Single monolayer ferromagnetic perovskite SrRuO3 with high conductivity and strong ferromagnetism

Achieving robust ferromagnetism and high conductivity in atomically thin oxide materials is critical for advancing spintronic technologies. Here, we report the growth of a highly conductive and ferromagnetic single monolayer SrRuO3 having a high Curie temperature of 154 K on DyScO3 110 substrates. The SrTiO3 capping layer effectively suppresses surface reactions, which typically hinder ferromagnetism in atomically thin films. X ray absorption spectroscopy and X ray magnetic circular dichroism measurements revealed strong orbital hybridization between Ru 4d and O 2p orbitals in the SRO monolayer, which contributes to enhancement of the conductivity and ferromagnetic ordering of both the Ru 4d and O 2p orbitals. The resistivity of the single monolayer SrRuO3 on the better lattice matched DyScO3 substrate is approximately one-third of that of previously reported single monolayer SrRuO3 grown on an SrTiO3 substrate. This study highlights the potential of monolayer SrRuO3 as a platform for two dimensional magnetic oxide systems, offering new opportunities for the eploration of spintronic devices and quantum transport phenomena.

💡 Research Summary

In this work the authors demonstrate that a single‑monolayer (1 ML) film of the itinerant ferromagnet SrRuO₃ (SRO) can retain robust metallic conductivity and a high Curie temperature when grown on a DyScO₃ (110) substrate and capped with a thin SrTiO₃ (STO) layer. The growth was performed by machine‑learning‑assisted molecular‑beam epitaxy (ML‑MBE) in which Bayesian optimization was used to locate the optimal temperature, flux, and stoichiometry parameters. Because the lattice constant of bulk SRO (3.93 Å) matches that of DyScO₃ (3.944 Å) much better than that of SrTiO₃ (3.905 Å), the interface contains far fewer misfit dislocations, leading to a high‑quality epitaxial film with atomically flat step‑and‑terrace morphology (RMS roughness ≈ 0.14 nm).

Electrical transport measurements reveal a resistivity of 253–339 µΩ·cm for the 1 ML SRO, which is roughly one‑third of the lowest values reported for 1 ML SRO grown on STO without a capping layer. A clear kink in the resistivity curve appears at ≈154 K, which the authors identify as the ferromagnetic transition temperature (T_C). This T_C is comparable to that of a 25 ML thick SRO film (≈164 K) and dramatically higher than the ≈25 K reported for uncapped monolayers on STO. Below T_C the magnetoresistance shows anisotropic behavior with a hysteresis loop, confirming ferromagnetic domain formation; even above the hysteresis regime a non‑parabolic MR persists up to ≈150 K, indicating that magnetic correlations survive well into the paramagnetic regime.

X‑ray absorption spectroscopy (XAS) at the Ru M₂,₃ edges and the O K‑edge demonstrates strong hybridization between Ru 4d t₂g and O 2p orbitals. The O K‑edge spectra display a pronounced coherent peak at 529 eV, which the authors associate with long‑lived quasiparticles in the hybridized band and attribute to the high conductivity of the monolayer. When the STO cap is omitted, this coherent feature disappears and the film becomes insulating, underscoring the protective role of the cap in preserving the Ru‑O hybrid network.

X‑ray magnetic circular dichroism (XMCD) measurements provide element‑specific magnetic information. At 14 K and 1.92 T, the total Ru magnetic moment reaches 0.19 µ_B per Ru atom, about 30 % of the moment in bulk‑like 60 nm SRO films. Importantly, a finite XMCD signal is observed even at zero field, corresponding to a spontaneous moment of 0.06 µ_B/Ru, which is a clear signature of intrinsic ferromagnetism in the monolayer. The O K‑edge XMCD shows a sizable orbital moment on the oxygen sites that follows the field dependence of the Ru moment, indicating a strong magnetic coupling mediated by the Ru‑O hybridization.

Overall, the study shows that three synergistic factors—(i) lattice‑matched DyScO₃ substrate, (ii) an STO capping layer that suppresses surface reactions and disorder, and (iii) ML‑MBE growth optimized by machine learning—enable a 1 ML SRO film to simultaneously achieve (a) a high Curie temperature of ~154 K, (b) a resistivity an order of magnitude lower than previously reported monolayers, and (c) a measurable spontaneous magnetic moment. These results open a pathway toward two‑dimensional magnetic oxide platforms for spintronic devices, quantum transport studies (e.g., 2D Weyl semimetal behavior), and scalable heterostructure engineering. Future work may explore electric‑field or strain tuning of T_C, integration with other functional oxides, and the realization of device concepts such as spin‑filter tunnel junctions or topological magnonic circuits based on atomically thin ferromagnetic perovskites.