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A new subsurface growth mode in the Co-Cu system is reported. This mode provides a direct subsurface growth of Co nanoclusters by depositing Co atoms on the Cu(001) surface in a single stage. The resulting subsurface Co nanoclusters are located 2 monolayers (ML) deep below the atomically flat surface of Cu(001). Although these hidden nanoclusters cannot be directly accessed by a scanning tunneling microscopy/spectroscopy (STM/STS) probe, their shape could be deduced using STM/STS via a careful analysis of the local deformation of the Cu(001) surface as well as local variations of surface electron density induced by the subsurface clusters. A strongly asymmetric shape of the nanoclusters is deduced: they are typically 5-10 nm in lateral size but only 2 to 3 ML in thickness. The thickness of the nanoclusters does not evolve significantly under a heat treatment. A simple model is implemented to describe the growth kinetics. The results in this study reveal that intense processes of diffusion, nucleation, and growth take place in a region 1 nm deep, thus defining the near-surface region.

The role of chirality is becoming more important for new applications in spintronics, especially in ultrathin magnetic films. [1][2][3][4][5][6] In magnetic racetrack applications for example, the chirality directly determines how magnetic domain walls and skyrmions interact with the spin-orbit torques. 2,3,7-10 It is therefore important to investigate the key contributing factors to this chirality. The underlying interaction that is believed to stabilize the chirality is the Dzyaloshinskii-Moriya interaction (DMI). As shown by a wealth of theoretical and experimental reports this interaction requires the breaking of inversion symmetry and originates from the interface between a heavy metal and a ferromagnet for the thin film systems investigated in this paper. 4,11 The DMI also helps to stabilize skyrmions because it favours non-collinear spin configurations, 12 which are envisaged to be used in areas ranging from magnetic racetrack memory and logic applications, to radio frequency devices and neuromorphic computing. 5,6Very recently, however, it was realized that DMI is not the only interaction that can stabilize a specific chirality. [8][9][10][13][14][15] Actually, already 40 years ago it was shown that the presence of dipolar fields leads to the formation of chiral Néel caps. 16,17 Here, the stray fields originating from magnetic domains align the spins inside the domain walls at the top of the film to form clockwise (CW) Néel walls and and at the bottom of the film to form counterclockwise (CCW) Néel walls, providing an optimized flux closure state. Dipolar interactions can often be ignored for the thin-film systems used for domain-wall studies.Because of the increase in magnetic volume and reduced coupling across the non-magnetic spacer layers this is no longer the case for the multilayer repeat systems often used to stabilize room-temperature magnetic skyrmions. [15][16][17][18][19] Both theoretical 8-10 and experimental work 8,13,14 suggests that in these multilayer repeat systems the DMI is in direct competition with the dipolar fields. Without DMI, the dipolar interactions introduce Néel caps. Including DMI, however, leads to a larger fraction of the layers being occupied by the Néel cap of the chirality favoured by the DMI. The other cap will be reduced in size and occupy fewer layers. This happens until the DMI is so large that it is no longer energetically favourable to accommodate a Néel cap not favoured by the DMI.The energetics and dynamics of both skyrmions and domain walls are affected by this competition because it determines the net chirality of the magnetic textures, which in turn influences the interaction with, for example, spin-orbit torques. 8-10 A fundamental understanding of this competition is therefore needed to properly tailor the interactions such that