Atomic-scale determination of spontaneous magnetic ... form for studying spin-dependent transport in
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Atomic-scale determination of spontaneous magnetic reversal in oxide heterostructures M. Saghayezhiana,1, Summayya Kouserb,c,d,1, Zhen Wanga,e, Hangwen Guoa, Rongying Jina, Jiandi Zhanga, Yimei Zhue, Sokrates T. Pantelidesb,c,d, and E. W. Plummera,2
aDepartment of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803; bDepartment of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235; cDepartment of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235; dMaterials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830; and eCondensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973
Contributed by E. W. Plummer, April 4, 2019 (sent for review November 15, 2018; reviewed by Jian Liu, Xiaoqing Pan, and David Vanderbilt)
Interfaces between transition metal oxides are known to exhibit emerging electronic and magnetic properties. Here we report in- triguing magnetic phenomena for La2/3Sr1/3MnO3 films on an SrTiO3 (001) substrate (LSMO/STO), where the interface governs the mac- roscopic properties of the entire monolithic thin film. The interface is characterized on the atomic level utilizing scanning transmission electron microscopy and electron energy loss spectroscopy (STEM- EELS), and density functional theory (DFT) is employed to elucidate the physics. STEM-EELS reveals mixed interfacial stoichiometry, sub- tle lattice distortions, and oxidation-state changes. Magnetic mea- surements combined with DFT calculations demonstrate that a unique form of antiferromagnetic exchange coupling appears at the interface, generating a novel exchange spring-type interaction that results in a remarkable spontaneous magnetic reversal of the entire ferromagnetic film, and an inverted magnetic hysteresis, per- sisting above room temperature. Formal oxidation states derived from electron spectroscopy data expose the fact that interfacial ox- idation states are not consistent with nominal charge counting. The present work demonstrates the necessity of atomically resolved elec- tron microscopy and spectroscopy for interface studies. Theory dem- onstrates that interfacial nonstoichiometry is an essential ingredient, responsible for the observed physical properties. The DFT-calculated electrostatic potential is flat in both the LSMO and STO sides (no internal electric field) for both Sr-rich and stoichiometric interfaces, while the DFT-calculated charge density reveals no charge transfer/ accumulation at the interface, indicating that oxidation-state changes do not necessarily reflect charge transfer and that the concept of polar mismatch is not applicable in metal−insulator polar−nonpolar interfaces.
magnetism | thin films | electron microscopy | oxide interfaces | density functional theory
Transition metal oxides (TMOs) exhibit a wide range ofelectrical, magnetic, and optical properties largely because they can be easily alloyed and the 10 slots in the transition metal atom d orbitals allow for very diverse spin arrangements. Adding strong electron−lattice coupling and easily formed oxygen vacancies to this mix, the result is a large number of coupled degrees of freedom. Therefore, it is not surprising that TMO heterostructures exhibit highly unusual behavior, induced by in- terfaces between different oxides where symmetry discontinuities occur, leading to properties that are absent in bulk (1–5). A well- known example is the LaAlO3/SrTiO3 (001) heterostructure, where a 2D electron gas with high mobility forms at the interface between two insulating oxides (6), including the appearance of superconductivity (7) and background ferromagnetic (FM) or- dering (8). In many cases, the symmetry discontinuity at the in- terface strongly modifies the transition metal−oxygen octahedra network, inducing changes in local structure (bond geometry) and local stoichiometry. These changes have been shown to re- sult in exotic magnetic properties such as antiferromagnets (AFM) built from FM layers (9), interface-driven magnetic phases absent
in bulk (5), antiparallel spin alignment at manganite−ruthenate interfaces (10), and fine control over magnetic anisotropy by in- terfacing manganites and iridates (11). Advances in epitaxial syn- thesis provide a fine control over interfaces that is necessary to realize novel magnetic systems hitherto unseen in TMOs, such as spontaneous magnetic reversal (SMR) and exchange spring (12–14). La2/3Sr1/3MnO3 is a material with interesting magnetic prop-
erties, such as high Curie temperature (15) and nearly perfect spin polarization (16), used for the realization of magnetic tunnel junctions (17), magnetoelectric devices (18), and spin injection into cuprate superconductors (19) as well as a plat- form for studying spin-dependent transport in organic materials (20). In this material, like other oxides, the interface-induced magnetic properties are usually discussed in terms of charge transfer (21–24), while the atomic-scale role of interface struc- ture and intermixture are not explicitly investigated (22, 25). The La2/3Sr1/3MnO3/SrTiO3 (001) (LSMO/STO) interface is composed of a stacking sequence SrO/TiO2−La2/3Sr1/3/MnO2/La2/3Sr1/3 and so on (26). By viewing formal oxidation states as physical charges on the atomic layers, it has been concluded that interface inter- mixing in LSMO/STO is driven by polar mismatch, thus degrading magnetic and electrical properties near the interface (27–30). However, the Thomas−Fermi screening length in LSMO is just
Transition metal oxide interfaces have shown extraordinary promise in the quest to design materials with custom elec- tronic, magnetic, and optical properties. In rare cases, inter- faces exhibit electronic and magnetic properties that are radically different from those of the components. An example is the emergent two-dimensional electron gas between two insulators. In this work, we show that the interface of a non- magnetic oxide substrate and a ferromagnetic metallic thin film possesses a local antiferromagnetic coupling which con- trols the reversal of the entire film’s ferromagnetic ordering. Electron microscopy and quantum calculations elucidate the atomic-scale origin of the observed phenomena, and demon- strate that local nonstoichiometry and structure are the key factors in interfacial phenomena.
Author contributions: M.S., H.G., J.Z., and E.W.P. designed research; M.S., S.K., Z.W., H.G., R.J., J.Z., Y.Z., and S.T.P. performed research; M.S., S.K., Z.W., H.G., R.J., J.Z., Y.Z., S.T.P., and E.W.P. analyzed data; and M.S., S.K., Z.W., H.G., R.J., J.Z., Y.Z., S.T.P., and E.W.P. wrote the paper.
Reviewers: J.L., University of Tennessee; X.P., University of California, Irvine; and D.V., Rutgers, The State University of New Jersey.
The authors declare no conflict of interest.
Published under the PNAS license. 1M.S. and S.K. contributed equally to this work. 2To whom correspondence should be addressed. Email: email@example.com.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 1073/pnas.1819570116/-/DCSupplemental.
Published online May 8, 2019.
www.pnas.org/cgi/doi/10.1073/pnas.1819570116 PNAS | May 21, 2019 | vol. 116 | no. 21 | 10309–10316
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0.31 nm (smaller than a unit cell) (31). The short screening length is not consistent with a buildup of an electrostatic potential and thus disagrees with the notion of electrostatic potential divergence (28). Contradictory past studies question the validity of divergence of energy at polar-mismatched metal−insulator interfaces (27). On the other hand, lack of atomic-scale studies limits the trans- ferability of experimental findings and does not provide useful input for or direct comparison with first-principles calculations. For example, Lee et al. (32) and Chen et al. (33) reported “hidden magnetic properties” in LSMO/STO heterostructures. The former work proposes mixed Mn3+/Mn4+ oxidation state, and possible electronic reconstruction at the interface as the reason behind the phenomena, while the latter relates unexpected magnetic proper- ties to the presence of an interfacial AFM-coupled pinned magnetic layer. In the absence of atomically resolved structure, stoichiometry, and oxidation state information, the data could only be discussed in terms of phenomenological models and cannot be evaluated the- oretically by density functional theory (DFT) calculations. In this paper, we report the intricate interplay between mag-
netic properties, structure, and chemical composition at the in- terface of LSMO/STO (001) heterostructures, revealing fascinating interface-induced magnetic behavior such as SMR and inverted hysteresis (IH), that are persistent above room temperature (TC = 337 K). Such behavior resembles exchange spring interactions which, in the past, have never been observed in monolithic films (2, 14). Atomically resolved electron mi- croscopy and spectroscopy reveal two Sr-rich interface atomic layers in LSMO, and significant changes in the interfacial structure (octahedral tilt and elongation of lattice constant) and in formal oxidation state at the interface. The formal oxidation