Magnetic instability and oxygen deficiency in Na-doped TbMnO3

Oxygen vacancies in TbMnO3 were created by monovalent Na+ doping. The effects of the interruption of the superexchange paths on the spin orderings were investigated. Chemical doping resulted in a much higher ordering temperature, which is probably associated with enhanced Tb-Tb coupling because of a reduction in Tb-Tb interatomic distances. The oxygen vacancies turned the modulated Mn spin structure into a simple commensurate one, presumably due to interruptions of the in-plane (Mn-O)-(O-Mn) superexchange paths, which reduced the significance of the next nearest-neighbor interactions of the Mn ions. The modulated spin structure of the Mn moments was found to reappear in systems with high Na doping, indicating that the anisotropic magnetic couplings had reemerged.