Material Reveals Intrinsic Magnetism at Low Temperatures

Scientists investigate the intriguing magnetotransport properties of strontium ruthenate (SrRuO3) films, a material possessing a unique triangular lattice structure with potential for advanced spintronic devices. Harunori Shiratani, Yuki K. Wakabayashi, Yoshiharu Krockenberger, and Masaki Kobayashi from Basic Research Laboratories, NTT, Inc., working with Kohei Yamagami from the Japan Synchrotron Radiation Research Institute (JASRI), Takahito Takeda, Shinobu Ohya, and Masaaki Tanaka from the Department of Chemical System Engineering and Department of Electrical Engineering and Information Systems, The University of Tokyo, and Yoshitaka Taniyasu also of Basic Research Laboratories, NTT, Inc., present a comprehensive study of high-quality SrRuO3 thin films grown using molecular beam epitaxy. Their research, achieving a record residual resistivity ratio of 45.5, reveals an intrinsic low-spin state of ruthenium and demonstrates how epitaxial strain can effectively tune the anomalous Hall effect, offering a pathway towards controlling spin-orbit-coupled transport and furthering the development of novel electronic technologies.

Scientists are unlocking new control over electron behaviour in complex materials, edging closer to more efficient electronic devices. High-quality films of strontium ruthenate exhibit unusual magnetic properties, responding predictably to external strain. This discovery offers a pathway to tailor materials for advanced applications without resorting to chemical doping.

Scientists have long sought materials exhibiting both high conductivity and tunable magnetic properties for advanced electronic devices. A new study details the growth and characterisation of high-quality SRO thin films, revealing a residual resistivity ratio of 45.5, the highest yet reported for this crystal orientation.

This achievement unlocks access to the material’s intrinsic electronic and magnetic behaviour, previously obscured by defects and impurities. These films, grown using a machine-learning-assisted molecular beam epitaxy technique, demonstrate Fermi-liquid transport at low temperatures, a hallmark of clean, unhindered electron flow. Detailed analysis of the anomalous Hall effect separates intrinsic and extrinsic contributions, demonstrating that strain can effectively tune the dominant mechanisms. This control over the Hall effect is vital for spintronic applications, where electron spin, rather than charge, carries information.

The study further resolves a long-standing debate regarding the electronic state of ruthenium within SRO, confirming an intrinsically low-spin ground state for both strained and relaxed films. This finding, supported by X-ray magnetic circular dichroism and SQUID magnetometry, provides a solid foundation for understanding and ultimately engineering quantum transport within this complex oxide.

These detailed characterizations not only advance fundamental knowledge of SRO but also pave the way for its integration into novel devices, potentially bridging oxide electronics with the emerging field of topological transport and quantum computing. Fabricating high-quality films of this orientation has proven challenging due to the inherent instability of the surface structure.

Previous attempts yielded residual resistivity ratios far below those achievable with other crystal orientations. To overcome this hurdle, the researchers employed machine-learning to optimise the growth process, carefully controlling parameters like substrate temperature and deposition rates. This innovative approach resulted in films with remarkably smooth surfaces and minimal defects, as confirmed by high-resolution X-ray diffraction and cross-sectional scanning transmission microscopy.

The resulting films exhibited coherent strain for thinner layers (10 and 20 nanometres) and strain relaxation at 60 nanometres, offering a pathway to tailor material properties. At temperatures below approximately 15 Kelvin, the electrical resistivity of the SRO films displayed a characteristic quadratic dependence, confirming Fermi-liquid behaviour.

This indicates that electron-electron interactions are minimal, allowing for a clear observation of intrinsic transport phenomena. Consequently, detailed magnetotransport measurements were performed, revealing the robust, non-saturating linear positive magnetoresistance. Further analysis of the anomalous Hall effect, using temperature scaling, successfully separated the contributions from Karplus-Luttinger and side-jump mechanisms, demonstrating the tunability of these effects through epitaxial strain. The combination of SQUID magnetometry and soft X-ray spectroscopy definitively established the low-spin ground state of ruthenium, resolving ambiguities present in earlier reports.

Molecular beam epitaxy and substrate preparation for strontium ruthenate film growth

Epitaxial strontium ruthenate (SRO) films, possessing a distinctive triangular-lattice geometry, were grown to investigate the interplay between magnetism and electronic transport. Films with thicknesses ranging from 1.2 to 60 nanometers were deposited on strontium titanate (STO) substrates using molecular beam epitaxy (ML-MBE), a technique allowing precise control over film composition and structure.

Prior to growth, STO substrates underwent etching in a hydrochloric-nitric acid solution, a 3:1 volume ratio of 36% HCl and 61% HNO3, followed by a 10-hour anneal at 1000°C in an oxygen atmosphere, ensuring atomically smooth surfaces for subsequent film deposition. Oxidation during SRO growth was achieved using a gas mixture of approximately 15% ozone and 85% oxygen.

A growth rate of 1.1 Å/s was determined from STEM measurements of a 60nm thick film and the deposition time, providing a precise calibration for thickness control. All films were grown under consistent conditions, a substrate temperature of 818°C, strontium flux of 0.98 Å/s, and ruthenium flux of 3.5 Ås-1, but were further refined by Bayesian optimisation, a machine learning approach used to maximise the residual resistivity ratio (RRR).

This optimisation yielded an exceptionally high RRR of 45.5 for the 60nm film, indicating high material quality and minimal scattering. Magnetometry employed a superconducting quantum interference device (SQUID), while Hall effect and magnetotransport measurements were conducted using a physical property measurement system (PPMS). Detailed structural characterisation involved X-ray diffraction (XRD) using a Bruker D8 Discover system with a Cu Kα1 line source and a germanium monochromator. Atomic force microscopy (AFM) with a Bruker Dimension FastScan provided insights into surface morphology, and high-angle annular dark-field (HAADF) and annular bright-field (ABF) STEM imaging, performed with a 20nm film, confirmed single-crystalline epitaxial growth with an abrupt substrate/film interface.

High quality strontium ruthenate films enable investigation of quantum spin effects

Scientists have long sought materials where electron behaviour isn’t dictated by simple charge, but by more subtle quantum properties linked to their spin. Strontium ruthenate, a complex oxide, presents a compelling, if challenging, case. Recent work detailing the growth and characterisation of high-quality thin films of this material represents a genuine step forward, not because of any single spectacular result, but because it establishes a firm foundation for exploring these exotic effects.

Achieving consistently reliable films has been a persistent hurdle; imperfections disrupt the delicate balance needed to observe the physics researchers desire. This study reports a remarkably high level of crystalline quality, evidenced by a substantial residual resistivity ratio, a measure of purity, exceeding previous attempts. The significance extends beyond simply making a cleaner material.

Understanding how electrons move within strontium ruthenate requires disentangling various contributing mechanisms, particularly when a magnetic field is applied. Now, with a well-defined baseline, more complex investigations into manipulating spin-based transport can begin. Limitations remain. While the films demonstrate behaviour consistent with theoretical predictions, fully controlling and exploiting these properties for practical applications is a distant prospect.

The observed effects are subtle, requiring extremely low temperatures and high magnetic fields. Scaling up production and achieving room-temperature operation are significant obstacles. The precise role of strain, intentionally applied to modify the material’s properties, needs further exploration. Future work will likely focus on combining this material with others, creating heterostructures designed to amplify these quantum effects. Ultimately, this research isn’t about a single breakthrough, but about building the necessary toolkit to unlock a new generation of spintronic devices.