Spin reorientation and Ce-Mn coupling in antiferromagnetic oxypnictide CeMnAsO

Structure and magnetic properties of high-quality polycrystlline CeMnAsO, a parent compound of the 1111-type oxypnictides, have been investigated using neutron powder diffraction and magnetization measurements. We find that CeMnAsO undergoes a C-type antiferromagnetic order with Mn2+(S = 5/2) moments pointing along the c axis below a relatively high Neel temperature of T-N = 347(1) K. Below T-SR = 35 K, two simultaneous transitions occur where the Mn moments reorient from the c axis to the ab plane preserving the C-type magnetic order, and Ce moments undergo long-range AFM ordering with antiparallel moments pointing in the ab plane. Another transition to a noncollinear magnetic structure occurs below 7 K. The ordered moments of Mn and Ce at 2 K are 3.32(4) mu(B) and 0.81(4) mu(B), respectively. We find that CeMnAsO primarily falls into the category of a local-moment antiferromagnetic insulator in which the nearest-neighbor interaction (J(1)) is dominant with J(2) < J(1)/2 in the context of J(1) - J(2) - J(c) model. The spin reorientation transition driven by the coupling between Ce and the transition metal seems to be common to Mn, Fe, and Cr ions, but not to Co and Ni ions in the isostructural oxypnictides. A schematic illustration of magnetic structures in Mn and Ce sublattices in CeMnAsO is presented.