Unveiling the Emergent Traits of Chiral Spin Textures in Magnetic Multilayers

Abstract

Magnetic skyrmions are topologically wound nanoscale textures of spins whose ambient stability and electrical manipulation in multilayer films have led to an explosion of research activities. While past efforts focused predominantly on isolated skyrmions, recently ensembles of chiral spin textures, consisting of skyrmions and magnetic stripes, are shown to possess rich interactions with potential for device applications. However, several fundamental aspects of chiral spin texture phenomenology remain to be elucidated, including their domain wall (DW) structure, thermodynamic stability, and morphological transitions. Here the evolution of these textural characteristics are unveiled on a tunable multilayer platform-wherein chiral interactions governing spin texture energetics can be widely varied-using a combination of full-field electron and soft X-ray microscopies with numerical simulations. With increasing chiral interactions, the emergence of Néel helicity, followed by a marked reduction in domain compressibility, and finally a transformation in the skyrmion formation mechanism are demonstrated. Together with an analytical model, these experiments establish a comprehensive microscopic framework for investigating and tailoring chiral spin texture character in multilayer films.

Thermal evolution of skyrmion formation mechanism in chiral multilayer films

“…Subsequently, non-magnetic contributions were removed by subtracting a background image acquired above saturation (see SM3). Expectedly, we observe a labyrinthine configuration at ZF, which transforms, with increasing field, into stripes, and then into 50-80 nm sized Néel skyrmions (see SM3) [30]. First, we examine the microscopic field evolution of an individual Fe(3)/Co( 7) stripe (Fig.…”

“…In the absence of fission, e.g. for κ < 1 multilayers, stripes would instead shrink into individual skyrmions [30].…”

“…Temperature variations are known to affect textural transitions in chiral multilayers via (a) thermal fluctuations [23], and/or (b) magnetic parameter evolution [21]. To delineate these effects, we perform similar experiments over 100 -350 K on two more samples -Fe(2)/Co (8) and Fe(4)/Co(6) -providing a wide range of magnetic parameters [29,30]. Consistently across samples, the saturation magnetization M s follows the T 3/2 Bloch's law (Fig.…”

“…The effective anisotropy, K eff ≡ K u − µ 0 M 2 s /2 includes uniaxial (K u ) and shape anisotropy (−µ 0 M 2 s /2) contributions, where M s is the saturation magnetization. Recently, it has been shown that for chiral multilayers with κ > 1, skyrmions may be formed from the break-up, or "fission" of stripes [29,30]. Temperature may influence the transition energetics and kinetics, and therefore merits systematic investigation.…”

“…The four samples studied here -sputter deposited on Si/SiO 2 substrates (see SM1) -have 1 nm thick FM layers, and are referred to by their Fe(x)/Co(y) composition. They were shown to host sub-100 nm Néel-textured skyrmions whose RT structure and stability have been studied extensively [29,30]. Magnetization data were acquired using an EZ11 vibrating sample magnetometer (VSM) from Mi-croSense™ in out-of-plane (OP) geometry.…”