First-Principles Simulation of Magnetic Exchange Force Microscopy on Fe/W(001)

Abstract

Based on density functional theory, we analyze the contrast mechanisms in magnetic exchange force microscopy (MExFM) and present first-principles calculations of the magnetic exchange force between an iron tip and an Fe monolayer on W(001), which is a model system of a two-dimensional antiferromagnet. Our study reveals that a single Fe atom is not an adequate tip model and we use Fe pyramids of five and 14 atoms. We find that the magnetic exchange forces on the tip atoms in the nearest layer from the apex atom are non-negligible and can be opposite to that on the apex atom. In addition, the apex atom interacts not only with the surface atoms underneath but also with the nearest-neighbors in the surface. We show that structural relaxations of tip and sample due to their interaction depend sensitively on the magnetic alignment of the two systems. As a result, the onset of significant magnetic exchange forces is shifted toward larger tip–sample separations which facilitates their detection in MExFM.