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

Abstract

Progress in nanotechnology requires new approaches to materials synthesis that make it possible to control material functionality down to the smallest scales. An objective of materials research is to achieve enhanced control over the physical properties of materials such as ferromagnets1, ferroelectrics2 and superconductors3. In this context, complex oxides and inorganic perovskites are attractive because slight adjustments of their atomic structures can produce large physical responses and result in multiple functionalities4,5. In addition, these materials often contain ferroelastic domains6. The intrinsic symmetry breaking that takes place at the domain walls can induce properties absent from the domains themselves7, such as magnetic or ferroelectric order and other functionalities, as well as coupling between them. Moreover, large domain wall densities create intense strain gradients, which can also affect the material’s properties8,9. Here we show that, owing to large local stresses, domain walls can promote the formation of unusual phases. In this sense, the domain walls can function as nanoscale chemical reactors. We synthesize a two-dimensional ferromagnetic phase at the domain walls of the orthorhombic perovskite terbium manganite (TbMnO3), which was grown in thin layers under epitaxial strain on strontium titanate (SrTiO3) substrates. This phase is yet to be created by standard chemical routes. The density of the two-dimensional sheets can be tuned by changing the film thickness or the substrate lattice parameter (that is, the epitaxial strain), and the distance between sheets can be made as small as 5 nanometres in ultrathin films10, such that the new phase at domain walls represents up to 25 per cent of the film volume. The general concept of using domain walls of epitaxial oxides to promote the formation of unusual phases may be applicable to other materials systems, thus giving access to new classes of nanoscale materials for applications in nanoelectronics and spintronics.

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Figure 1: Atomic-resolution domain structure of strained TbMnO3.
Figure 2: Structure and chemistry of the domain walls.
Figure 3: Magnetic behaviour of the strained TbMnO3 films.
Figure 4: Crystal structure of the new 2D phase.
Figure 5: Simulated magnetic order of the new 2D phase.

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Acknowledgements

We are grateful to B. Kooi, T. Palstra, J. Fontcuberta, E. Canadell and the members of the Leverhulme Trust network ‘International Network on Nanoscale Ferroelectrics’, in particular J. F. Scott and F. Morrison, for discussions. This work is supported by NanoNextNL, a micro- and nanotechnology consortium of the Government of the Netherlands and 130 partners. It is also part of the research program NWO-Nano and is funded by the Foundation for Fundamental Research on Matter (FOM), which is financially supported by the Netherlands Organization for Scientific Research (NWO). C.M. and E.S. acknowledge the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragon, Universidad de Zaragoza, where the aberration-corrected TEM studies were conducted, and the support of the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2. C.d.G. obtained financial support from the Spanish Administration (project CTQ2011-23140) and the Generalitat de Catalunya (project 2009SGR462). J.I. received financial support from MINECO-Spain (grants nos MAT2010-18113 and CSD2007-00041). D.R. is a fellow of CONICET. S.V., A.M., M.D. and C.S. acknowledge financial support from the German Science Foundation (DFG) via the Cluster of Excellence NIM. We made used of the facilities provided by the CESGA supercomputing centre.

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Authors and Affiliations

Authors

Contributions

C.J.M.D. and B.N. initiated the work. S.F. and C.J.M.D. grew the films and performed the structural and magnetic characterization. D.R. helped with the magnetic analysis. C.M. and E.S. performed the TEM measurements reported here. S.V., A.M., M.D. and C.S. performed preliminary TEM measurements and analysis that led to the discovery of the novel 2D phase. J.I. performed the density functional theory calculations. C.d.G. performed the embedded cluster calculations. M.M. simulated the magnetic structure. B.N., C.M., S.F., J.I. and M.M. wrote the paper. B.N. coordinated the activities. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to C. Magén or B. Noheda.

Extended data figures and tables

Extended Data Figure 1 AFM and XRD characterization of the films.

a, AFM images of the topography of TbMnO3 films with three different thicknesses, showing flat surfaces and the presence of the substrate steps up to a thickness of 85 nm (film thickness is denoted in the left lower corner). b, XRD reciprocal-space maps around the substrate (103) reflection for four different thicknesses. The structural characterization of the films confirms the twin-domain configuration, constant out-of-plane lattice parameter and partial coherence with the substrate of the films used in this study, in agreement with those reported in ref. 17. The axes represent the components of the scattering wavevectors parallel (Kpar) and perpendicular (Kperp) to the film surface in units of 2Ko = 4π/λ, where λ is the X-ray wavelength.

Extended Data Figure 2 Reference EELS at the O K edge.

Electron energy loss spectrum at the O K edge of crushed powder extracted from the TbMnO3 target used to grow the sample studied in this work. This image includes the two Gaussian fits used to estimate the Mn oxidation state.

Extended Data Figure 3 Domain wall EELS at the O K edge.

Local determination of the Mn oxidation state from a spectrum image obtained around a TbMnO3 domain wall following the analysis of the O K edge previously described. a, Simultaneously acquired HAADF signal. b, Mn oxidation state map. c, O K edges obtained by integrating spectra in the upper domain (orange), in the domain wall (green) and in the lower domain (red).

Extended Data Figure 4 Domain wall EELS at the Mn L2,3 edge.

Map of the fine structure of the Mn L2,3 edge around a TbMnO3 domain wall, obtained from the spectrum image analysed in Extended Data Fig. 3. a, Simultaneously acquired HAADF signal. b, Energy difference between the maxima of the L3 and L2 peaks. c, Mn L2,3 edges obtained by integrating spectra in the upper domain (blue), in the domain wall (red) and in the lower domain (green).

Extended Data Figure 5 Substrate magnetic contribution.

The M–H loop corresponding to the bare SrTiO3 substrate is added to the curves in Fig. 3c, for direct comparison.

Extended Data Figure 6 Out-of-plane magnetization.

Out-of-plane magnetic M–H curves for the samples shown in Fig. 3b. The contribution of the substrate has been subtracted.

Extended Data Figure 7 Inverse susceptibility.

Inverse magnetic susceptibility of a 55 nm film showing deviation downwards from the Curie–Weiss law, which is indicative of the onset of a net magnetic moment below 45–50 K.

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Farokhipoor, S., Magén, C., Venkatesan, S. et al. Artificial chemical and magnetic structure at the domain walls of an epitaxial oxide. Nature 515, 379–383 (2014). https://doi.org/10.1038/nature13918

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