We achieved an improvement in the TMR ratio at RT by precisely controlling the interface between the magnetic layer and barrier of a tunnel magnetoresistance (TMR) device. Specifically, as shown in the figure on the left, we fabricated thin-film devices using epitaxial growth techniques to enhance their quality. By improving the crystallinity of the interfaces at the atomic scale, such as by inserting a ultra-thin magnesium layer at the lower barrier interface and additional oxidation process at the upper barrier interface, we succeeded in realizing a single-crystal device with sharp interfaces on both the upper and lower sides. As a result, we achieved a TMR ratio of up to 631% at RT, setting a new world record for the first time in 15 years (see figure on the right). Furthermore, we clearly observed a phenomenon in which the TMR ratio oscillate periodically as a function of the barrier thickness (TMR oscillation), and found that the oscillation amplitude is as large as 141% (previously, a few percent to several tens of percent). By investigating the mechanism of this oscillation phenomenon and clarifying its relationship with the TMR ratio, we expect to achieve even higher TMR ratios at RT. The demonstration of this high RT TMR ratio is expected to lead not only to applications in high-sensitivity magnetic sensors, but also to its use in new memory technologies that would have been difficult to achieve with conventional TMR ratio levels.