• Open Access

Robust Ferromagnetism in Highly Strained SrCoO3 Thin Films

Yujia Wang, Qing He, Wenmei Ming, Mao-Hua Du, Nianpeng Lu, Clodomiro Cafolla, Jun Fujioka, Qinghua Zhang, Ding Zhang, Shengchun Shen, Yingjie Lyu, Alpha T. N’Diaye, Elke Arenholz, Lin Gu, Cewen Nan, Yoshinori Tokura, Satoshi Okamoto, and Pu Yu
Phys. Rev. X 10, 021030 – Published 7 May 2020
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Abstract

Epitaxial strain provides important pathways to control the magnetic and electronic states in transition-metal oxides. However, the large strain is usually accompanied by a strong reduction of the oxygen-vacancy formation energy, which hinders the direct manipulation of their intrinsic properties. Here, using a postdeposition ozone annealing method, we obtain a series of oxygen stoichiometric SrCoO3 thin films with the tensile strain up to 3.0%. We observe a robust ferromagnetic ground state in all strained thin films, while interestingly the tensile strain triggers a distinct metal-to-insulator transition along with the increase of the tensile strain. The persistent ferromagnetic state across the electrical transition therefore suggests that the magnetic state is directly correlated with the localized electrons, rather than the itinerant ones, which then calls for further investigation of the intrinsic mechanism of this magnetic compound beyond the double-exchange mechanism.

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  • Received 24 September 2019
  • Revised 17 January 2020
  • Accepted 27 February 2020

DOI:https://doi.org/10.1103/PhysRevX.10.021030

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yujia Wang1,2, Qing He3,*, Wenmei Ming4, Mao-Hua Du4, Nianpeng Lu1,5, Clodomiro Cafolla3, Jun Fujioka6, Qinghua Zhang5,7, Ding Zhang1,2, Shengchun Shen1,2, Yingjie Lyu1,2, Alpha T. N’Diaye8, Elke Arenholz8, Lin Gu5, Cewen Nan7, Yoshinori Tokura9,10, Satoshi Okamoto4,†, and Pu Yu1,2,9,‡

  • 1State Key Laboratory of Low Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing, 100084, China
  • 2Frontier Science Center for Quantum Information, Beijing, 100084, China
  • 3Department of Physics, Durham University, Durham DH1 3LE, United Kingdom
  • 4Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
  • 5Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, China
  • 6Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki, Japan
  • 7State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
  • 8Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
  • 9RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan
  • 10Tokyo College and Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan

  • *qing.he@durham.ac.uk
  • okapon@ornl.gov
  • yupu@mail.tsinghua.edu.cn

Popular Summary

When thin films of transition-metal oxides are fabricated on substrates with mismatched atomic lattices, the film develops a strain, which provides an important pathway to manipulate the film’s magnetic and electronic states. One such film, SrCoO3, has garnered particular research interest: While gently strained samples reliably produce a ferromagnetic metallic state, highly strained samples exhibit an enormous diversity of states. These results have shown that researchers do not understand the intrinsic magnetic and electronic states of SrCoO3 nor the mechanisms at play when it is highly strained—therefore its magnetic and electronic properties remain unclear. To that end, we experiment with samples of SrCoO3 strained to varying degrees and find a robust ferromagnetic ground state, in stark contrast with previous work reporting a transition to an antiferromagnetic state.

The reason that highly strained SrCoO3 samples have exhibited so much diversity thus far is because the tensile strain energetically favors the accumulation of oxygen vacancies, leading to extrinsic magnetic and electronic states. To circumvent this problem, we employ a special method in which strained thin films of SrCoO2.5 are first fabricated on lattice-mismatched substrates and then subjected to an ozone annealing process that triggers a phase change from SrCoO2.5 to SrCoO3. In thin films with tensile strain as high as 3%, we find an unexpected ferromagnetic ground state. Interestingly, the increase in the tensile strain also triggers a distinct metal-to-insulator transition.

The persistent ferromagnetic state across the electrical transition suggests that the magnetic state is directly correlated with the localized electrons, rather than the itinerant ones, which calls for further investigation of the intrinsic mechanism of this magnetic compound. Finally, these results clearly highlight the importance of excellent oxygen stoichiometry for the studies of strained engineered complex oxides.

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Vol. 10, Iss. 2 — April - June 2020

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