Exploring the antiferromagnetic ground states and domain walls of Mn on Ir(111) by SP-STM

Conventional magnetic skyrmions are susceptible to unwanted phenomena such as the skyrmion Hall effect, hindering their applications in spintronic devices. An alternative are antiferromagnetic (AFM) skyrmions which do not show the skyrmion Hall effect.

In the quest to explore systems that can host AFM skyrmions, we have studied the magnetism of Mn on Ir(111) using spin- polarized scanning tunneling microscopy (SP-STM). Having an AFM spin coupling on a hexagonal periodic lattice induces geometric frustration leading to a Néel state. However, also other magnetic states have been observed, such as the row-wise AFM state (1Q) or a superposition of the 1Q states resulting in a 2Q or 3Q state (hexagonal magnetic superstructure), depending on the stacking of Mn as observed for Mn on Re(0001) [1].

In the present work, the magnetic properties of the monolayer, bi-layer, and the tri-layer of Mn on Ir(111) are studied. The first monolayer of Mn exhibits the Néel state as the magnetic ground state. The bi-layer is shown to exhibit a row-wise AFM magnetic ground state. Several different types of structural defects on the bilayer induce a certain type of orientation of the 1Q states thus leading to many domains and domain walls. Within the domain walls, a hexagonal pattern is observed, similar to the superposition state observed previously [2]. We find that the details of such domain walls depend on their direction relative to the adjacent magnetic states. A good understanding of AFM domain walls is desirable with regard to applications for domain wall transport-based memory devices. Apart from the domain walls observed on the bilayer, the third layer exhibits a hexagonal magnetic superstructure, indicative of a most likely 3Q state. Hence, the 2nd – 4th layers of Mn/Ir(111) prove to be a good system to study the role of higher order interactions that give rise to exotic

antiferromagnetic ground states such as the 2Q and 3Q.

1. [1]  J. Spethmann, S. Meyer, K. von Bergmann, R. Wiesendanger, S. Heinze, and A. Kubetzka, Discovery of magnetic single-and triple-q states in Mn/Re (0001), Phys. Rev. Lett. 124, 227203 (2020).

2. [2]  J. Spethmann, M. Grünebohm, R. Wiesendanger, K. von Bergmann, and A. Kubetzka, Discovery and Characterization of a New Type of Domain Wall in a Row-Wise Antiferromagnet, Nature Commun. 12, (2021).

Figure 1: Constant-current STM image, showing the 2nd, 3rd, and 4th Mn layers on Ir(111) exhibiting different magnetic ground states. The 2nd ML shows the different orientations of the 1Q row-wise AFM states separated by a domain wall (DW). The 3rd ML exhibits a clear hexagonal magnetic structure hinting towards a 3Q state.