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Spin friction of single atoms investigated by scanning tunneling microscopy

Subject Area Experimental Condensed Matter Physics
Term from 2018 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 408119516
 
Final Report Year 2023

Final Report Abstract

Atom manipulation imaging is a variant of scanning tunneling microscopy (STM), where an adatom on the surface follows the tunneling tip while the tip scans an atomically flat surface area. The manipulated adatom can be seen as an extension of the tip probing forces right at the surface. When tip, adatom and surface are magnetic, the acquired image contains both atomic-scale structural and magnetic information. Within the project this technique was used as a sensitive tool to identify new atomic-scale spin textures. Further objectives were to identify the main (magnetic) tip-adatom and adatom-surface interactions and to employ adatom manipulation as a tool to write (alter) atomic-scale spin textures locally. In Manganese (Mn) monolayers on Re(0001) we found three new magnetic states, which are degenerate with respect to the Heisenberg exchange interaction: a row-wise antiferromagnetic state (1Q) in fcc Mn, a hexagonal so-called 3Q state, with 109.47° between neighboring spins in hcp Mn, and a transient 2Q state within domain walls in fcc Mn. Density functional theory shows that the degeneracy of these states is lifted by 4thorder exchange terms, favoring either the 1Q or the 3Q state, whereas the 2Q state cannot be a ground state based on 4th-order exchange. The 2Q domain walls in fcc Mn/Re(0001) are energetically saddle points and establish a new type of antiferromagnetic domain wall which has no analog in ferromagnets. Surprisingly, the widths of these walls depend on the 4th-order exchange term. This type of novel domain wall can be expected for any system for which the symmetry of the magnetic texture is lower than the symmetry of the atomic lattice. In Fe layers on Rh/Ir(111) a double-row-wise antiferromagnetic state and a hexagonal mosaic state were found. Surprisingly, the mosaic state is collinear and is stabilized by the three-site four-spin interaction, which is a 4th-order exchange term. The strength of magnetic atom manipulation imaging lies in the unavoidable atomic resolution combined with a strong magnetic signal (often an order of magnitude stronger than in spinpolarized STM), which for instance allows to "see" directly whether a spin texture is commensurate with the atomic lattice or not. The goal to disentangle different forces acting on the manipulated adatom and to draw conclusions for free adatom diffusion on magnetic surfaces turned out to be too ambitious within the time frame of the project. The manipulation of spin textures by adatoms, on the other hand, was successful in two cases: the position of 2Q domain walls in fcc Mn/Re(0001) was altered by positioning Co adatoms in close proximity, exploiting an attractive interaction, and the antiferromagnetic double-rows in Fe/Rh/Ir(111) showed a tendency to align along the Co adatom manipulation path, allowing to reconfigure the domain pattern. All these results provide valuable information for the growing field of antiferromagnetic spintronics, which aims to exploit the fast dynamics of antiferromagnets for device applications.

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