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SCIENCE CHINA Materials, Volume 62, Issue 4: 577-585(2019) https://doi.org/10.1007/s40843-018-9344-5

Hidden metal-insulator transition in manganites synthesized via a controllable oxidation

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  • ReceivedJul 16, 2018
  • AcceptedAug 23, 2018
  • PublishedSep 10, 2018

Abstract

Oxygen usually plays crucial roles in tuning the phase structures and functionalities of complex oxides such as high temperature superconductivity, colossal magnetoresistance, catalysis, etc. Effective and considerable control of the oxygen content in those functional oxides could be highly desired. Here, using perovskite manganite (La0.5Sr0.5)MnO3 as a paradigm, we develop a new pathway to synthesize the epitaxial thin films assisted by an in-situ chemical process, where the oxygen content can be precisely controlled by varying oxidative activity tuned by the atmospheric temperature (Tatm) during the growth. A hidden metal-insulator transition (MIT) emerges due to the phase competition, which is never shown in the phase diagram of this classic manganite. The oxygen-mediated interaction between Mn ions together with the change of carrier density might be responsible for this emerging phase, which is compatible with the results of first-principle calculations. This work demonstrates that, apart from traditional cation doping, a precise modulation of anion (O2−, S2−, etc.) may provide a new strategy to control phase structures and functionalities of epitaxial compound thin films.


Funded by

the National Key Research and Development Program of China(2016YFA0302300)

National Natural Science Foundation of China(51332001)

the Fundamental Research Funds for the CENTRAL Universities(2017EYT26)


Acknowledgment

This work was financially supported by the National Key Research and Development Program of China (2016YFA0302300). Zhang J also acknowledges the support from the National Natural Science Foundation of China (51332001) and the Fundamental Research Funds for the Central Universities (2017EYT26).


Interest statement

The authors declare no conflict of interest.


Contributions statement

Zhang J and Song C conceived the experiments and prepared the manuscript. Song C prepared the samples and performed the electronic transport, magnetic, XPS measurement. Malik IA performed the MFM measurement. Duan W and Li M performed the DFT calculation. Gu L and Zhang Q designed and performed the (S)TEM measurements. Wang L, Wang J, Chen R, Zheng R, Dong S and Nan CW were involved in the revision of the manuscript. All authors were involved in the analysis of the experimental and theoretical results.


Author information

Chuangye Song obtained his Bachelor’s degree in physics from Zhengzhou University in 2014. After that, he continued his education as a PhD candidate under the supervision of Prof. Jinxing Zhang at the Department of Physics, Beijing Normal University. His research interest now is the epitaxial growth of correlated-electron oxide film and the surface novel electronic phases at atomic scale by scanning tunneling microscopy in extreme conditions.


Jinxing Zhang obtained his PhD from The Hong Kong Polytechnic University in 2009 under the supervision of Prof. Helen Chan. After that he continued his research work at the Department of Physics of University of California, Berkeley as a post-doc scholar at Professor R. Ramesh’s group. In 2012, he joined the Department of Physics, Beijing Normal University as a professor. The central goal of Zhang’s group is the pursuit of the emerging phenomena and exotic physical behaviors behind the coupling and control of multiple order parameters (e.g., lattice, spin, orbital, charges) at a reduced dimension. His team is striving to create a bridge between those fundamentally scientific discoveries in functional nano-systems and future possible applications such as sensing, actuation, data storage, energy conversion, quantum manipulation, etc.


Supplement

Supplementary information

Experimental details and supporting data are available in the online version of the paper.


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  • Figure 1

    High-quality crystallographic structure of the epitaxial LSMO films by laser-ablated CVD. (a) Schematic of our home-designed laser-ablated CVD. (b) X-ray diffraction pattern of 120 nm LSMO thin film grown on LAO substrate, indicating a typical (001)-oriented epitaxial oxide thin film. (c) HAADF micrograph of the LSMO film on LAO substrate taken by an aberration-corrected STEM. (d) Zoom-in image of the rectangular solid area in (c). The yellow-dashed line indicates the LSMO/LAO interface location. (e) Intensity line profile of the region marked by the purple arrows in (d).

  • Figure 2

    Tatm-dependent lattice and valence structure. (a) An expanded view of the (002) peak of LSMO films acquired at Tatm = 1,173, 1,073 and 973 K. (b) Mn 3s XPS spectra of the LSMO films. (c) The XPS peaks splitting analysis of Mn 2p of the three LSMO films after subtraction of the background. (d–f) Schematics of the nonstoichiometric oxygen content (δ) distribution in LSMO films.

  • Figure 3

    Hidden MIT and corresponding CMR. (a) Temperature-dependent resistance of films grown at varying Tatm(973, 1,073 and 1,173 K). (b) Temperature-dependent magnetization measured during warming under 1,000 Oe after zero field cooling. Inset in (b) shows magnetic-field-dependent magnetization at 4 K after zero field cooling. (c) Temperature-dependent magnetoresistance (MR) of LSMO/LAO film grown at Tatm = 1,073 K by laser-ablated CVD system.

  • Figure 4

    Direct visualization of phase separation using MFM at 4 K. (a) Magnetization as a function of magnetic field along in plane and out of plane of film grown at Tatm = 1,073 K. Inset in (a) is the temperature-dependent magnetization measured during warming under 1,000 Oe after zero field cooling. (b–e) Magnetic-field-dependent MFM for direct visualization of FM and AFM regions. External magnetic field is applied perpendicular to the film surface in the same direction of initial magnetization of tip.

  • Figure 5

    Temperature-δ phase diagram of LSMO3+δ and mechanism of phase transition. (a) Temperature-oxygen stoichiometry (δ) phase diagram in LSMO films. The electronic states (metal: M, insulating: I) are indicated as well as magnetic structures [paramagnetic: PM (dark yellow), ferromagnetic: FM (yellow), antiferromagnetic: AFM (orange)], red rectangular area indicates the hidden metal-insulator transition. (b) The perovskite structure of LSMO where the red dotted circle denotes the position of an oxygen vacancy. (c) Schematic illumination for the physical mechanism, showing that oxygen in the LSMO films has a significant influence on the Mn–O–Mn orbital hybridization through double exchange and superexchange.

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