Structure and magnetic properties of LnMnSbO (Ln=La and Ce)

A neutron powder diffraction (NPD) study of LnMnSbO (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 P4/nmm space group down to 2 K, and whereas magnetization measurements do not show obvious anomaly at high temperatures, NPD reveals a C-type antiferromagnetic (AFM) order below TN=255K for LaMnSbO and 240 K for CeMnSbO. While the magnetic structure of LaMnSbO is preserved to base temperature, a sharp transition at TSR=4.5K is observed in CeMnSbO due to a spin-reorientation (SR) transition of the Mn2+ magnetic moments from pointing along the c axis to the ab plane. The SR transition in CeMnSbO is accompanied by a simultaneous long-range AFM ordering of the Ce moments, which indicates that the Mn SR transition is driven by the Ce-Mn coupling. The ordered moments are found to be somewhat smaller than those expected for Mn2+ (S=5/2) in insulators, but large enough to suggest that these compounds belong to the class of local-moment antiferromagnets. The lower TN found in these two compounds compared to the As-based counterparts (TN=317 for LaMnAsO, TN=347K for CeMnAsO) indicates that the Mn-Pn (Pn=As or Sb) hybridization that mediates the superexchange Mn-Pn-Mn coupling is weaker for the Sb-based compounds.