Orientation Control and Mosaicity in Heteroepitaxial RuO₂ Thin Films Grown through Reactive Direct Current Sputtering

With the emergence of altermagnetism, a theoretical spin state has been revealed, which could address the long-standing scaling and frequency challenges of modern spintronics in high-performance computing. The experimental realization of high-speed and low-power spintronics has brought about an especially high renewed interest in the preparation and characterization of antiferromagnetic materials. Among the candidates for future altermagnetic applications is RuO2, which has been modeled to boast a large spin–orbit coupling-related band splitting, a high Néel temperature, and a robust inherent sheet conductivity. Investigation of this material for future altermagnetic applications necessarily involves the ready preparation of single-crystal films with controlled orientations, which to date is almost exclusively accomplished through the use of oxide molecular beam epitaxy or pulsed metal organic chemical vapor deposition. Within this report, the crystalline qualities of RuO2 films prepared through high-temperature direct current reactive sputtering are investigated. It is shown that utilization of TiO2 substrates with different orientations [(001), (101), and (110)] provides a means to template the orientation of RuO2 and that the crystalline quality of the deposited film depends strongly on the lattice match between the two materials. Through X-ray diffraction, atomic force microscopy, and electron back scatter diffraction measurements, it is shown that high-quality epitaxial films can be grown on all three substrates, while the (101)-oriented TiO2 presents the best template for the strain-relaxed and low-roughness single-crystal RuO2 unit cell. This fast and accessible process is enabling for the experimental investigation of the altermagnetic properties of this material, which is a requirement ahead of its application in future spintronics devices.