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Description
Ferroelectric materials possess a spontaneous polarisation that can be electrically switched between different orientations. Compared to ferromagnetics, the domain walls are small, allowing memory cells of higher storage density. Thin films of ferroelectric materials allow to increase the storage density even further. Exemplarily, incipient ferroelectricity is found in strontium titanate SrTiO3 (STO) at a low transition temperature. This ferroelectricity can be stabilized by a variety of external factors such as doping, strain, electric field, isotope substitution etc., even up to room temperature.
Recently, we studied the effect of the electroformation of STO and the accompanied creation of the migration-induced field-stabilized polar (MFP) phase [1] extensively, i.e. with the newly developed approach of Resonantly Suppressed Diffraction (RSD) [2] at Beamline P23 of PETRA III. Because of the breakdown of Friedel's law under resonant conditions, RSD can be used to obtain information about the polarization state of polar materials, namely by monitoring the Bragg intensity while scanning the energy of the incident beam through the absorption edge of strontium.
Here, we transferred this methodology to ferroelectric STO thin films and measured the energy dependent intensities for different Bragg reflections. As underlying structure model, we combined the structure of the low temperature antiferrodistortive (AFD) phase and of a generalized MFP phase. The resulting structure is ferroelectric and rather unknown within the perovskites. The energy dependent intensities of the chosen reflections of simulated data were fitted against the experimental values. Additionally, we performed Density Functional Theory (DFT) calculations to determine the atomic displacements from the lattice parameters at the different low temperatures. The results show that the major structural components within those STO thin films are the AFD displacements, while the MFP displacements are negligible.
[1] J. Hanzig, M. Zschornak, F. Hanzig, E. Mehner, H. Stöcker, B. Abendroth, C. Röder, A. Talkenberger, G. Schreiber, D. Rafaja, S. Gemming, D. C. Meyer: Migration-induced field-stabilized polar phase in strontium titanate single crystals at room temperature, Physical Review B 88, 024104 (2013).
[2] C. Richter, M. Zschornak, D. Novikov, E. Mehner, M. Nentwich, J. Hanzig, S. Gorfman, D. C. Meyer: Picometer polar displacements in strontium titanate determined by a new approach of resonant x-ray diffraction, Nature Communications 9, 178 (2018).
