宮坂 茂樹

Transition metal oxides are a platform for exploring strain-engineered intriguing physical properties and developing spintronic or flexible electronic functionalities owing to the strong coupling of spin, charge, and lattice degrees of freedom. In this study, we exemplify the strain-engineered magnetism of La2/3Sr1/3MnO3 in freestanding and rippled membrane forms without and with process-induced strain, respectively, prepared by the epitaxial lift-off technique. We find that the deposition of the Pt/Ti stressor suppresses the crack formation in the lift-off process and induces a ripple structure in the La2/3Sr1/3MnO3 membrane. Laser micrograph and Raman spectroscopy show a ripple period of about 30 μm and a height of a few μm, where alternating convex and concave structures are subjected to a tensile strain of 0.6% and a compressive strain of 0.5%, respectively. While the freestanding La2/3Sr1/3MnO3 membrane exhibits room-temperature ferromagnetism, the macroscopic magnetic transition temperature (TC) of the rippled membrane is reduced by as much as 27%. Temperature-variable Kerr microscopy observation in the rippled membrane reveals that the spatial variation of TC is ∼4% of the macroscopic TC, which coincides with the local strains at convex and concave structures. The large reduction of macroscopic TC in the rippled membrane may be ascribed to the lattice disorders due to the strain gradient. Our demonstration of tuning ferromagnetism by the ripple structure validates the high potential of the process-induced strain in the epitaxial lift-off technique and paves the way for strain-mediated emerging physical properties in various transition metal oxides.

Transition metal oxides (TMOs) exhibit a broad spectrum of functional electronic, magnetic, and optical properties, making them attractive for various technological applications. The scale and impact of surface defects and inhomogeneity can extend many unit cells below the surface. Overlooking this aspect of TMO surfaces can result in an incorrect interpretation of their physics and inhibit their maturation into device technology. Soft x-ray absorption spectroscopy (XAS) is a common technique for TMO studies, and different XAS acquisition modes can be used to measure different depth regimes in the sample. Here, we demonstrate a substantial disparity between the near-surface region and the “bulk” of the prototypical TMO SrVO3. By driving the system across two scenarios of orbital polarization, we illustrate how a common XAS surface-sensitive acquisition technique fails to detect the intrinsic orbital polarization. By stark contrast, a “bulk”-sensitive technique successfully captures this effect, elucidating the expected orbital occupation inversion. These results not only underscore the impact of the near-surface region on the correct interpretation of TMO fundamental physics, but further highlight the scale of surface inhomogeneity, a critical aspect of nanoscale functional devices.