PARADIM Highlight #87—In-House Research (2023)
D.G. Schlom, L.F. Kourkoutis, and K.M. Shen (Cornell University)
Molecular beam epitaxy is ideal for growing stacks of functional materials to utilize interfaces to create emergent phenomena such as magnetic skyrmions. Strontium ruthenate (SrRuO3) and SrRuO3-based heterostructures have been at the center of a debate on whether a hump-like feature appearing in Hall resistivities is sufficient evidence to prove the presence of skyrmions in a material.
Figure 1: (Top left) structure of the thin film layers of the device. (Top center) Measurement of the Anomalous Hall Effect at 40K showing a hump feature. (Top right) Cover of the journal APL Materials June 2023 issue. (Bottom) Various tests for skyrmions that should be affected by: (1) the current density, but is not seen in transverse resistance, Rxy, at various currents, where all measurements almost perfectly overlap (left); (2) the canting angle, expecting a significant drop off in Rxy , but is not seen for angles up to 50° (center); and (3) minor loops of Rxy, measured with various Bmin, but hysteretic behavior like this is not consistent with the presence of skyrmions (right).
Here, the PARADIM In-House Team synthesized a simple bilayer combining a positive anomalous Hall effect (AHE) layer (Sr0.6Ca0.4RuO3) with a negative AHE layer (SrRuO3). The bilayer shows a hump-like feature in the Hall resistivity that closely resembles the one often attributed to skyrmions. Multiple tests for skyrmions were performed, but no evidence of their existence was found.
In related work, the team demonstrated the highest residual resistance ratio, RRR = ρ [300 K]/ρ [4 K] = 205, for SrRuO3 thin films ever achieved. RRR is a measure of film quality, and our results exceed the best films grown by others using machine learning (RRR = 80) as well as the best single crystals (RRR = 162). This achievement shows the power of a team-based approach to synthesis science.
Molecular-beam epitaxy enables ultrathin functional materials to be combined in heterostructures to create emergent phenomena at the interface. Magnetic skyrmions are an example of an exciting phase found in such heterostructures. SrRuO3 and SrRuO3-based heterostructures have been at the center of the debate on whether a hump-like feature appearing in Hall resistivities is sufficient evidence to prove the presence of skyrmions in a material. To address the ambiguity, the PARADIM In-House Team synthesized a model heterostructure with engineered Berry curvature that combines, in parallel, a positive anomalous Hall effect (AHE) channel (a Sr0.6Ca0.4RuO3 layer) with a negative AHE channel (a SrRuO3 layer). Combining the two opposite AHE channels can artificially reproduces a “hump-like” feature, which closely resembles the hump-like feature typically attributed to the topological Hall effect and the presence of chiral spin textures, such as skyrmions. Measuring the current dependence, angle dependence, and minor loop dependence of Rhump in the heterostructure, the team could not find evidence of skyrmions, despite the clear hump.[1]
Epitaxial untwinned SrRuO3 thin films were grown on (110)-oriented DyScO3 substrates by molecular-beam epitaxy achieving an exceptional sample with a residual resistivity ratio (RRR), ρ [300 K]/ρ [4 K] of 205 and a ferromagnetic Curie temperature, TC, of 168.3 K. We compare the properties of this sample to other SrRuO3 films we have grown on DyScO3(110) with RRRs ranging from 8.8 to 205, and also compare it to the best reported bulk single crystal of SrRuO3 (with RRR=162). We determine that SrRuO3 thin films grown on DyScO3(110) have an enhanced TC as long as the RRR of the thin film is above a minimum electrical quality threshold. This RRR threshold is about 20 for SrRuO3.[2]
Ruthenates are important quantum materials with intrinsic behaviors that include ferromagnetism, metamagnetism, and superconductivity. Nonetheless, as demonstrated in prior PARADIM work, the properties of ruthenates are quite sensitive to disorder including the presence of out-of-phase boundaries, dislocations, and ruthenium vacancies. Such defects can completely suppress superconductivity, suppress conductivity (especially in ultrathin SrRuO3 films), and lead to hump-shaped features in Hall resistivity measurements that others have interpreted as indicative of the presence of skyrmions. Some controversy has emerged in the literature regarding the meaning of such humps and whether seeing a hump is sufficient to conclude the presence of skyrmions. In our past PARADIM work we showed that less defective films of the same type as those reported in the literature to contain humps do not contain humps and that altering our growth conditions to induce ruthenium vacancies or dislocations results in humps. The present work provides more information on this controversy by showing that a bilayer of high structural quality, made under conditions suppressing ruthenium vacancies and other defects, can be engineered to show a hump. Multiple tests for skyrmions performed on the resulting bilayer do not show any evidence of skyrmions. We thus conclude that although skyrmions give rise to a hump feature that the inverse of this statement (hump feature implying skyrmions) does not always hold. Our related work to continue to improve the perfection and properties of ruthenates is relevant to establishing the intrinsic properties of ruthenates and ruthenate-based heterostructures. This improvement comes from our application of thermodynamic understanding to synthesis. By doing so we have broken our own prior records for quality as well as all records in the literature. Recent studies by others using the same growth technique (MBE) aided by machine learning also do not come close (RRR = 80 for artificial intelligence vs. RRR = 205 for human intelligence).
PARADIM’s community of practitioners and in particular the thermodynamics of MBE (TOMBE) diagrams calculated in the collaboration between PARADIM’s in-house team with the group of Prof. Zi-Kui Liu at Penn State was pivotal to the underlying understanding enabling the advances in MBE growth of ruthenates [see prior PARADIM publications, especially H.P. Nair, Y. Liu, J.P. Ruf, N.J. Schreiber, S-L. Shang, D.J. Baek, B.H. Goodge, L.F. Kourkoutis, Z.K. Liu, K.M. Shen, and D.G. Schlom, “Synthesis Science of SrRuO3 and CaRuO3 Epitaxial Films with High Residual Resistivity Ratios,” APL Mater. 6, 046101 (2018)]. Further, PARADIM’s expertise in electron microscopy with state-of-the-art instrumentation was useful for this study.
The work is part of the core research theme of the PARADIM In-House Research Team.
- N.J. Schreiber, L. Miao, H.P. Nair, J.P. Ruf, L. Bhatt, Y.A. Birkholzer, G.N. Kotsonis, L.F. Kourkoutis, K.M. Shen, and D.G. Schlom, "Enhanced TC in SrRuO3/DyScO3(110) Thin Films with High Residual Resistivity Ratio," APL Mater. 11, 111101 (2023).
- N.J. Schreiber, L. Miao, B.H. Goodge, L.F. Kourkoutis, K.M. Shen and D.G. Schlom, "A Model Heterostructure with Engineered Berry Curvature," APL Mater. 11, 061117 (2023).
Data Availability: Access to data associated with the MBE growth of the structures is available through the PARADIM Data Collective at DOI: 10.34863/36tx-c606.
- This work was supported by the National Science Foundation Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM) under Cooperative Agreement No. DMR-2039380. This research was funded in part by the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant Nos. GBMF3850 and GBMF9073 to Cornell University. Sample preparation was, in part, facilitated by the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant No. NNCI-2025233). This work made use of the Cornell Center for Materials Research (CCMR) Shared Facilities, which are supported through the NSF MRSEC Program (Grant No. DMR-1719875). The FEI Titan Themis 300 was acquired through Grant No. NSF-MRI-1429155, with additional support from Cornell University, the Weill Institute, and the Kavli Institute at Cornell. The Thermo Fisher Helios G4 UX FIB was acquired with support by NSF Grant No. DMR-1539918. The authors thank Sean Christopher Palmer for his assistance with substrate preparation.
- This work was supported by the National Science Foundation Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM) under Cooperative Agreement No. DMR-2039380. This research was funded in part by the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant Nos. GBMF3850 and GBMF9073 to Cornell University. Sample preparation was, in part, facilitated by the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant No. NNCI-2025233). This work also made use of the Cornell Energy Systems Institute Shared Facilities, partly sponsored by the NSF (Grant No. MRI DMR-1338010). This work made use of the Cornell Center for Materials Research (CCMR) Shared Facilities, which are supported through the NSF MRSEC Program (Grant No. DMR-1719875). The Thermo Fisher Helios G4 UX FIB was acquired with support from NSF Grant No. DMR-1539918. The authors thank Sean Christopher Palmer for his assistance with substrate preparation.
Recognitions:
- 2nd Prize 2023 APL Materials Excellence in Research Award (https://pubs.aip.org/aip/apm/pages/excellence).
- Selected for the cover of APL Materials: Issue June 2023, https://pubs.aip.org/aip/apm/issue/11/6.
