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Here we study the effect of an additional interfacial spin-transfer torque,as well as the well established spin-orbit torque and bulk spin-transfertorque, on skyrmion collections - group of skyrmions dense enough that they arenot isolated from on another - in ultrathin heavy metal / ferromagneticmultilayers, by comparing modelling with experimental results. Using a skyrmioncollection with a range of skyrmion diameters and landscape disorder, we studythe dependence of the skyrmion Hall angle on diameter and velocity, as well asthe velocity as a function of diameter. We show the experimental results are ingood agreement with modelling when including the interfacial spin-transfertorque, and cannot be reproduced by using the spin-orbit torque alone. We alsoshow that for skyrmion collections the velocity is approximately independent ofdiameter, in marked contrast to the motion of isolated skyrmions, as the groupof skyrmions move together at an average group velocity. Moreover, thecalculated skyrmion velocities are comparable to those obtained in experimentswhen the interfacial spin-transfer torque in included, whilst modelling usingthe spin-orbit torque alone shows large discrepancies with the experimentaldata. Our results thus show the significance of the interfacial spin-transfertorque in ultrathin magnetic multilayers, which is of similar strength to thespin-orbit torque, and both significantly larger than the bulk spin-transfertorque. Due to the good agreement with experiments, we conclude that theinterfacial spin-transfer torque should be included in numerical modelling forcorrect reproduction of experimental results.

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