Interfacial atomic layers for full emergence of interfacial Dzyaloshinskii–Moriya interaction

Interfacial phenomena play decisive roles in modern science and technology as the scale of the material shrinks down to a few atomic layers. Such minute nanostructures require a more comprehensive understanding beyond the conventional concepts of interfaces and interfacial phenomena generated at interfaces. From a series of a few-atomic-layer-thick magnetic films, we experimentally demonstrate that, contrary to the common notion, interfacial phenomena require a finite thickness for their full emergence. The layer-thickness dependences reveal that the interfacial Dzyaloshinskii–Moriya interaction (DMI) begins to appear with increasing thickness, and emerges completely at a thickness of 2–3 atomic layers, at which the magnitude is maximized. This result implies that the DMI is suppressed when the “bulk” layer adjacent to the interface is thinner than the threshold thickness. The existence of the threshold thickness indicates the need to refine conventional perspectives on interfacial phenomena, and imposes the lowest structural bound and optimum thickness to maximize interfacial effects for technological applications. Magnetic layers need to have a minimum thickness to be at their most effective in magnetic memories, scientists from South Korea have shown. The Dzyaloshinskii–Moriya interaction (DMI) is a key mechanism in magnetic films that can lead to magnetic structures useful in novel magnetic memories. DMI is particularly generated at the interface between a magnetic and nonmagnetic material, and it was believed that the effect gets larger as the magnetic material gets thinner. Sug-Bong Choe from the Seoul National University, and co-workers have shown that this is not the case. The team created a series of multilayer samples, each including a film of cobalt of different thickness. Their results demonstrated that, for very thin films, the DMI increases with increasing thickness and achieves a maximum magnitude at a thickness of two to three atomic layers. Since the interfacial effect has been commonly understood to be generated entirely at the boundary between the two atomic layers, the strength of the interfacial effect has been considered independent of the thickness of each layer. In this study, contrary to the common concept, we show that the interfacial phenomenon requires threshold thickness for full emergence and is suppressed when it is too thin, by providing the result of the peculiar ferromagnet layer thickness dependence of the Dzyaloshinskii-Moriya interaction (DMI). Our result refines conventional perspectives on interfacial phenomena and provides the optimum thickness to maximize DMI for technological applications.