattern of ${\text{Co}}^{2+}/{\text{Co}}^{3+}$ charge order at temperatures below $\sim 800\text{K}$. Magnetic correlations are observed at temperatures below $\sim 60\text{K}$ but the magnetic order only becomes established at 31 K, a temperature at which a kink is observed in the susceptibility. On warming above 31 K we observed a change in the magnetic correlations which we attribute either to a spin canting or to a change in the proportion of inequivalent magnetic domains. The magnetic excitation spectrum is dominated by an intense band extending above a gap of approximately 3 meV up to a maximum energy of 16 meV. A weaker band exists in the energy range of 20–30 meV. We show that the excitation spectrum is in excellent quantitative agreement with the predictions of a spin-wave theory generalized to include the full magnetic degrees of freedom of high-spin ${\text{Co}}^{2+}$ ions in an axially distorted crystal field, coupled by Heisenberg exchange interactions. The magnetic order is found to be stabilized by dominant antiferromagnetic ${\text{Co}}^{2+}–{\text{Co}}^{2+}$ interactions acting in a straight line through ${\text{Co}}^{3+}$. No evidence is found for magnetic scattering from the ${\text{Co}}^{3+}$ ions, supporting the view that ${\text{Co}}^{3+}$ is in the $S=0$ state in this material.

• Received 6 July 2009

DOI:https://doi.org/10.1103/PhysRevB.80.134414