Pinning and movement of individual nanoscale magnetic skyrmions via defects

Abstract

An understanding of the pinning of magnetic skyrmions to defects is crucial for the development of future spintronic applications. While pinning is desirable for a precise positioning of magnetic skyrmions it is detrimental when they are to be moved through a material. We use scanning tunneling microscopy (STM) to study the interaction between atomic scale defects and magnetic skyrmions that are only a few nanometers in diameter. The studied pinning centers range from single atom inlayer defects and adatoms to clusters adsorbed on the surface of our model system. We find very different pinning strengths and identify preferred positions of the skyrmion. The interaction between a cluster and a skyrmion can be sufficiently strong for the skyrmion to follow when the cluster is moved across the surface by lateral manipulation with the STM tip.Magnetic skyrmions are particle-like knots in the spin texture which are considered as promising candidates for future spintronic applications [1-3]. They typically arise in materials which have a spin spiral ground state due to the Dzyaloshinskii-Moriya (DM) interaction [4, 5] at zero magnetic field; upon application of an external magnetic field a hexagonal skyrmion lattice phase can occur before the magnetization is saturated. From an analysis of the energies it is known that near the boundary between the lattice phase and the ferromagnetic state also isolated skyrmions can be stabilized, and it has been demonstrated experimentally that also reduced temperature facilitates the preparation of individual skyrmions [6]. Skyrmion lattices have been moved through a material by laterally imprinted currents due to spin transfer torques [7][8][9]. While below a critical current density the periodic magnetic texture is pinned, for larger lateral currents pinning forces were found to be negligible and the skyrmion velocity was proportional to the current [8]. Simulations for skyrmion lattices support this finding and suggest that randomly distributed impurities do not have a significant impact on the velocity under applied currents [10]; however, an influence of the density of magnetic skyrmions on their lateral movement has been proposed [11]. For individual magnetic skyrmions several different pinning mechanisms have been considered theoretically, ranging from a combined increase of magnetic exchange and DM interaction [12], over vacancies in the magnetic material [13], to repulsive interaction due to areas with higher magnetic anisotropy [14]. The conclusion from these studies is that depending on the parameters both the movement around a defect and the capturing of a skyrmion at a pinning site is possible.Here we study pinning in the model system of the PdFe atomic bilayer on Ir(111) [6,15,16]. Using scanning tunneling microscopy (STM) we investigate the interaction of non-collinear magnetic states with different atomic-scale defects. We find that single magnetic skyrmions can be significantly distorted due to their interaction with clusters adsorbed on the surface. Furthermore the ...

Energy surface and lifetime of magnetic skyrmions

“…After overcoming a small energy barrier, a lower energy minimum is obtained corresponding to a stable state where the defect is near the border of the skyrmion, where the magnetic moments have a large in-plane component. This is in agreement with experimental observations [15]. The saddle point on the energy surface between these two states determines the activation energy for pinning and dissociation of the skyrmion from the defect.…”

“…Experiments have indicated that atomic defects can act as nucleation and pinning sites for skyrmions [13]. There is a repulsive interaction with non-magnetic defects but if this energy is overcome the skyrmion tends to be pinned by such defects [15,16]. It is important to characterize the energy surface for a skyrmion near an impurity, and assess the activation energy for the attachement and dissociation as well as the effect of the impurity on the lifetime of the skyrmion.…”

Manipulating and trapping skyrmions by magnetic field gradients

“…Finally, we would like to point out that the trapping of magnetic skyrmions has attracted much attention recently. Different techniques have been proposed, for example, using a hole (defect effect) [42,46,47] or electric field gradient [30] to guide the skyrmion motion. Here we would like to point out that a radially gradient magnetic field may be generated by different experimental techniques, such as a magnetic tip or a magnetic dot deposited on top of a thin film.…”

Magnetic skyrmions on a two-lane racetrack

“…Such obstacles can, however, hinder the motion of the skyrmions towards read or write elements. Due to the complex pulsed motion of the skyrmions, disorder effects may also become more important [32,33].…”