\textbf{Magnetism in}\textbf{\textit{ Ln}}\textbf{MnSbO (}\textbf{\textit{Ln }} $=$\textbf{ La or Ce)}

Neutron diffraction of polycrystalline (PND) \textit{Ln}MnSbO (\textit{Ln }$=$ La or Ce) reveals differences between the magnetic ground state of the two compounds due to the strong Ce-Mn coupling compared to La-Mn. The two compounds adopt the \textit{P4/nmm} space group down to 1.5 K and whereas magnetization measurements do not show any anomaly at high temperatures, PND reveals a C-type antiferromagnetic (AFM) order below $T_{\mathrm{N}} \quad =$ 255 K for LaMnSbO and 240 K for CeMnSbO. While the magnetic structure of LaMnSbO is preserved to base temperature, a sharp transition at $T_{\mathrm{SR}} \quad =$ 5 K in CeMnSbO due to a spin-reorientation (SR) transition of the Mn$^{\mathrm{2+}}$ from pointing along the $c$-axis to the \textit{ab}-plane is found. The SR transition in CeMnSbO is accompanied by a simultaneous long-range AFM ordering of the Ce moments. This indicates that the Mn SR transition is driven by the Ce-Mn coupling similar to recent observation in the isostructural CeMnAsO. The ordered moments are found to be somewhat smaller than those expected for Mn$^{\mathrm{2+}}$ (S $=$ 5/2) in insulators, but large enough to suggest that these compounds belong to the class of local-moment antiferromagnets. The lower $T_{\mathrm{N\thinspace }}$found in this compound compared to the As-based counterpart ($T_{\mathrm{N}} \quad =$ 347K for CeMnAsO) indicates that the Mn-\textit{Pn} (\textit{Pn} $=$ As or Sb) hybridization that mediates the exchange Mn-Mn coupling is weaker for the Sb-based compounds.