Observation of stable Néel skyrmions in cobalt/palladium multilayers with Lorentz transmission electron microscopy

Abstract

Néel skyrmions are of high interest due to their potential applications in a variety of spintronic devices, currently accessible in ultrathin heavy metal/ferromagnetic bilayers and multilayers with a strong Dzyaloshinskii–Moriya interaction. Here we report on the direct imaging of chiral spin structures including skyrmions in an exchange-coupled cobalt/palladium multilayer at room temperature with Lorentz transmission electron microscopy, a high-resolution technique previously suggested to exhibit no Néel skyrmion contrast. Phase retrieval methods allow us to map the internal spin structure of the skyrmion core, identifying a 25 nm central region of uniform magnetization followed by a larger region characterized by rotation from in- to out-of-plane. The formation and resolution of the internal spin structure of room temperature skyrmions without a stabilizing out-of-plane field in thick magnetic multilayers opens up a new set of tools and materials to study the physics and device applications associated with chiral ordering and skyrmions.

Interface Dzyaloshinskii-Moriya interaction in the interlayer antiferromagnetic-exchange coupled Pt/CoFeB/Ru/CoFeB systems

“…It is noticeable that ∆F for mode 2 (not observed) is slightly IEC dependent but its variation would not be experimentally detected, according to figure 4a insert. Moreover, the ∆F value for mode 1 is the half of that for a single layer (according to [13], [22] complemented with iDMI boundary conditions of ref [21], using the other parameters of figure 3. Inserts of (a) and (b) are the frequency differences corresponding to modes 1 and 2. .…”

“…Indeed, Shawn et al [21] have reported on the direct imaging of chiral spin structures including skyrmions in an exchange coupled thick ferromagnetic Co/Pt multilayer at room temperature with Lorentz transmission electron microscopy. Moreover, it is of utmost importance to investigate the spin waves spectrum in the presence of both DMI and IEC.…”

Spin-wave propagation in the presence of inhomogeneous Dzyaloshinskii-Moriya interactions

“…All these three prerequisites are simultaneously satisfied in material systems such as B20 structures [3][4][5] and ferromagnet/heavy metal bilayer structures [6][7][8][9][10][11][12]. The DMI affects the equilibrium spin texture and consequently magnetization dynamics by stabilizing chiral domain walls [13][14][15][16] or magnetic skyrmions [17][18][19][20][21][22][23]. The DMI also causes the nonreciprocal spin-wave propagation [24][25][26], which is widely used to estimate the strength of DMI [27][28][29][30][31].…”

Eigenmodes of Néel skyrmions in ultrathin magnetic films

“…8,9 Investigations into their static spin structures reveal that the bulk DMI stabilizes vortex-type (or Bloch-type) skyrmions in B20 crystallographic structures, 3,10 while the interfacial DMI leads to the formation of hedgehog-type (or Néel-type) skyrmions in ultrathin ferromagnetic films deposited on a strong spin-orbit metal. [5][6][7][10][11][12] Extensive studies have been done on the collective dynamics of skyrmions under external fields. Among them are investigations into the current-induced motion of skyrmions and the associated skyrmion Hall effect, 7,13,14 the collective rotation of the skyrmion lattice, 15 and the pinning of skyrmions.…”