Effects of oxygen deficiency on the magnetic ordering of Mn in Tb _0.9Na_0.1MnO_2.9

Monovalent Na⁺ ions have been incorporated into a multiferroic TbMnO₃ compound. Structural studies, using high-resolution neutron diffraction patterns, show that monovalent Na⁺-doping results in a noticeable oxygen deficiency in the compound. The alternations in crystalline and magnetic structures due to oxygen deficiency are investigated by neutron diffraction and magnetic susceptibility measurements. Na-doping turns the modulated incommensurate Mn spin structure, observed for the parent compound, into a simple commensurate one. The observed magnetic diffraction patterns agree very well with that calculated assuming a D-type arrangement for the Mn spins, with the moments pointing along the c-axis direction. No significant alternation in the ordering temperature of the Mn moments is found.