Additive interfacial chiral interaction in multilayers for stabilization of small individual skyrmions at room temperature

Abstract

Facing the ever-growing demand for data storage will most probably require a new paradigm. Magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin films in nature. We designed cobalt-based multilayered thin films where the cobalt layer is sandwiched between two heavy metals providing additive interfacial Dzyaloshinskii-Moriya interactions, which reach about 2 mJ/m 2 in the case of the Ir|Co|Pt multilayers. Using a magnetization-sensitive scanning x-ray transmission microscopy technique, we imaged magnetic bubble-like domains in these multilayers. The study of their behavior in magnetic field allows us to conclude that they are actually magnetic skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. This discovery of stable skyrmions at room temperature in a technologically relevant material opens the way for device applications in a near future.A major societal challenge is related to the continually increasing quantity of information to process and store. The hard disk drives, in which information is encoded magnetically, allow nowadays the storage of zettabytes (10 21 ) of information, but this technology should soon reach its limits. An up-and-coming avenue has been opened by the discovery of magnetic skyrmions [1], i.e. spin windings that can be localized within a diameter of a few nanometers and can move like particles [2]. These magnetic solitons, remarkably robust against defects due to the topology of their magnetic texture [3], are promising for being the ultimate magnetic bits to carry and store information. The topology of the skyrmions also appears to further underlie other important features such as their current-induced motion induced by small dc currents that is crucial for real applications but also the existence of a specific component in Hall Effect [4][5][6] that can be used advantageously for an electrical read-out of the information carried by nano-scale skyrmions. We proposed recently that these skyrmions could be used in future storage devices and information processing [2].The existence of skyrmion spin configuration has been predicted theoretically about thirty years ago [1] but it was only recently that skyrmion lattices have been observed in crystals with noncentrosymmetric lattices, e.g. B20 crystallographic structure in MnSi [7][8][9] FeCoSi [10] or FeGe [5] crystals. In 2011, skyrmions have also been identified in single ultrathin ferromagnetic films with out-of-plane magnetization (Fe and FePd) deposited on a heavy metal substrate such as Ir(1 1 1) [11,12]. Thin magnetic films appear to be more compatible with technological developments, though the observation of skyrmions in thin films has been limited up to now to low temperature and also needs, in some cases, the presence of a large applied magnetic field [12]. The study of these new magnetic phases associated with chiral interactions has generated a

Geometric Hall effects in topological insulator heterostructures

“…By applying a field-effect transistor structure (FET, see Methods), the Fermi energy (E F ) of the TI heterostructures can be precisely controlled in a large energy range over the bulk bandgap. In particular, the quantum AHE realized in the magnetic TIs offers an interesting scenario where the two different physical mechanisms merge-namely, skyrmion formation around the doped carrier into the quantum Hall ferromagnet 23 and skyrmion formation at the interface of magnets [24][25][26][27] . First, we investigate the gate voltage (V G ) and temperature (T ) dependence of the Hall effect for a 2-nm CBST/5-nm BST heterostructure as shown in Fig.…”

“…The surface state of the TI exhibits inversion symmetry breaking and a fairly strong spin-orbit interaction, as exemplified by the spin-momentum-locking in the Dirac dispersion. Thus, the electrons at the surface state mediate the Dzyaloshinskii-Moriya (DM) interaction where the DM vector is pointing in the in-plane direction [24][25][26][27]31 . In the heterostructure, therefore, such an in-plane DM vector originating from the top CBST surface favours the formation of a Néel-type skyrmion [24][25][26][27] .…”

“…Thus, the electrons at the surface state mediate the Dzyaloshinskii-Moriya (DM) interaction where the DM vector is pointing in the in-plane direction [24][25][26][27]31 . In the heterostructure, therefore, such an in-plane DM vector originating from the top CBST surface favours the formation of a Néel-type skyrmion [24][25][26][27] . However, in the single-layer CBST, DM vectors exist both at the top and bottom surface, each pointing along opposite directions, so that skyrmion formation is disfavoured owing to the frustration.…”

Emergent geometric frustration of artificial magnetic skyrmion crystals

“…The presence and nucleation of individual/isolated skyrmions in confined magnetic nanostructures as well as the evolution of the skyrmion size has also been demonstrated in circular dots [13,[15][16][17][18]74] and nanowires [15,16].…”

“…Pt/CoB/Pt [13], Pt/Co/Ta [15], Pt/CoFeB/MgO [15], Ir/Co/Pt [16], Pt/Co/MgO [17], and Ir/Fe/Co/Pt [18]. Furthermore, the presence and nucleation of individual/isolated skyrmion in confined magnetic nanostructures as well as the evolution of the skyrmion size has also been demonstrated in circular dots [13,[15][16][17][18] and nanowires [15,16].…”