Bulk TbMnO3 is a classic multiferroic material that combines antiferromagnetic ordering below 42 K and, below 27 K, a spin spiral transition with inversion symmetry breaking below 27 K that causes ferroelectricity . However, it was recently demonstrated that strained TbMnO3 thin films grown on SrTiO3 (001) display an induced ferromagnetic behavior absent in the bulk material. This net magnetic moment arises from a two-dimensional ferromagnetic phase synthesized at the ferroelastic (orthorhombic) domain walls (DW), which nucleate to accommodate the huge epitaxial strain induced by the substrate, and thus, scales with the DW density . These ferromagnetic DWs are characterized by a spatially-ordered substitution of alternate Tb sites by Mn atoms along the pseudocubic  growth direction, inducing magnetic frustration and a spin canted ferromagnetic ground state. First-principle calculations point to two different types of Mn atoms occurring at the boundary planes, Mn(I) and Mn(II), with different magnetic moments and crystal environments, i.e. tetrahedral and a quasi-square-planar O coordinations, respectively [see Fig.1(a)].
This work presents the detailed aberration-corrected Scanning Transmission Electron Microscopy (STEM) study of the different structural environments and electronic properties of Mn cations located at the Tb sites of the DW structure. For this purpose, atomically resolved High Angle Annular Dark Field (HAADF), Annular Bright Field (ABF) imaging and fine structure Electron-Energy Loss Spectroscopy (EELS) in plane view and cross sectional configurations have been combined. ABF imaging evidences that the two first neighbor O coordinations for Mn(I) and Mn(II) sites predicted theoretically are present in plane-view specimens, see Fig. 1(b). Furthermore, the analysis of the O K edge fine structure has allowed us to map the nominal Mn oxidation state  using atomic-resolution STEM-EELS, see Figure 2. This experiment has confirmed the decrease of the overall Mn valence at the domain wall previously reported and a fine modulation of the electronic state between the tetrahedrally-coordinated Mn(I) and the square-planar-coordinated Mn(II) replacing Tb cations, as predicted by DFT calculations.
 Y. Kimura et al., Nature 426, 55 (2003).
 S. Farokhipoor, C. Magén, et al., Nature 515, 379 (2014).
 M. Varela et al., Physical Review B 79, 085117 (2009).
To cite this abstract:Roger Guzman, Jorge Iñiguez, Saeedeeh Farokhipoor, Beatriz Noheda, César Magén; Valence states of new Mn coordination sites at the ferromagnetic domain walls of TbMnO3 thin films. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/valence-states-of-new-mn-coordination-sites-at-the-ferromagnetic-domain-walls-of-tbmno3-thin-films/. Accessed: September 20, 2021
EMC Abstracts - https://emc-proceedings.com/abstract/valence-states-of-new-mn-coordination-sites-at-the-ferromagnetic-domain-walls-of-tbmno3-thin-films/