Neutron scattering investigation of the magnetic correlations in Sr$_2$CoO$_3$Cl and Sr$_3$Co$_2$O$_5$Cl$_2$

The magnetic interactions in the layered cobalt (III) oxychlorides, Sr$_{2}$CoO$_{3}$Cl and Sr$_{3}$Co$_{2}$O$_{5}$Cl$_{2}$, have been studied using constant wavelength neutron powder diffraction and inelastic time-of-flight neutron scattering. The materials crystallize with Ruddlesden-Popper structures and contain layers of CoO$_{5}$ pyramids that form CoO$_{2}$ sheets in the \textit{ab}-plane separated along $c$ by SrCl rocksalt layers. The phases display contrasting magnetic properties despite the probable presence of high spin Co$^{3+}$ (S = 2) in both compounds. Sr$_{2}$CoO$_{3}$Cl undergoes a transition to long range 3D antiferromagnetic (AFM) order at a T$_{N}$ = 330 K, preceded by strong diffuse scattering from 2D spin coherence. Sr$_{3}$Co$_{2}$O$_{5}$Cl$_{2}$ does not exhibit the expected long range ordered AFM ground state, and instead the spin correlation is limited to short range 2D interactions. The material's magnetism is rationalized based on competing FM and AFM coupling.