Towards spin-polarized two-dimensional electron gas at a surface of an antiferromagnetic insulating oxide

Rohan Mishra, Young-Min Kim, Qian He, Xing Huang, Seong Keun Kim, Michael A. Susner, Anand Bhattacharya, Dillon D. Fong, Sokrates T. Pantelides, and Albina Y. Borisevich

Phys. Rev. B 94, 045123 – Published 18 July 2016

Abstract

The surfaces of transition-metal oxides with the perovskite structure are fertile grounds for the discovery of novel electronic and magnetic phenomena. In this article, we combine scanning transmission electron microscopy (STEM) with density functional theory (DFT) calculations to obtain the electronic and magnetic properties of the (001) surface of a ${(LaFe O_{3})}_{8} / {(SrFe O_{3})}_{1}$ superlattice film capped with four layers of $LaFe O_{3}$ . Simultaneously acquired STEM images and electron-energy-loss spectra reveal the surface structure and a reduction in the oxidation state of iron from $F e^{3 +}$ in the bulk to $F e^{2 +}$ at the surface, extending over several atomic layers, which signals the presence of oxygen vacancies. The DFT calculations confirm the reduction in terms of oxygen vacancies and further demonstrate the stabilization of an exotic phase in which the surface layer is half metallic and ferromagnetic, while the bulk remains antiferromagnetic and insulating. Based on the calculations, we predict that the surface magnetism and conductivity can be controlled by tuning the partial pressure of oxygen.

Received 11 April 2016
Revised 20 May 2016

DOI:https://doi.org/10.1103/PhysRevB.94.045123

Physics Subject Headings (PhySH)

Ferromagnetism Surface reconstruction

Two-dimensional electron gas

DFT+U Scanning transmission electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Rohan Mishra^1,2,3,*, Young-Min Kim^2,4,5, Qian He², Xing Huang³, Seong Keun Kim^6,7, Michael A. Susner², Anand Bhattacharya^6,8, Dillon D. Fong⁶, Sokrates T. Pantelides^1,2, and Albina Y. Borisevich²

¹Deparment of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
²Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
³Department of Mechanical Engineering and Materials Science, and Institute of Materials Science and Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, USA
⁴Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Republic of Korea
⁵Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea
⁶Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
⁷Center for Electronic Materials, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea
⁸Nano Science and Technology Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

^*Author to whom all correspondence should be addressed: rmishra@wustl.edu

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Vol. 94, Iss. 4 — 15 July 2016

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Images

Figure 1
Thin film structure, surface termination, and octahedral tilt profile. (a) HAADF STEM image of ${[110]}_{pc}$ -oriented ${(LaFe O_{3})}_{8} / (SrFe O_{3})$ superlattice thin film grown on $SrTi O_{3}$ substrate. The inset shows the atomic structure of the surface the film. (b) Set of HAADF and ABF STEM images showing atomic column positions of the thin film up to the outermost surface layer (the arrow points toward the surface) and the profile of alternating oxygen octahedral tilts measured from the ABF STEM image.
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Figure 2
Surface reduction from STEM EELS. (a) HAADF image of the ${[100]}_{pc}$ -oriented superlattice structure selected for core-loss EELS with the line showing the region from where the spectra was obtained. (b) The EEL spectra of Fe $L$ edge for the top five unit cells from the surface. (c) Change in the ratio of the $L_{3}$ to $L_{2}$ peak intensity of Fe ( $L_{3} / L_{2}$ ratio) along the EELS line scan. The horizontal regions shaded in green and pink show the $L_{3} / L_{2}$ ratio of standard specimens having $F e^{3 +}$ and $F e^{2 +}$ , respectively. The vertical regions shaded in gray highlight the $SrFe O_{3}$ layers in the superlattice. The black arrows point toward the surface.
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Figure 3
The EELS analysis of O $K$ edge. Comparison of O $K$ edges obtained at the $Fe O_{2}$ atomic layers from the inside of the film (numbered 1) to the outmost surface (numbered 5) from an EELS line scan. The inset shows the decrease in separation between Peak B and A $(Δ E)$ on moving toward the surface.
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Figure 4
Electronic structure of $LaFe O_{3}$ slabs with insulating surface. (a) Structure of LaO-terminated $LaFe O_{3}$ slab showing the top three layers from the surface along with Fe- $3 d$ DOS for the corresponding layers. (b) Spin isosurface showing charge ordering of Fe atoms in the subsurface Fe-O layer of the slab shown in (a). (c) Structure of the $LaFe O_{3}$ slab with an oxygen vacancy concentration $n_{V} = 0.25$ in the subsurface Fe-O layer along with Fe- $3 d$ DOS for the corresponding layers. (d) Spin isosurface of the subsurface Fe-O layer with $n_{V} = 0.25$ .
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Figure 5
Half-metallic ferromagnetism at the surface. (a) Magnetic configuration of the $LaFe O_{3}$ slab with a vacancy concentration $n_{V} = 1$ in the subsurface Fe-O layer. (b) Fe $3 d$ DOS of the top three unit cells from the surface with the stable surface ferromagnetic configuration. (c) Difference in energy between the ferromagnetic and antiferromagnetic coupling of the surface Fe spins as a function of vacancy concentration in the subsurface Fe-O layer. A negative energy corresponds to stable ferromagnetic configuration. (d) Spin isosurface of the subsurface Fe-O layer with $n_{V} = 1$ and with ferromagnetic ordering of surface Fe spins.
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