Artificial chemical and magnetic structure at the domain walls of the epitaxial oxide TbMnO3

C. MAGÉN; S. FAROKHIPOOR; C. DAUMONT; S. VENKATESAN; E. SNOECK; J. INIGUEZ; D. RUBI; M. MOSTOVOY; B. NOHEDA

Conferencia; 20th International Conference on Magnetism; 2015

Strain engineering can improve the functional properties of thin films or induce new unexpected Physics by means of epitaxial growth, particularly in complex oxides such as perovskites (ABO3) [1]. The application of this strategy to multiferroic materials can produce dramatic and unexpected changes in the physical properties. The terbium manganite TbMnO3 (TMO) is an example of a multiferroic material in which epitaxial strain sets in a new functional property [2]. This material exhibits an antiferromagnetic (AFM) ordering below 42 K and a spin spiral state below 27 K, the latter inducing the inversion symmetry breaking that causes ferroelectricity [3]. In this work we show that epitaxially-strained TMO on SrTiO3 (001) grown by Pulsed Laser Deposition (PLD) induces the formation of a novel chemical and magnetic structure, which orders ferromagnetically at low temperature with average magnetic moments up to 0.15 μB/f.u. at 15 K [2]. This new phase is synthesized at the domain walls (DWs) that TMO forms to accommodate the epitaxial strain caused by the large lattice mismatch. This rising ferromagnetic order is strongly correlated with the strain-induced microstructure of twin domains. As thickness is reduced, a higher density of twin domain walls (DW) is found and magnetization increases. In fact, magnetization scales with the DW density [2]. Aberration-corrected Scanning Transmission Electron Microscopy (STEM) combined with Electron-Energy Loss Spectroscopy (EELS) has been carried out in an FEI Titan at 300 kV to characterize locally the structural and chemical properties this novel phase. High-angle annular dark field (HAADF) imaging evidences the abundance of this [001]-oriented DWs and the local relaxation of strain at the DW positions. Atomic-resolution STEM-EELS chemical mapping demonstrates that the new structure synthesized at the DWs is characterized by the presence of spatially ordered Tb-deficient positions along [001] which are in fact almost entirely replaced by Mn atoms. The substitution of alternate Tb positions by Mn in the A sites of the perovskite gives rise to exotic atomic coordinations for these Mn atoms at the wall with respect to ?bulk? Mn. DFT+U as well as embedded cluster calculations demonstrate that these wall Mn atoms present different magnetic parameters than the ?bulk? Mn and perturbs the local magnetic coupling around the DW to induce long range ferromagnetism.