  Resting on multi-scale modelling simulations, we explore dynamical aspects
characterizing skyrmions driven by spin-transfer-torque towards repulsive and
pinning 3d and 4d single atomic defects embedded in a Pd layer deposited on the
Fe/Ir(111) surface. The latter is known to host sub-10 nm skyrmions which are
of great interest in information technology. The Landau-Lifshitz-Gilbert
equation is parametrized with magnetic exchange interactions extracted from
first-principles. Depending on the nature of the defect and the magnitude of
the applied magnetic field, the skyrmion deforms by either shrinking or
increasing in size, experiencing thereby elliptical distortions. After applying
a magnetic field of 10 Tesla, ultrasmall skyrmions are driven along a straight
line towards the various defects which permits a simple analysis of the impact
of the impurities. Independently from the nature of the skyrmion-defect complex
interaction a gyrotropic motion is observed. A repulsive force leads to a
skyrmion trajectory similar to the one induced by an attractive one. We unveil
that the circular motion is clockwise around pinning impurities but counter
clockwise around the repulsive ones, which can be used to identify the
interaction nature of the defects by observing the skyrmions trajectories.
Moreover, and as expected, the skyrmion always escapes the repulsive defects in
contrast to the pinning defects, which require a minimal depinning current to
observe impurity avoidance. This unveils the richness of the motion regimes of
skyrmions. We discuss the results of the simulations in terms of the Thiele
equation, which provides a reasonable qualitative description of the observed
phenomena. Finally, we show an example of a double track made of pinning
impurities, where the engineering of their mutual distance allows to control
the skyrmion motion with enhanced velocity.
