Enhancement of ferromagnetic correlations on multiferroic TbMnO3 by replacing Mn with Co

The structural, electronic and magnetic properties of TbMn1-xCoxO3 (0.1 <= x <= 0.9) compounds are reported. The samples are isostructural to TbMnO3 adopting the orthorhombic distorted perovskite structure (Pbnm), except for x = 0.4, 0.5 and 0.6, where an ordered double perovskite structure (P2(1)/n) is found. X-ray absorption spectra at the Mn and Co K edges show an incomplete charge transfer between Mn and Co atoms yielding a mixed valence state Mn3+/Mn4+ and Co3+/Co2+ for the whole series. Neutron powder diffraction measurements show the development of a ferromagnetic ground state for the intermediate compositions (0.3 <= x <= 0.6) indicating that the ferromagnetic superexchange Mn4+-O-Co2+ interaction is the strongest among a wide set of competitive interactions. The ferromagnetic ordering is, however, not fully achieved and coexists with glassy magnetic properties. With increasing concentration of Co (x >= 0.7) the long range ordering vanishes and only a glassy magnetic behavior with slow dynamics is found. These properties could be related to the existence of magnetically inhomogeneous small clusters arising from competitive magnetic interactions.