Magnetic Weyl semimetals have novel transport phenomena related to pairs of Weyl nodes in the band structure. Although the existence of Weyl fermions is expected in various oxides, the evidence of Weyl fermions in oxide materials remains elusive. Here we show direct quantum transport evidence of Weyl fermions in an epitaxial 4d ferromagnetic oxide SrRuO3. We employ machine-learning-assisted molecular beam epitaxy to synthesize SrRuO3 films whose quality is sufficiently high to probe their intrinsic transport properties. Experimental observation of the five transport signatures of Weyl fermions—the linear positive magnetoresistance, chiral-anomaly-induced negative magnetoresistance, π phase shift in a quantum oscillation, light cyclotron mass, and high quantum mobility of about 10,000 cm2V−1s−1—combined with first-principles electronic structure calculations establishes SrRuO3 as a magnetic Weyl semimetal. We also clarify the disorder dependence of the transport of the Weyl fermions, which gives a clear guideline for accessing the topologically nontrivial transport phenomena.

Materials informatics exploiting machine learning techniques, e.g., Bayesian optimization (BO), has the potential to offer high-throughput optimization of thin-film growth conditions through incremental updates of machine learning models in accordance with newly measured data. Here, we demonstrated BO-based molecular beam epitaxy (MBE) of SrRuO3, one of the most-intensively studied materials in the research field of oxide electronics, mainly owing to its unique nature as a ferromagnetic metal. To simplify the intricate search space of entangled growth conditions, we ran the BO for a single condition while keeping the other conditions fixed. As a result, high-crystallinequality SrRuO3 film exhibiting a high residual resistivity ratio (RRR) of over 50 as well as strong perpendicular magnetic anisotropy was developed in only 24 MBE growth runs in which the Ru flux rate, growth temperature, and O3-nozzle-to-substrate distance were optimized. Our BO-based search method provides an efficient experimental design that is not as dependent on the experience and skills of individual researchers, and it reduces experimental time and cost, which will accelerate materials research. The itinerant ferromagnetic perovskite SrRuO3 is one of the most promising materials for oxide electronics. 1 -10 Owing to its compatibility with other perovskitestructured oxides, as well as its high conductivity and chemical stability, 3 SrRuO3 is widely used as an epitaxial conducting layer in oxide heterostructures. However, a thorough understanding of its transport properties, electronic structure, and origin of its ferromagnetism remains elusive despite tremendous efforts for over five decades. While high-quality specimens are indispensable for exploring electronic states, it is difficult to make high-quality bulk single crystals of SrRuO3, and hence, thin film specimens have been making a significant contribution to such research. The residual resistivity ratio (RRR), which is defined as the ratio of resistivity at 300 K [ρ(300 K)] to that at 4 K [ρ(4 K)], is a good measure of the purity of a metallic system, and accordingly, the quality of single-crystalline SrRuO3 thin films: RRR is very sensitive to defects and offstoichiometry. 8,11,12 More specifically, only SrRuO3 films with high RRR values above 40 and 60 have enabled observation of sharp dispersive quasiparticle peaks near the Fermi level by angle-resolved photoemission spectroscopy (ARPES) and of quantum oscillations in the electrical resistivity, respectively. 13,14 Such high-quality films maintain their metallic and ferromagnetic character even when the thickness is reduced to a monolayer, 15 providing a rare example of two-dimensional ferromagnetism. This means that extremely thin SrRuO3 films can serve as a two-dimensional spin-polarized electron system, as the existence of a spin-polarized electron current has been established in thicker SrRuO3-based magnetic tunnel junctions. 16,17 Accordingly, high-quality SrRuO3 thin films are also promising for future spintronics application...

Magnetic insulators have wide-ranging applications, including microwave devices, permanent magnets and future spintronic devices. However, the record Curie temperature (TC), which determines the temperature range in which any ferri/ferromagnetic system remains stable, has stood still for over eight decades. Here we report that a highly B-site ordered cubic double-perovskite insulator, Sr3OsO6, has the highest TC (of ~1060 K) among all insulators and oxides; also, this is the highest magnetic ordering temperature in any compound without 3d transition elements. The cubic B-site ordering is confirmed by atomic-resolution scanning transmission electron microscopy. The electronic structure calculations elucidate a ferromagnetic insulating state with Jeff = 3/2 driven by the large spin-orbit coupling of Os6+ 5d2 orbitals. Moreover, the Sr3OsO6 films are epitaxially grown on SrTiO3 substrates, suggesting that they are compatible with device fabrication processes and thus promising for spintronic applications.

We investigate the local electronic structure and magnetic properties of the group-IV-based ferromagnetic semiconductor, Ge1−xFex (GeFe), using soft X-ray magnetic circular dichroism. Our results show that the doped Fe 3d electrons are strongly hybridized with the Ge 4p states, and have a large orbital magnetic moment relative to the spin magnetic moment; i.e., morb/mspin ≈ 0.1. We find that nanoscale local ferromagnetic regions, which are formed through ferromagnetic exchange interactions in the high-Fe-content regions of the GeFe films, exist even at room temperature, well above the Curie temperature of 20–100 K. We observe the intriguing nanoscale expansion of the local ferromagnetic regions with decreasing temperature, followed by a transition of the entire film into a ferromagnetic state at the Curie temperature.

We report the annealing-induced enhancement of ferromagnetism and nano-particle formation in group-IV-based ferromagnetic-semiconductor GeFe. We successfully increase the Curie temperature of the Ge 0.895 Fe 0.105 film up to ~220 K while keeping a single ferromagnetic phase when the annealing temperature is lower than 500°C. In contrast, when annealed at 600°C, single-crystal GeFe nano-particles with stacking faults and twins, which have a high Curie temperature nearly up to room temperature, are formed in the film. Our results show that annealing is quite effective to improve the magnetic properties of GeFe for high-temperature-operating spin-injection devices based on Si or Ge.