First-principles investigation of magnetic exchange force microscopy on adatoms adsorbed on an antiferromagnetic surface

Using density functional theory (DFT), we calculate the magnetic short-ranged exchange forces between a magnetic tip and an adatom adsorbed on the antiferromagnetic Mn monolayer on the W(110) surface [Mn/W(110)]. These exchange forces can be measured in magnetic exchange force microscopy allowing atomic-scale imaging of spin structures on insulating and conducting surfaces. We consider two types of 3d transition-metal atoms with intrinsic magnetic moments: Co and Mn and Ir as an example of a 5d transition-metal atom exhibiting an induced magnetic moment on Mn/W(110). The tips are modeled by Fe pyramids and terminated either with an Fe or a Mn apex atom. From our total energy DFT calculations for a parallel and antiparallel alignment between tip and adatom magnetic moments we obtain the exchange energy Eex(d) as a function of tip-adatom distance d. The exchange forces, Fex(d), are calculated based on the Hellmann-Feynman theorem. We show that structural relaxations of tip and sample due to their interaction need to be taken into account. Due to the exchange interaction the relaxations depend on the alignment between tip and adatom magnetization—an effect which will affect the tunneling magnetoresistance that can be measured by a spin-polarized scanning tunneling microscope. A maximum in the exchange energy and force curves is obtained for magnetic adatoms at tip-adatom separations of about 3 to 4 Å. The exchange forces with an Fe terminated tip reach a maximum value of up to 0.2 and 0.6 nN for Co and Mn adatoms, respectively, and prefer an antiferromagnetic coupling. Surprisingly, we also find an exchange force of up to 0.2 nN for Ir adatoms. We analyze the exchange interaction between tip and adatom based on the spin-polarized electronic structure of the coupled system. A competition occurs between long-range Zener-type indirect double exchange favoring ferromagnetic coupling and short-range direct d−d antiferromagnetic exchange. For the Ir adatom the interaction can be explained from the spin-dependent hybridization with the tip apex atom. Our results show that magnetic adatoms on Mn/W(110) are a promising system to study exchange forces at the single-atom level via magnetic exchange force microscopy. Published by the American Physical Society 2025