Ab-initio simulation of magnetic exchange force mycroscopy of the antiferromagnetic Fe monolayer on W(001)

Magnetic exchange force microscopy (MExFM) is a promising new technique to perform magnetic imaging with atomic resolution by measuring the magnetic exchange force between a magnetically coated tip and a magnetic sample [1]. Here, we apply density functional theory using the full-potential linearized augmented plane wave (FP- LAPW) method to investigate the exchange forces on the antiferromagnetic monolayer Fe on W(001) [2]. We use an Fe cluster as a tip model and include relaxations of the cluster and the surface. Surprisingly, relaxation effects of tip and sample depend sensitively on the local magnetic configuration. Therefore, relaxations play a crucial role for the magnetic signal. In particular, the onset of the exchange forces is shifted to larger distances, which facilitates their experimental observation. Based on the calculated force-distance curves we simulate MExFM images which display a competition of chemical and magnetic forces. Our simulations can explain the experimentally observed magnetic contrast [3]. [1] U. Kaiser \emph{et al}., Nature 446, 522 (2007). [2] A. Kubetzka \emph{et al}., Phys. Rev. Lett. 94, 087204 (2005). [3] R. Schmidt, C. Lazo, \emph{et al}., submitted (2007).