Interplay of Fe and Nd magnetism in NdFeAsO single crystals

The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific-heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at TS~142 K and the onset of antiferromagnetic (AFM) Fe order below TN~137 K. The Fe moments order AFM in the well-known stripelike structure in the $(ab)$ plane but change from AFM to ferromagnetic (FM) arrangement along the c direction below T^~15 K accompanied by the onset of Nd AFM order below TNd~6 K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO.