Skyrmions at the Edge: Confinement Effects in Fe/Ir(111)

Abstract

We have employed spin-polarized scanning tunneling microscopy and Monte-Carlo simulations to investigate the effect of lateral confinement onto the nanoskyrmion lattice in Fe/Ir(111). We find a strong coupling of one diagonal of the square magnetic unit cell to the close-packed edges of Fe nanostructures. In triangular islands this coupling in combination with the mismatching symmetries of the islands and of the square nanoskyrmion lattice leads to frustration and triple-domain states. In direct vicinity to ferromagnetic NiFe islands, the surrounding skyrmion lattice forms additional domains. In this case a side of the square magnetic unit cell prefers a parallel orientation to the ferromagnetic edge. These experimental findings can be reproduced and explained by Monte-Carlo simulations. Here, the single-domain state of a triangular island is lower in energy, but nevertheless multi-domain states occur due to the combined effect of entropy and an intrinsic domain wall pinning arising from the skyrmionic character of the spin texture.Magnetic skyrmions are particle-like states [1] which can occur in magnetic systems with broken inversion symmetry due to the Dzyaloshinskii-Moriya interaction [2,3]. Skyrmions can either form lattices at certain field ranges [4-10] or they can be created and manipulated individually [9], thereby offering great potential for data storage, transfer and processing [11,12]. Since the geometric layout is an essential part of a device, theoretical investigations explored the effect of boundaries and confinement in skyrmionic systems [13,14]. For instance, skyrmion movement in racetracks [12] and the effect of notches [15] on their trajectories were studied and lateral confinement was suggested as a means to stabilize skyrmions without an external magnetic field [16]. However, the relative orientation of skyrmion lattices in regard to the geometry of the boundary of magnetic systems is still an open question. Most experiments focused on extended films and on their temperature and fielddependence while finite size effects were explored only very recently, e.g. in nanostripes [17] or in disk-shaped structures [18,19].Here, we employ spin-polarized scanning tunneling microscopy (SP-STM) and Monte-Carlo (MC) simulations to investigate the effect of two types of boundaries onto the nanoskyrmion lattice in the Fe atomic monolayer on Ir(111). This skyrmion lattice has four-fold symmetry, a period of about 1 nm, and exists already in zero field due to the four-spin interaction [6]. At T = 4.2 K it is insensitive to fields of up to 9 Tesla [20], which shows that it has a vanishing net moment. In extended films, three rotational domains can be found [6], where a diagonal of the magnetic unit cell aligns with one of the three symmetry equivalent close-packed atomic rows of the hexagonal Fe layer. Our new data show that at a close-packed edge of an Fe nanostructure, one diagonal of the magnetic unit cell is preferentially oriented parallel to the edge, i.e. the edge selects one of the three rotational ...

Dynamical Defects in Rotating Magnetic Skyrmion Lattices

“…The coordination number of the defects depends on the geometry of the magnetic lattice as can be seen from a comparison to recent investigations by Hagemeister et al concerning defects in a skyrmion square lattice [38]. A single 5-7 defect is shown in Figure 1a • .…”

Magnetic field gradient driven dynamics of isolated skyrmions and antiskyrmions in frustrated magnets

“…Moreover, the change of J 1 /J 2 ratio and other parameters will slightly change the value of the magnetic charge q, and in turn changes the velocity of the skyrmion. However, the confinement effects are less dependent of materials and external stimuli, and the acceleration is probably available in other confined materials such as chiral magnets, which deserves to be checked in future experiments [46].…”

Controlled Creation of Quantum Skyrmions

“…Being characterized by non-trivial winding characteristics of the magnetization [2], magnetic skyrmions show a remarkable stability against local perturbations [6]. Skyrmion lattices have been experimentally realized in bulk magnetic systems such as MnSi [7], Fe 0.5 Co 0.5 Si [8] and FeGe [9] and at interfaces of, e.g., Fe/Ir(111) [10][11][12][13]. Furthermore, single magnetic skyrmions can be created as metastable excitations via the injection of currents [14][15][16], magnetic field pulses [17] and by tailored boundary conditions [18,19].…”