PROBING STRUCTURAL DEFECTS IN BIFEO3 THIN FILMS AT ATOMIC RESOLUTION

Marta D. Rossell

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Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstr. 129, 8600 Dübendorf, Switzerland

Multiferroic materials that exhibit simultaneous -and strongly coupled- magnetic and ferroelectric order above room temperature offer exciting potential for room-temperature device integration. In particular, magnetoelectric multiferroic films are ideal candidates for applications in next-generation memory devices, which utilize low consuming electric fields to control magnetic order. However, structural defects can dramatically change the way multiferroic materials behave. Crystal imperfections can, for example, induce local polarization reversal, alter the domain kinetics or even modify the phase transition temperatures. Thus, a better understanding of the structure and properties of structural defects is required to help drive multiferroic-based devices toward technological application. To this end, we use advanced analytical imaging methods based on (scanning) transmission electron microscopy (S/TEM) in combination with energy dispersive x-ray (EDX) spectroscopy and electron energy-loss spectroscopy (EELS).

In this talk, we will provide a review of zero-, one- and two-dimensional lattice defects typically found in one of the most widely studied multiferroic systems, BiFeO3 thin films. We will first show the formation of a dopant-controlled polar pattern in BiFeO3 leading to a spontaneous instauration of periodic polarization waves exhibiting giant polarization gradients as large as 70 μC cm−2 across 30 Å thick domains [1]. Further, the influence of edge misfit dislocations on the local electronic properties of BiFeO3 thin films will also be discussed. We will show that small amounts of Fe atoms are present at the BiFeO3 dislocations cores, which result in uncompensated Fe spins along the dislocations giving rise to a magnetic signal [2]. Lastly, we will demonstrate that charged domain walls comprise a core region where most of the screening charge is localized. In particular, an accumulation of oxygen vacancies occurs at tail-to-tail charged domain walls as the leading charge screening process and is responsible for their enhanced conductivity [3].

References

[1] M. Campanini et al. Nano Lett. 18, 717-724 (2018).

[2] P. Agrawal et al. Phys. Rev. Materials 3, 034410 (2019).

[3] M. Campanini et al. Nanoscale 12, 9186-9193 (2020).