Role of an additional interfacial spin-transfer torque for current-driven skyrmion dynamics in chiral magnetic layers

Abstract

Skyrmions can be driven by spin-orbit torques as a result of the spin Hall effect. Here we model an additional contribution in ultra-thin multilayers, arising from the spin accumulation at heavy metal / ferromagnetic interfaces and observe the effects on a large range of skyrmion diameters. The combination of the interfacial spin-transfer torque and the spin-orbit torque results in skyrmion motion which helps to explain the observation of small skyrmion Hall angles for skyrmion diameters less than 100 nm. We show that this additional term has a significant effect on the skyrmion dynamics and leads to rapidly decreasing skyrmion Hall angles for small skyrmion diameters, as well as a skyrmion Hall angle versus skyrmion velocity dependence nearly independent of the surface roughness characteristics. Also, the effect of various disordered energy landscapes, in the form of surface roughness, on the skyrmion Hall angle and velocity is shown to be largely drive-dependent. Our results show good agreement with those found in experiments thus concluding that the interfacial spin-transfer torque should be included in micromagnetics simulations for the reproduction of experimental results.

Boris computational spintronics—High performance multi-mesh magnetic and spin transport modeling software

“…17,70 Moreover, when a spin accumulation is generated at magnetization gradients, such as a skyrmion, the resulting imbalance in spin accumulation either side of the HM/FM interface generates vertical spin currents, which leads to an additional type of interfacial spin torque, termed interfacial STT (ISTT). 10,11 In many cases, it is sufficient to run simulations with direct expressions for spin torques (e.g., STT and SOT) augmenting the LLG equation; however, the drift-diffusion spin transport solver is still useful for calculating the strength of these spin torques in the first place from spin transport parameters. Here, we further verify the spin transport solver reproduces the expected spin torques in a spin valve structure shown in Fig.…”

Collective skyrmion motion under the influence of an additional interfacial spin-transfer torque

“…Modelling results shown in Fig. 3 values is larger for skyrmion collections, compared to isolation skyrmion motion [25], since skyrmions interact not only with the local landscape disorder, but also with neighbouring skyrmions. As noted above, the spread in SkHA is reduced for the SOT-only model, since skyrmion velocities are significantly larger at the same current density (skyrmion velocities and comparison with experimental results are discussed later in relation to Fig.…”

“…For the work presented here we have |P|  1 and ||  1, obtained by fitting the ISTT expression in Equation (3) to the self-consistent spin torque, Equation (1), computed using the drift-diffusion modelfor details see Ref. [25] and the Supplementary Information.…”

“…Moreover, the interaction of a skyrmion collection with the boundaries of a multilayered track was recently shown to result in a reshaped SkHE [31]. Driving individual skyrmions through a disordered landscape has been extensively studied under the effects of SOT from a HM layer [5,32], spin transfer torque (STT) in the FM layer [33], and more recently ISTT [24,25].…”

Electric-field driven stability control of skyrmions in an ultrathin transition-metal film

“…Magnetic skyrmions -topologically nontrivial swirling spin structures 1,2 -show great promises as information carriers in future magnetic memory and logic devices due to their nanoscale size and ultralow current-driven ma-nipulation, achieved by spin transfer torque (STT) [2][3][4][5][6][7] and spin orbit torque. [8][9][10][11][12][13][14] The main drawback of these techniques to realize a low-energy-dissipation device is Joule heating, which destabilizes the skyrmionic bits. The electric field induced manipulation offers an efficient route for creating, deleting and controlling skyrmions avoiding the heating problem.…”