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 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.
- 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)
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, , 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 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 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 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 are first fabricated on lattice-mismatched substrates and then subjected to an ozone annealing process that triggers a phase change from to . 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.