Skyrmions at room temperature in magnetic multilayers

Abstract

Magnetic skyrmions are arguably the smallest stable magnetic configuration in films, and therefore could be the ultimate magnetic storage bit [1,2] . They have also triggered a wide interest due to the new fundamental phenomena related to their topology . Numerical simulations have shown that the interfacial Dzyaloshinskii-Moriya interaction (DMI) can stabilize such skyrmions in nanoscale disks or tracks for a rather large range of DMI amplitudes for which the skyrmion can either be the ground state or metastable relative to the uniform state [4,5,6] . Here, we demonstrate experimentally the presence of skyrmions in metallic multilayers structures engineered to exhibit a strong DMI interaction . In this work, beyond the study of skyrmions in thin films [3], we focus our investigation on sputtered multilayers with vertical magnetization consisting of stacks of trilayers composed of 0 .6-nm-thick Co layers sandwiched between 5d transition metal layers, namely Pt, Ir and W . Asymmetric sandwiches were designed in order to introduce additive DMI from the top and bottom interfaces of Co [2,5,6] while also obtaining a considerable perpendicular magnetic anisotropy . We will present our results on two types of metallic multilayers grown at room temperature: |Pt10|-Co0 .6|Pt1|{Co0 .6|Pt1}x10|Pt3 and |Pt10|Co0 .6|Pt1|{Ir1|Co0 .6|Pt1}x10|Pt3 (thickness in nm) . The magnetic anisotropy is determined using standard magnetometry, while the DMI amplitude is estimated by two original methods . Based on detailed mapping of the magnetization obtained using scanning transmission X-ray microscopy (STXM) combined with the XMCD effect, STXM allows the magnetic imaging of patterned structures in a non-invasive way with nanoscale resolution (<50 nm) [7] . We acquired such images at different perpendicular magnetic fields in both symmetric Pt|Co|Pt and asymmetric Pt|Co|Ir multilayers [8] . From the analysis of the magnetic domain configurations, either at zero field after demagnetization (not shown) or following the evolution of the size of bubbles with perpendicular magnetic field (Figure 1), we evaluate consistently a DMI amplitude D as large as 2 mJ/ m 2 in Pt|Co|Ir . The process for estimating the skyrmion radius is displayed in Figure 2 . A direct consequence of having such a large DMI is that the bubble-like domains that we have identified are indeed isolated magnetic skyrmions . In micromagnetic simulations, trivial bubbles (winding number equal to zero) are not stable and vanish in a few nanoseconds or less, while skyrmions (winding number equal to one) are stable . The good agreement of the size dependence as well as the stability of the bubble domains is strong evidence that these domains are, in fact, skyrmions . The value of D is further confirmed by studying the typical width of the worm domains, which are observed at zero magnetic field, indicating consistency in the analysis .In conclusion, we demonstrate the presence, at room temperature, of skyrmions stabilized by interfacial DMI in metallic multilayers, opening the...

Degeneracies and fluctuations of Néel skyrmions in confined geometries

“…of Co and Pt with PMA [48]. The skyrmions observed in these measurements are rather large and stabilized as a result of both dipole-dipole and DM interactions, and are therefore in a somewhat different regime from the skyrmions that we have studied in this article.…”

Complex magnetic phase diagram and skyrmion lifetime in an ultrathin film from atomistic simulations

“…The identification of the A phase of the cubic B20 system MnSi as a physical realization of the skyrmion lattice [7] increased interest in finding further materials with similar spin configurations. In the last few years, skyrmion states were also observed experimentally in bulk systems and thin films of FeGe [8,9] and FeCoSi [10,11]; in Fe monolayer [12] and PdFe bilayer [13] on Ir(111); in bulk Cu 2 OSeO 3 [14] and GaV 4 S 8 [15]; and in Pt|Co|Ir multilayer [16]. Since skyrmions are stable spin configurations which can be moved around by spin-polarized current, these systems have promising applications for spintronics devices [17,18].…”

Shapiro steps for skyrmion motion on a washboard potential with longitudinal and transverse ac drives

“…In contrast, the skyrmions that were recently discovered in chiral magnets have particle-like properties and many similarities to superconducting vortices, but have the important distinction that there is a strong non-dissipative Magnus force in their motion [36][37][38][39][40][41][42][43][44][45] . The skyrmions can be set into motion by an applied current and are observed to have a very small depinning threshold [38][39][40][41]46,47 , in part because the effectiveness of the Magnus force can be up to ten times stronger than the dissipative force component.…”