Spin‐state transitions are an important research topic in complex oxides with the diverse magnetic states involved. In particular, the low‐spin to high‐spin transition in LaCoO3thin films has drawn a wide range of attention due to the emergent ferromagnetic state. Although various mechanisms (e.g., structural distortion, oxygen‐vacancy formation, spin‐state ordering) have been proposed, an understanding of what really underlies the emergent ferromagnetism remains elusive. Here, the ferromagnetism in LaCoO3thin films is systematically modulated by varying the oxygen pressure during thin‐film growth. Although the samples show dramatic different magnetization, their cobalt valence state and perovskite crystalline structure remain almost unchanged, ruling out the scenarios of both oxygen‐vacancy and spin‐ordering. This work provides compelling evidence that the tetragonal distortion due to the tensile strain significantly modifies the orbital occupancy, leading to a low‐spin to high‐spin transition with emergent ferromagnetism, while samples grown at reduced pressure demonstrate a pronounced lattice expansion due to cation‐off‐stoichiometry, which suppresses the tetragonal distortion and the consequent magnetization. This result not only provides important insight for the understanding of exotic ferromagnetism in LaCoO3thin films, but also identifies a promising strategy to design electronic states in complex oxides through cation‐stoichiometry engineering.
BaTiO3is a technologically relevant material in the perovskite oxide class with above‐room‐temperature ferroelectricity and a very large electro‐optical coefficient, making it highly suitable for emerging electronic and photonic devices. An easy, robust, straightforward, and scalable growth method is required to synthesize epitaxial BaTiO3thin films with sufficient control over the film's stoichiometry to achieve reproducible thin film properties. Here the growth of BaTiO3thin films by hybrid molecular beam epitaxy is reported. A self‐regulated growth window is identified using complementary information obtained from reflection high energy electron diffraction, the intrinsic film lattice parameter, film surface morphology, and scanning transmission electron microscopy. Subsequent optical characterization of the BaTiO3films by spectroscopic ellipsometry revealed refractive index and extinction coefficient values closely resembling those of stoichiometric bulk BaTiO3crystals for films grown inside the growth window. Even in the absence of a lattice parameter change of BaTiO3thin films, degradation of optical properties is observed, accompanied by the appearance of a wide optical absorption peak in the IR spectrum, attributed to optical transitions involving defect states present. Therefore, the optical properties of BaTiO3can be utilized as a much finer and more straightforward probe to determine the stoichiometry level present in BaTiO3films.
more » « less- Award ID(s):
- 1905861
- NSF-PAR ID:
- 10495024
- Publisher / Repository:
- 10.1002/admi.202300018
- Date Published:
- Journal Name:
- Advanced Materials Interfaces
- Volume:
- 10
- Issue:
- 11
- ISSN:
- 2196-7350
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/admi.202300018
