Double-cell superstructure and vacancy ordering in tensile-strained metallic thin films of $\mathrm{P}{\mathrm{r}}_{0.50}\mathrm{C}{\mathrm{a}}_{0.50}\mathrm{Co}{\mathrm{O}}_{3}$ on $\mathrm{LaAl}{\mathrm{O}}_{3}$

Double-cell superstructure and vacancy ordering in tensile-strained metallic thin films of $P r_{0.50} C a_{0.50} Co O_{3}$ on $LaAl O_{3}$

Jessica Padilla-Pantoja, Xavier Torrelles, Jaume Gazquez, Juan Rubio-Zuazo, Javier Blasco, Javier Herrero-Martín, and Jose Luis García-Muñoz

Phys. Rev. Materials 3, 104407 – Published 10 October 2019

Abstract

The Pr-based cobaltate $P r_{0.5} C a_{0.5} Co O_{3}$ (PCCO) presents in bulk form a singular simultaneous valence and spin-state transition that turns the metallic state into insulator, and displays a large and ultrafast photoresponse in the insulating phase. Epitaxial thin films of PCCO have been grown by deposition on $LaAl O_{3}$ (001) (LAO) substrate, chosen to minimize the mismatch with the film. The films grow epitaxially with two times the substrate periodicity $(2 a_{0} \times 2 a_{0} \times 2 a_{0})$ and the long perovskite axis perpendicular to the surface. We report a reduction of the structural symmetry from Pnma (bulk) to $P 2_{1} 2_{1} 2_{1}$ (film). The $P 2_{1} 2_{1} 2_{1}$ symmetry revealed by synchrotron x ray remains at low temperatures. These PCCO films are metallic, and ferromagnetic below $T_{C} = 170 K$ , confirming the stabilization of excited $C o^{3 +}$ spin states and the suppression of the concurrent Co spin-state, valence, and metal-insulator transitions. $Z$ -contrast imaging and electron-energy-loss spectroscopy elemental maps reveal long-range ordered oxygen vacancy planes unexpectedly stacking parallel to the interface, in spite of the tensile character of the PCCO/LAO heterostructure. In contrast to the general tendency reported for strained $L a_{0.5} S r_{0.5} Co O_{3 - δ}$ (LSCO) films, we show that a nominal tensile strain can be also compatible with the presence of alternating O vacancy planes parallel to the interface, instead of perpendicular to it. That occurs thanks to the double cell of the film and the formation of the (1/2, 1/2, 1/2) superstructure studied in this work. These results expand the possibilities of controlling interfacial physical properties via engineering of ordered oxygen-defect structures in strongly correlated oxides.

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Received 12 March 2019
Revised 22 July 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.104407

Physics Subject Headings (PhySH)

Ferromagnetism

Cobaltates Thin films

Grazing-incidence X-ray diffraction High-resolution electron energy loss spectroscopy High-resolution transmission electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jessica Padilla-Pantoja¹, Xavier Torrelles¹, Jaume Gazquez¹, Juan Rubio-Zuazo^2,3, Javier Blasco⁴, Javier Herrero-Martín⁵, and Jose Luis García-Muñoz ^1,*

¹Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus universitari de Bellaterra, E-08193 Bellaterra, Spain
²SpLine Spanish CRG Beamline at the ESRF Grenoble cedex ESRF, BP 220–38043, France
³Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC, 28049 Madrid, Spain
⁴Instituto de Ciencia de Materiales de Aragón, CSIC-Univ. de Zaragoza, 50009 Zaragoza, Spain
⁵ALBA Synchrotron Light Source, 08290 Cerdanyola del Vallès, Barcelona, Spain